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J {'7}: ”9...: 31'";- ”23‘ 'n.‘ I", o‘r'v J1.- ""4115 ,.::1'.g...,3' VA "'2': '.':...--’::~ l". «row :5 ”3957:? l MICHIGAN 3| %W75%%H7 HWWWWWWWMWMW LIBRARY Michigan State University This is to certify that the thesis entitled MANAGEMENT ALTERNATIVES 0F WEEDS, SPECIFICALLY COMMON LAMBSQUARTERS (Chenopodium— album L. ), IN SOYBEANS figmby— max (L) Merr. ) TERESA M . CROOK has been accepted towards fulfillment i of the requirements for v . . Master 5 degree 1n Solence "1%Mj @4411/ Major professor Date IDA/Mg! fig 0-7639 MSU is an Affirmnfiw ‘ ' " ’ “,4, ... J, Institution '1 PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE m' 579% . PfEXLALm math ‘ ‘ u Hmu MSU Is An Attirmative Action/Equal Opportunity Institution MANAGEMENT ALTERNATIVES OF WEEDS, SPECIFICALLY COMMON LAMBSQUARTERS (Chenogodium album L.), IN SOYBEANS (Glycine mg! (L) Merr.) BY Teresa Marie Crook A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Crop and Soil Sciences 1989 (200 I750 ABSTRACT MANAGEMENT ALTERNATIVES OF WEEDS, SPECIFICALLY COMMON LAMBSQUARTERS (Chenopodium album L.), IN SOYBEANS (Glycine mg; (L) Merr.) Bv Teresa Marie Crook Weed control impacts soybean production management. Tillage, row spacing, herbicides, and cultivation impact weeds such as: common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium 91939 L.), redroot pigweed (Amaranthus retroflexus L.), velvetleaf (Abutilon theophrasti Medic.), yellow nutsedge (Cygerus esculentus L.), and yellow foxtail (Setaria lutenscens Hubb.). Tillage influences weed spectrum. Soybeans planted in narrow rows (increased plant populations) have higher yields in both conventional and no—tillage systems when weeds are controlled. If risk of herbicide failure is high, the option of cultivation in wide row spacing may be important for effective weed control. Soybean yield was not reduced when lambsquarters were removed by hand within 7 to 8 weeks following soybean emergence. Lambsquarters were not controlled with a postemergence application of bentozon plus acfluorfen when lambsquarters surpassed 15 cm height resulting in reduced yield and 480 million seed/A produced by remaining lambsquarters' plants. DEDICATION TO MY MOTHER AND FATHER MARCELLA CATHERINE (WENNMACHER) CROOK AND ADRIAN CHARLES CROOK For giving me the freedom to pursue my own goals and dreams by lending their own quiet support and love to all of my many endeavors ACKNOWLEDGEMENTS The accomplishment of this degree has been very rewarding. Much knowledge has been acquired from the experiences in both the classroom and field with the help of many people who need to be recognized for their support, assistance, and understanding. First and foremost is Dr. Karen A. Renner, my major professor, for giving me the opportunity to come to Michigan State University and do research in the subject matter of my interest. In addition to giving me guidance and freedom, she has retained my deepest respect in her ability to balance her professional and private life. I also want to express my gratitude to my two other committe members, Dr. Maury Vitosh, for his statistical expertise and knowledge of soybean production and Dr. Gerry Schwab, for his guidance on the economic analysis and interpretation in the tillage study (maybe I can get him to plant narrow row soybeans at home). I want to thank the Michigan Soybean Association for their financial support of both the common lambsquarters and tillage studies. A very special thanks is given to Mark J. VanGessel who helped for many long hours in the field for two years, who will always have a unique (and loud) laugh, and who remains a special friend. And one person I could not forget is "Dr. Gary Powell", our technician, whose expertise in the field is many times not recognized or acknowledged, but the project would just not be the same without him. iv TABLE OF CONTENTS CHAPTER 1 — INTRODUCTION ................................. 1 LITERATURE REVIEW ...................................... 2 COMMON LAMBSQUARTERS (Chenopodium album L.) ......... 2 I. Characteristics ........................... 2 II. Seed Production ........................... 6 III. Seed Germination .......................... 7 IV. Germination Physiology .................... 8 PLANT COMPETITION .................................. 11 I. General Principles ....................... 11 II. Common Lambsquarters Competition ......... 12 III. Soybean Competition ...................... 16 TILLAGE ............................................ 21 HERBICIDES ......................................... 25 BENTAZON ........................................... 26 I. Mechanism of Action ...................... 27 ACIFLUORFEN ........................................ 28 I. Mechanism of Action ...................... 30 BENTAZON and ACIFLUORFEN ........................... 30 ECONOMICS .......................................... 31 LITERATURE CITED ................................... 35 CHAPTER 2 - ABSTRACT .................................... #5 WEED MANAGEMENT ALTERNATIVES IN CONVENTIONAL AND NO—TILLAGE DRILL AND ROW SOYBEANS ..................... 46 INTRODUCTION ....................................... #6 MATERIALS AND METHODS .............................. 48 RESULTS AND DISCUSSION ............................. 58 Conventional tillage system weed control and soybean yields ................................. 58 No-tillage system weed control and soybean yield .......................................... 71 LITERATURE CITED ................................... 83 CHAPTER 3 - ABSTRACT .................................... 86 COMMON LAMBSQUARTERS COMPETITION AND TIME OF REMOVAL IN SOYBEANS ........................................... 88 INTRODUCTION ....................................... 88 MATERIALS AND METHODS .............................. 9# Influence of common lambsquarters on soybeans ....................................... 9h Seed production and germination ................ 98 RESULTS AND DISCUSSION ............................. 99 Handweed Study ................................. 99 Postemergence Study ........................... 110 Seed Production and Germination ............... 124 LITERATURE CITED ................................... 130 vi Table LIST OF TABLES Page CHAPTER 2 Soil information, herbicide applications, and cultivation dates as described by tillage system ........................ 49 Planted and final populations in each year .................. 51 Monthly rainfall totals for the 3 years of research ......... 54 Total variable costs as described for both tillage systems. 55 Herbicide treatments, application rates and methods, and herbicide costs for both tillage systems .................... 56 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the conventional tillage system in 1986 .......... 59 Soybean yield as affected by weeds, cultivation, and row spacing in the conventional tillage system .................. 62 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the conventional tillage system in 1987 .......... 65 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the conventional tillage system in 1988 .......... 68 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the no-tillage system in 1986 .................... 72 Soybean yield as affected by weeds, cultivation, and row spacing in the no-tillage system ............................ 73 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the no—tillage system in 1987 .................... 76 Weed control, soybean yield, and breakeven price as affected by herbicide treatments, cultivation, and row spacing in the no—tillage system in 1988 .................... 79 CHAPTER 3 Monthly rainfall and mean temperature in 1987 and 1988 ...... 97 Total biomass production, percent soybean and percent common lambsquarters composition as influenced by density for the handweed study in 1987 and 1988 .................... 100 Seed production and germination of common lambsquarters as influenced by density in the handweed study in 1987 and 1988. Plants were harvested in August only from the all season competition plots only .............................. 111 Common lambsquarters control as influenced by postemergence herbicide application in 1987 and 1988 ..................... 116 The number of common lambsquarters remaining/1O m of soybean row after postemergence herbicide application in 1987 and 1988 .............................................. 123 Common lambsquarters' seed production per plant as influenced by the time of postemergence herbicide application and density in 1987 and 1988 ................... 125 Common lambsquarters’ seed production per hectare as influenced by time of postemergence herbicide application and density in 1987 and 1988 ............................... 126 Common lambsquarters' seed germination as influenced by time of postemergence herbicide application and density in 1987 and 1988 ........................................... 127 Common lambsquarters' dry weight per plant as influenced by the time of postemergence herbicide application and density in 1988 ............................................ 129 viii LIST OF FIGURES Figure Page CHAPTER 3 1 Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the handweed study in 1987. (weed—free yield 2854 kg/ha) ...... 103 2 Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the handweed study in 1987. (weed—free yield 2854 kg/ha) ...... 105 3 Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the handweed study in 1988. (weed-free yield 1659 kg/ha) ...... 107 4 Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the handweed study in 1988. (weed—free yield 1659 kg/ha) ...... 109 5 Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the postemergence study in 1987. (weed—free yield 3067 kg/ha) ................................................ 113 6 Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the postemergence study in 1987. (weed-free yield 3067 kg/ha) ................................................ 115 7 Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the postemergence study in 1988. (weed—free yield 1752 kg/ha) ................................................ 119 8 Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the postemergence study in 1988. (weed-free yield 1752 kg/ha) ................................................ 121 ix CHAPTER 1 INTRODUCTION Soybeans rank third in production value to Michigan’s agriculture. Three trends are impacting this state’s soybean production: minimum tillage, decreased row spacing, and an increasing numbers of herbicide options. Minimum tillage decreases soil erosion and input costs. Decreased row spacing increases the producers ability to increase yield. The increasing number of herbicides available provides increased weed control options. The main goal of the producer is profitability or economic return. Weeds are a major pest in soybean production and can reduce soybean yield and quality. Common lambsquarters (Chenopodium glbgm L.) is present throughout Midwest agriculture, including Michigan. If soil- applied herbicides fail to control common lambsquarters, bentozon and acifluorfen provide some suppression in soybeans. As stated on their labels, these postemergence broadleaf herbicides must be applied before the weed has surpassed five cm in height. If the weed exceeds this height, these uncontrolled weeds can produce seeds which are added to the 3011’s seed reserve and may become potential weed problems in the future. LITERATURE REVIEW COMMON LAMBSQUARTERS (Chenogodium album L.) I. Characteristics Common lambsquarters is one of the most widely distributed weed species in the world. Common lambsquarters grows from 70°N to 50°S latitude, except for extreme desert conditions (65). It is found in forty—seven countries and forty crops, but is native to Europe (47). Chenopodium glbgm is a hexaploid (2n=54), with 34 subspecies which are minor variants from g. glbum (6). Common lambsquarters is a good colonizer occurring in habitats that have become open by disturbance (132). It is rarely found as a single plant (132). The species has no special seed dispersal mechanism, and therefore, most seeds are deposited near the mother plant, resulting in patchy growth (47). The plant is an erect pale green summer annual having alternate leaves with a white mealy appearance to the flowers and leaves, especially in young plants (132). The canopy shape is conical with regular distribution of branches along the main axis (60). Pronounced anthocyanin pigment in the stem sometimes provides a red color (132). The inflorescence is a spiked panicle containing perfect flowers with superior seeds (47), with one seed produced per flower (60). Flowers are self-pollinated or cross- pollinated by the wind (6, 132). 9. glbgm has epigeal germination from the optimum soil depth of 2 to 2.5 cm (132). At the four—leaf stage, the fleshy cotyledons shrivel and the primary root is formed and then replaced by shallow adventitious roots (132). Common lambsquarters is extremely tolerant to wide soil pH variations and grows well in most soils except very acidic ones (132). Common lambsquarters requires the presence of iron and boron to grow. It accumulates potassium, very high levels of ascorbic acid (132), and has a luxury consumption of nitrate (48). Very low levels of potassium depress dry matter production (132). Common lambsquarters cannot withstand cutting, trampling, or heavy frost (132). Q. glbgm requires no mycorrhizae (6). Q. glbum is unique in that it accumulates triterpenes (pentacyclic oleanolic acid) only in the floral parts of mature plants (71L Common lambsquarters requires 658 grams of water to produce 1 gram of dry matter (9). Common lambsquarters’ dry matter yield/m2 will increase to a density of 576 plants/m2 at which point yield will plateau because of increasing intraspecific competition. Common lambsquarters plants respond to density by changing the number of plant parts, not the size of the parts (133). Therefore, there is an inverse relationship between the number of plant parts and density. As the density increases, the stem diameter, number of branches, size and number of leaves, leaf area per plant, and weight per plant decrease (36, 60). Also, an increase in the number of leaf deaths may affect fruit set more than flowering (60). At high densities, common lambsquarters can produce seeds at 50 mm in plant height (41). Common lambsquarters can allocate 68% of its dry weight into reproduction (60). Competition from other plants, either inter or intraspecific may also delay flowering of the common lambsquarters plant (83), but flowering is continuous once initiated (60). The amount and duration of shade influences g. glbgm growth. Branch and tiller number decrease under shaded conditions (72). Main stem length is inhibited under 84% shade (72). Shade has to be greater than 90% to diminish common lambsquarters overall growth (72). Shade also delays heading and flowering of the mother plant, and seed ripening (72). Plants have different metabolic pathways for fixing carbon dioxide (C02) into carbon containing compounds (2). The first compound when 002 is fixed can have a three or four carbon structure, each involving different pathways of carbon assimilation. Therefore, plants are designated as C3 or C4 plants, dependent on their specific metabolism (41). The type of metabolism may influence the optimum environment for plant performance, because the Cu pathway provides for more efficient water usage (41). Common lambsquarters is a C3 plant, having optimal growth under lower temperatures and higher relative humidity conditions (15). Germination, growth, 602 exchange (19) and light utilization are more efficient (118) early in the season when cooler air and soil temperatures prevail. These lower temperatures may give common lambsquarters a competitive advantage compared to a C# plant which germinates later in the season when the soil and air temperatures are warmer. Environmental factors may have an impact on the initial establishment of C. 91939. Early germination may be an important determinant in competitive interactions, primarily through events prior to the actual initiation of competition under field conditions (81). The most competitive weeds appear to be the earliest to emerge. Emergence timing of a seedling population is more important than the spatial arrangement of the seedlings (63, 100). The growth rate of individual plants may be directly related to the time at which the individual plants emerge, rather than the absolute time of each plant's emergence (63). Emergence time has a greater effect on biomass than on plant survival; individuals emerging first achieve a greater biomass (63). Emergence time is important in determining the survival and growth of individual plants. Plants which emerge near the optimum emergence time have a disproportionate advantage over the other individuals and contribute large numbers for offspring to the next generation (63). The individual plants potential for capturing resources is dictated by the number and proximity of neighbors already capturing resources (100). Thus, factors influencing future growth rate of individual plants is dependent on the density of seedlings already emerged (100). By the time of plant maturity, the zone of influence of individual plants is extensive and embraces many neighbors. Therefore, the underlying relationships between individual plants is completely masked (100). Competition encountered by one individual plant is dependent on the density, distribution, duration, and species of competitor plants (10). Plants can compete for a supply of nutrients, light, or water simultaneously or in rapid succession. Therefore, plant growth integrates the situation of justifying the use of plant weight as an index of competition (10). Plant size, weight and height, suggest the potential for the capture of light (101), although climatic and edaphic conditions have a modifying effect on these results. II. Seed Production Chenopodium glbum has “somatic polymorphism", a condition where a plant produces more than one seed type in terms of morphology and/or behavior (41, 48, 131, 134). The lambsquarters seed’s testa or seed coat may be smooth or reticulate with raised lines and may be either black or brown in coloring. Therefore, the seeds are broken down into four categories: brown-smooth, brown-reticulate, black—smooth, and black— reticulate. The brown seeds germinate immediately, while the black seeds tend to be dormant (131, 134). The daylength at the time of seed production is believed to influence the common lambsquarters seed coat development, and consequently, the germination of these common lambsquarters seeds in the field (37). Fourteen hours of daylight is required by the mother plant before the induction of flowers (48). Common lambsquarters will set seed under adverse conditions (133). The total seed production is also dependent on the nitrate level in the soil (134). Common lambsquarters grown at low plant densities produce more seed per plant than plants at higher density (36). The average 9. glbum plant produces approximately 72,450 seeds (6), while a single large common lambsquarters plant has the potential to produce 500,000 seeds/plant (47). Density does not affect seed yield per area since seed yield per plant decreases as density increases (36). Low temperatures prolong vegetative growth and promote greater seed production (47). III. Seed Germination The peak germination of common lambsquarters is in April and May, continuing in lesser numbers throughout the summer and autumn (5, 73, 93, 94, 96). Tillage changed the magnitude of germination responses, but not the emergence pattern (73, 94). The largest percentage of seed germination followed April and May tillage (98). Seedlings that germinated accounted for less than 5% of the total viable seeds in the top 10 cm of soil (98). Cultivation increased the rate of loss of common lambsquarters seeds, but there was little difference between two and four cultivations per year (93). Habitat disturbances such as plowing would promote germination of common lambsquarters since light promotes germination (5). Soil disturbances result in a flush of emerging seedlings, 90% of which appear within the first ten weeks after a soil disturbance in early spring, and within three weeks after cultivation in the summer (97). A higher percentage of seed germination occurs when soil temperatures first become favorable. Consequently, germination during the summer occurs mainly from seeds brought to the soil surface by disturbances such as cultivation (5, 73). The number of viable seed in the top 23 cm of soil in successive years followed a pattern of exponential decay, decreasing at a constant rate of 22% per year. Therefore, 1% of the initial population would potentially remain viable after 18 years (93). Common lambsquarters seeds can survive up to 40 years in the soil (47). Other research has shown that common lambsquarters seed had 6% germination after 39 years (2). After six years in cultivated soil, common lambsquarters had 6% germination while 53% germination occurred in undisturbed soil (2). It has also been noted that harvesting of slimleaf lambsquarters (Chenopodium leptophyllum) seed with a commercial type combine enhanced germination compared to a hand—harvested seed (25). The impact of seed on metal surface and cylinder speed damaged the weed’s seed coat which influenced seed germination (25). IV. Germination Physiology The physiological process of Chenopodium glbgm’s germination is complex, involving the light—activated phytochrome system (24, 49, 50, 88, 92), alternating temperatures (37), and the nitrate level in the soil (92, 102). These three factors can in part substitute for each other (44). Common lambsquarters' seed have higher germination in light with a red/far—red ratio similiar to sunlight. Consequently, germination is restricted in areas shaded by green plants (24), because the quality of light has been spectrally changed (117). The leaf filtered light affects the ratio of inactive/active phytochrome (Pr/Pfr) in the underlying seeds (117). Other structures could also influence seed germination by affecting the spectral quality of light. The maternal tissues that surround the developing seeds retain a high concentration of chlorophyll. During seed maturation and desiccation, this could also act as a far—red filter, influencing the potential germination of the seed because it influences the seed coat development (37). As common lambsquarters’ seed ages, the response to light also changes (44). Individual seeds of a population require varying Pfr levels for germination to begin. The reason for the difference in the Pfr requirement may be a function of the seed coat thickness (49). Common lambsquarters does not germinate in darkness at any temperature (5). In other words, weediness in the Chenopodium spp. had been positively correlated with the absence of germination sensitivity to photoperiodic responses (25). Karssen (50) reported that common lambsquarters requires active Pfr during all of its germination process. Therefore, germination of common lambsquarters is prevented at soil depths where light penetration is not sufficient to change phytochrome red (Pr) to phytochrome far—red (Pfr) (37). In Karssen's experiments, the emergence of the growing radicle from the seed coat was used as the first visible sign of germination. However, radical emergence must be proceeded by a chain of metabolic processes requiring light (52) and involving two sites of hormonal action (51). In common lambsquarters seed, gibberellic acid (GA 4+7) and ethylene act as inducers of germination in the presence of light, resulting in visible growth of the inner seedcoat layers which form the second site of hormonal control. The active form of Pfr influences both of these phases. A high level of Pfr is required for the gibberellic acid and ethylene induction which indirectly removes the second block in germination by increasing seed cytokinin content (52) which breaks seed dormancy. Abscisic acid (ABA) does not function in regulating the dormancy in the light—dependent induction of radicle growth, but it can inhibit growth of the radicle before final protrusion from the seed coat (51). g. glbgm‘s germination is stimulated by alternating temperatures and nitrate. Alternating temperatures increase the sensitivity to light 10 and/or nitrate (44). In other words, the response to light without nitrate and to nitrate without light was enhanced by alternating temperatures (44). Extreme temperature differences result in higher germination percentages with light (24). Germination of young seeds less than a year old treated with nitrate was promoted with light, but at a constant temperature both light and nitrate were needed to promote the germination of young seed (44). Young seeds germination response was determined more by nitrate than by alternating temperatures (44). The low threshold temperature for common lambsquarters' germination is 6°C (129). Ethylene also stimulates germination (116). The dormancy-breaking action of ethylene in common lambsquarters seed is dependent on available nitrate (103). The nitrate can be present either as an endogenous constituent of the seed or supplied exogenously (102). The endogenous content of seed nitrate is a direct function of the nitrate nutrition received by the parent plant (102). Stages of incomplete germination of seeds which have a ruptured testa and no longer a light requirement may be of adaptive value, especially under dry conditions. Seeds with incomplete germination remain viable for prolonged periods, whether kept in moist or dry conditions. These seeds germinate very rapidly when transferred to optimum conditions (24). ll PLANT COMPETITION I. General Principles Plant interactions can be broken down into three categories: allelomediation, allelopathy, and competition (87). Allelomediation is the relationship between a third party and the species of concern. Allelopathy is the growth inhibition of one plant by another through chemical means. Competition is mutually adverse effects of an organism which utilizes a resource in short supply (87). Competition itself can be divided into two categories: interspecific and intraspecific competition (87). Intraspecific competition is the negative interaction between plants of the same species. Interspecific competition is the adverse interaction between two different plant species. There are several methods to study the relationship between plants growing in mixed cultures. Three of these methods are additive, substitution, and systematic interactions. The additive method involves growing two plant species together. The density of one species is then varied, while the density of the other species remains constant (86). In most studies, the crop density remains constant and the weed density is varied. This simulates an agronomic field situation where crop density is constant, but weed density varies throughout the field. As weed density increases, crop productivity decreases curvilinearly to a point at which crop yield will no longer be reduced as weed population increases. The degree of yield loss associated with a specified weed density can be determined from this model, but not the competitive 12 ability of the crop versus the weed since the total plant density (crop plus weed) is not constant. The substitutive or replacement method is used to predict the competitiveness of one species with another, as the total plant density is held as a constant. The law of constant final yield--total plant yield is independent of density--is applied in the replacement method (86). The total plant density remains constant while the proportion of the two species is varied (101). The replacement method is valuable in assessing the competitive ability of plants at a constant total plant density (86). The systematic method utilizes a parallel row or fan design to determine intraspecific competition effects on crop yields (87). The advantage of this design is the wide arrangement of plant densities without changing the pattern of arrangement, but it is also difficult to achieve under field conditions. Competition research often is focused primarily on the association of the weeds and crop, rather than on the underlying processes of the interaction, especially with the additive model. The additive model does not adequately account for the influence of density and species proportion on the outcome of competition. Total plant density is always varied, while proportion amoung the species also is changed simultaneously with total density, thus making interpretation of either factor difficult (86). II. Common Lambsquarters Competition Common lambsquarters competes in forty crops grown throughout the world. Worldwide, common lambsquarters is the most important weed in l3 potatoes (Solanum tuberosum L.) and sugar beets (Egtg vulgaris L.) and ranks number seven in corn (leg mgys L.) (65). It is a major weed problem in a wide range of additional agronofiic crops, such as barley / (Hordeum vulgare L.), soybeans (Glycine mg; (L.) Merr.), alfalfa (Medicago sativa), and tomatoes (Lycopersicon esculentum). Research in California by Rousch (101) found common lambsquarters to rank third in relative competitive index after barnyardgrass (Echinochloa cggs-gglli L.) and redroot pigweed (Amaranthus retroflexus L.) but it was more competitive than black nightshade (Solanum nodiflorum L.). This heirachy of these four weed species occurred when grown in a relatively warm climate. Therefore, the redroot pigweed and the barnyardgrass which are C“ plants appeared to be more competitive in the warmer climate than the common lambsquarters and the black nightshade, which are both 03 plants. This competitive advantage may be due to the characteristics of the specific carbon pathway metabolism (2, 9, 48). The interaction among these species pairs was competitive rather than mutually stimulatory or mutually antagonistic (101). Environmental temperature plays an important role in differential suppression of specific metabolic activities in plant Species (2, 9, 19, 48). Common lambsquarters, a 03 plant, had better germination, general growth, and higher photosynthetic rate (1, 19, 81) at lower temperatures when compared to C4 plants, such as corn and barley. Common lambsquarters’ competitive ability was dependent on air temperature and soil temperature (19). For instance, common lambsquarters is one of the first weeds to emerge in the spring under Wisconsin conditions (129). Under cool wet spring conditions, common lambsquarters emerged first and l4 established a dominant population (19). Therefore, this competitive advantage may be gained primarily through events prior to the actual initiation of competition (81). Vegetative development of common lambsquarters’ total plant weight in the field was maximum at 42 to 49 days after emergence (133). Consequently, control measures must start early (133). Common lambsquarters’ plant size, weight and height give it a potential advantage for light capture (101, 133). Relative seed weight reflected the initial energy reserves for early growth and possibly provided a source for resource exploration before the seedling was fully autotrophic (101). Common lambsquarters' aggressiveness in competition was also attributed to its ability to strongly compete for nutrients (133). Holm stated that common lambsquarters is a strong competitor with corn for nitrogen, potassium, calcium, and magnesium (47). In recent research, common lambsquarters density did not alter the concentration of potassium, calcium, and phosphorus in tomato leaves (8). However, nitrogen concentrations in tomato leaves were unaltered at the vegetative and flowering stages, but decreased regardless of common lambsquarters density at early fruit and harvest stages (8). The impact of common lambsquarters on crop yield and quality varies greatly. Sugar beets are in the same botanical family as common lambsquarters (Chenopodiceae or the goosefoot). This impacts sugar beet weed management, since a limited number of herbicides are available that will preferentially control common lambsquarters and not injure sugar beets. In sugar beets, eight plants/10 m of sugar beet row decreased root yield 48% and recoverable sucrose yield 46% (104). In other research, a common lambsquarters’ population of 170 plants/m2 decreased 15 the yield of sugar beets 86% (47). Common lambsquarters also utilized nitrogen more efficiently than sugar beets (107). When nitrogen fertilizer was increased from 75 to 150 kg/ha, common lambsquarters doubled its dry weight while sugar beet dry weight remained the same (107). Maximum sugar yield in sugar beets occurred when nitrogen was applied at 63 kg/ha, and the addition of more nitrogen decreased sugar yield by stimulating the growth of common lambsquarters (107). Seed yield of alfalfa infested with broadleaf weeds was reduced 90% compared to the weed free check (26). The broadleaf weed population was composed of 92% common lambsquarters (26). The severe competition from a dense population of common lambsquarters killed alfalfa plants by decreasing the number of seed producing stems per plant (26). The common lambsquarters seed was harvested with the alfalfa seed and produced an estimated 400 kg/ha of weed seed (26). The weeds in the plots also slowed the harvest and increased the post-harvest processing needed to clean the alfalfa seed (26). Common lambsquarters reduced barley yield 20% when both species were seeded at the same time (100). When barley was planted seven weeks after the common lambsquarters, the yield reduction of the barley reached 45% (100). Common lambsquarters reduced the yield of spring barley 23% and 36% at 150 and 300 to 400 plants/m2, respectively, in research utilizing a rectangular hyperbola model (21). Season long interference of common lambsquarters reduced marketable tomato fruit number (8). The marketable fruit weight also decreased 17% for 16 common lambsquarters plants/m row and 36% for 64 plants/m row (8). A curvilinear relationship existed between the fruit fresh weight and the common lambsquarters’ density (8). 16 Season long interference of common lambsquarters in corn decreased yields curvilinearly with increasing weed density, resulting in a maximum yield loss of 12% at 4.9 plants/m of row (7). The curvilinear response observed suggests that as weed density increases, the individual effect of each additional weed on yield diminishes, probably from the increasing intraspecific competition (7). Some of the yield loss from common lambsquarters was attributed to a reduction in the total number of corn kernels produced (7). Corn yield was decreased when common lambsquarters density was greater than 46 and 109 plants/m2 in a two year Canadian study (108). Common lambsquarters impacted corn yield by decreasing ear length and kernel size (108). In North Carolina field studies, 16 common lambsquarters plants/10 m row reduced soybean yield 15% (109). However, nine common lambsquarters plants/10 m row decreased soybean yield 33 and 23% in 1986 and 1987, respectively, in an Ohio study (43). With densities of 22 plants/10 m row, soybean yields were not reduced if common lambsquarters were removed prior to five weeks after emergence (43). In greenhouse studies, soybean dry-matter production was reduced when common lambsquarters was planted two weeks before soybeans (110). The observed growth stimulation of common lambsquarters when planted prior to soybeans suggested that this species may compete with soybeans for available resources when the common lambsquarters' root system is established prior to the soybean (110). III. Soybean Competition Resources such as light, water, and nitrogen are the limiting factors in plant growth and crop production (2). Since soybean are legumes, nitrogen is fixed into the organic form by the plant, and 17 therefore, nitrogen is not required, except in starter fertilizer for initial growth. Competition with weeds for nitrogen is usually not a limiting factor for soybean growth. Weed control is essential in soybean production; the total annual yield loss in soybeans in the United States due to weeds was 8% (84). Weeds compete for light and moisture with soybeans, although recent research found interspecific competition between soybeans and velvetleaf could not be attributed to water limitations (67). In soybeans, moisture is a critical factor for maximum yield. The amount and duration of seasonal rainfall are the two most important factors in determining soybean yield (126), with the distribution of water being more important than the amount (99). Soybean yields were reduced more by weed competition under water stress (39), than under high soil moisture conditions (38). Soybean yields were reduced more when soil moisture was abundant early in the growing season and limited in late summer than when moisture was limited in the early season and above average until soybeans matured (31). Under water stress, plants decrease partitioning of photosynthate into leaves and increase partitioning into roots (80). Under drought conditions, soybean yield reduction did not occur until weeds were allowed to compete for 12 to 16 weeks after soybean emergence (42). Adequate soil moisture was critical during the podfill stage of soybeans, emphasizing the detrimental effect of weed competition on plants when soil moisture was limiting (127). The greatest competitive effect of giant foxtail (Setaria faberi Herrm.) occurred after initiation of the soybean reproductive stage or after the grass become sufficiently 18 dense to reduce the light requirement for normal growth (54). Research with venice mallow competition in soybeans, reported that the most severe weed competition occurred when the soybean reproductive stage was initiated (31). The time when weeds are competing is a critical factor in the degree of competition the soybean plant experiences. Yield reduction may be due to the combined effect of light and moisture competition (127). Weed control during the first month after planting is most critical to obtain maximum yields, regardless of planting date (17, 68), though soybean morphology was not affected by interspecific competition eight weeks after emergence (67). Early weed removal also aids in soybean stand establishment (17). An inverse relationship exists between soybean stand and the production of weed top growth (17). The weight of weeds at harvest is inversely correlated to soybean yield (119). The magnitude of decrease in soybean growth is dependent on weed density (38). As weed growth increased, the soybean seed weight and numbers of seed per plant decreased (17). Soybean seed size was correlated with yield; greater soybean yield had the largest soybean seed (33). Other soybean growth parameters such as dry weight of the leaves, stems, roots, pods, seeds, and pod number, and leaf area index are also reduced by weed competition (38). The number of pods per plant is often influenced more than any other yield component by competition over the entire growing season from numerous weed species (68). For instance, venice mallow competition reduced the number of seed pods per soybean plant more than any other yield component (31). Wild oat competition decreased the number of soybean pods per plant and the 19 number of seeds per plant, with little influence on the number of seeds per pad or seed weight (89). Planting date was also an important factor effecting weed competition. In Arkansas, one velvetleaf plant/30 cm of row reduced soybean yield 27% in mid May planted soybeans, and 14% in June planted soybeans (74). However, the period of weed—free maintainance required to prevent soybean yield reductions was not affected by planting date (68). Competition within the row is studied more frequently because weeds are believed to be more competitive for light and moisture in the soybean row. Secondly, weeds cannot be removed from within the row by cultivation, and therefore can remain for the duration of the growing season. However, competition was greater from sicklepod (Cassia obtusifolia L.) planted 15 to 30 cm from the 102 to 107 cm soybean rows than from sicklepod seeded within the row (119). In contrast, Eaton (32) found no differences in soybean yield reduction when velvetleaf (Abutilon theophrasti Medic.), prickly sida (Sigg spinosa L.), and venice mallow (Hibiscus trionum L.) were planted either in or between the rows of 75 cm planted soybeans. Cultivation and row spacing influence the type of competition that takes place in the soybean row. Under cultivation, weeds present in wide (76 cm) soybean rows had an increased ability to interfere with soybeans, since intraspecific competition was decreased (78). However, narrow (38 cm) row soybeans allowed more interspecific interference between weeds, thus reducing weed growth more than wide rows (78). The reverse situation occurs under no cultivation. More intraspecific interference within weed species occurred in wide rows, thus allowing greater weed growth than in narrow soybean row spacing (78). Yield data indicate the 20 same potential for controlling weeds in narrow-row soybeans without cultivation and in wide rows with cultivation (123). However, narrow rows at 51 cm had the advantage of at least one cultivation compared to 25 cm rows (124). At row spacings less than 51 cm, two cultivations were needed for good weed control and high soybean yields (82). When herbicides effectively controlled all weeds in 18 cm rows without cultivation, soybean yield increased 9% compared to yields of soybeans in 76 cm rows when planted to the same population (123). However, cultivation increased soybean yield when few or no weeds were present (82). Narrow (31 cm) rows generally have higher yields compared to wide (61 cm) rows if plant population is increased (18, 40, 57, 62, 82, 124). Another advantage of narrow rows is more rapid canopy closure with increased shading, resulting in a shorter weed control period required (18, 82, 124). Soybeans have the ability to compensate when placed under stress. When a stand reduction occurs, remaining plants will branch and little yield reduction will occur (45). Branch number decreased as soybean row spacing within the row decreased (57). A narrow row spacing increased the number of soybean branches per plant compared to a wide row spacing (78). Soybean cultivars vary in their ability to compete with weeds (16). Soybean cultivars also influence the level and duration of weed competitiveness, dependent on the weed species present (66). A limited amount of plant biomass is needed to support seed production in any species, and in some instances, biomass may be slightly reduced even though yield is not reduced (66). 21 The competitiveness of weeds in soybeans is species dependent, with other grass and broadleaf weeds being more competitive than common lambsquarters. Grassy weeds such as wild oat (Avgflg fgtgg L.) at 300 plants/1O m of row reduced soybean yield 51% with season long competition in a 76 cm row spacing (89). However, if the wild cats were removed by four weeks after crop emergence, no yield reduction occurred (89). In Michigan, annual grass interference for the duration of the growing season at 70 plants/m2 decreased soybean yield 46 and 55% on a loam and sandy loam soil, respectively, when grown in a dry year in 76 cm row soybeans (69). Broadleaf weeds, such as common cocklebur (Xanthium gensylvanicum L.) reduced soybean yield 3 to 12% with three plants/10 m of soybean row when competing for the duration of the growing season (11). In contrast, common ragweed (Ambrosia artemisiifolia L.) reduced soybean yield 8% when four plants/10 m of soybean row were present all season (20). Jimsonweed (Datura stramonium L.) decreased soybean yield 24% when 16 plants/10 m of row were present all season (40). TILLAGE Tillage systems impact weed management alternatives in the soybean production. Not all conventional production practices can be incorporated into reduced tillage systems. In conventional systems, spring or fall moldboard plowing is followed by disking or field cultivating to prepare a seedbed. In minimum tillage systems, soil disturbances are reduced because primary tillage is not practiced. Minimum tillage does not allow the incorporation of certain soil—applied herbicides, resulting in fewer weed control options. It also eliminates 22 plowing as a practical means of eliminating phytotoxic residues of shoot— absorbed herbicides when soil residue problems exits (16). Tillage impacts on soybean yields are conflicting. A two year study in Illinois on a silty clay loam found grain yields in a corn/soybean rotation were similiar for a moldboard plow, chisel plow, and spring disk system, but were consistently lower for no-till (121). Another study in Alabama on a sandy loam found soybean yields in strip and no-till corn/soybean rotation to have the most consistent yield increase when compared to conventional tillage in four years (33). The same study also found the conventional soybean yields were more affected by rainfall than the strip and no-till (33). Tillage did not affect the yields between soybean cultivars; the best yielding cultivars in the tilled system were the best yielding in the no-till system on a silty loam when moisture was above normal (34). However, soybeans planted in conventional tillage systems have greater dry matter production than no—till system soybeans (126). The conventional tillage system soybeans also had greater growth and matured more rapidly than the no-till system (126). This growth increase was possibly due to the more ideal seedbed in the conventional tillage system with warmer soils and less restriction to soybean emergence than the no- tillage system which had greater plant residues and a more dense surface soil (126). Adequate soil moisture conditions may be sufficient in no-till for weed seeds to germinate, but not sufficient under other tillage systems. This would result in increased weed pressure in a no-till field (137). No difference was observed between tillage systems for the total number of broadleaf weed species (137). However, the number of grasses in the 23 no-till field was significantly higher than in either the disk or plow fields (137). The no-tillage system always had greater total soil moisture, and thus may benefit in dry years from greater soil moisture and the soybeans’ ability to use soil moisture from deeper in the soil profile (120, 126). Soil moisture depletion at deeper depths in no- tillage systems indicates greater soybean rooting lower in the soil profile under no—till compared to conventional tillage (127). Soil temperatures influenced germination of both crop and weed seed (73). In minimum tillage systems, soil temperature in the spring is lower than in the conventional plowed system because of decreased soil aeration and increased plant residue on the soil surface. Tillage influences seed positioning in the soil (128). With reduced plowing, an increasing number of weed seed remains in the upper portion of the plow layer (128, 137). Eighty—five percent of all weed seed in a reduced—tillage system was in the 0 to 5 cm depth soil layer, compared to 28% in the conventional-tillage system (79). Tillage systems also changed the magnitude of response of weed emergence due to seed positioning, but not the pattern (73). Cultivation can assist soybean producers weed management by decreasing the risk of only using herbicide control measures. Herbicides plus cultivation were the most reliable and economic means of controlling weeds in cotton (111). Herbicides plus cultivation provided more consistent and acceptable weed control, yield, and net return in peanuts (130). In corn, if adequate mechanical weed control was achieved by cultivation, herbicides offered very little yield increase if the mechanical control methods were timely (4). However, if poor mechanical 24 weed control methods were used, herbicides offered considerably more net income potential (4). Soybean producers can rely on a combination of soil—applied and postemergence herbicides in both conventional and and minimum tillage systems. Postemergence herbicides can be used in reduced tillage systems and have the advantage of the producer being able to identify the weed problem prior to the herbicide application. Timing of application is critical when using postemergence herbicides, because the weed growth stage at the time of application is critical for effective control. Secondly, just as with soil—applied herbicides, rainfall is important for postemergence herbicide efficacy. Weeds need to be actively growing at the time of application for postemergence weed control to be effective. A combination of soil applied and postemergence herbicides may result in higher soybean yields (99). Combination programs can be advantageous to soybean producers, because of reduced risk of herbicide failure and a lower cost/acre compared to a total postemergence program. Herbicides also impact the seed reserve in the soil. Residual soil— acting herbicides are effective in restricting the weed population. The number of seed species and their abundance in relation to other species remains the same, but the total numbers of viable seeds are reduced in the herbicide treated plots when compared to untreated plots (95). An intensive system of weed management may be initiated for the first few years where large weed seed reserves in the soil exists, but once the weed seed reserve is reduced, moderate levels of herbicide and tillage may be employed (106). The principle reason for the reduction in the weed seed reserve was due to minimizing the production of new weed seed during the six year study (107). Cropping and tillage effectively 25 decreased the number of wild mustard (Brassica kaber) seeds. When seed bank replenishing was prevented, the population of wild mustard seed declined rapidly, and the decline was hastened by tillage (122). The contribution of sicklepod seed to the soil was also studied under different tillage practices (14). Intensive weed management inputs resulted in excellent control of sicklepod regardless of cropping or tillage systems. However, permitting subcompetitive densities of sicklepod to reach maturity each year increased the seed number in the soil, with the most dramatic increase occurring in the conventional system (14). Cropping and weed management systems changed the number of sicklepod seeds in the soil. Crop rotation, particularly in no—till, resulted in a more stable and downward trend in seed number. Long—term control or management of sicklepod may be more easily obtained with minimum tillage than conventional tillage. Seed distribution was not affected by the type of primary tillage. The number of sicklepod in pods produced per plant was greatest in the tilled system. HERBICIDES Common lambsquarters is a difficult weed to control with postemergence herbicides. The leaf surface has a waxy covering making it difficult for the herbicide to penetrate into the plant, giving inconsistent control or suppression (115). The two postemergence herbicides presently labeled for common lambsquarters control in soybeans are bentozon (3—(1—methylethyl)—(1H)~2,1,3—benzothiadiazin—4(3H)—one 2,2- dioxide) and/or acifluorfen (5-[2—chloro-4-(trifluoromethyl)—phenoxy]—2— 26 nitrobenzoic acid). These herbicides should be applied to common lambsquarters when it is less then 5 cm tall in accordance with their labels. BENTAZON Bentazon is a contact postemergence herbicide used primarily to control broadleaf weeds and yellow nutsedge (Cyerus esculentus L.) in soybeans. Postemergence herbicides, such as bentozon, are important in minimum tillage systems where farmers can neither incorporate a herbicide nor cultivate for additional weed control. Certain soil—applied herbicides such as chloramben (3-amino-2,5-dichlorobenzoic acid), metribuzin (4—amino—6—(1,1—dimethylethyl)-3—(methylthio)—1,2,4—triazin— 5(4H)-one) and linuron (N’-(3,4—dichlorophenyl)—N—methoxy-N-methylurea) control Chenopodium glbum L.. Bentazon at the maximum application rate of 1.12 kg ai/ha is labeled for partial control of common lambsquarters in the 4 to 8 leaf stage or when the weed is less than 5 cm (label). Even at the full labeled rate, common lambsquarters’ control obtained by bentozon was erratic (3). Postemergence herbicides for control or suppression of common lambsquarters are limited because of the difficulty of absorption through the waxy surface of the leaf (115). The common lambsquarters wax forms a homogeneous covering over the entire leaf surface, though platelets are less dense over the mid—ribs, large veins, and stomata vicinity (115). This wax contains a substantial proportion of aldehydes which present a barrier to penetration of polar molecules through the cuticle (115). Temperature, humidity, and additives influence bentozon efficacy. Improved weed control with bentozon occurred under conditions of low 27 relative humidity (115). Weed control was greater at lower temperatures than at higher temperatures (70, 115). Crop oil concentrate increased bentazon efficacy regardless of environmental conditions (115), and therefore the addition of oil to a bentozon application would improve weed control under adverse climatic conditions (70). By providing better coverage of the leaf’s surface, the oil is believed to result in a greater portion of the leaf’s stomata being exposed to the bentozon (115). With dry environmental conditions, the bentozon dries on the leaf without being absorbed (70). Absorption could be increased by wetting the leaf with a low volume of water. This additional moisture for increasing bentozon efficacy must occur soon after application for maximum effectiveness (70). The time of day also influences application effectiveness. Application at mid-day is less effective than at early morning or evening (70, 114). This may be due to stomatal aperture being reduced at midday, and therefore reducing herbicide absorption (114). I. Mechanism of Action Bentazon is believed to inhibit photosystem II between the unknown quencher Q and plastoquinone by primarily uncoupling the electron transport system from water to ferricyanide or methylriologen (113). Bentazon also may inhibit photosynthetic COZ—fixation with symptoms similiar to decreased cellular photosynthesis (113). The longer the incubation with bentozon the more irreversible this photosynthetic inhibition. Photosynthesis was inhibited more rapidly as the dose of bentozon increased. Necrosis developed in cocklebur seven hours after photosynthesis ceased. Light was required for necrosis to develop in 28 treated leaves. These symptoms support the hypothesis that photo-induced toxic by-products result when photosynthesis is inhibited (29, 85). Bentazon also changed the shape and distribution of the chloroplasts within the leaves (85). This change in chloroplast structure was not considered to be a toxic response (85). In other research, bentozon injury was prevented by endogenous or exogenously supplied carbohydrates (64). The principle route of entry of bentazon into the plant is believed to be through the wax—free antechamber in the open stomata that is vulnerable to the bentozon (115). In 9. glbgm epidermal peels, bentozon opened the stomatal apparatus over a temperature range of 9 to 29°C. In this case, the entry of bentazon through the stomata appeared to be very important. Stomata opening is dependent on the potassium concentration in the guard cells surrounding the stomata. Low levels of light and reduced 002 concentration cause stomatal opening in the presence of potassium chloride (KCl). Bentazon closed stomatas regardless of the KCl availability (29). Bentazon may effect stomatal movement by interfering with the guard cell ion pump activity which may itself be regulated by 002 (30). ACIFLUORFEN Acifluorfen is a diphenyl ether herbicide used to control broadleaf weeds as a contact postemergence application in soybeans. Acifluorfen is labeled to control common lambsquarters at the maximum application rate of 0.56 kg ai/ha when the weed is less than three true leaves or less than 2.5 cm in height. Spray additives are required for maximum effectiveness, and cultivation in three to seven days will usually assist 29 in control (label). Control of common lambsquarters varied between 75 to 100% with 0.28 kg ai/ha, and control increased linearly as application rate increased to 0.84 kg ai/ha (135). Penetration and translocation of acifluorfen occurs at varying rates in different plant species. Acifluorfen followed a basipetal direction from treated leaf to stem and then acropetal to the apical regions of the leaf (91). The degree of penetration and translocation coupled with the rate of metabolism in a particular plant species may contribute to the susceptibility of a plant to acifluorfen (91). Acifluorfen’s weed control activity is affected by relative humidity, temperature, and additives. Under conditions of high humidity, acifluorfen applications resulted in increased phytotoxicity and reduced weed dry-weight (90). Temperature also played a role in acifluorfen activity, and was plant species dependent (90). Postemergence activity of acifluorfen on susceptible weed species is usually enhanced by the use of additives (55), but the addition of surfactant had no effect on common lambsquarters control (135). However, in another study, the use of additives with acifluorfen on common lambsquarters increased the herbicidal activity by two to four times. Acifluorfen in combination with a non-ionic additive provided the greatest control and reduction in dry- weight of common ragweed, especially when applied at 85% relative humidity and high temperature (90). Increasing the additive rate resulted in a strong dose—response relationship in the effectiveness of acifluorfen (55). As the control of weeds increased with the increasing concentration of additives, the soybean injury also increased (55, 56). I. Mechanism of Action Initial response to acifluorfen in plants is expressed as an increase in membrane permeability (76). Light was required for this to occur (76), and this occurred after a delay of one to two hours (28). Decreasing the quantity of light delayed the effect of the herbicide (76). Neither the photosynthetic electron transport chain nor chlorophyll appeared to be involved in the response (76). It was hypothesized that light was absorbed by the carotenoids which are yellow plant pigments and photorecepters. This absorption caused the production of a toxic species that had action on the outer plastid envelop of the mitochondria (28). The toxic species produced are the superoxide radical and hydrogen peroxide (28). The free radical then initiated a chain reaction with molecular oxygen (77) which affected the polyunsaturated fatty acids in the membrane, resulting in increased membrane permeability. This free radical reaction, however, was not believed to cause the direct reduction and reoxidation of the diphenyl ether herbicide molecule (35). Instead, a ubiquinone, a hydrogen carrier in the electron transport chain, was totally oxidized due to either an inhibition of the reduction of the quinone or its’ facilitated oxidation (46). BENTAZON and ACIFLUORFEN Postemergence herbicides are often used in combination. When applying acifluorfen and bentozon together, a significant decrease in percent moisture of common lambsquarters indicated greater phytotoxicity for the combination of acifluorfen and bentozon than to either herbicide alone (112). Percent moisture and fresh and dry weight of common 31 lambsquarters generally decreased as herbicide rates increased. However, another study showed combinations of bentozon plus acifluorfen at 0.4 and 0.2 kg/ha, respectively, did not increase common lambsquarters control when the rate of bentozon or acifluorfen was increased or if an additive was included, when common lambsquarters was less than 5 cm in height at the time of application (12). Common lambsquarters also showed a synergistic response to all combinations of acifluorfen and bentozon if no crap oil concentrate was added (112). The response was additive if the crop oil concentrate was present. Radiolabel studies indicated that both acifluorfen and bentozon uptake into common lambsquarters was reduced in the presence of the other herbicide by 15 and 17%, respectively (112). ECONOMICS Increased input costs and decreased crop value have resulted in reduced profitability in soybean production. When specific interactions between soybeans, weeds species, and weed densities are determined crop loss estimates can be calculated. Numerous studies have been conducted to determine the feasibility of herbicide application for weed control in soybeans. Herbicide application cost and probable benefit can be incorporated into an "index of competition“ which calculates weed thresholds that justify economic expenditure for herbicides (27). There are two types of thresholds: the competitive threshold and the economic threshold. The competitive threshold pertains to the weed density and duration of interference above which the crop yield is reduced (75). The economic threshold is when the monetary yield loss exceeds the cost of control (75). When combining the two threshold concepts, the thesholds 32 together predict when weed control measures should be implemented for maximum benefits. The harmful effects of weeds on soybean production increase as weed density increases until a yield plateau is reached, where any additional increase in weed density will not reduce crop yield. The "classical" critical time period for weed control has two important components: 1. length of time after planting during which weeds must be controlled in the crop to prevent yield reduction, and 2. the last time period in the season when the crop must be kept free of weeds so that it can escape yield reductions from any weed that subsequently emerges. Competitive indices assume similiar plant counts with a higher seeding rate of the crop resulting in higher yields, and removal time of the weed affecting the competitive ability of the crop (27). Crop population can significantly alter the relationship between yield losses and weed density. Seeded tomatoes in twin rows had greater yields than single rows, although tomato yields declined significantly with increasing nightshade density at all crop population densities (125). The relationship between weed density and yield reduction can be modeled by two different approaches: multiple regression and a hyperbolic model. The multiple regression model assumes the species (crop and weed) emerge at the same time (23). With multiple linear regression, a sigmoidal relationship implies a minimum number of weeds which can be tolerated by the crop before yield reduction occurs (1). The curvilinear relationship which fits a hyperbolic model (23) implies no minimum number of weeds which can be tolerated (1). The initial slope of the curve represents the percentage of yield loss per unit weed density and is relatively constant until intraspecific interference 33 reduces the competitive impact of each additional weed (22). The hyperbolic model can also be used to relate the weed density and the relationship of emergence time to crop yield (23). Coble applied the index of competition over a spectrum of weeds and assigned a competitive rating to each specific weed. Common cocklebur was considered the most competitive and was given a rating of 10. Common lambsquarters received a 4.7. This model can be utilized in determining economic weed thresholds in soybeans by setting minimum weed population densities that justify a postemergence herbicide treatment (61). Weed management can be a profitable component of a soybean production system (106). The probability that the return above variable costs will differ between two management systems is dependent on the product prices and herbicide costs (106). Higher herbicide costs favor standard weed management systems based an alternating herbicides and product prices (58). Higher crop prices favor weed management systems with higher yield adjusted for quality (58). The economic sensitivity or the order of profitability of the management systems depended on the unique ratios of the crop price and variable costs (59). The impact of escaped weeds and their potential for seed production provides future weed control concerns on subsequent crops (106). Wilson found weed seed numbers in the soil to be correlated to the number of plants growing in the field (136). Seeds found most frequently in soil also had the highest and most consistent germination (136). Hence, weed seed production is important to incorporate into the threshold model. In recent research with tomatoes, nightshade (Solanum spp.) seed production was a function of the nightshade density (125). Large quantities of seed 34 were produced even for nightshade densities at which little actual tomato yield loss occurred (125). The soybean producer would benefit from a threshold model that incorporated all of the following: specific crop yield and value, crop growth stage, crop population, herbicide options with degree of control or suppression for specific weeds, herbicide cost, weed species, weed growth stage, weed densities with their impact on the specific crop, weed seed production potential, and weed seed germination potential in the following years. 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Interrelations of tillage and weed control for soybean (Glycine max) production. Weed Sci. 35:830—836. Wicks, G. A. and B. R. Somerhalder. 1971. Effect of seedbed preparation for corn on distribution of weed seed. Weed Sci. 19:666-668. Wiese, A. M. and L. K. Binning. 1987. Calculating the threshold temperature of development for weeds. Weed Sci. 35:177—179. Wilcut, J. W., G. R. Wehtje, and R. H. Walker. 1987. Economics of weed control in peanuts (Arachis hypogaea) with herbicides and cultivations. Weed Sci. 35:711-715. Williams, J. T. 1962. Dormancy in Chenopodium album L.. Ann. Appl. Biol. 50:352. Williams, J. T. 1963. Biological flora of the British Isles. Chenogodium album L. J. Ecol. 51:711—725. Williams, J. T. 1964. A study of the competitive ability of Chenopodium album L. I. Interference between kale and g. album grown in pure stands and in mixtures. Weed Res. 4:283.293. Williams, J. T. and J. L. Harper. 1965. Seed polymorphism and germination I. The influence of nitrates and low temperature in the germination of Chenopodium album. Weed Res. 5:141—150. Wilson, H. P. and T. E. Hines. 1987. Snap bean (Phaseolus vulgaris) and common lambsquarters (Chenopodium album) response to acifluorfen. Weed Tech. 1:18-21. Wilson, R. G., E. D. Kerr, and L. A. Nelson. 1985. Potential for using weed seed content in the soil to predict future weed problems. Weed Sci. 33:171—175. Wrucke, M. A. and W. E. Arnold. 1985. Weed species distribution as influenced by tillage and herbicides. Weed Sci 33 853—856. CHAPTER 2 ABSTRACT Soybean weed control programs in narrow and wide row spacings with and without cultivation in both conventional and no—tillage systems in a corn/soybean rotation were studied. Weed control treatments consisted of soil—applied herbicides, total or partial postemergence herbicide treatments, and a weedy and weed-free check. Herbicide options, cultivation, and row spacing impact on weed control, soybean yield, and breakeven price were determined. Soil moisture condiditons were critical for both postemergence and soil—applied herbicide activity. Weeds impacted soybean yield and breakeven price in the conventional and no— tillage system. Increased soybean yield in the narrow row spacing justified the higher plant populations when weed control exceeded 80%. Cultivation was important only when weed control with herbicides failed or weeds were difficult to control. Cultivation also increased soybean yield in the weed—free soybeans in the no-tillage field in 3 of 3 years of research. Therefore in a dry spring, cultivation provided better weed supression than planting soybeans in narrow rows. 45 46 WEED MANAGEMENT ALTERNATIVES IN CONVENTIONAL AND NO-TILLAGE DRILL AND ROW SOYBEANS INTRODUCTION Tillage, row spacing, cultivation, and herbicide programs impact soybean yield and economic return. Soybeans planted in a conventional tillage system had faster growth rates, increased dry matter production, and more rapid maturity than in a no-tillage system (26). In Alabama on a sandy loam soil, soybeans planted in the no-tillage system gave the most consistent yield increase compared to a conventional tillage system in a corn/soybean rotation (6). However, in Illinois on a silty clay loam soil, soybeans yields were lower in the no-tillage system compared to a conventional tillage system in a corn/soybean rotation (23). Rainfall may be a critical requirement for higher yields in conventional tillage systems (6), and no-tillage may be beneficial in a dry year due to greater soil moisture retention (22, 26). Row spacing also influences soybean yield and economic return. Soybeans planted in narrow rows (51 cm or less) had higher yields compared to soybeans planted in wide rows (76 cm) when plant populations per acre were increased (4, 7, 8, 11, 25). Soybeans in narrow rows suppressed weed growth by faster shading between the soybean rows (3, 12, 16, 25). At a row spacing of 51 cm or greater, cultivation remains a weed control option (25). Soybeans planted in row spacings greater than 51 cm may require two cultivations for adequate weed control (16). Tillage systems also influence weed emergence and growth (27, 31). The emergence of weed seedlings may be restricted in no-tillage systems due to greater crop residue (31) and dense surface soil (27). Tillage 47 may change the magnitude of weed emergence (14) due to the changes in seed distribution in the soil profile (28, 31). In a reduced-tillage system, 85% of all weed seed was in the 0 to 5 cm soil depth, compared to 28% of the total weed seed in the conventional tillage system (15). Seeds of grassy weeds increased in a no-tillage system at shallow depths, while broadleaf weed distribution was similiar in both tillage systems (31). Weed management is an important profit component of soybean production systems (20). The probability that the return above variable costs will differ between two weed management systems is dependent on the commodity price, the weed control achieved, and the herbicide cost (9). Higher herbicide costs favor standard weed management systems based an alternating herbicides and commodity prices, while higher crop prices favor weed management systems with higher yield adjusted for quality (9). Economic differences are dependent on market conditions (9). The economic sensitivity or the order of profitability of the management systems is dependent on the unique ratio of the crop price to variable costs (10). Soybean producers rely on combinations of mechanical and chemical practices. Herbicide applications followed by cultivation were the most reliable and economic means of controlling weeds in cotton (21), and provided more consistent and acceptable weed control, yield, and net return in peanut production (29). Timely cultivations of corn with no prior herbicide applications produced corn yields similiar to plots treated with herbicides prior to cultivation (2). Combining weed control practices such as herbicide applications followed by cultivation, or soil-applied followed by postemergence herbicide applications may result 48 in higher soybean return because the risk of herbicide failure is reduced and the herbicide cost per acre is lower compared to a total postemergence herbicide program (19). Postemergence herbicides provide an advantage to the producer, since the weed problem can be identified prior to herbicide application. The timing of postemergence herbicide applications is critical, because applications delayed past the optimum weed growth stage reduce herbicide efficacy. Rainfall is also important for postemergence, as well as soil—applied, herbicide activity. Weeds must be actively growing at the time of postemergence herbicide application for effective weed control, and rainfall is also required for soil-applied herbicide activation. Research evaluated various weed control programs in soybeans planted in narrow and wide row spacings in both conventional and no—tillage systems in a rotation of soybeans following corn. The objective of this research was to determine the impact of herbicide treatments, cultivation, and row spacing on weed control, soybean yield, and breakeven price in both conventional and no—tillage soybean production systems. MATERIALS AND METHODS In 1986, 1987, and 1988, soybeans (Glycine mgx (L.) Merr. cultivar ‘Corsoy 79’) were grown in two tillage systems without irrigation in two adjacent fields in Michigan on a complex of Aubbeenaubbee—Capac soils (Aeric Ochraqualfs). Soil texture, soil pH, and percent soil organic matter are shown in Table 1. Individual plots were 3.0 by 12.2 m in 1986, and 3.0 by 10.7 m in 1987 and 1988. Plots were replicated four .mocomLeEmowoa ouap .hmOQ .4 neocomLoEouwoa .hwod neocomLoEooLQ .wma “councoacoocv pcmpaoco .Hdd .Ewumzw omappwaro: L04 opnmowpaom no: oumowocw wozmmo .Emop xmpo steam —o usmop .P 4 can m. 360 ow boo m >oz m. 300 _N 600 ”mama omw>tmz _. >_:e o spas m_ s_:w w. spas N span. me spas N co_om>,opzo 4— was: mu mass m >_:e 4F 6:34 AN 6:36 o span _ cospm>mu.:o . nmcwevh :o_pm>wp~:o m 0:35 A_ mass mm mass m mean 0, mass an acne Amen .5 AN sex o_ mass m_ oczw hm sax _ 6:31 m_ was: emoa 9 8 sex G Am: m_ sm: 4 sex 8 sex n. so: mad 4 1 u I m A“: m so: ~_ s5: Han “wcowumompaa< onwownLoI m.~ m.~ ~.. A.~ N.N m.. Axe Leases o_:mmco pow F p0 . Po pow Pow ouxu Fmow o.o ~.o 5.0 N.@ mew 0.0 :6 __ow . ”covumELOCCH —_ow mom, Fem. mom. . mmm_ Amm_ mom— Eopmxm omappmuroz Eoumxw ommrpvu pmcowoco>coo .Eouwxw mam—P?» so nonwLowoc we woumn.cowuc>wup:o ace .wCOTDMOWFoam oowownco; .cowmeLOCCT ”mow "_ oFome 50 times. The experimental design was a randomized complete block with a two-factor factorial arranged as a split-plot. Herbicide treatments were split on row spacing. The conventional tillage system was fall plowed in 1987 and 1988 and spring plowed in 1986 due to wet fall soil conditions in 1985. Secondary tillage (disking in 1986 and field cultivation in 1987 and 1988) was made prior to a broadcast fertilizer application in the conventional tillage system. Fertilizer (6—24—24) at 224 kg/ha was applied to both tillage systems. A second pass with the field cultivator was made prior to soybean planting in the conventional tillage system. Soybeans in the no— tillage system were planted directly into the previous year's corn stubble, and a burndown herbicide glyphosate (N-(phosphonomethyl)glycine) at 1.1 kg ai/ha was applied to control emerged weeds. Soybeans were planted on May 12, 1986, May 5, 1987, and May 4, 1988 in both tillage systems. The three main plots included a narrow (25 cm in 1986 and 20 cm in 1987 and 1988) row spacing, a wide row (76 cm) soybean spacing, and wide rows (76 cm) with cultivation. Wide row soybeans were cultivated twice in both the conventional and no—tillage systems in all three years (Table 1). The narrow row soybeans were replanted in 1987 on June 21 because of poor emergence in the 17 cm spacing. Soybean populations at planting for each row spacing are given in Table 2. Herbicide sub—plot treatments included soil-applied and postemergence herbicides in various combinations (Table 5). The herbicides common to both tillage systems included: metolachlor [2—chloro—N—(2—ethly—6-methylphenyl)—N—(2-methoxy- 1—methylethyl)acetamide], metribuzin [4—amino-6-(1,1-dimethylethy1)—3— (methylthio)-1,2,4-triazin—5(4H)-one], linuron [N'—(3,4-dichlorophenyl)- l 5 voucmpaoL * .umo>La£ o» LowLQ coxau :owuopsaoa acmpa cemnxow chmu .coxap uoc covuopsooa acmFQ :monxow pacww mumomocw magmas .oowéZ Swimm— : 80.7.: 03.43 .. zoL . 08.3w *oom.mmw : 0358 08;: 1 :Ce n<\:o_pm_:aou coonxow —mcvu 80.03 80.8. 08.9: 88.3; 08.2: 08.03 :3 000.000 . Aucmpava ooo.om~ 80.2: 25.08 08.03.. 25.2: 0868 :76 u<\co_uapzoou :mwnxom noose—a 33 2:: 2:: mom. 53 of: Eoumxm camp—+0102 Eouwxm comp—v0 cho+uco>coo .Lnox some :4 mcowpcpzoca Pmcwm ace vou:m_a ”N opnme 52 N—methoxy-N-methylurea], imazaquin [2-[4,5—dihydro—4-methyl—4—(1- methylethyl)-5-oxo—1fl-imidazol-2-yl]-3—quinolinecarboxylic], chlorimuron [2-[[[[(4—chloro-6-methoxy-2-pyrimidinyl)amino]carbonyl]amino] sulfonyl]- benzoic acid], bentazon [3-(1-methylethy1)—(1fl)-2,1,3-benzothiadiazin- 4(3H)—one 2,2—dioxide], acifluorfen [5-[2—chloro-4- (trifluoromethyl)phenoxy]-2-nitrobenzoic acid], and sethoxydim [2-[1- (ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3—hydroxy—2—cyclohexene-1— one]. Nonionic surfactant1 and crop oil concentratez were added to postemergence treatments in accordance to label. The conventional tillage system also included preplant incorporated treatments of trifluralin [2,6—dinitro—N,N-dipropyl-4—(trifluoromethyl)benzenamine] and clomazone [2—[(2—chlorophenyl) methyl]—4,4-dimethyl-3-isoxazolidinone]. Incorporation was accomplished in one pass using a Triple K3 set to a depth of 11 cm at 9.6 km/hr. The no—tillage system also included two herbicide treatments containing chloramben [3-amino-2,5-dichlorobenzoic acid]. Soil-applied herbicides were broadcast in a spray volume of 215 L/ha at 207 Kpa at standard rates for soil type, soil pH, and organic matter content. Postemergence broadleaf herbicides (bentozon and acifluorfen) were applied at 271 L/ha and 345 Kpa. Tank mixed postemergence broadleaf and grass herbicide treatments were applied at 215 L/ha and 207 Kpa, and were timed for application at the proper 1 Surfactant nonionic Triton Ag-98, alkylaryl polyoxethylene glycols, Rohm and Haas Co., Springhouse, PA 2 Surfactant Crop oil Concentrate. Herbimax, petroleum hydrocarbons (83%) — light paraffinic distillate, odorless alaphatic petroleum solvent, surfactant (17%) — mono and diesters of omeega hydroxypoly oxyethylene, Loveland Indust., Loveland, IO. 3 Triple K Danish s—tine field cultivator with rolling baskets. Manufacturer Kongskilde, Exeter, Ontario. 53 broadleaf weed stage. The late postemergence grass herbicide (sethoxydim) was applied at 79 L/ha at 345 Kpa. All herbicide applications were made with a tractor-mounted compressed air sprayer with a 2.9 m boom having a 51 cm nozzle spacing. Application dates for each year are given in Table 1. Plots were infested with common ragweed (Ambrosia artemisiifolia L.), velvetleaf (Abutilon theoghrasti Medic.), redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenogodium giggm L.), annual grasses (primarily yellow foxtail (Setaria lutescens (Weigel) Hubb.)), and yellow nutsedge (Cyperus esculentus L.). Rainfall data are given in Table 3. Plots were evaluated three times during the growing season for weed control, with only the late season evaluation presented. Final soybean plant populations were counted prior to soybean harvest in 1987 and 1988 (Table 2). Soybeans were harvested with a plot combine by cutting a 1.2 m swath from the middle of each plot, after trimming a minimum of 1.5 m from each plot end. Samples were cleaned to remove weed seeds as needed, moistures recorded, and yields in bu/A adjusted to 13% moisture. The economic analysis was based on a comparison of breakeven prices. The breakeven price is defined as the soybean value (in $/bu) that will justify the variable input costs. Breakeven price is calculated by dividing the total variable cost (including herbicide and application costs) by the soybean yield. Variable input costs4 include: tillage, planting, burndown herbicide, fertilizer, soybean seed (Table 4), herbicides (Tables 5), and application costs. Tillage was a cost unique to the conventional tillage system. Planting costs for the no—tillage 4 Cost values were taken from Ext. Bul. E-2131. "Custom Work Rates in Michigan". Michigan State University, 1988. 54 00.0 00.0 00.0 _N.. 00020>oz 0_.~ 04.4 00.0 40.0 twoouoo 40.0 00.4 00.4 00.0 Lensmoamw 40.0 00.4 40.0 40.0 00300< 05.0 04.0 .0.0 04.0 >400 00.0 00.0 00.4 F0.0 0:00 “0.0 00.0 .4.F 00.0 >02 00.0 00.4 00.0 00.0 _0L0< moxocw IIIIIIIIIIIIII ; o>< 000_ ~00. 000s mzezoz LND) 0m .zocaomoc to wLmox 0 any Low wpmuou rpmwcwmc xrzucoz "0 means 55 .AmoL< 0:00:040 000— F4L0< —mpwlm .430 .uxm xuwwco>wc0 mbmpm comwzowz .cmm0zovz cw wouam xLoz 500030 020 EoLw coxwu wmopm> 0~.m_ <\9 pwo>Lmz 00.N <\w Acowpmovpaamv 00.0w <\w Lonpprmu 00.0 <\0 A00 00000>00_0o 00.0 mooow 00000—\0 0000 00.0 (\0 Acompmovpaumv 04.NF <\0 oo_oanw: czoncczm 00.00 <\0 0000.00-02 00.0 <\w Pacovuco>coo mcwucmpa 04.0 <\» cowumcoaLoocH o—.o (\w .mum>wppzo Upowd 00.» (\w x040 0w.—. (\0 zo—d ommp_we A<\00 ospa> more: EouH .wEoomxm omoppwu 2000 Low nonwcomoo on 00000 opnawca> F000» “4 o_nme 5 (5 00° .0. .0 ...0 .0....__... .0.0..... .0._0=_ 0.. ..00 ...0 00.__.0-00 0.. .000_.=..000 000. 0. 00. .00....» _.00_0=..000 0. .00 0.....0 00.....-00 0. 0.0 .00._._0 _.=0..00.=00 .. .0. 0...... 000.... 00 0.00 00.00.00.000. 000—0.. .00 0.00 .0. .0000. 00.0.0__000 .000 0.000 00.0 000 0000 _-‘_0 00.0 0.0..00000 .000 _.\_0 00.0 0._.00.__0. 00.00 .000 .0000 00.0 000.0000 00 0000 .0 _-\00 00.. . .00. .0 .0000 0... .00. _-\_o 00.0 .000 0.0.0 00.0 000000...00 00.00 .000 000.0 00.. 0000.0. 0. .00. .0 ._.0_0 0... 000 .00. .0 _.\_0 0... 0.02.00... 0000 __\_0 00000.. 0.. 00.00 0000 _.\_0 00.. 00.000___0. 0. 0000 .0 _-\_0 .0.— 000 0000 .0 0.0.0 0... 0.00.0... 0000 0.0.0 00.. 000 .00.00 .000 _.\.0 00.. 00~..=.0 0. .000 _-\_0 00 0 000 . 0000 0.0.0 00.0 0.0.00...00 0000 000—0 0... 000.0000 00.00 0.0 0.000 00.0 00.000.00- 0. 0000 000.0 00000 0 0.- 32 =:0 :3. .0:32=:. 00.00 0.0 _=\_0 00.0 00.000.000 0. 0000 _-\_0 00.. 000 .000 .0000 00.. 000.000. 0_.0~ 000 .0000 00.0 00.00._0... 0. 1E: E 0000 0 .00— 0.00.0 .0_0.0__..0 00....— 0.. .u.:oo 0m aHan .00. .0 _.\_0 000 .000 .0 00000 0.0.000... 0..." 0.0 .00.. 0_.. .._.=__ 0. .000 .0 0.0.0 00.. 000 .002 .0 ..\_0 0... ._0..°.... .0.00 00. ..\_0 .n.. ._..._.0.. 0. . 0.. _. 0... .. ...0_.. 0.. _. 0... =0..=,. 00.00 00. _. .0.0 00_.0._.... _. .0. _. 0... ..0 ._._.0 0.. _. .0.. ._~..__... 0.. _. 00.0 .._gu._.,.. ._ 000 00‘.» 00.0 0000-_00_.0 0.. _.\_0 0... .._.=__ 0..00 0.. 0.0.0 .0.0 .0_.u._0... 0 000 .0 00.0 00.00.00.00 00. 0. 00.0 0.000_00.. 00.0. 0.. .— 00.. 00.00...“ . . 0.. _.\_0 0... .0..... 0.... 0.. 0.0.0 .0.” . 0._.0._.... _ . 0...... ..0_0 00.. .__.._.. . 0.... 0...... _.000 00.0 .0_.0._.... ..0 0...... _.0_0 . ._. c... 00.00 0.0.... _.\_0 .°_00._., . ... 0o 0_..0_000. 00.00 .0 .000..o_00 0 .2. _0 ._~..__... .0... .0. .0 ..l........ _ .00.. .00.. ._. 00.0 .000000. 00.___0 .000 .00 0.000 00_0_.00. 000 . 00.0-00.00. .0.0000 .00 0 0_0_0..0 .0 0.... 57 system were higher compared to the conventional tillage system due to specialized planting equipment. The burndown herbicide ’s value (unique to the no-tillage system) was an application of paraquat (1,1’-dimethyl— 4,4'bypyridinium as the dichloride salt) at 0.42 kg/ha rate because paraquat is the herbicide most often used to control emerged weeds in no— tillage systems. Soybean seed cost varied with the plant population as noted in Table 2. Variable costs do not include costs associated with land or management such as taxes, depreciation. and interest. Weed control. yield data, and breakeven price were subjected to analysis of variance. Soybean yield and the breakeven price of various herbicide treatments were compared with and without cultivation in the wide row spacing for both tillage systems. Soybean yield and breakeven price were also compared between the narrow and wide row (without cultivation) spacings in both tillage systems. The weed—free and weedy check were included as a reference. but were not statistically analyzed with the herbicide treatments for soybean yield and breakeven price analysis, but were included in the weed control analysis. Treatment means were compared using a least significant difference (LSD) test at the P<=0.05 when treatment effects were significant. Data were not combined over years because of significant year by treatment interactions. The statistical package used was MSTAT5 (Microcomputer STATistical program). 5MSTAT (Microcomputer STATistical program), East Lansing, MI. 58 RESULTS AND DISCUSSION Conventional tillage system weed control and soybean yields. 1g§§. In 1986, the common ragweed pressure was high and averaged 20 plants/ftz. Common ragweed control in cultivated wide row soybeans was significantly greater than in the narrow row spacing for the herbicide treatments of trifluralin plus metribuzin (trt. 1), metolachlor plus linuron (trt. 7), linuron followed by sethoxydim (trt. 13), bentazon following metolachlor (trt. 14), and bentazon, acifluorfen, or bentozon plus acifluorfen followed by sethoxydim (trt. 17, 18, 19) (Table 6). Postemergence broadleaf herbicide applications were made at the 4 to 8 leaf stage of common ragweed, and bentozon did not control common ragweed at this growth stage. Common ragweed control was greater when bentazon followed preemergence metolachlor (trt. 14) applications compared to treatments where bentazon was followed by sethoxydim (trt. 17) in the narrow row spacing and the uncultivated wide row spacing. Applications of metribuzin followed by sethoxydim (trt. 12), and metolachlor followed by either an acifluorfen (trt. 15) or acifluorfen plus bentazon (trt. 16) application provided common ragweed control equal to the weed-free check in the narrow row plots. Acifluorfen alone or acifluorfen plus bentazon treatments in the narrow row and uncultivated wide row spacing provided greater common ragweed control when metolachlor (trt. 15, 16) was applied preemergence than when sethoxydim (trt. 18, 19) was applied for annual grass control. Research has shown that acifluorfen controls common ragweed at the 6 to 7 leaf stage under conditions of both high temperature and high relative humidity (17). It appeared that metolachlor either reduced the common ragweed pressure, increased the 59 .___..~°_ _°__.._ z... :5.» .33... ...;:;.3.=_ 5.5.8. .3: 5.22.2. .55 23...: .523. 5322...: :35... is: 2 z 2 2 E 3 3.. . -, .2 .3. .3. - - - 3. =3 .... . - . 2 z - - - .5: = - - - z 2 z . - - .2332. .... . - z 2 2 - - - :33; 2.. . .3 . . 2 2 3. a: :4 3.. .2 3 2 3.. z... 2 a 3 .. 3 3 .2 = 2 _... 2.. 2.. 2 2 2.. N... z 2 2 3 z 3 2 z 2 3.. 2.. 2.. z 2 3.. 3.. . z 2 z .2 2 2 = : 2 2.. 2.. 2.2 2 2 3.. 1.. : z ._ N. N. a 3 2 : 3.. 3.. 3.. z 2 :4 3.. 2 2 . z 2 3 3 a z 2 2.. 2.. 3.. z a 3.. 2.. 2 a z 2: = 3 2: 3 = 1. 3.. 3.. z 2 .3 3.. z s z .2 a .2 2 3 : 2.. 2.. 3.. z 2 2.. 2.. z 2 z 3 a. s 2 2 2 :5 3.. 3.. a n 3.. 2.. 2 z N. a 3. 2 2 z : :.. 2.. 2.. z 2 3.. 2.. 2 2 z 8. 2: 2 = 3 . 3.. 2.. 8.. a 2 a... 2.. 2 3 2 3 2 2 2: m. . :.. 2.. m... 2 2 z. :3 _ z 2 2 z .2 2 2 3 m 3.. 2.. N... : 2 N... 3.. 2 z : 2 2: 2 : 2 _ 3: z... 2 . z. 2.. ._ ._ N. e a . . o :21...— .2 3.. 2 m. 2.. 3.. 2 z 2 2: 8. 3 3 .2 3.2:: .32........ - 3x 5;... n ._=9. ._=.. ._=.. ._=.- ._=u. ._=c- “co....__ ... .._. .o._.= .._. .o._ = .._. ..__ . .>. .._. .._. .o...= a... “.2. .._._..~= 3:. 3:. .------:.=.::.-: ..__. e >...._. ._._. =....om .9... =.>...._. e...» =..g.om .. .e..... .o. =o_..._._=. _°_.=°. .0.— .Szfizs 2: 5 .52.: x2 2: 5225—3 .3553: .2933; 3 .301: a. 8.3 5.2:; 2: .2... 53.3 :828 2; .2: E .3... 3.2: H. .3: 6O susceptibility of the weed to bentazon applications, or slowed the weed’s growth, such that it was more susceptible to the bentozon application. Common lambsquarters and yellow foxtail pressure was moderate in 1986 at 4 and 8 plants/ftz, respectively. Common lambsquarters control was equal to that of the weed—free check for all herbicide treatments except for the acifluorfen or acifluorfen plus bentazon followed by sethoxydim treatments (trt. 18, 19) (Table 6). Acifluorfen alone provided variable control of common lambsquarters in previous research (30). The common lambsquarters pressure in the field in 1986 was moderate, but potential problems could arise if it was the predominant weed since it is difficult to control with postemergence herbicide applications (30). All herbicide treatments provided greater than 92% control of yellow foxtail, except for the preplant incorporated treatment of trifluralin plus metribuzin (trt. 1) which gave only 82% control. Herbicides impacted weed control which in turn, impacted soybean yield in the wide row spacing when averaged over cultivated and uncultivated plots (Table 6). Weeds reduced the soybean yield in both the cultivated (53%) and uncultivated (63%) wide row soybeans when averaged across herbicide applications (Table 6). The degree of control achieved from a herbicide application in the wide row soybean was reflected in yield due to high common ragweed pressure. Soybean plots treated with metribuzin followed by sethoxydim (trt. 12), and metolachlor followed by either acifluorfen or bentozon plus acifluorfen (trt. 15, 16) had significantly greater soybean yields than soybean treatments with metribuzin plus trifluralin (trt. 1) or metolachlor plus metribuzin (trt. 5, 6), and also for metolachlor plus linuron (trt. 7), bentazon following metolachlor (trt. 14) preemergence or followed by sethoxydim (trt. 17) 61 and bentazon plus acifluorfen followed by sethoxydim (trt. 19) application. However, uncultivated plots had a significantly higher breakeven price than the cultivated plots where sethoxydim followed bentozon (trt. 17) application or where trifluralin plus metribuzin (trt. 1) was preplant incorporated (Table 6). Soybeans treated with bentazon followed by sethoxydim (trt. 17) had a significantly higher breakeven price than all other soybeans in both the cultivated and uncultivated plots. The ability to cultivate soybean fields has been shown to be very important if herbicides fail to control a predominant weed species (25). The lower soybean yield coupled with the higher cost of the postemergence broadleaf herbicide bentazon resulted in the higher breakeven price. Weeds reduced soybean yield in both narrow (80%) and wide (62%) row spacing in the non—herbicide treated plots (Table 7). and both row spacing and herbicide treatments impacted soybean yield (Table 6). Narrow row soybeans provide greater weed control possibly due to faster shading between the rows (A. 12, 16, 25). However, a crusted soil surface in 1986 resulted in poor soybean emergence and lower soybean plant populations per acre which resulted in slower canopy closure in the narrow row soybeans. Consequently. common ragweed was competitive and caused a greater yield reduction in the narrow row spacing in 1986. Narrow row soybeans yielded significantly lower compared to the wide row soybeans for herbicide treatments containing linuron (trt. 7. 13). Soybeans treated with acifluorfen (trt. 15, 18), metribuzin preemergence (trt. 6. 12), or bentazon plus acifluorfen plus sethoxydim (trt. 20) yielded significantly greater than other soybeans in the narrow row spacing. In the wide row spacing, soybeans receiving application of trifluralin plus metribuzin (trt. 1) preplant incorporated or bentazon 62 m ..m.: o v m n mo. owe mm .v o. uxuocu sumo: om mm es ”xooco sawmz .v he em ”amic1ummz we we mm Hootc-uwmz .m.: m .m.: .m.: .m.: .m.: no. em; mm mm mm ov.z so mm mm noom>_o.:o on om .N zottmz mm mm mm noom>e»_:o:: mommuo>< .m.c .m.c .m.c .m.c .w.: .w.c mo. am; mu «m N. one: _m an o_ noom>_opzo m_ me p zottmz mm an N. voum>¢u_:oc: "xooco xummz .e «v mm ou_z me vv em noon>wu_:o .v on no zottmz .v «e um nmom>iasaoca ”oocwlnooz "mmcmlvomz «\nn «\an mmm_ pom. oom_ mam. some cam. UP¢v> 3500.530 30L vco .covum>vupzo .Eoumxw omopp_u Pocowuco>coo as» c? mc_omam .wuooz xn nouomwuo ma npavx :monxow ”N o_nme 63 followed by sethoxydim (trt. 17) yielded significantly less than soybeans where metribuzin was followed by sethoxydim (trt. 12), or where metolachlor was followed by acifluorfen (trt. 15), or bentozon plus acifluorfen (trt.16). Previous research in soybeans planted in wide row spacing found no relationship between soybean yields and soybean stands (27). Therefore, under high common ragweed pressure and low soybean plant populations, soybean yield reflected the weed control achieved with the herbicide treatments in both the narrow and wide row spacings. The breakeven price for soybeans planted in the wide row spacing was lower than these in the narrow row spacing when averaged over herbicide treatments (Table 6). Thus, the additional seed cost was not justified in the narrow row spacing in 1986. Crusting of the soil reduced the narrow row soybean stands and soybean yields, irregardless of weed control measures. The breakeven price for soybeans treated with bentozon followed by sethoxydim (trt. 17) was significantly higher compared to all other herbicide treatments when averaged over row spacing. The higher cost of these two postemergence herbicides, coupled with the lower yield due to poor common ragweed control with the bentazon, resulted in the high breakeven price. legz. In 1987, common ragweed, velvetleaf, and common lambsquarters weed pressures averaged 1 plant/ftz. Redroot pigweed and yellow foxtail pressures averaged 3 plants/ft2 while yellow nutsedge pressure averaged 6 plants/ftz. Postemergence herbicide applications were made at the optimum growth stages for maximum weed control. Common ragweed was in the 1 to 4 leaf stage at the time of postemergence broadleaf application. Consequently, common ragweed control was 92% or greater for all herbicide treatments except metolachlor plus imazaquin preplant incorporated (trt. 64 3) and linuron followed by sethoxydim (trt. 13) (Table 8). Velvetleaf control was 92% or greater in plots treated with bentozon, acifluorfen, or bentazon plus acifluorfen (trt. 14 -20) (Table 8). Preplant incorporation of clomazone plus metribuzin (trt. 2) provided excellent control of velvetleaf. Other soil-applied broadleaf herbicides provided velvetleaf control ranging from #8 to 89%, but velvetleaf pressure was low in the field, and poor control did not influence soybean yield. All herbicides treatments gave greater than 86% redroot pigweed control, except for the linuron or bentozon applications when followed by sethoxydim (trt. 13, 17) (Table 8). Bentazon does not adequately suppress redroot pigweed (1). However, control increased under high humidity conditions when air temperatures were lower than 10°C and the weed was only 5 cm in height (13), or at the 3 leaf stage (1). Annual grass control was greater than 92% for all herbicide treatments except metribuzin plUs either trifluralin (trt. 1) or metolachlor (trt. 5, 6) (Table 8). The application of metolachlor followed by bentazon plus acifluorfen (trt. 16) gave superior yellow nutsedge control compared to all other herbicide treatments except for metolachlor plus either imazaquin (trt. 3) or metribuzin preplant incorporated (trt. 5), metolachlor plus linuron plus chlorimuron (trt. 9), bentozon followed by sethoxydim (trt. 17), and bentazon plus acifluorfen followed by or plus sethoxydim (trt. 19, 20). Both metolachlor and bentozon provided good suppression of yellow nutsedge, while imazaquin (trt. 3) and linuron plus chlorimuron (trt. 9) provided fair to good suppression. Moderate weed pressure in 1987 decreased soybean yields in both the cultivated and uncultivated wide row soybeans in the non-herbicide 65 u_c..x._. .oo.v.x. .=..__°.a._ ..~,=oug== .o...»_ n=g=~_=Um. m=_.a»o .wwm»u u.__.9.o_ .o...». n=~.=_ ._..Q~m .=d—wm w=;“=._.s< .m¢‘=< u._.~_go>_u>. _u...;go~g9 =°__o=a‘ .=_=m< “Aesoxa._ =oesau. .__o.__m_.w._. ._mo_n=< .dwm=‘ .~ ~_ . - ~_ _._ >9 .°.° m .. = ...= ...= .n.= ...= .m.= no. owl a: s .. .7 . . : . a s 2 a .35. - - .n.n .n ._ on _. n~ mo ._=u..v__ _..~ .. _..N _v o_ o. .. n. .¢ ._=U..v_. .un- u-K .u.= ...= .. = NN . ._ ~_ ~_ mo. and ...~ ._.~ on a. .m “a.” an." N.," _. on a. o. .. .. oo— oo. °~ ...~ nn.~ on m. mm mm.“ _... ...~ _. Kn . a. n. N. .o «a Na ,— n..~ .n.~ _. _. an .o.n aw." _..~ 9. .n _. .. co. co. 0. co. ._ .~.~ __.~ °m a. _. ~n.n 6..” _..~ .3 ”a a. .a oo. N. 09. a. h. um.~ an.“ on n. .m 0°.n _~.n ...~ _. an n. .9 .. .. co. cc. .- m_.~ o..~ a. _n a. ._.n ¢~.n .9.” .m .n .n .5 oo— oo. m. co. “— ..." .o.~ .. .n o. m..n o~.n ...n an o. .n .. .. .. «a co_ .— ...~ .n.~ .. .9 o. ._.n no." .N.. an _. .m .. 0°. ~_ .. .. n. ...~ ._.N m. .n .. “N.” .n.» .0." .n N” .n ~m co. .. m. .. ~_ .”.. .o.~ "a m. a. o..~ -.n m..~ a. o. m. .. . n. co. ._ n. __ .n.~ .°.~ .. _~ on _..~ .a.~ .‘.~ o. o. _. .. Nu .. .. no k ~..~ .~.~ ~. an .. ~_.n o_.o ._.n N" an an - a. .. .~ .a . m..~ oo.~ a. o. .m .9." ~_.n °¢.~ a. a. o. .o .. _. N. no m . ...~ ON.“ on .. .m .9." on." ._.~ _. on .. .. n. .Q .~ ~. n .a.~ _n.~ ~. ~. ~w .9.” om." ...~ .. an N. _. a. e. mm 0, ~ .~.~ .o.~ .. .. No .o.~ “N." ...~ 9. on .. a. .a .n a. .. _ .~.~ .n.~ 9a.. _. «a a. mo.“ _..~ ~..~ Na Na ~u . o . . a o .uwgu .9uu. _.._ n... .O.. _. .. on .°.~ °~.~ N... .. .. .. m. o. oo— n“ =9. ow...v-. (.3: r :5. - a_=u. ._=u- “.39. ._=u- gem—a...— .>. .u_. a». .u_. so...= a). .v.. .v_. .>. oe_. .¢_. : .9_u_n_~= wwm»g =dpww w¢<=< =_=m‘ .wm=< ._._> =.~..°w .u..g =~>.‘.... u_._> =~ua.om . .=_u.nm .9: =a_g.>_9_=u _o_~=oo a. . .3: s 5:: .22: .=o_.=.>=°u .g. e. ac.“ . .o. v:. _=°_..>...=u ..0=~.Q ._g .t.u_a..= .a togum~_. a. .9..a =.>o‘...a as. .c_._. =.~..om __o..cou gem. n. ._D._ 66 treated plots (Table 7). However, the main effects of either cultivation or herbicide treatment were not significant for either soybean yield or breakeven price (Table 8) in 1987, reflecting little impact of weed control on soybean yield and not justifying higher priced herbicides. Soybeans planted in the narrow row spacing had higher soybean yield compared to those in wide row spacing when averaged over the weed—free and weedy check (Table 7). Weeds did not reduce the yield of soybeans planted in narrow rows when compared to the weed—free check. Consequently, variable weed control of the herbicide treatments was not reflected in soybean yield. Soybeans planted in narrow rows had higher yields than soybeans planted in the wide row spacing when averaged over herbicide treatments (Table 8). This is in agreement with other research where soybeans planted in narrow row spacings had the potential for higher yields compared to soybeans planted in the wide row spacing (4, 8, 16, 24, 25). Consequently, breakeven price was significantly lower for soybeans in the narrow row spacing compared to the wide row soybeans when averaged over herbicide treatments (Table 8), thus justifying the additional seed cost for the narrow row spacing. lggg. In 1988, moisture was below average early in the growing season, and weed pressure was low. Moisture is a critical factor for maximum soybean yields. The amount and duration of seasonal rainfall influences soybean yield (26), and distribution of water over time may be more important than total rainfall or irrigation (19). Common ragweed, velvetleaf, yellow foxtail and yellow nutsedge were present at 1, 1, 2, and 3 plants/ftz, respectively. No interaction was present; and the main effect of herbicide treatments and cultivation was shown. Cultivated wide row plots had greater common ragweed control compared to the 67 uncultivated wide row and the narrow soybean spacing when averaged over herbicide treatments (Table 9). Preemergence applications of either metribuzin (trt. 6, 12) or linuron (trt. 7,13) provided poor common ragweed control (Table 9). Dry weather reduced the soil-applied herbicides’ effectiveness on this small seeded broadleaf weed. Postemergence applications of either bentozon or acifluorfen (trt. 14, 15, 17) provided good control of common ragweed with the exception of the acifluorfen followed by a sethoxydim application (trt. 18) (Table 9). Poor control noted in this treatment cannot be explained, since common ragweed is susceptible to acifluorfen (18). ‘ Cultivation improved velvetleaf control when averaged over herbicide treatments (Table 9). Velvetleaf control was equal to the weed-free check for all herbicide treatments except metolachlor plus linuron (trt. 7) or acifluorfen followed by sethoxydim (trt. 18) (Table 9). Annual grass control was equal to the weed—free check for all herbicide treatments except linuron plus metolachlor (trt. 7) or metribuzin followed by sethoxydim (trt. 12) (Table 9). This poor grass control in the sethoxydim treated plots was due to a late flush of yellow foxtail which emerged after the sethoxydim application. Planting soybeans in narrow rows and wide rows with cultivation improved yellow nutsedge control. Herbicide treatments containing either linuron (trt. 7, 13) or acifluorfen (trt. 15, 18) provided the least control of yellow nutsedge (Table 9). Under limited moisture conditions early in the growing season, weeds and soybeans compete for moisture. In previous research, soybean yield reductions were greater when soil moisture was abundant early in the spring and limited later in the season, than when limited early and then (323 .......o. .o._... .:.=._=u.. ......u ...... "...—.... .o..... .=..=. ...—... .=.... ........>_.>. ...—...o... =o__.=.< _=_=.. ..coox... cons... ...o.__._:.... ......n. ....x. .. .. .. N. ... .9 .... .... .... N. .... . .. ... ... . ... . .... . o. - - .. .. .. .. .....z - - N... .. .. m. m. .. ..=u..._. .... .. .... m. o. .. .. .. ......... .un- $3.. .... . .... . n .N .. NN .N ... ... .... .... ...N .. .. N. .N.. NN.. .... .. o. .. .. ... .. .. .N .... .... ...N .. m. N. .... ...N .... .. .. m. .. .. .. m. ._ .... .... .... .. N. e. .N.. ...N .... .. N. N. .. .. .. ._ .. ...N ...N N..N e. .. _. ...N ...N ...N N. m. .. .. ... .. o. ._ .... .... .N.. .. .. .. .... .... .... .. e. .. .. ... o. .. .. .... .... .... .. .. .. .... .... .... .. .. .. .. .. .. .. m. ...N ...N ...N N. o. .. ...N N..N ...N .. .. o. .. .. .. .. . .... N... ...N .. .N e. .... .... N... .. .. .N .. .. .. _. .— ._.. .... ...N .. .. .. .... ...N .... .N .. .. .. .. .. .. N. .... . .... N... .. .. .. .... .... .... .. .. .. . N. .. .. o. .. .... .... .... N. .N .. .... .... .... N. .. .N _. .. N. .. . .... .... .... .. .. .. .... .... .... .. .. .. N. .. o. .. . . ...N .... ...N .. .. .. .... .... .... .. o. .. .. o. .. .. m N... .... ...N .. .. N. N... .... .... .. .. .. .. .. .. .. . ...N ...N ...N .. .. N. ...N ...N ...N N. .. .. N. .. N. .. N ...N .... ...N .. .. N. ...N N..N .... .N .. m. N. .. .. .. . N... .... ..... .N .N .. .... .... .... .N .. .N .N .. .. .N ....u ....» ...N .... ...N ... .. .. ...N ...N .... N. .. .. .. ... ... ... ........_ :3: ...—.3 a: .0 E.,: . I 1. . .. 1 ...: - ...... - ._=u. ..au. ..au. ..=u- r .c....... ... .... .... = . . .... .... = ... .._. .... ... .... .... ...o_._.= . .N... =._.. =.:.. ...=. on... ......o.. a...» =....o. .9... =.>...... ....> .....o. . .=.u... so. go...>.._=. .....o. .... ..... c. ...... ..._._. .=o_.=.,¢°u ... .. .... .. so. ... .ca...>.._=. ...c. ..... ......... .. ........ .. .9... =........ ... .._... .....o. .....=o. .... U. ..... 69 remaining above average until soybean maturity (5). Weeds in 1988 reduced soybean yields in cultivated and uncultivated wide row spacing, and in narrow rows when compared to the the weed-free check (Tables 7). A 28% yield reduction occurred due to weeds in the cultivated plots, 32% in the uncultivated plots, and 56% in the narrow rows, when compared to the weed-free check. Therefore in a dry spring, cultivation provided better weed supression than planting soybeans in narrow rows. Cultivation did not increase soybean yield, when averaged over herbicide treatments. However, herbicide treatments impacted soybean yield when averaged over cultivated and uncultivated plots (Table 9). Soybeans treated with bentozon (trt. 14, 17) had greater soybean yield than soybeans treated with metolachlor plus linuron (trt. 7). Low yields in this treatment were a reflection of poor weed control. However, applications of metolachlor plus either imazaquin (trt. 3) or linuron plus chlorimuron (trt. 9) provided excellent weed control which was not reflected in greater soybean yield. Possibly stress caused by these herbicide treatment coupled with below average moisture conditions may have reduced the yield of these plots in 1988. Imazaquin and chlorimuron are root inhibitors and may reduce root biomass, which in turn, may have lowered the soybean plant's ability for water and nutrient uptake in a dry year. Cultivation did not effect the breakeven price so the herbicide treatments were combined over cultivation treatments. The breakeven price for herbicide treatments containing linuron (trt. 7, 13) or imazaquin (trt. 3) was significantly higher than the breakeven price for both preplant incorporated treatments of either metribuzin plus clomazone (trt. 2) or metribuzin plus trifluralin (trt. 1), and for the application 70 of metolachlor followed by bentozon (trt. 14) (Table 9). Low yields in the linuron (trt. 7, 13) and imazaquin (trt. 3) treated plots resulted in the higher breakeven price even though herbicide costs were lower. When narrow row spacings were compared with wide row spacing in 1988, soybeans treated with metolachlor followed by bentozon (trt. 14) yielded significantly higher than the metolachlor plus linuron (trt. 7) treated soybeans (Table 9). The breakeven price for the metolachlor followed by bentozon (trt. 14) treatment was lower than for treatments containing linuron (trt. 7) or metolachlor followed by acifluorfen (trt. 15) (Table 9), reflecting the greater herbicide cost and reduced soybean yield. In summary, weed control impacted the breakeven price in the conventional tillage system. Narrow row soybeans had a lower breakeven price compared to the wide row soybeans, when weed control was 80% or greater, thus justifying the additional seed cost of the increased plant populations. Cultivation, however, had a lower breakeven priced if weeds were not controlled by herbicide applications prior to cultivation. The cost of two cultivations at $9.00 would be justified if the soybeans yield increased 2 bu/A at $4.50/bu soybeans. Weed pressure in the conventional tillage system consisted primarily of small—seeded broadleaf weeds, such as common ragweed, redroot pigweed, and common lambsquarters. The low cost of preplant incorporated herbicides such as trifluralin plus metribuzin (trt. 1) resulted in low breakeven price, since the small seeded broadleaf weeds were adequately controlled. The combination of soil—applied grass herbicides followed by a postemergence broadleaf herbicide gave consistent weed control in all three years of research, which resulted in lower breakeven prices. 71 Postemergence broadleaf herbicide applications must be made at the optimum growth stage of the weed for maximum effectiveness. Weed control was dependent on the ability to apply a postemergence herbicide that had activity on the weed species present (proper weed seedling identification), and to time the postemergence herbicide application at the optimum weed growth stage. Soil moisture conditions were critical for both postemergence and soil—applied herbicide activity. No-tillage system weed control and soybean yield. lggg. In 1986, no interactions between herbicide treatment, cultivation or row spacing occurred, and therefore only main effects were presented. Common ragweed pressure was moderate at 2 plants/ftz. Annual grass pressure consisted primarily of yellow foxtail, and was moderate at 18 plants/ftz. All herbicide treatments provided similiar control of common ragweed and yellow foxtail except for the metolachlor plus linuron plus chloramben (trt. 11) application (Table 10). Common ragweed and yellow foxtail control increased when soybeans were planted in narrow spacing or when cultivated in wide rows (Table 10). Uncultivated wide rows had lower annual gross control than the cultivated wide row spacing, but control in both the cultivated and uncultivated row spacing was equal to that of the narrow row spacing when averaged over herbicide treatments. Soybean yield in the weed—free and weedy check plot did not differ due to cultivation nor the presence of weeds (Table 11). However, when herbicide treatments were evaluated, cultivation significantly increased the soybean yield of the acifluorfen followed by sethoxydim (trt. 18) treatment (Table 10). When acifluorfen followed a preemergence metolachlor (trt. 15) application, soybean yield was significantly greater than when acifluorfen was followed by a sethoxydim (trt. 18) 72 Tahle 10: Head control, seyhean yield, and treekeven price as affected by nerhicide treetaents. cultivation, and l’OI spacing in the no-tillege systee in 1885. weed Central Cultivation Roe Spacing Soybean Yield Breakeven Price Soyheen Yield Breakeven Price AHBEL SETLU -----—— Herbicide ride ride lid! lide avg nerrne lide narree lid: Treateent -cult écult -cult +cult " Z hull: 31'5". "_hu"'-°‘ “*5/01?“ deed-free 96 89 38 42 .18 54 39 1 Ieedy check 43 38 36 43 2 10 1 86 .03 41 35 2.10 5 84 87 40 40 2.54 2.85 2.58 57 40 1.83 2.54 7 80 85 42 44 2.40 2.45 2.43 51 42 2.08 2.40 10 87 88 38 43 3.03 2.82 2.87 51 38 2.32 3.03 11 82 81 35 41 3.32 3.04 3.18 52 35 2.28 3.32 12 85 80 43 38 2.54 2.84 2.74 48 43 2.35 2.54 13 88 83 43 42 2.45 2.57 2.55 53 43 2.05 2.45 14 88 83 45 41 2.38 2.80 2.54 51 45 2.17 2.38 15 88 88. 45 37 2.35 3.38 2.87 48 48 2.31 2.35 15 88 88 42 38 2.74 3.11 2.82 45 42 2.55 2.74 17 85 84 45 43 2.34 2.71 2.52 44 45 2.57 ' 2.34 18 88 88 34 45 3.50 2.58 3.08 51 34 2.30 3.60 18 88 88 35 38 3.71 3.10 3.40 54 35 2.14 3.71 20 100 83 43 41 2.50 2.85 2.77 55 43 2.08 2.50 LSD .05 13 15 3 . n.s. 8 0.84 A‘veuga. Vida-cult 86 85 - 2.17 - ‘ lide+cult 85 85 - 2.88 - - Harrow 83 80 - - - - L511 .05 4 5 - - 11.5. CY (11 13 21 12 22 AMBEL, Ambrosia artemisiifolia (Cannon rageeed); SETLU. Setarie lutens (Yelloe foxtail). 73 “xoozo xuooz "mogwivomz mo. ow; "xooco xnwmz ”moguluooz mo. om; umpm>wu_:o vmam>.p_:o:: any: zoggmz any: zoLLmz ”mogmiuooz mo. om; uwum>*u—:o umum>vupsocz "xomco xummz umum>¢u_ao um»m>.u.:o:: ”mwcuivmwz m .w.: ~ an mu NV .m.c n m— on ON 0? .w.: .w.c @ on p —v vw an NM vm « .n mwm— oom— uPo—> connman .Eouwxm ommp—_ulo: mzu cw mcwomam :0; van .:o_um>_upzo .mnwoz >5 nouoomwm ma 3pm?» cmmnxom n__.m_nm~ 74 application in both cultivated and uncultivated plots. These results are similiar to the conventional tillage system where metolachlor may have reduced common ragweed pressure or slowed the weed's growth. Neither cultivation nor herbicide treatments impacted breakeven price in 1986 (Table 10)l due to the moderate weed pressure not impacting soybean yield. and in turn not impacting the breakeven price. Soybeans planted in narrow rows had higher yields than the wide row spacing for each herbicide treatment except metolachlor plus linuron (trt. 7). metribuzin followed by sethoxydim (trt. 12), and bentozon, acifluorfen, or bentozon plus acifluorfen, followed by sethoxydim (trt. 17, 18, 19) (Table 10). Soybeans planted in narrow rows had significantly higher yields compared to the wide row spacing in the weed- free plots (Table 11) and when combined over herbicide treatments (Table 10). Soybean yield in the narrow row plots receiving a metribuzin plus metolachlor (trt. 3) application was significantly greater than where metribuzin was followed by a sethoxydim (trt. 12). In the wide row spacing, soybeons treated with metolachlor followed by acifluorfen (trt. 15) had significantly greater yield than when acifluorfen was followed by a sethoxydim (trt. 18) application. This cannot be explained since both herbicide treatments had greater than 98% control of both common ragweed and yellow foxtail at the end of the soybean growing season. The breakeven price of metolachlor plus linuron plus chloramben (trt. 11) and bentazon plus acifluorfen followed by sethoxydim (trt. 19) treated plot was significantly higher for soybeans planted in the wide row spacing compared to the narrow row spacing. not justifying the additional seed cost (Table 10). In the narrow row soybeans the breakeven price of the bentozon followed by sethoxydim (trt. 17) treated plot was significantly 75 higher than metolachlor plus metribuzin (trt. 6) treated plot. The higher cost of the postemergence herbicide bentozon, along with the lower yield of this plot treatment increased the breakeven price. In the wide row spacing, plots receiving either acifluorfen or bentazon plus acifluorfen followed by sethoxydim (trt. 18, 19) applcations had significantly higher breakeven prices than other herbicide treatments, with the exception of those containing chloramben (trt. 10, 11). The high cost per acre of chloramben, bentozon, and acifluorfen was not justified by increased soybean yield in 1986. legz. In 1987, weed pressure was low to moderate in the no—tillage field with common ragweed, redroot pigweed, and yellow nutsedge present at 1, 1, and 6 plants/ftz, respectively. Common ragweed control was greater than 90% for all herbicide treatments with the exception of metolachlor plus metribuzin (trt. 6), metolachlor plus imazaquin (trt. 4), and linuron followed by sethoxydim (trt. 13) (Table 12). Soil—applied herbicides were not as effective as postemergence herbicides, as applications of acifluorfen or bentozon provided excellent common ragweed control because the weed was smaller than the 8 leaf stage at the time of postemergence herbicide application. Redroot pigweed control was equal to the weed—free check for all herbicide treatments, except for metolachlor followed by bentozon (trt. 14), bentazon followed by sethoxydim (trt. 17),linuron followed by sethoxydim (trt. 13), and bentozon plus acifluorfen plus sethoxydim (trt. 20) (Table 12). Bentazon is not an effective redroot pigweed herbicide, and should not be applied postemergence for redroot pigweed control. The herbicide treatment of metolachlor plus linuron plus chlorimuron (trt. 9) gave superior control of yellow nutsedge compare to all other herbicide treatments except that ..NNN.»N== :°__NN_ m=N=~_=UN. m=.NNNu .NNNNQ N.v~N.N_N NOO.NNN. m=.o__o.N~. 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N. .. a..N .N._ nm .N N. NN.N .N.N NN.N N. _. .N N. N. NN N N... N..N .N .0 NN NN.N _N.N NN._ N. N. N» N. _. N. N NN.N .N.N .N N. _. NN.N m..N NN.N .. N. N. N. oo— .. N N... N... o... a» N. N» NN._ N... .... N. N. N. N. .N NN Nuogu News. N... N... .N.. NN _. N. N... N... NN.. N. on _. N. co. NN -_N.N~y_ . :N:: .5: 1::— I :.J 4‘. - A I N_=u. N_=u- ._=u. N_=u- NascN...N ... .N.. .o__ N . .u_. .a.. = .>. .g.. .n_. .N. .v_. .e_. NN_U_9_~= NNNNN NNNNN NNNIN .0... =.,.N ... a...» =..a.om .u_.N =.>.N...N ...NN =..NN°m N=NUNNm .ox =o_N.N.N_=o _oNN=oo Now. .NNNN =_ .aNmNm NNN___N.O= .g. :. .=_u N. .o. 9:. .=o_N.>NN_=u ..N=¢.NNuNN .u.u.a.u; N. vNNQNNN. . .u_NN =.NNN—NNN ec- .u_._N =NNaNou ._°_N=°u sum. “N. ..pNN 77 of metolachlor plus either imazaquin (trt. 4) metribuzin plus chlorimuron (trt. 8), or metolachlor followed by a postemergence broadleaf herbicide (trt. 14-16) (Table 12). As in the conventional tillage system, metolachlor provided good yellow nutsedge control, and chlorimuron (trt. 8, 9) increased the effectiveness of this treatment. In 1987, neither cultivation nor weeds (without herbicide treatments) influenced soybean yield as weed pressure was low (Table 11). Consequently, all herbicide treatments provided good to excellent weed control, regardless of cultivation, and no difference in soybean yield or breakeven price occurred (Table 12). Soybeans planted in the narrow row spacing yielded significantly greater than those planted in the wide row spacing in both the weed-free and weedy plots (Table 11), and also when averaged over herbicide treatments (Table 12). Recall, the narrow row spaced soybeans were replanted in 1987 resulting in a higher soybean plant population per acre (Table 2). The breakeven price of the narrow row spacing soybeans was significantly lower than the wide row spacing soybeans when averaged over herbicide treatments (Table 12). However, small differences in yield, coupled with the larger relative differences in herbicide prices, resulted in the bentozon followed by sethoxydim (trt. 17) and metolachlor plus linuron plus chloramben (trt. 11) treated plots having significantly higher breakeven prices than all other herbicide treatments except metolachlor plus metribluzin plus chloramben (trt. 10), metolachlor followed by bentozon (trt. 14), and acifluorfen or acifluorfen plus bentozon followed by sethoxydim (trt. 18, 19). 1988. In 1988, common ragweed pressure was moderate at 4 plants/ftz. Postemergence broadleaf herbicide applications were made when the common 78 ragweed was in the 2 to 6 leaf stage. However, below average rainfall conditions were not conducive for active plant uptake of the postemergence broadleaf herbicide. Therefore, control of common ragweed in both the cultivated and uncultivated wide row spacing was good except for plots treated with linuron (trt. 7, 13) or imazaquin (trt. 4). The preemergence metolachlor application made prior to the postemergence broadleaf herbicides (trt. 14—16) (Table 13) application had increased common ragweed control. Again, metolachlor appeared to increase the susceptibility of the common ragweed to postemergence herbicide applications by reducing either the numbers or the size of the weed. In the narrow row spacing, common ragweed control was lower in plots treated with metolachlor plus metribuzin (trt. 6), metolachlor plus linuron (trt. 7), linuron followed by sethoxydim (trt. 13), metolachlor followed by bentozon (trt. 14), and either acifluorfen or acifluorfen plus bentozon followed by sethoxydim (trt. 18, 19) when compared to the weed—free check (Table 13). Weeds reduced soybean yield in both the cultivated and uncultivated wide row spacing check plots in 1988 (Table 11). When soybean yields of the herbicide treated plots were averaged across cultivation, all the postemergence broadleaf herbicide treatments yielded less than the metolachlor plus metribuzin (trt. 6) or the metolachlor plus metribuzin plus chlorimuron treatments (trt. 8) (Table 13). This lower yield reflected the poor common ragweed control possibly due to the droughty conditions at the time of postemergence herbicide application. The soybeans treated with metolachlor plus imazaquin (trt. 4) or the linuron followed by sethoxydim (trt. 13) also yielded significantly less than the metolachlor plus metribuzin (trt. 6) or the metolachlor plus metribuzin 79 leble 13: leed control, soybean yield, and breekeven price es ailected by herbicide met-eats, cultivation, end roe spacing in the no-tillege systee in 1888. Ieed Control Cultivation Roe Spacing Soybeen Yield Breakeven Price Soybean Yield Breakeven Price WHEEL-"m , Herbicide lide eide nerro’e llde eide eve lide lide eve nerroe eide narrol lide lreetnent -cult ecult' -cu1t 1cult -c11|t ecult : bull. 4/5. ---bu/1--— ”Who-- Yeed°free 88 80 88 24 28 25 ' 3.14 3.04 3.08 32 24 2.54 3.14 Yeedy check 0 3 60 6 12 5 20.20 15.41 11.84 1 6 16.04 20.20 4 78 50 51 18 21 20 5.14 5.48 5.81 26 ’ 18 4.38 6.14 5 51 71 78 25 28 25 4.11 4.08 4.10 25 25 4.41 4.11 1 25 55 51 23 22 22 4.51 - 5.10 4.58 15 23 1.34 4.61 8 80 73 81 28 25 27 3.52 4.55 4.04 31 28 3.53 3.52 8 83 76 83 23 24 23 5.61 4.52 5.12 28 23 3.80 5.51 10 80 85 81 25 28 25 4.82, 4.51 4.57 27 25 4.87 4.52 11 75 73 80 28 20 24 4.11 31.44 11.78 32 25 3.85 4.11 12 58 80 81 25 24 24 4.18 4.81 4.55 28 25 4.78 4.18 13 38 48 43 21 18 18 5.53 7.01 6.27 18 21 13.12 5.53 14 84 41 61 17 23 20 5.75 5.23 5.00 32 17 3.58 5.75 15 51 28 52 15 23 18 8.21 5.13 1.20 28 15 4.12 8.27 15 78 38 50 17 22 18 7.72 5.58 5.58 30 17 4.08 7.72 17 81 35 55 15 23 20 8.14 5.23 7.18 27 .15 4.43 8.14 18 55 18 53 12 20 18 10.21 5.03 8.15 25 12 4.71 10.27 18 50 48 51 22 20 21 5.15 5.41 5.28 25 22 4.58 5.15 20 80 48 50 22 18 20 5.73 6.53 5.13 28 22 ‘ 4.20 5.73 1.50 .05 31 8 11.8. 11 4.55 :mqe. Yide-cult - - - 21 1.11 - ’ — Yidelcolt - - - . 22 8.88 - - 1mm - - - - ' - - - 150 .05 ~ - - 11.3. 11.x. - . CV 15) 25 148 23 57 AXBEL. Ambrosia erteeisiiiolie (Cannon ragweed). 80 plus chlorimuron (trt. 8) (Table 13), reflecting the poor common ragweed control provided by linuron (trt. 7, 13) and imazaquin (trt. 4). However, neither herbicide treatments nor cultivation influenced the breakeven price (Table 13), showing the difference in soybean yield was offset by the same relative difference in herbicide price . Weeds significantly decreased soybean yield when compared to the weed—free check in both the narrow and wide row spacing in 1988 (Table 11). Soybeans planted in narrow row spacing require effective early weed control until canopy closure (4). Soybeans which received postemergence broadleaf herbicide applications yielded significantly higher in the narrow row spacing compared to the wide row spacing with the exception of the bentozon plus acifluorfen application when combined with or followed by sethoxydim (trt. 19, 20). Soybeans planted in narrow rows receiving metolachlor plus linuron (trt. 7) yielded significantly less than all other herbicide treatments except for those treated with linuron followed by sethoxydim (trt. 13) because of the poor common ragweed control. In the wide row spacing, soybeans treated with metolachlor plus metribuzin plus chlorimuron (trt. 8) had a higher yield than soybeans treated with acifluorfen followed by sethoxydim (trt. 18) (Table 13). Soil-applied compounds have residual activity in the soil and were able to provide additional weed control later in the soybean growing season. Postemergence broadleaf herbicides which were applied only once during the season provided no residual activity to stop subsequent weed germination and growth. The breakeven price of the linuron followed by sethoxydim (trt. 13) treatment in the narrow row spacing plot was significantly higher than in the wide row spacing, and this treatment also had a significantly higher breakeven price than soybeans in all I, s 81 other herbicide treatments in the narrow row spacing. The herbicide treatments of bentozon or acifluorfen followed by sethoxydim (trt. 17, 18) had a significantly higher breakeven price in the wide row spacing than in the narrow row spacing. The high cost per acre of acifluorfen coupled with the lower soybean yields resulted in the higher breakeven price both between and within narrow and wide row spacings. In summary of the no—till system, weeds in the untreated check decreased soybean yield when compared to the weed-free check in 1988 only. Cultivation did not increase soybean yield in any year of research in the weedy no-till plots. However, in the weed—free soybean plots, yields tended to increase when cultivated (3, 9, 4 bu/A in 1986, 1987, 1988, respectively), possibly due to cultivation aerating the soil or increasing water infiltration in the no—tillage fields. Narrow row widths did not increase weed control, but did increase soybean yield in two years of research. Therefore, herbicide treatments impacted weed control, which in turn impacted soybean yield and breakeven price. Broadleaf weed pressures were less in the no—tillage system compared to the conventional tillage system. Weeds consisted primarily of small— seeded broadleaves with varying grass pressures. The herbicide treatments which gave the lowest breakeven price in the three years of research were the metolachlor plus metribuzin (trt. 6) application, or metolachlor plus metribuzin plus chlorimuron (trt. 8). In conclusion, consideration of the weed spectrum is important when choosing a tillage system. In the no—tillage system, preplant incorporated herbicides cannot be applied. Soybeans planted in narow row spacing with increased plant populations result in higher yields when weeds are controlled. If the risk of herbicide failure is high (ie. 82 weeds are difficult to control), the option for cultivation in wide row spacing may be important for effective weed control. Literature Cited Anderson, R. N., W. E. Lueschen, D. D. Warnes, and W. W. Nelson. 1974. Controlling broadleaf weeds in soybeans with bentozon in Minnesota. Weed Sci. 22:136-142. Amstrong, D. L., J. K. Leasure, and M. R. Corbin. 1968. Economic comparison of mechanical and chemical weed control. Weed Sci. 16:319-571. Burnside, 0. C. 1979. Soybean (Glycine mg!) growth as affected by weed removal, cultivar, and row spacing. Weed Sci. 27:562—565. Burnside, 0. C. and W. L. Colville. 1964. Soybean and weed yields as affected by irrigation, row spacing, tillage, and Amiben. Weed Sci. 12:109—112. Eaton, B. J., K. C. Feltner, and 0. 6. Russ. 1973. Venice mallow competition in soybeans. Weed Sci. 21:89—94. Edwards, J. H., D. L. Thurlow, and J. T. Eason. 1988. Influence of tillage and crop rotation in yields of corn, soybeans, and wheat. Agron. J. 80:76-80. Hammerton, J. L. 1972. Effects of weed competition, defoliation, and time of harvest on soybeans. Expl. Agric. 8:333-338. Lehman, W. F. and J. W. Lampert. 1960. Effects of spacing of soybean plants between and within rows on yield and its components. Agron. J. 52:84-86. Lybecker, D. W., R. P. King, E. E. Schweizer, and R. L Zimdahl. 1984. Economic analysis of two weed management systems for two cropping rotations. Weed Sci. 32:90—95. Lybecker, D. W., E. E. Schweizer, and R. P. King. 1988. Economic analysis of four weed management systems. Weed Sci. 36:846—849. McWhorter, C. G. and W. L. Barrentine. 1975. Cocklebur control in soybeans as affected by cultivars, seeding rate, and methods of weed control. Weed Sci. 23:386-390. Murdock, E. C., P. A. Banks, and J. E. Toler. 1986. Shade development effects on pitted morningglory (Ipomoea lacunosa) interference with soybeans (Glycine max). Weed Sci. 34:711—717. Nalewaja, J.D., J. Pudelko, and K. A. Adamczewski. 1975. Influence of climate and additives on bentozon. Weed Sci. 23:504—507. Ogg, A. G. Jr., and J. H. Dawson. 1984. Time of emergence of eight weed species. Weed Sci. 32:327-335. 83 19. 20. 21. 22. 23. 2h. 25. 26. 27. 84 Pareja, M. R., D. W. Staniforth, and G. P. Pareja. 1985. Distribution of weed seed among soil structural units. Weed Sci. 33:182-189. Peters, E. J., M. R. Gebhardt, and J. F. Stritzke. 1965. Interrelations of row spacings, cultivations and herbicides for weed control in soybeans. Weed Sci. 13:285-289. Ritter, R. L. and H. D. Coble. 1981. Influence of temperature and relative humidity on the activity of acifluorfen. Weed Sci. 29:480- 485. Ritter, R. L. and H. D. Coble. 1981. Penetration, translocation, and metabolism of acifluorfen on soybean (Glycine mgx), common ragweed (Ambrosia artemisiifolia) and common cocklebur (Xanthium strumarium). Weed Sci. 29:474—480. Robinson, E. L., G. W. Langdale, and J. A. Stuedemann. 1984. Effect of three weed control regimes on no-till and tilled soybeans ( (Glycine max). Weed Sci. 32:17-19. . Schweizer, E. E. and R. L. Zimdahl. 1984. Weed seed decline in irrigated soil after six years of continuous corn (Egg mays) and herbicides. Weed Sci. 32:76—83. Snipes, C. E., R. H. Walker, T. Whitwell, G. E. Buchanan, J. A. ‘ McGuire, and N. R. Martin. 1984. Efficay and economics of weed control methods in cotton (Gossypium hirsutum). Weed Sci. 52:95— 100. Tyler, D. D. and J. R. Overton. 1982. No—tillage advantage for soybean seed quality during drought stress. Agron. J. 74:344—347. Vasilas, B. L., R. W. Esgar, W. M. Walker, R. H. Beck, and M. J. Mainz. 1988. Soybean response to potassium fertility under four tillage systems. Agron. J. 80:5—8. Wax, L. M., W. R. Nave, and R. L. Cooper. 1977. Weed control in narrow and wide-row soybeans. Weed Sci. 25:73-78. Wax, L. M. and J. W. Pendleton. 1968. Effects of row spacing on weed control in soybeans. Weed Sci. 16:462—465. ' Webber, C. L. III, M. R. Gebhardt, and H. D. Herr. 1987. Effect of tillage on soybean growth and seed production. Agron. J. 79:952— 956. Webber, C. L. III, H. D. Herr, and M. R. Gebhardt. 1987. Interrelations of tillage and weed control for soybean (Glycine mgx) production. Weed Sci. 35:830—856. 28. 29. 30. 31. 85 Wicks, G. A. and B. R. Somerhalder. 1971. Effect of seedbed preparation for corn on distribution of weed seed. Weed Sci. 19:666-668. Wilcut, J. W., G. R. Wehtje, and R. H. Walker. 1987. Economics of weed control in peanuts (Arachis hypogaea) with herbicides and cultivations. Weed Sci. 35:711—715. Wilson, H. P. and T. E. Hines. 1987. Snap bean (Phaseolus vulgaris) and common lambsquarters (Chenopodium album) response to acifluorfen. Weed Tech. 1 18-21. Wrucke, M. A. and W. E. Arnold. 1985. Weed species distribution as influenced by tillage and herbicides. Weed Sci 33:853-856. CHAPTER 3 ABSTRACT Field studies were conducted in 1987 and 1988 to evaluate the control and competitiveness of common lambsquarters in soybeans at four densities and four times of removal including all season competition. Lambsquarters were removed at certain growth stages either by hand or with a postemergence herbicide application of bentozon plus acifluorfen plus crop oil concentrate. A 20% soybean yield reduction (compared to the weed-free check) occurred when lambsquaters were removed by hand for 32 plants/10 m row for all season competiton in 1987 and for 32 plants/10 m row at 10 weeks after soybean emergence in 1988. However, when removal was attempted with a postemergence herbicide, a 20% yield reduction occurred if lambquarters were greater than 15 cm in height at the time of application in 1987. In 1988, all postemergence herbicide applications failed to control lambsquarters as a result of the droughty conditions. The postemergence herbicide’s degree of control was reflected in both the number and the size of the remaining lambquarters' plants. In 1987, the first herbicide application timing reduced lambsquarters’ densities and plants were removed. The second and third postemergence application timings did not reduce the number of lambsquarters remaining. In 1988, plants were not removed by any herbicide application. Neither seed production per hectare nor seed germination was affected by postemergence herbicide applications. A strong correlation existed between increasing dry weight of the remaining uncontrolled plants and seed production per 86 87 plant. The minimum number of lambsquarters plants in the handweed study is higher and could remain longer where weeds were entirely removed compared to the postemergence study where the weeds were only partially removed. Lambsquarters has to be controlled before 5 to 10 cm in height, or the cost of the postemergence application will not be economically justified because of poor weed control, no increase in soybean yield, and uncontrolled weed seed production. 88 COMMON LAMBSQUARTERS COMPETITION AND TIME OF REMOVAL IN SOYBEANS INTRODUCTION Common lambsquarters (Chenopodium album L.) is a competitive weed in forty crops throughout the world (18). Worldwide, common lambsquarters is the most important weed in sugar beets (Beta vulgaris L.) and ranks seventh in importance in corn (Z29 mgys L.) (22). It is a major weed problem in agronomic crops such as barley (Hordeum vulgare L.), tomatoes (Lycopersicon esculentum Mill.), and soybeans (Glycine max (L.) Merr ), and can impact both crop quality and yield. Sugar beets are in the same botanical family as common lambsquarters (Chenopodiceae), and therefore a limited number of herbicides are available that will preferentially control common lambsquarters and not injure the sugar beet crop. In sugar beets, eight plants/10 m of sugar beet row decreased root yield 48% and recoverable sucrose yield 46% (35). In other sugar beet research, a common lambsquarters population of 170 plants/m2 decreased sugar beet yield 86% (18). Season long interference of common lambsquarters in corn decreased yields curvilinearly with increasing weed density, resulting in a maximum yield loss of 12% at 49 plants/10 m of row (3). Some of the yield loss from common lambsquarters was attributed to a reduction in the total number of corn kernels produced (3). Corn yield was decreased when common lambsquarters density was greater than 46 and 109 plants/m2 in a two year Canadian study (36). Common lambsquarters impacted corn yield by decreasing ear length (36) and kernel size (3, 36). 89 Common lambsquarters reduced barley yield 20% when both species were seeded at the same time (32). When barley was planted seven weeks after common lambsquarters, the barley yield was reduced 45% (32). Common lambsquarters reduced the yield of spring barley 23% and 36% at 150 and 300 to 400 plants/m2, respectively, in research utilizing a rectangular hyperbola model (10). Season long interference of common lambsquarters reduced marketable tomato fruit number (4). Marketable fruit weight also decreased 17% for 160 common lambsquarters plants/10 m row and 36% for 640 plants/10 m row (4). A curvilinear relationship existed between the fruit fresh weight and the common lambsquarters’ density (4). In North Carolina field studies, 16 common lambsquarters plants/10 m row reduced soybean yield 15% (37). However, nine common lambsquarters plants/10 m row decreased soybean yield 33 and 23% in 1986 and 1987, respectively, in Ohio (17). With densities of 22 plants/1O m row, soybean yields were not reduced if common lambsquarters were removed prior to five weeks after soybean emergence (17). In greenhouse studies, soybean dry matter production was reduced when common lambsquarters was planted two weeks prior to soybeans (38). The observed growth stimulation of common lambsquarters when planted prior to soybeans suggested that this species competed with soybeans for available resources when the common lambsquarters’ root system was established prior to the soybean (38). Competitiveness of weeds in soybeans is species dependent, as other grass and broadleaf weeds are more competitive than common lambsquarters. Wild oats (Ayeflg fatug L.) reduced yield of soybeans planted a 76 cm row spacing 51% when present at 300 plants/10 m of row all season (27). 90 However, when wild oats were removed by four weeks after crop emergence, no yield reduction occurred (27). In Michigan research, annual grass at 70 plants/m2 decreased yield of soybeans planted in 76 cm row spacing 46 and 55% on a loam and sandy loam soil, respectively, when the grass competed all season under dry conditions (24). Broadleaf weeds, such as common cocklebur (Xanthium pensylvanicum L.), reduced soybean yield 3 to 12% when 3 plants/10 m of soybean row competed for the duration of the growing season (5). In contrast, common ragweed (Ambrosia artemisiifolia L.) reduced soybean yield 8% when 4 plants/1O m of soybean row were present all season (9). Jimsonweed (Datura stramonium L.) decreased soybean yield 24% when 16 plants/10 m of row were present all season (21). Therefore, the competitive hierachy of these weeds was: cocklebur>common ragweed>jimsonweed. Researchers at North Carolina have defined an index of competition for a spectrum of weeds, and assigned a competitive index rating to each weed species (Coble and Gunsolus). Common cocklebur was considered to be the most competitive weed, and assigned a competive index value of 10. Common lambsquarters was assigned a competitive index value of 4.7. Data used in determining the competitive index for the various weed species was obtained from the southern soybean production areas. In southern soybean production areas, warmer temperatures and longer daylengths may favor plants with a C4 carbon metabolism (16). Common lambsquarters and soybeans are both C3 plants. 03 plant germination, growth, carbon dioxide exchange (8), and light utilization (A1) are more efficient at lower temperatures early in the season when cooler air and soil temperatures prevail. In northern soybean production areas, lower 91 temperatures in May at the time of soybean planting may give common lambsquarters a competitive advantage compared to C4 plants which prefer warmer soil and air temperatures. In soybeans, moisture is a critical factor for maximum yield. Amount and duration of season rainfall influences soybean yield (43). Distribution of water over time may be more important than the total rainfall or irrigation for the growing season (31). Soybean yields were reduced more by weed competition when under water stress (15), than when soil moisture conditions were high (14). Soybean yield reductions were greatest when soil moisture was abundant early in growing season and limited in late summer than when moisture was limited early in the growing season and above average for the duration of the growing season (11). Competition within the soybean row is studied more frequently because weeds are believed to be more competitive for light and moisture when present in the soybean row. Secondly, weeds cannot be removed from within the row by cultivation, and therefore, remain for the duration of the growing season. Cultivation and row spacing influence the type of competition that occurs in the soybean row. Under cultivation, weeds present in 76 cm soybean rows had an increased ability to interfere with soybeans since intraspecific competition was decreased (25). Early weed removal aids in soybean stand establishment (7). Also, shading by the soybean canopy may surpress late germinating weeds (5,23). Weeds germinating 20 to no days after soybean emergence were greatly surpressed due to soybean canopy closure, and the weed impact on soybean yield was reduced (11). An inverse relationship existed between the production of weed top growth and both soybean stand (7) and soybean 92 yield (42). Therefore, weed control during the first month after planting is critical to obtain maximum soybean yields (7,23). Weeds, including common lambsquarters, can be controlled during the first month after planting with soil—applied herbicides if adequate rainfall is available for herbicide activation. However, if adequate rainfall does not occur, soil—applied herbicide failure may occur. Timeliness of postemergence herbicide applications are critical, and control of certain weed species including common lambsquarters is limited. Common lambsquarters’ leaf surface has a waxy covering making it difficult for postemergence herbicides to penetrate into the plant, resulting in inconsistent common lambsquarters' control or suppression (40). Two postemergence herbicides, bentozon (3—(1—methylethyl) S—[2— [(phenylsulfonyl)amino] ethyl] phosphorodithioate) and acifluorfen (5—[2— chloro-4-(trimethyl)phenoxy —2-nitrobenzoic acid) are labeled for common lambsquarters suppression in soybeans (label)°. However, adequate suppression of common lambsquarters can be achieved only if the weed is less than 5 cm in height and has not surpassed the 8 leaf stage (label)°. When bentozon and acifluorfen were tank mixed, a significant decrease in the moisture content of common lambsquarters indicated greater phytotoxicity for the combination of bentozon and acifluorfen than for either herbicide used singularly (39). The response to bentozon plus acifluorfen was additive if crop oil concentrate was included in the tank mix (39). However, in other research (6) common lambsquarters control did not increase when common lambsquarters was less than 5 cm in height 0 Label for bentozon (BASF); and acifluorfen (BASF; Rohm and Haas). 93 at the time of application, and the rate of bentozon or acifluorfen was either increased or an additive included. Common lambsquarters infestations in a given year may be so low, that a postemergence herbicide application may not be economically justified. The increase of soybean yield value would not be greater than the cost of a postemergence herbicide application. The minimum number of common lambsquarters in 10 m soybean row which would justify a postemergence herbicide application has not been determined. However, if common lambsquarters were not controlled, seeds would be produced. A strong correlation exists between the weed seed present in the soil, and the emerged weeds in a field (44). Seeds found most frequently in the soil also had the highest and most consistent germination (44). The number of seeds produced by uncontrolled weeds contribute to the soil seed reserve and add to future weed problems in a field (44). The number of viable seeds in the top 23 cm of soil follows a pattern of exponential decay in subsequent years, decreasing at a constant rate of 22% per year (29). Therefore, 1% of the initial population would potentially remain viable after 18 years (29). Common lambsquarters seeds can survive up to 40 years in the soil (18). However, common lambsquarters’ seedlings which germinate each year account for less than 5% of the total viable seeds in the top 10 cm of soil (30). The average 9. album plant produces approximately 72,450 seeds (2), while a single large common lambsquarters plant has the potential to produce 500,000 seeds/plant (18). Common lambsquarters produces more seed when planted at low densities (12). However, density does not 94 affect seed yield per area since seed yield per plant decreases as density increases (12). Studies were initiated to determine the competitiveness of common lambsquarters in northern soybean production, and to compare the efficacy of a postemergence herbicide application to hand removal of common lambsquarters in soybeans. Seed production and viability of the seeds produced from the uncontrolled common lambsquarters’ plants were also determined. MATERIALS AND METHODS The influence of common lambsquarters on soybean growth and soybean reproduction was determined in field studies in 1987 and 1988. The two experiments included a handweeded study and a postemergence study. The handweeded study consisted of removing the entire weed at one of three specific times during the growing season. The postemergence study attempted to remove common lambsquarters with a postemergence herbicide application at one of three times during the growing season. Influence of conunon lambsquarters on soybeans: Field plots were established each year on a Marlette sandy loam (Glossoboric Hapludalfs) with an organic matter content of 1.6% and a soil pH of 6.5. Both the handweed and the postemergence experiments were a two—factor factorial arranged in a randomized complete block design with densities of 8, 16, 32, and 64 common lambsquarters/10 m of soybean row and four removal times of the common lambsquarters. Removal times of the handweed study were when common lambsquarters reached 10 to 20 cm in height (3 weeks after emergence (WAE)), 15 to 31 cm (5 WAE), and greater than 46 cm (7 WAE) in 1987; 15 to 31 cm (6 WAE), 95 greater than 46 cm (8 WAE), and greater than 61 cm (10 WAE) in 1988; and full season competition in 1987 and 1988. Removal of the common lambsquarters at greater than 61 cm (10 WAE) was added in 1988 because removal of the common lambsquarters up to 46 cm (7 WAE) in 1987 did not reduce soybean yield. Each density and time of removal was replicated four times. The postemergence study's removal times were when common lambsquarters reached 10 to 20 cm in height (3 WAE), 15 to 31 cm (5 WAE), and greater than 46 cm (7 WAE) in 1987; and 5 to 13 cm (3.5 WAE), 15 to 31 cm (6 WAE), and greater than 46 cm (8 WAE) in 1988; and full season competition in 1987 and 1988. A postemergence herbicide application consisting of bentozon, acifluorfen, and crop oil concentrate (COC)1 at 0.84 kg/ha, 0.14 kg/ha, and 2.34 L/ha, respectively, was applied at 271 L/ha and 345 Kpa at each time of removal in the designated plots. Each density and time of removal was replicated three times. Control of common lambsquarters was visually evaluated at 7, 14, and 30 days after treatment (DAT). Inoculated soybeans (cultivar ‘Corsoy ’79‘) were planted at 335,000 plants per hectare into spring plowed alfalfa ground on May 11, 1987 and plowed soybean stubble on May 4, 1988 with conventional tillage practices. One hundred ninety-one kg/ha of starter fertilizer (6—24-24) was applied at planting. The four row plot size was 3.05 by 10.67 and 3.05 by 9.14 square meters for the postemergence and handweed study, respectively, with a soybean row spacing of 76 cm in both 1987 and 1988. 1 Surfactant Crop Oil Concentrate, Herbimax, petroleum hydrocarbons (83%) —light paraffinic distillate, ordorless aliphatic petroleum solvent, surfactant (17%) - mono and diesters of omega hydroxypoly oxyethylene, Loveland Indust. Loveland, I0 A. 96 Common lambsquarters2 was seeded at a 0.5 to 1.0 cm depth at the proper spacing the day following soybean planting by stretching a marked string along each soybean row. (Viability of the common lambsquarters seed at planting was approximately 10% based on growth chamber germination tests). Fifty to 100 seeds were planted into the soybean row at each appropriate mark along the string. At the densities of 8 and 16 plants/10 m of soybean row, the common lambsquarters’ spacing was staggered in adjacent soybean rows to provide an alternating pattern of common lambsquarters in the plot. Soybean emergence was on May 20 and May 18 in 1987 and 1988, respectively. When the handweed study’s soybeans reached the unifoliolate to first trifoliate growth stage (23 and 27 days after planting in 1987 and 1988, respectively), a second planting of common lambsquarters was seeded at the four densities in the appropriate plots. Common lambsquarters’ seedlings were thinned to the proper density by four weeks after soybean emergence in 1987 and 1988. Plots were maintained free of undesirable weeds between and within the soybean rows by hoeing and cultivation. Plots were cultivated on June 11, June 25, and July 8, 1987 and June 14, June 28, and July 26, 1988. In the postemergence study, cultivation did not occur within five days of postemergence herbicide application. Soybean canopy closure occurred on July 29, 1987. However, in 1988 the soybean canopy did not close due to the drought. Rainfall data and monthly mean temperatures for both years are presented Table 1. In the handweed study, dry matter production (common lambsquarters plus soybeans) per square meter was measured on August 7, 1987 and August 2 F & J Seeds (Woodstock, IL) 97 m.m ..sF s.o m m m Loo5o>oz e.m o.~. o.m as s s Loooooo m6 5: 5: t m: D Loosoooow s.n s.~F ..sF so mm .m ow=o=< ..s ..o m.m mm em em >H=o o.o o.o n.op as ou Pm meow ..o s.F a.» s? we he >o2 n.s o.~F n.e s a or Hoto< Eu oo oooto>o moo, hoop omoto>o ammo sum. ozocoz Leo> on Loo> on Haemcaaz oczuocoaeoh zoos .mmmp uco swap ca oczuatonsou come one Haamcfioc >azuzoz .P mane» 98 19, 1988. Samples were removed from outside the harvest area when soybeans were in the early reproductive stage in the full season competition plots only. All remaining common lambsquarters plants in the harvest area of both studies were removed in late October in 1987 and 1988 so weeds would not interfere with mechanical harvest. Dry weight of the removed common lambsquarters plants was recorded in 1988 only. The two center soybean rows of each plot were harvested on October 26, 1987 and November 15, 1988. The harvest area was 1.52 by 7.14 m2 for the handweed study in 1987, and the harvest area was 1.52 by 7.62 m2 for the postemergence study in 1987, and for both studies in 1988. Soybean seed was cleaned, percent moisture at harvest recorded, and plot yields converted to kg/ha at 13% moisture for statistical analysis. Seed production and germination: The number of common lambsquarters plants removed from each plot prior to harvest was recorded in 1987 and 1988. The total number of seeds per plot was determined on a dry weight basis by removing all seed from the air—dried plants and weighing the total number of seeds produced (0.1 grams = 200 seeds). Seeds were stored in the dark at room temperature until germination was measured. Twenty-five seeds from each plot were placed onto 9 cm Whatman #1 filter paper in disposable petri dishes (100 by 15 mm) and 10 mls of distilled water added. The petri plates were placed in a growth chamber at 12 to 15°C for a 12 hour photoperiod with an average quantum flux density of 102 uE m"2 sec—1. After 14 days, the germination percentage was determined by the radical protrusion from the seed coat. Four replicate samples from each plot were germinated in 1987 and 1988. 99 Yield data was subjected to analysis of variance. Linear, quadratic or cubic relationships between soybean yield and time of removal and/or density were determined with equations from Steele and Torrieb. Treatment means were compared using a least significant difference (LSD) test at P<=0.05 when treatment effects or interactions were significant. Data were not combined over years because of significant year by treatment interactions. The statistical package used was MSTAT3 (Microcomputer STATistical program). RESULTS AND DISCUSSION Handweed Study: In 1987 and 1988, the total plant dry matter biomass (soybean plus common lambsquarters) produced per square meter did not increase as common lambsquarters density increased (Table 2). However, the portion of the total plant biomass composed of soybeans decreased as common lambsquarters' density increased. Common lambsquarters contribution to total plant biomass increased as common lambsquarters’ density increased. Neither the percentage of common lambsquarters’ biomass composition nor soybean yield decreased when weed density increased from 32 to 64 plants/1O m soybean row in 1988. This lack of reduction in both plant composition and soybean yield may be due to the droughty conditions in 1988. The second planting of common lambsquarters (when soybeans reached the unifoliate to first trifoliate growth stage) did not germinate in either 1987 or 1988. Common lambsquarters’ seeds requires light, b Steel, R and J. Torrie. 1980. Priciples and Procedures of Statistics: A Biometrical Approach. McGraw—Hill Inc. New York, NY. 3 MSTAT, E.Lansing, MI. Table 2. Total biomass production, percent soybean and percent common lambsquarters composition as influenced by density for the handweed study in 1987 and 1988. Total dry Common matter Soybean lambsquarters Density production Composition Composition plants/ 10 m raw 1987 1988 1987 1988 1987 1988 -—--g/m2 ----- --- x ----- ---- as ----- 8 589 499 90 86 10 14 16 601 469 82 77 18 23 32 687 506 73 58 27 42 64 671 568 54 66 46 34 LSD 05 - n.s. n.s. 10 23 10 23 lOl (1,19,20,26) adequate nitrogen (33,34), moisture, and alternating temperatures (13) of 10 to 30°C to break innate dormancy and trigger germination. Lack of germination in the field could possibly be due to the high soil temperatures and lack of moisture in early June in both years for the second planting of common lambsquarters. In 1987, delaying the time of common lambsquarters’ removal did not reduce soybean yield compared to season long competition (Figure 1). When common lambsquarters’ competed for the entire growing season, soybean yield was reduced. Percent yield reduction increased linearly as the time of removal was delayed. Higher common lambsquarters’ densities reduced soybean yield more than lower densities when removal times were compared (Figure 2). In 1988, both a delay in the time of common lambsquarters’ removal (Figure 3) and an increase in common lambsquarters' density (Figure 4) reduced soybean yield. Yield was reduced linearly as common lambsquarters’ removal time was delayed (Figure 3). As common lambsquarters density increased, yield decreased in a quadratic response when averaged over removal timing (Figure 4). Yield reductions were greater in 1988 than in 1987 for each removal time possibly due to below average rainfall during the months of May, June, and July in 1988. Soybean yields were reduced more by weed competition when under water stress (15), than when soil moisture conditions were high (14). The economic threshold in soybean was chosen at 20% yield reduction. A soybean yield of 2000 kg/ha with 20% yield reduction is 400 kg/ha and $0.18/kg, the cost of the postemergence herbicide would have to be below $75/ha for an economical application. A 20% soybean yield reduction (compared to the weed—free check) occurred when common lambsquaters were 102 Figure 1. Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the handweed study in 1987. (weed—free yield 2854 kg/ha). 103 AHQBV communes aeoaem hog 3.33 an h m n E: 5.353 3...! E: 5:55... am .i E: .5155... e. .i E: 5:35.... a 1: lam :3 law new [can @23er so coo—am some eaofiom 104 Figure 2. Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the handweed study in 1987. (weed-free yield 2854 kg/ha). 105 32 a 335.3 £20: .795 3 mm 5:. w as: a: .3. 1. E43 5 .5 3A ..1 -3 A; 8 :8 2.2:: A; n E 3.3.--. :2: 32983.... i .853. 33.» neofiom Figure 3. Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the handweed study in 1988. (weed—free yield 1659 kg/ha). 107 $593 35325— :aoémom 5&4 v3.35 3.: 5135.: 3.1.. E: sense...— 8:1 E: 5:35.... 3 .i E: 5335..— u law a” 1i: [can 92:33 is .835. 29.5 :5...» Figure 4. 108 Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the handweed study in 1988. (weed—free yield 1659 kg/ha). 109 9...: a 3353 has: QED 3. mm 3 w a? a: .md. ...... Ag? :6 Eu GA .-I 553.. 8 Eu 3A II A; 3 Eu Hm.m—.-l 1.x: $33.82 5 .353. 229 :Bfiom llO removed entirely by hand for 32 plants/10 m row for all season competiton in 1987 and for 32 plants/10 m row at 10 WAE in 1988. As the density of common lambsquarters increased, the number of common lambsquarters seeds produced per plant did not decrease in 1987 or 1988 (Table 3). Total number of seeds produced per plant ranged from 22,700 to 55,900 seeds. A curvilinear relationship revealed an increase then plateau in seed production per hectare as density increased from 0 to 64 plants/10 m row in 1987 and 1988 (Table 3). Research has shown weed density to have littly effect on seed yield per area since seed yield per plant decreased as density increased (12). Density of common lambsquarters had no effect on seed germination in 1987 and 1988 and germination averaged 31 and 23% in 1987 and 1988, respectively. Dry weight of each common lambsquarters plant harvested in 1988 (Table 3) did not decrease as density increased. However, seed production increased as dry weight per plant increased in 1988. One gram of plant dry weight produced 1017 seeds in 1988 (r2=97). Postemergence Study: Soybean yields were reduced linearly as the postemergence herbicide application was delayed in 1987 (Figure 5), and as the density of common lambsquarters increased (Figure 6). The postemergence application of bentozon plus acifluorfen plus COC provided excellent control of common lambsquarters at all densities when applied to common lambsquarters at the 10 to 20 cm in height (3 WAE) in 1987 (Table 4), and yields in these plots were equal to that of the weed-free check (Figure 5). However, when common lambsquarters reached 15 to 31 cm (5 WAE) in 1987, the postemergence herbicide application provided only fair common lambsquarters control (Table 4) and soybean yield was reduced (Figure 6). Herbicide application resulted in poor common lambsquarters lll .mmnmc x mm.om + «x Nm.ou I > ”mmm— .ocouooz can comm n .mw-NL x mm.mw + Nx Fm.o: . > "nmm— .ocouoo: Lon comm o .m.: .m.: .w.: n.w.: comm .m.= .m.: n mo.om4 —.on an em meow Pam? oompn comma :0 :.Nn Pm Fn nanp m—mF oonnm comp: mm :.mm my hm emoF mmm oommm com—m mF F.o¢ on an own mm: oomNn ooom:_ m osotm A o_va ucoaa Lon acoocoa m: can ucoaa Lon mmm— mmm— hemp wmmr nwmr mmm— snow u 30; E or s >ufimcou ocofio Hoogo usage: >Lo :ofiuocascoo :ofiuoauoca uoow mcoucoacmnEoH :oEEoo .>H:o muoHQ :ofiufiumaeoo comomm ago on» 50;» >Hco umzmz< ca ooumo>coz ego: mucofio .mmms oco sows :H >u=oo ooozuco; on» :3 >ouwcoo >9 noocozficCH mo mcoucazcwnEoH :05500 no cofiuo:«ELom uco :ofiuozuoca umow "m oHnoh 112 Figure 5. Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the postemergence study in 1987. (weed—free yield 3067 kg/ha). 113 firm-43v QUEQWHQE flaws—havw hOfi mum—00g .- Ian 13. 19% E: 5:35... 3!. 2.8 £3351 um .... -3 E: 323.51. 3:1 E: sausage a -2: 322.8: so 53.3. 2.5 as...» Figure 6. Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the postemergence study in 1987. (weed—free yield 3067 kg/ha). 115 $2. a 335.3 92.8 flan-5 3. mm on w A; a: .3. 1. figs. 3 .5 9x ..I -8 A; my .5 8.2.-.. A; a an 3.3..-. -2: sea-.59... so .353 22% 53.2Kn 116 .wmmp :a so nrum .nmm— ca Eo omlop Ha>oEoc no uzmfioz mcoacoacwneoa :oEEoo a acoeuoocu Loved m>au n hoeot . Ho>oEmL no mo oEw» pn neocoafimcfi mo Hogucoo mcoucozcmnEoH :05500 .9 canoe 117 control when lambsquarters' height exceeded 46 cm (7 WAE) in 1987 and soybean yields equalled that of where lambsquarters had competed all season. Therefore, "partial" control from herbicide applications at 5 and 7 WAE resulted in the common lambsquarters remaining competitive for the remainder of the growing season. The economic threshold of 20% yield reduction in 1987 was 32 plants/10 m row, if the herbicides were applied before the actively growing lambsquarters reached 10 to 15 cm in height. In 1988, the application of bentozon plus acifluorfen plus 000 provided only fair to poor common lambsquarters’ control when applied to the common lambsquarters at 5 to 13 cm in height (3.5 WAE) (Table 4). Soybean yields were low, with no difference in yield due to the time of herbicide application (Figure 7). Higher densities of common lambsquarters were more difficult to eradicate compared to lower densities when averaged over all postemergence herbicide application times in 1988 (Figure 8). Possibly inadequate spray coverage at higher weed densities and droughty conditions at the time of postemergence herbicide application caused the reduced control in 1988. Environmental conditions at the time of postemergence herbicide application influence efficacy of both bentozon (40) and acifluorfen (28) with drought reducing the herbicide effectiveness. All postemergence application timings failed to control common lambsquarters in 1988, resulting in an average soybean yield reduction of 68% compared to the weed-free check. Common lambsquarters' plants remaining for the duration of the growing season after herbicide application were counted to determine if the herbicide treatments had removed the weeds or only suppressed their growth. The postemergence herbicide’s degree of control was reflected in 118 Figure 7. Soybean yield reduction as influenced by time of removal for each common lambsquarters density in the postemergence study in 1988. (weed—free yield 1752 kg/ha). 119 E... a 235.5 been .135 we am e. n _ _ _ _ _ _ ~ r p _ b _ _ F _ _ A. a? c .3. .1 A33 8 .5 3A In . on A; 8 a... 3.2:: _ A; ms in mmm . A... 1:. on -2: -3. sec-e8: so .353— Eofi as...n 120 Figure 8. Soybean yield reduction as influenced by common lambsquarters density for each time of removal in the postemergence study in 1988. (weed—free yield 1752 kg/ha). 121 Rage: cog—cubes :52.sz hog mace? 3 a e v _ _ _ _ _ _ p _ _ — — _ h _ _ _ _ _ a E: 5.553 3.1.. a E: £3555 fill . 1; E: 51553 3 .... E: 59:55.: a A... la. ervlzan-uurxruvxxux A.” $3 in. Ase-woo: so .525: 25» 52: 122 both the number and the size of the remaining common lambquarters’ plants, dependent on the time of application. At the first herbicide application timing in 1987 (Table 5), common lambsquarters' densities of 32 and 64 plants/10 m row were reduced to 25 and 29 plants/10 m row, respectively, and the plants were removed. However, at the second and third postemergence application timings, the number of common lambsquarters remaining was not reduced. In 1988, delayed postemergence herbicide application and higher common lambsquarters density, increased the number of common lambsquarters plants remaining in the plot in 1988 (Table 5). Therefore, in 1988 plants were not removed by any herbicide application, and since the soybean canopy did not close, late germinating common lambsquarters were present at harvest. With adequate moisture, soybeans are a more competitive crop than corn (3, 36), tomatoes (4), or sugar beets (35). Common lambsquarters is not as competitive as other weeds such as cocklebur (5), but is equally competitive as common ragweed (9) or jimsonweed (21), and more competitive than foxtail (24). In North Carolina, common lambsquarters reduced soybean yield 15% with 16 plants/10 m row in season long competition (37). Research in Ohio showed 22 lambsquarters plants/10 m row did not reduced yield if removed (entirely and by hand) prior to five weeks after soybean emergence (17). The economic threshold for the minimum number of common lambsquarters plants/10 m row which would cause a 20% yield reduction is higher and could remain longer in the handweed study the weeds were entirely removed compared to the postemergence study where the weeds were only partially removed. Response and time of removal thresholds are meaningless if the weed cannot be rem0ved by that time with a herbicide application. The postemergence herbicide 123 Table 5. The number of common lambsquarters remaining/10 m of soybean row after postemergence herbicide application in 1987 and 1988. Time of Application Density averaged over time of Cheal planted 1 2 3 all season removal density 16 m raw 1987 1988 1987 1988 1987 1988 1987 1988 1987 1988 ----------- ------- common lambsquarters remaining/1D m lu— 8 1s 23 12 1a 17 33 8 3a 13 26 1s 35 14 19 23 13 25 22 31 22 23 32 25 13 3a 29 33 3a 25 4s 28 31 fl 1 s4 19 29 ea 62 a3 79 ea 59 54 57 ' Time of removal averaged over density 24 21 32 33 36 42 35 41 } 1987 1988 Significance Time LSD.BS 18 16 linear n.s. ”*b quadratic n.s. n.s. Density LSD.BS n.s. 16 linear n.s. *“c quadratic n.s. ”*c Time x Density **° n.s. a Common lambsquarters remaining interaction between time of postemergence herbicide application and common lambsquarters density, 1987; v-a.a1 x2 - 6.26 x + 1.99 (WAE) + 15.59 r2-36 b Linear equation for common lambsquarters remaining, 1988: V- 1.69 WAE + 24 r2-24 C Quadratic equation for comnon lambsquarters remaining, 1988: Y--D.61 X2 + 1.52 X r2-37 124 application study showed the height of common lambsquarters and the weather conditions at application were critical for herbicide effectiveness. This degree of weed control, in turn, greatly impacted soybean yield reduction. Also, any uncontrolled plants which remain will produce seed which contribute to the soil’s seed reserve and subsequent weed pressures. Seed Production and Germination: In 1987, neither seed production per plant nor seed production per hectare was influenced by common lambsquarters' density or the time of postemergence herbicide application (Tables 6 and 7). The average number of common lambsquarters seeds produced per plant was 14,400 and the average number of seeds produced per hectare was 480 million. In 1988, however, increasing lambsquarters‘ density decreased seed production per plant (Table 6). Dry conditions in 1988 may have influenced the common lambsquarters’ seed production by stressing the mother plant. Seed production per hectare was not affected by the postemergence herbicide application timing or the density of common lambsquarters (Table 7). Seed production increased as plant dry weight increased with one gram of plant dry weight producing 829 seeds (r2=96). Neither the time of postemergence herbicide application nor the density of common lambsquarters affected germination of the seed produced by the common lambsquarters in 1987 or 1988 (Table 8). Common lambquarters' germination is influenced by light (19, 20, 26), temperature, moisture (13), and nitrate level (34). Nitrate can be present as an endogenous constituent of the seed or supplied exogenously (34), and the endogenous content of seed nitrate is a direct function of the nitrate nutriton received by the parent plant (34). It appears that 125 .NNINL “x scm.s1u> “camp .ucoua Lon cofiuuauocq neon mcoucoacmneofi coeeoo cos cofluoscm oduocuoso c oHnoHuo>o so: can: + .m.c .m.: .m.: .m.c .m.c .m.c c.m.c .m.c u msdm.d ssN—e ssem . sstF + ssFmN sst . sssse ssmmp em ss—om + ssemm + sssmm ssmsp sscem ssmm Nm sssme sssem ssmsm ss—sN sseme ssssm sspms ssmm— w— ss—mm + ssswm ssmmu ssmmn ssmF— ssmsc sst m 111111 1-1u-ucofia can couuosuocq noon n_nu r 1 . ::==5; wmm— moms mmm— mom? swap Romp msm— Ramp 30L E sr >usmcmu concom “do n N. F noscosa Howzo cosucousaq< so wash .msm— use msmp cs >usmcwu use cofiucodflaao wcdouncoc mocwmcoEmuwoa so med» oz» >3 smocmsaccs mo acofln Lon coauosuoca noon .mcwucoavmneoH coEEoo .s msnoh 126 oflncafic>o no: case + ms? x F as cosuosuocq comm Hugo» o .w.c Nmsw «swr N—mw + - Pemp rum mpsw + .m.= .m.= .m.c .m.= .m.c .m.c "ms.omu sass. + QANN “as - mes, mAN em meme + msm_ -e New. QNN NM mam, New mam, .me Ase Few w_ -m. mam sums ems were em, a n 1111111111 -omceuowz cog :osuosuoca umom mcoucozamneofi :oEEoo 11111111 swap mums msm— Ramp mmmp Romp comp swap :5; e sr >asmcmu comeom “H: m m — smucosa Hooco cofiuoofisaa< so mash .mmmp uco «amp cfi >ufimcmu uco couuaoaaaqo oufiosncmc oocmemEmumoa so was» >n umoca:~ec« we ocoaomz con cosuosuoca comm .mcoacozvaan :oEEoo .m manop 127 .m.: .m.: .m.: .m.: .m.c .m.c .m.c .m.: "ms.om4 mN Nm em mm mN «N mm N: am mu sN m— mm mm mm Pm + mm Pu mm s~ —m mw Fe m— em w_ mp mm m— we ms mm ew + s K \ ssm— msme som—. Asm— omm— Romp smmp Asap 3oc 5 sp >usmcmu comomm “so umucoso Hooco Ho>oEmc so mesa ----1 ....... 111-cesuocHELwo uwmm meoucoaamQEo~ season-1:11:11 ........ .smmp use nmm— cu >u«mcou use cosuoofigaao musowncoc mocmmcosoumoq so mafia >3 noocossscs mo cosQOsEEms umom .wcoucoacmneoH coEEoo .s mHnop 128 neither high densities of common lambsquarters did not deplete the nitrogen supply to the seed, nor postemergence herbicide applications alter the nitrogen uptake by the parent plant. In 1988, common lambsquarters dry weight per plant was greatest as the time of postemergence herbicide application was delayed and as the density of common lambsquarters was reduced (Table 9). Weed seed produced by common lambsquarters could contribute to the soil's seed reserve and increase the possibility of greater common lambsquarters pressure in the following years. Viable common lambsquarters seed in soil decreased at a constant rate of 22% per year (29) and only 5% of the viable seeds in the top 10 cm of soil germinated in one year (30), which left 1% of the potential seed in the soil in any one year. If the uncontrolled common lambsquarters’ in this study averaged 480 million seeds produced per hectare with 25% viable seeds, then over 1 million seeds per hectare could germinate the following year. 129 mpumc x F¢.a 1 u<3 ms.mm + ~m<3 wn.m 1 mm<3 Km.su> “smm— .>afimcwu nematoscmnecs coEEoo ace cosuoosflaao oufiofincm; mocmmcmswumoa so mega cwmzumn couuoocoucq pecan Lon unmdmz >Lu mcmucoacmnEus coEEoo c “.ms .- ~.m~ _.~m m.~m em w.mm F.we s.s¢ ~.ms Nm m.mm m.me «.mm m.m¢ w— e.ms m.~¢ m.mm m.mm o 3.541;..— A ..i\ 30.. E E- >ufimcmu comomm -e m m . F coucofia scone Ho>osmc so mega 11111111111111 seeds can uzmumz >cu mcmucoacmnEofi :oEEoo-u-111111111-1 .mmmp cs >usmcwv use cosucofiflaac muuofincmc mucmmLmEmumoa so was» on» >3 umocmsaecs no cause Lea usage; >Lu .mcmacoaamneoH :oEEoo .m manH LITERATURE CITED Baskin, J. M. and C. C. Baskin. 1977. Role of temperature in the germination ecology of three summer annual weeds. Oecologia 30:377— 382. Bassett, E. J. and C. W. Crompton. 1978. The biology of Canadian weeds. Chenopodium album. Can. J. Plant Sci. 58:1061—1072. Beckett, T. H., E. W. Stoller, L. M. Wax. 1988. Interference of four annual weeds in corn (leg mays). Weed Sci. 36:764-769. Bhowmik, P. C. and K. N. Reddy. 1988. Interference of common lambsquarters (Chenogodium album) in transplanted tomatoes (Lycopersicon esculentum). Weed Tech. 2:505-508. Bloomberg, J. R., B. L. Kirkpatrick, and L. M. Wax. 1982. Competition of common cocklebur (Xanthium pensylvanicum) with soybean (Glycine Egg). Weed Sci. 30:507-513. Blumhorst, M. 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Effect of the photoperiod during growth and development of the plants on the dormancy of the produced seeds. Acta. Bot. Neerl. 19(1):81-94. Karssen, C. M. 1970. The light promoted germination of the seeds of Chenopodium album L. VI. Pfr requirement during different stages of the germination process. Acta. Bot. Neerl. 19(3):297—312. Kirkpatrick, B. L., L. M. Wax, and E. W. Stoller. 1983. Competition of jimsonweed with soybean. Agron. J. 75:833—836. Mitich, L. W. 1988. Intriguing world of weeds—common lambsquarters. Weed Tech. 2:550—552. Murphy, T. R. and B. J. Gossett. 1981. Influence of shading by soybeans (Glycine mgx) on weed suppression. Weed Sci. 29:610-615. Mutch, D. R. 1986. Dectection, influence, and economics of annual gross interference on soybeans (Glycine max (L.) Merr.) PhD. Dissertation. Michigan State University. pp. 102. Orwick, P. L. and M. M. Schreiber. 1979. Interference of redroot pigweed (Amaranthus retroflexus) and robust foxtail (Seteria viridis var. robusta—alba or var. robusta—purpurea) in soybeans (Glycine mgx). Weed Sci. 27:665-674. Ramakrishnan, P. S. and P. Kapoor. 1974. Photoperiod requirements of seasonal populations of Chenopodium album L. J. Ecol. 62:67-73. Rathmann, D. P. and S. D. Miller. 1981. Wild oat (Avena fatua) competition in soybean (Glycine max). Weed Sci. 29:410—414. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 132 Ritter, R. L. and H. D. Cable. 1981. Influence of temperature and relative humidity on the activity of acifluorfen. Weed Sci. 29:480— 485. Roberts, H. A. and P. A. Dawkins. 1967. Effect of cultivation on the numbers of viable weed seeds in soil. Weed Res. 7:290-301. Roberts, H. A. and M. E. Ricketts. 1979. Quantitative relationship between the weed flora after cultivation and the seed population in the soil. Weed Res. 19:269—275. Robinson, E. L., G. W. Langdale, and J. A. Stuedemann. 1984. Effect of three weed control regimes on no-till and tilled soybeans (Glycine mgx). Weed Sci. 32:17-19. Ross, M. A. and J. L. Harper. 1972. Occupation of biological space during seedling establishment. J. Ecol. 60:77—88. Saini, H. S., P. K. Bassi, and M. S. Spencer. 1985. Seed germination in Chenopodium album L. further evidence for the dependence of the effects of growth regulators in nitrate availability. Plant Cell and Environ. 8:707—711. Saini, H. S., P. K. Bassi, and M. S. Spencer. 1986. Use of ethylene and nitrate to break seed dormancy of common lambsquarters (Chenopodium album). Weed Sci. 34:502—506. Schweizer, E. E. 1983. Common lambsquarters (Chenopodium album) interference in sugarbeets (Beta vulgaris). Weed Sci. 31:5-8. Sibuga, K. P. and J. D. Bandeen. 1985. Effects of green foxtail and lamb’s-quarters interference in corn. Can. J. Plant Sci. 60:1419-1425. Shurtleff, J. L. and H. D. Cable. 1985. Interference of certain broadleaf weed species in soybeans (Glycine mgy). Weed Sci. 33:654— 657. Shurtleff, J. L. and H. D. Cable. 1985. The interaction of soybean (Glycine mgx) and five weed species in the greenhouse. Weed Sci. 33:669—672. Sorensen, V. M. 1984. The interaction of acifluorfen and bentozon in herbicidal combinations. PhD. Thesis. Michigan State University. Taylor, F. E., L. E. Davies, and A. H. Cobb. 1981. An analysis of the epicuticular wax of Chenopodium album leaves in relation to environmental change, leaf wettability, and the penetration of the herbicide bentazone. Ann. Appl. Biol. 98:471—478. 41. 42. 43. 44. 133 Tenhumen, J. D. 1982. The diurnal course of leaf gas exchange of the C4 species Amaranthus retroflexus under field conditions in a "cool" climate: Comparison with the C3 species Glycine mg; and Chenopodium album. Oecologia 53:310-316. Thurlow, D. L. and G. A. Buchanan. 1972. Competition of sicklepod with soybeans. Weed Sci. 20:379—384. Webber, C. L. III, M. R. Gebhardt, and H. D. Herr. 1987. Effect of tillage on soybean growth and seed production. Agron. J. 79:952- 956. Wilson, R. G., E. D. Kerr, and L. A. Nelson. 1985. Potential for using weed seed content in the soil to predict future weed problems. Weed Sci. 33:171-175. NICHIan 579 E ‘ 111 (WWW(Hi!11111111111 006251916 111111111 12