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VanGessel has been accepted towards fulfillment of the requirements for M.S. Crop and Soil Sciences degree in {mflm I 77 hhfiupnmuun Date December 22, 1988 0-7639 MS U i: an Waive Action/Equal Opportunity Institution RETURNING MATERIALS: MSU Place in book drop to remove this checkout from your record. FINES LIBRARIES will be charged if book is returned =— after the date stamped below. WEED MANAGEMENT FOR IMPROVED POTATO INTEGRATED PEST MANAGEMENT (IPM) BY Mark J. vanGessel A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Crops and Soil Sciences 1989 66.14578 ABSTRACT WEED MANAGEMENT FOR IMPROVED POTATO INTEGRATED PEST MANAGEMENT (IPM) BY Mark J. vanGessel Field studies with redroot pigweed (Amaranthus retroflexus) and barnyardgrass (Echinochloa crus-galli) seeded at l, 2, and 4 plants/m, either within or between the potato (Solanum tuberosum) row resulted in one weed/m seeded within the crop row at planting time causing a yield reduction of at least 20%. weeds seeded between the crop row after hilling (40 - 49 days after planting) did not reduce yield. In other field research, 'Atlantic' variety of potato was a better competitor with a natural infestation of weeds than 'Russet Burbank' on coarse textured mineral soil, but 'Russet Burbank' was a better competitor on muck soil. When this study was monitored for pest insects, larval Colorado potato beetles were most abundant in early hilled and weed free plots. ACKNOWLEDGMENTS A project of this sort requires a lot of help from many people. I realize the importance of these people and how limited my research would have been without them. This list is not a complete list, but it includes those whose contributions were most obvious. Dick Kitchen and Ron Gnagey for their assistance in planting, irrigating, maintaining, and harvesting the crop at the respective research farms. The undergraduates who had to endure my humor and unending phrase of 'only a few more': Scott Williams; Eric Green; Marvin Ries; Kelly Veit; and particularly Teresa Petersen. All the weed science graduate students, particularly Fred Salzman and especially Teresa Crook whose assistance was invaluable. Dr. Gary Powell, whose contributions were many, although some are not printable in this type of forum. Jackie Schartzer for her assistance in producing the tables and explanations of WOrdStar. My committee members: Dr. Ed Grafius for his expertise in entomology and his encouragement; Dr. Dick Chase for his vast knowledge and assistance in the area of potato production, and allowing me to contribute millions of weed seeds to the soil at the Montcalm Potato Research Farm; and my major professor Dr. Karen Renner who was not only an excellent advisor, but more importantly, a friend. The support of Kate Everts while I was frantically trying to produce this manuscript was greatly appreciated. Finally, I would also like to acknowledge my family for their support and encouragement. To know their love and comfort was there whenever I needed kept me going. 111 TABLE OF CONTENTS LIST OF TABLES vi REVIEW OF LITERATURE l Interference 1 Interference studies 4 studies on weed interference in potatoes 9 Competitive characteristics 11 Redroot pigweed - Amaranthus retroflexus 12 Barnyardgrass - Echinochloa crus-galli 13 barnyardgrass and redroot pigweed 14 Potatoes - Solanum tuberosum 16 Influence of potato production on weeds 17 Literature cited 21 REDROOT PIGWEED (Amaranthus retroflexus L.) AND BARNYARDGRASS (Echinochloa crus-galli L. Beauv.) INTERFERENCE IN POTATOES (Solanum tuberosum L., var. 'Atlantic') Abstract 27 Introduction 28 Materials and methods 31 Results and discussion 36 Literature cited 58 iv TABLE OF CONTENTS EFFECT OF SOIL TYPE, BILLING TIME, AND POTATO VARIETY ON WEED INTERFERENCE IN POTATOES Abstract 61 Introduction 62 Materials and methods 65 Results and discussion 69 Literature cited 90 INFLUENCE OF WEEDS ON INSECT DIVERSITY AND POPULATION DYNAMICS IN POTATOES (Solanum tuberosum L.) Abstract 93 Introduction 94 Materials and methods 97 Results and discussions 99 Literature cited 110 LIST OF TABLES REDRDOT PIGWEED (Amaranthus retroflexus L.) AND BARNYARDGRASS (Echinochloa crus-galli L. Beauv.) INTERFERENCE IN POTATOES (Solanum tuberosum L., var. 'Atlantic') Table 1. Competitive indexes (CI) and relative competitive ability (RCA) for redroot pigweed, barnyardgrass, and two potato varieties ('Atlantic' and Russet Burbank'). Determined by greenhouse replacement studies, 1988. .....38 Table 2. Height of weeds seeded within the crop row, measured at hilling time and canopy closure in 1988. Data was combined over weed densities. 0.0.039 Table 3. Dry weight of individual weeds measured at potato senescence in 1987 and 1988. Data is combined over weed densities. 0.00.41 Table 4. Total weed dry weight/plot when measured at potato senescence in 1987 and 1988. 0.00.43 Table 5. weed density regressed on total weed biomass/plot in 1987 and 1988. 0.00.44 Table 6. Potato height measured at billing time and canopy closure in 1987 and 1988. 0.00.46 Table 7. PAR absorption by the potato canopy and potato plus weed canopy measured at crop senescence in 1987. PAR absorption data was combined over weed species and densities. 0.0.048 Table 8. Measured growth parameter for each species that had the greatest correlation with leaf area in 1987 and 1988. 0.00.50 Table 9. Yield of graded tubers and the percent reduction of marketable tuber yield combined over weed species in 1987 and 1988. 00.0.51 vi LIST OF TABLES Table 10. Yield of tubers in 1988, averaged across weed densities and location. 0.0.053 Table 11. Yield regressed on percent weeds in total plant biomass for weeds seeded within the crop row, 1987 and 1988. 0.00.54 EFFECT OF SOIL TYPE, BILLING TIME, AND POTATO VARIETY ON WEED INTERFERENCE IN POTATOES Table 1. Need dry weight/m2 for early and conventionally billed treatments, Montcalm, combined over potato varieties. 00.0.70 Table 2. Aboveground biomass of individual potato plants, Montcalm, August 17, 1987 and August 12, 1988. . 0.00.73 Table 3. Yield and specific gravity, averaged over hilling time, Montcalm, 1987. 0.00.74 Table 4. Yield and specific gravity, Montcalm, 1988. 00.0.76 Table 5. Internal defects in 15 Grade A tubers, averaged over hilling time, Montcalm. 00.0.78 Table 6. weed dry weight/m2 for early and conventionally billed treatments, combined over potato variety, Rose Lake. 00.0.80 Table 7. Aboveground biomass of individual potato plants, Rose Lake, August 14, 1987 and September 7, 1988. 0.00.82 Table 8. Yield at Rose Lake, 1987. 0.00.83 Table 9. Yield and specific gravity, Rose Lake, 1988. 00.0.85 vii LIST OF TABLES Table 10. Internal defects in 15 Grade A tubers, averaged over hilling times, Rose Lake. 0000088 INFLUENCE OF WEEDS ON INSECT DIVERSITY AND POPULATION DYNAMICS IN POTATOES (Solanum tuberosum L.) Table 1. Treatment effects on various stages of CPB when totaled for four measurement times, in 1987. .....100 Table 2. Influence of billing time on CPB at successive stages of development, in 1988. Data combined over potato variety and weed pressure. 00000102 Table 3. CPB counts totaled for the six observation times in 1988. Data combined over potato variety. 0000.104 Table 4. Flea beetle, tarnished plant bug, and aphid population counts, totaled for the entire growing season, in 1987 and 1988. .....105 Table 5. Beneficial insect counts in 1987 and 1988, totaled for the entire growing season. 0.00.107 Table 6. Beneficial insect counts in the redroot pigweed and barnyardgrass study, in 1987. .....108 viii REVIEW OF LITERATURE INTERFERENCE Interference is a broad term that encompasses all factors that may reduce plant growth. Putnam (53) lists three sub-disciplines which fall under the category of interference: allelopathy; allelomediation; and competition. Allelopathy is defined as the interaction of interspecific and intraspecific allelochemicals of both higher plants and microbes. These interactions may be beneficial or detrimental to plant growth. Allelomediation refers to the selective harboring of an herbivore by a plant which selectively attacks other plants, thus giving the harboring plant an advantage over neighboring plants. In agriculture, allelomediation is important in relation to microbes and arthropods. Competition is a common term that implies one organism actively seeking to control, or controlling, the growth requirements of another organism (53). Competition generally results from one plant being better suited for growth and survival than another. The more competitive plants tend to germinate faster and exhibit vigorous early growth of both above and below ground parts (27). In relation to agriculture, competition occurs as a result of a finite system that contains a limited amount of resources for sustaining optimum growth at a specific density. If this density is exceeded, growth of one or more of the less competitive plant species will be hindereded (67). Plants respond to changes in density via phenotypical 2 plasticity, which refers to the plant's ability to alter its growth i.e., amount of tillering, branching, and height, depending on the density (27). In a finite, competitive system plants compete for moisture, nutrients, light, or carbon dioxide (1, 27, 30). Carbon dioxide is ubiquitous and seldom a factor limiting plant growth. Since it is extremely difficult to manipulate C02 concentration in the field it is usually neglected in the literature pertaining to competition (30, 55). Plants which utilize the C4 photosynthetic pathways have different characteristics than plants with C3 photosynthetic pathways. Ribulose bisphosphate carboxylase (RuBP carboxylase) is the main enzyme in carbon fixation and has a high affinity for both 02 and C02. Conditions of high 02 concentration favor photorespiration which limits carbon fixation. C4 plants separate RuBP carboxylase spatially within the plant whereas C3 plants do not. C4 plants have direct contact between the mesophyll cells and the bundle sheath separating 02 from RuBP carboxylase and is known as Kranz anatomy (26). Water is lost when a plant's stomates are open allowing C02 to diffuse into the plant. C4 plants limit the time stomates are open, and thus reducing water loss, by the ability of aspartate and malate to combine with C02 and transport it to the mesophyll cells where photosynthesis occurs. However, C3 plants cannot transport C02, thus the stomates remain open longer to achieve a higher 002 concentration for photosynthesis. Competition is not an intrinsic property of a specific plant, but rather a comparison between plants and within various environmental conditions (51). Black et a1. (6) have reported that C4 plants are generally better competitors than C3 plants. The biochemistry and 3 physiology of C4 plants provide a growth advantage under competitive conditions (6, 50, 51). Baskin and Baskin reviewed the literature and concluded that C4 plants do not have an inherent competitive advantage over C3 plants. Plants are most competitive when they are in their preferred environments, regardless of whether they are C4 or C3 plants (4). Moisture is an important factor that may limit plant growth (30, 67). Leaf expansion is very sensitive to water stress, and when leaf expansion is decreased, the surface area for photosynthetic assimilation decreases (51). Since plant roots absorb soil moisture, the most competitive plants appear to be those with root systems that thoroughly explore a volume of soil (8, 30). Plants compete for nutrients, with nitrogen frequently being the limiting nutrient (51). Due to nitrogen's mobility in the soil, the plants with the root system best adapted for its interception have the competitive advantage (30). Ozturk et a1. (50) showed neither C4 nor C3 plants are consistently more competitive for nitrogen. In a greenhouse competition study between redroot pigweed (Amaranthus retroflexus L.) and tomatoes (Lycopersicon esculentum Mill.), the dry weight of tomatoes was reduced when nutrients and moisture were below the optimum level (40). However, further decreases in tomato dry weight resulted when grown with redroot pigweed under the same stressed conditions. Light can also be a major factor limiting plant growth (30, 55). Radiant energy is critical to many plant processes such as transpiration, photomorphogenesis, photoperiodism, chlorophyll synthesis, chloroplast development, seed germination, stem elongation, leaf expansion, light induced plant movements, and light induced enzyme synthesis and 4 regulations (30). Most of these processes require a ratio of light from specific regions of the light spectrum and if the ratio is disrupted, these processes can be hindered. The light spectrum is altered when passed through a canopy of leaves and allows more far red light to pass through than red light (73). Seed germination requires a high ratio of red light to far red light, and since the canopy drastically reduces the ratio of red light to far red light, seed germination can be limited (73). This filtered light is also less photosynthetically effective (30). Shading can also disrupt the Kranz anatomy in a normal C4 leaf (51). INTERFERENCE STUDIES various experimental designs are available to study weed/crop competition in the field and under artificial conditions. This discussion will be limited to those designs which will be used in later chapters. An additive design experiment compares the relative aggressiveness of a series of competitors compared to an indicator species (30). This design simulates the field situation of weed infestations in a crop. The experiment uses an indicator plant (crop) at a fixed density, and the density of the competitor (weed) is varied. The growth parameters of both species are measured (30, 51, 67). A replacement design, or substitutive experiment, is another method of studying plants' interactions. This design involves two species, but the total density of plants remains constant and the ratio of the species is varied. Replacement designs help determine plant species interactions and group these interactions as either no effect, a strong competitor, 5 mutual antagonism, and symbiosis (30, 67). Both additive and replacement designs have been utilized in determining the effect mixed populations of . weeds have on a crop. Dawson (22) proposed two time periods where weed/crop interactions are critical to evaluate for optimum yields. Additive designs appear to be used most frequently when researching the critical periods of weed control to ensure maximum yield. The first time period involves how long the weeds emerging with the crop can compete before being removed without a reduction in yield. The second critical time period is the number of weeks into the season a crop must be kept weed free to avoid crop loss. A grower can then determine the necessary residual time a herbicide must have as well as the time period for control of escape or late germinating weeds to ensure maximum yield. The length of the first time period is dependent on the vigor of the weed species and its ability to capture the factors needed for growth. For example, if moisture is the competitive factor, the length of time that plants can compete without a yield reduction is shorter than if light is the competitive factor (21). Dawson (21) found in his period threshold studies that for each week a crop remained weed free, the crop yield increased and total weed weight decreased until a plateau was reached. weed growth was never zero. When half the crop was removed after a certain period of time, weed growth increased in the non-crop area, but growth remained suppressed in the cropped area (21). The second stage of the period threshold involves a grower controlling the weeds early in the season until the crop has reached a stage where weeds can be suppressed through interference (21). A crop's ability to compete is diminished by any factor that reduces the vigor of the crop or decreases the stand (21). A plant with an advantage in one growth requirement will, in time, compete with the other plants for other growth requirements, making it very difficult to determine which is the most limiting component of growth (21, 30). Critical period results vary depending on the crop, the weeds, and the environmental conditions. Therefore, generalized statements applying specific results to a wide range of circumstances may be invalid. Attempts have been made to define competitive relationships and to use these for threshold models. These models determine when a plant begins to be a detriment to other plants. Radosevich and Holt (55) reviewed plant characteristics they felt must be considered in developing threshold models. The first consideration is the plant's plasticity, i.e., the ability to vary vegetative growth depending on density. Greater plasticity means that the number of plants is less critical in relation to other parameters. Secondly, plasticity of weeds allows competition to result from low plant densities. Third, the seed bank in the soil makes predictions of what may germinate and compete very difficult. Fourth, natural weed communities are mixed species, and the models must consider the reaction of a particular species to others. Fifth, crop rotations must consider the weed threshold of subsequent crops not only the present season threshold. The following crop may be sensitive to herbicides needed to control a specific weed, thus maximum weed control may not be possible if a sensitive crop is in the rotation. Finally, thresholds should not be based only on simple economics of yield gains versus cost of treatment. Rather they should include such peripheral areas as ease of harvest, crop quality and impact of pests and beneficial organisms (55). Radosevich and Holt (55) further stated, competitive relationships 7 must include the spatial arrangement of the weeds, timing of germination, and the growth rate of the plants. The more competitive plants are separated from other plants, established before other plants, and grow quicker than their neighbors (55). Competitive characteristics can be modified or reversed by changes in growth parameters, e.g., precipitation, fertility or temperature (86). Dexter and Evans (23) found when predicting yield losses due to weed competition that measurements of precipitation, maximum-minimum soil' temperatures 10 centimeters below the soil surface, and weed density gave a much more accurate coefficient of determination than only using weed density. Oliver (49) explored the concept of a sphere of influence. Sphere of influence is the effect a single weed has on a crop plant at regular distance intervals away from the crop plant. Oliver concluded it was an accurate method in assessing the interference of low densities of weeds on a weed/crop relationship. Coble developed a model to evaluate mixed weed population situations, particularly weed problems arising from incomplete control. A competitive index was developed for individual weed species from a linear regression model of soybean (Glycine max L.) yield on weed densities. The competitive load is determined for each species by multiplying the competitive index times the average number of weeds per 10 meter of row. The competitive load for all individual species is summed to determine the total competitive load. Each unit of increase in the total competitive load resulted in approximately 5% decrease in soybean yield (16). Dawson's period threshold, Oliver's sphere of influence, and Coble's competitive index all allow for a tolerance of weeds in the crop. These models dictate weed management decisions to be made when a given weed species is over a specific threshold. The zero threshold concept, on the other hand, views any control less than 100% as unacceptable (46). The zero threshold concept is difficult to justify in modern agricultural practices on the basis of cost/benefit. Many growers however, place an intangible value on 100% weed control. Studies have also been conducted to examine weed density effects on yield, both for monoculture and mixed weed populations. Some researchers, however, feel that density is not as crucial as the total dry mass production of the weeds (45, 76). Thurlow and Buchanan (76) hypothesize that due to the plasticity of weeds, the density is not as good an indicator of yield loss as total dry matter. Mohammed and Sweet (39) found similar dry weight when 16 redroot pigweeds/m2 were grown or 256 redroot pigweeds/mz. Numerous competition studies have been conducted with pigweed species. Moolani et al. (41) found smooth pigweed (Amaranthus hybridus L.) infesting corn (Egg gays L.) reduced the dry weight of corn one unit for each unit of increase in the dry weight of smooth pigweed. Soybean dry weight was reduced 1 1/3 unit for each unit redroot pigweed increased (41). Buchanan et a1. (10) studied weed effects on cotton (Gossypium hirsutum L.) yield and reported that yield decreased as the density of redroot pigweed and sicklepod (Cassia obtusifolia L.) increased. Weeds did not interfere with harvest except at high densities (10). Schweizer (64) examined redroot pigweed interference with sugarbeets (Beta vulgaris . L.) and the yield of sugarbeets and sucrose content decreased as weed densities increased. Fennemore et a1. (25) conducted a replacement study with beans 9 (Pbaseolus vulgaris L.), barnyardgrass (Echinochloa crus-galli (L.) Beauv.), and black nightshade (Solanum nigrum L.). The beans suppressed the relative growth rate of barnyardgrass for the first 37 days after germination. The relative growth rate of barnyardgrass later increased and surpassed that of the beans. Yield reductions due to late season competition could therefore occur. Similarly, Dawson (20) found weeds that germinated soon after the crop caused the greatest reduction in irrigated bean yield, although the period of competithmmdid not occur for weeks afterwards. Shurtleff and Coble (66) examined numerous broadleaf weeds in soybeans. Leaf area of the soybeans increased as the distance from the weeds increased, and the researchers concluded the range of reduction in soybean leaf area caused by a weed's location was a good method of predicting an individual weed's competitiveness (66). Similarly, Thurlow and Buchanan (77) found sicklepod seeded in the drill at the same time as soybeans were usually less competitive than when seeded 15 cm or more from the drill row due to the competitive nature of the soybean plant. Both broadleaf and grass weeds grown with cotton had a greater reduction in yield when grown within the row than between the row (59). Most studies assume cultivation will remove the weeds from between the rows, thus weed pressure within the crop row is more detrimental in field situations. Studies On weed Interference in Potatoes vanHeemst (77) compared numerous crops and rated the crops according to their ability to compete with weeds. Only wheat (Triticum aestivum 10 L.) and peas (Pisum sativum L.) surpass potatoes (Solanum tuberosum L.) in their relative mean yields in weedy plots compared to weed-free plots. Research determining the critical period for weed control in potatoes has been conducted overseas as well as in the United States. In two Indian studies, 45 days and 4 to 6 weeks of a weed-free period were required for optimum growth (69, 75). A weed-free period of only 25 days was required in Chile (61). In the United States, Vitolo (81) found 6 to 8 weeks of competition from grass weeds could be tolerated in 'Superior' potatoes while only a two-week weed-free period was required for maximum yield. In North Dakota, yield reductions resulted after 8 weeks of mixed weed competition (45). A Lebanese study concluded that 9 weeks of competition with broadleaf weeds could be tolerated (63). Nelson and Thoreson (45) and Saghir and Markoullis (63), concluded that weeds reduced tuber yield due to a decrease in both the number of tubers and average size of tubers. A 10% increase in dry weight of weeds, decreased the fresh tuber yield 12% (45). However, the weeds did not affect the specific gravity of the potatoes (45). varieties may impact weed control due to their various growth habits and thus affect their competitiveness with weeds. Potato varieties with fast emergence, rapid early growth, and an upright dense canopy are best at suppressing weed growth. Potatoes that provide the maximum amount of shade the earliest and for the longest duration are the best competitors (87). 'Superior', an early maturing variety, appeared to be a weaker competitor over the full season when compared with late season potato varieties (54, 65). 'Katahdin' and 'Hudson', longer season varieties, without herbicide were able to suppress weeds similar to 'Superior' with herbicide treatments (65). 11 COMPETITIVE CHARACTERISTICS Roush and Radosevich (62) examined various growth parameters to determine which had the greatest influence on establishing a hierarchy of growth ability among four species, Echinochloa crus-galli (L.) Beauv., Amaranthus retroflexus L., Chenopodium album L., and Solanum nodiflorum Jacq.. Relative growth rate did not vary among the four species. However, unit leaf rate (ULR), leaf area ratio (LAR), and plant dry weight best fit the linear regression of aggressivity (A) (37, 62). Agressivity a 1/2 (W/X)-(Y/Z) where, W a yield of individual plant per species in monoculture . x a yield of individual plants per species in monoculture averaged over reps Y a yield of individual plant per species in mixed culture 2 a yield of individual plants per species in mixed culture averaged over reps. Kroh and Stephenson (35) developed the Competitive Index (CI) to determine the competitive ability of a plant species. CI is determined by: CI - mean plant weight of each species in monoculture divided by mean plant weight of the species in a mixed species treatment. CI is one for intraspecific competition. A CI greater than one indicates a plant is more competitive than its neighbors, and conversely, a CI less than one means that the plant is less competitive. A ranking of competitiveness is determined by summing the CI of each species to give an overall total and the species with the highest total is the most competitive. 12 REDROOT PIGWEED - Amaranthus retroflexus L. REdIOOt pigweed E an annual plant found in disturbed areas where annual weeds predominate (83). It is found on all soil types ranging from sandy loam to clay to muck, and appears to grow best in soils with pH above 6.0 (83). Weaver and McWilliams (83) reviewed the literature on three species of Amaranthus, including A; retroflexus. Redroot pigweed has growth characteristics which aid in its ability to compete with other weeds and crops. The stem of the plant is erect, up to 2 meters tall, and may be either simple of branched. The leaves are alternate and are either ovate or rhombic-ovate. The plant's height, branching, and dense leaves all contribute to increase the plant's light interception capabilities. The plant may take a more prostrate growth habit if it is greatly disturbed. Redroot pigweed has a shallow taproot system and small numerous flowers crowded into dense blunt spikes forming terminal panicles (83). Redroot pigweed is a C4 plant with typical Kranz leaf anatomy. It has a low C02 compensation point, high transpiration efficiency, and high light saturation for photosynthesis. Optimum temperature for photosynthesis is 300 to 400 C. Relative growth rate and leaf expansion increase with increasing temperature and irradiance (83). Tenhenen found that redroot pigweed had photosynthetic rates approximately equal to common lambsquarters (Chenopodium album L.) when compared at a maximum leaf temperature of 15° C. This temperature was less than optimum for pigweed, yet its highly efficient utilization of light and low rates of C02 respiration at night allowed it to have a l3 photosynthetic rate similar to a C3 plant in this study (74). Redroot pigweed is a faculative short-day flowering plant capable of producing 100,000 seeds per plant with 96% of the seeds viable (83). Siriwardana and Zimdahl (70) found the average redroot pigweed produces 67 times more seed than barnyardgrass. Studies looking at the longevity of the seeds in the soil have found seeds to survive from 18 months to 40 years (11, 83). Young plants, up to four weeks after emergence, are quite susceptible to cultivation. Older plants are often able to recover from cultivation (83). BARNYARDGRASS - Echinochloa crus-galli (L.) Beauv. Barnyardgrass, like redroot pigweed, has a C4 photosynthetic pathway, and therefore prefers high light intensity for photosynthesis, and high optimum temperature for growth (32). It is an annual weed that is favored by disturbed environments (32). Barnyardgrass is member of the Poaceae family. It is considered polymorphic due to its wide variety of morphologic variation. Several characteristics allow it to be competitive with other weed species and crops. Barnyardgrass has a stout stem which may reach 1.5 meters in height. One stem may produce up to 15 tillers and the main stem may produce up to eight leaves. The height, tillering, and leaves all contribute to the plant's ability to capture light. Barnyardgrass has a fibrous root system. The panicles are composed of numerous racemes, which may be either spreading, descending, or branched. A single plant may produce up to 7,000 seeds with 90% of the seeds viable (29, 32, 34). 14 There is a decline in the number of tillers and panicles produced when barnyardgrass is in a crowded, competitive situation as well as a reduction in height and dry weight of the plant (5, 47). Barnyardgrass will grow in a wide variety of soils but prefers moist or wet soils (70). Slightly compacted soils favor its emergence (32). weise (85) found barnyardgrass' competitiveness to be adversely affected under water stress conditions. Nussbaum et al. (47) cited barnyardgrass as an inefficient user of water. This finding is not consistent with C4 plant characteristics (6, 52). Barnyardgrass flowers over a wide range of photo-periods (32, 80). Reproductive phase can begin with four to five fully expanded leaves (34). Formation of reproductive shoots is negligible when under approximately 70% shade (5). . Echinochloa crus-galli var. Frumentacea (Roxb). has been cultivated as a forage grass and its seeds used for bird feed. This variety is characterized by its thick, appressed racemes and turgid, awnless spikelets (31). Barnyardgrass and Redroot Pigweed Barnyardgrass and redroot pigweed have a high optimum soil temperature range for germination, 30° to 40° C (32, 47, 83). Increasing soil temperatures decrease the time of emergence for both species (79, 83, 85). These two weeds, therefore, emerge in late spring, due to their high temperature requirement, and continue to emerge through late summer (3, 19, 48, 83). vengris discovered that barnyardgrass seedlings emerging on July 20 in Massachusetts produced mature seeds (79), while 15 redroot pigweed emerging after the first of August produced a negligible number of mature seeds (78). For both species, the earliest plants to emerge, produced the largest amount of dry matter and in turn were the best competitors. The number of days from emergence to maturity for barnyardgrass progressively decreased as the emergence date became progressively later (80). Emergence of barnyardgrass and redroot pigweed appears to be under the control of the phytochrome system, but is greatly enhanced by temperature (72, 83). For both barnyardgrass and redroot pigweed, greater depth of seed increases viability (72). As time of burial increases, viability decreases (72). Roche and Muzik (60) found that barnyardgrass could emerge from a six inch depth and give a competitive stand. Wiese (85) found no redroot pigweed to emerge from a four inch depth of either silty-clay loam or sandy loam soils. Barnyardgrass' ability to emerge from greater depths than pigweed may account for its emergence pattern not being affected by cultivation. Peak emergence of both weeds is at the l to 4 cm depth (19, 85). Emergence of both weed species was favored on a fine sandy loam compared to a silty clay loam in Texas (85), while in Nebraska results with redroot pigweed were contradictory (11). Cultivation results in bringing weed seed to the surface where the likelihood of germination is increased. Cultivation followed by rainfall resulted in a flush of germination of redroot pigweed (24, 48, 58). Shallow tillage of barnyardgrass after May did not have an appreciable influence on barnyardgrass emergence (48). Baskin and Baskin (3) concluded that soil disturbance brought redroot pigweed seeds to the surface and resulted in higher redroot pigweed emergence due to its high light requirements. Both weed species respond favorably to additions of nutrients to the 16 soil. The greatest response was due to an addition of nitrogen, and the least response was due to potassium (32, 33, 83). Both species show they can accumulate high levels of nitrates in their tissue even to levels toxic to wildlife (32, 83). In both additive and replacement studies, barnyardgrass was more competitive than redroot pigweed (38, 62). Siriwardana and zimdahl found redroot pigweed to emerge quicker than barnyardgrass at equal depths and concluded in this case early emergence did not lead to greater competitiveness. Barnyardgrass' competitiveness was favored by lessening the intraspecific competition of barnyardgrass via a smaller barnyardgrass to redroot pigweed ratio, deeper seed depth, or higher soil moisture (70). Gressel and Holm found that aqueous extracts from the seeds of both barnyardgrass and redroot pigweed exhibited some seed germination inhibiting properties. Barnyardgrass extract also decreased root growth of pepper (Capsicum frutescens L.) by more than 20% (28). POTATOES - Solanum tuberosum L. Potato varieties have varying growth characteristics, such as leaflet size, speed of early growth, and ability to maintain a dense canopy that allow them to effectively compete with weeds (71, 87). Collins (17) analyzed canopy size and found branching to have a major influence on a variety's relative size. 'Russet Burbank' is a late maturing variety and 'Atlantic' is a medium to late maturing variety. Both are classified as varieties with large amounts of biomass. 'Russet Burbank' has four pairs of primary leaflets and 'Atlantic‘ only three 17 pairs. 'Atlantic', however, has more secondary and tertiary leaflets (14, 84). Cultural practices are very similar for both varieties, however, 'Russet Burbank' is more sensitive to water stress and early die complex (Verticillium wilt and nematodes) (14). 'Atlantic' is susceptible to internal brown spots and both 'Atlantic' and 'Russet Burbank' are susceptible to hollow heart. INFLUENCE OF POTATO PRODUCTION ON WEEDS weeds are the greatest factor limiting potato yield (57). weed pressure in potato crops has increased with such improved potato production techniques as irrigation, optimum nutrient supply, better disease and insect control and the use of varieties that lack a dense canopy (18). The improved potato production techniques also favor weed growth. Nelson and Thoreson (45) reported that as the total dry weight of weeds is increased, the tuber yield is decreased. Herbicides and mechanical tillage are the conventional options available for weed control in potatoes (18). Herbicides for Michigan potato production are generally applied at planting or prior to the crop's emergence (56). Pre-emergence herbicides such as metribuzin (4— amino-6-(1,1-dimethylethyl)-3-(methylthio)-l,2,4-triazin-5(4H)-one), linuron (N'-3,4-dichlorophenyl)-N-methoxy-N-methlyurea), and metolachlor ( 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-l- methylethyl)acetamide) are usually applied for weed control in potatoes (56). Potatoes generally require at least two weeks from planting until emergence, thus, there is a period of two weeks that pre-emergence herbicides can be applied. The time required for a competitive canopy to 18 herbicides can be applied. The time required for a competitive canopy to develop depends on the potato variety. Therefore, growers using chemical means of weed control must apply a herbicide with soil residual activity to control weeds until the crop is able to compete with emerging weeds. Post-emergence herbicide options are limited to metribuzin and sethoxydim (21_l-(etboxyimino)butly ~51 2-(ethylthio)propyl -3-hydroxy-2-cyclohexen- 1—one) (56). No weed control options are available for broadleaf weeds appearing late in the growing season when the canopy begins to senesce and the foliage becomes sparser (18). Research on the effect of these late developing weeds on tuber yield is not available, but they do produce seeds which could detrimentally affect subsequent crops and these weeds may hinder the harvesting operation (13). Numerous studies have been conducted evaluating the effectiveness of cultivation on weeds and on crop growth. Studies on cultivation show no advantageous effects on tuber yield, therefore, these studies conclude that cultivation should only be used for weed control purposes (7, 13, 18, 42, 44, 57). Hilling potatoes serves as a cultivation to control weeds emerging between the crop rows (7, 44). Rioux et a1. (57) found that hilling potatoes just prior to emergence gave the best weed control. weeds emerged at various times, making it difficult to time the billing operation with weed emergence. Hilling time had no effect on the efficacy of herbicides. Rioux et. a1. recommended that hilling should be done to maximize the vegetative growth of the plant, not as a weed control method. Mechanical tillage and billing will disrupt the layer of herbicide 19 applied to the soil, thus permitting weeds to emerge. These operations will also bring new weed seeds to the surface, increasing the chances of a new flush of weed germination (24, 57, 58). In reviewing the literature on moisture and potato growth Singh (68) found that soil moisture was an important factor in potato production. Tuber yield was greatest when the moisture level of the soil remained above 50% of the field capacity. Maintaining high soil moisture levels eliminated moisture stress and increased both top growth and leaf surface area. Increased leaf surface area, in turn, increased the shading ability of the plant (67). Nitrogen is required for maximum potato production, particularly during the tuberization process (8, 9, 44). Bradley and Pratt concluded that if large amounts of irrigated water are needed, additional nitrogen may be required to avoid nitrogen stress (9, 43). Watson (82) found that the cultural practices such as fertility and adequate soil moisture, which increase yield, can result in an increase in leaf growth. Thus Watson concluded, to maximize yield the leaf area should be at its maximum when the environmental conditions are optimum for photosynthesis. The length of time that the maximum leaf area is present should also be increased (15, 82). Burstall (12) showed that a particular leaf area value will provide greater ground cover earlier in the season than late in the season due to lodging. Allen and Scott (2) examined yield and leaf area index and came to very similar conclusions as Watson. A linear relationship existed between both total dry weight and tuber dry weight and the amount of radiation intercepted by the potato canopy (2). Allen and Scott's article also led to an examination and discussion of cultural practices 20 to increase the leaf area index of the potato crop. Increasing yield as a result of greater leaf area also improved the potato plant's shading ability and may reduce weed germination and suppress weed growth. Potato production is generally on coarse textured soils. In Michigan, approximately 1/10 of the potato production is on high organic soils, or muck. Michigan has 1.8 million hectares of organic soils (third in total area in the United States) (36). Literature pertaining to potato production on muck soils is very limited. No publications were found investigating weed control or weed interference in potatoes on muck soils. 1. 9. 10. 11. 12. 13. 21 LITERATURE CITED Aldrich, R. J. 1987. Prediction crop yield reduction from weeds. Weed Tech. 1:199—206. Allen, E. J. and R. R. Scott. 1980. An analysis of growth of the potato crop. J. of Agric. Sci. 94:583-606. Baskin, J. M. and C. C. Baskin. 1977. Role of temperature in the germination ecology of three summer annual weeds. Oecologia. 30:377- 382. . 1978. A discussion on the growth and competitive ability of C3 and C4 plants. Castanea. 43:71-76. Bayer, G. H. 1964. Growth and Reproduction Studies with barnyardgrass (Echinochloa crusgalli). Abstr. weed Sci. Soc. Amer. p. 35. Black, C. C., T. M. Chen, and R. H. Brown. 1969. Biochemical basis for plant competition. Weed Sci. 17:338—344. Blake, G. R., G. W. French, and R. E. Nyland. 1962. Seedbed preparation and cultivation studies on potatoes. Amer. Potato J. 39:227-234. Bradley, G. A. and A. J. Pratt. 1954. The response of potatoes in irrigation at different levels of available moisture. Amer. Potato J. 31:305-310. . 1955. The effect of different combinations of soil moisture and nitrogen levels on early plant development and tuber set of the potato. Amer. Potato J. 32:254-258. Buchanan, G. A., R. H. Crowley, J. E. Street, and J. A. Moguire. 1980. Competition of sicklepod (Cassia obtusifolia) and redroot pigweed (Amaranthus retroflexus) with cotton (Gossypium hirsutum). Weed Sci. 28:258-262. Burnside, O. C., C. R. Fenster, L. L. Evetts, and R. F. Mumm. 1981. Germination of exhumed weed seed in Nebraska. weed Sci. 29:577-586. Burstall, L. and P. M. Harris. 1983. The estimation of percentage of light interception from leaf area index and percentage ground cover in potatoes. J. Agric. Sci. 100:241-244. Chitsaz, M. and D. C. 1983. Comparison of various weed control programs for potatoes. Amer. Potato J. 60:271-280. 14. 150 160 17. 18. 19. 20, 21. 22. 23. 24. 25. 26. 27. 28. 29. 22 Clark, C. F. and P. M. Lombard. 1946. Descriptions of and Key to American Potato varieties. USDA Circular No. 741. Clutterbuck, B. J. and K. Simpson. 1978. The interaction of water and fertilizer nitrogen in effects on growth pattern and yield of Coble, H. D. 1985. Multi—species number threshold for soybeans. Proc. weed Sci. Soc. Amer. p. 59. Collins W. B. 1977. Comparison of growth and tuber development in three potato cultivars with diverse canopy size. Can. J. Plant Sci. 57:797-801. Dallyn, S. L. 1971. Weed control methods in potatoes. Amer. Potato J. 48:116-128. Dawson, J. H. and v. F. Burns. 1962. Emergence of barnyardgrass, green foxtail, and yellow foxtail seedlings from various soil depths. Dawson, J. H. 1964. Competition between irrigated field beans and annual weeds. weeds 12:206-208. . 1970. Time and duration of weed infestations in relation to weed-crop competition. Proc. South. Weed Sci. Soc. 23:13-25. . 1985. The concept of period threshold. Proc. weed Sci. Soc. met 0 P0 600 Dexter, A. G. and R. R. Evans. 1985. Environmental factors affecting weed number threshold. Abstr. Weed Sci. Soc. Amer. p. 59. Egly, G. H., and R. D. Williams. 1979. Cultivation influences on weed seedling emergence. Abstr. weed Sci. Soc. Amer. p. 82. Fennimore, S. A., L. W. Mitich, and S. R. Radosevich. 1984. Interference among bean (Phaseolus vulgaris), barnyardgrass (Echinochloa crusgalli), and black nightshade (Solanum nigrum). weed Sci. 32:336-342. Fisher, D. G. and R. F. Evert. 1982. Studies on the leaf of Amaranthus retroflexus amaranthaceae morphology and anatomy. Amer. J. Bot. 69:1133-1147. Glaunginger, J. and W. Holzner. 1982. Interference between weeds and crops: a review of literaure. Pp. 149-159 in in W. Holzner and N. Numata ed. Biology and Ecology of weeds. Junk Publishers, The Hague. Gressel, J. B. and L. G. Holm. 1964. Chemical inhibition of crop germination by weed seeds and the nature of inhibition by Abutilon theophrasti. weed Res. 4:44-53. Hafliger, E. and H. Scholz. 1980. Grass weeds 1. Documenta:CIBA- 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 23 GEIGY. Harper, J. L. Pages 305-345 in Population Biology pf Plants. Academic Press, NY. Pp. 892. Hitchcock, A. S. 1971. Manual of the Grasses of the United States. 2nd Ed. V012. Dover Publications, Inc., New York. 2nd Ed. Revised by Agnes Chase. Pp. 712-716. Holm, L. G., D. L. Plucknett, J. V. Pancho and J. P. Herberger. 1977. Chapter 3 in The Werld's werst Weeds. East-West Center Book, Univ. Press of Hawaii, Honolulu, Hawaii. Hoveland, C. S., G. A. Buchanan, and M. C. Harris. 1976. Response of weeds to soil phosphorus and potassium. Weed Sci. 24:194-201. Kacperska-Palacz, A. E., E. C. Putala and J. Vengris. 1963. Developmental anatomy of barnyardgrass seedlings. Weeds. 11:311-316. Kroh, G. C. and S. N. Stephenson. 1980. Effects of diversity and pattern on relative yields of four Michigan first year fallow field plant species. Oecologia. 45:366-371. Lucas, R. E. 1982. Organic Soils (Histols); Formations, Distirbution, Physical and Chemical Properties and Management for Crop Production. Michigan State University Agricultural Experiment Station and Cooperative Extension Service. McGilchrist, C. A. and B. R. Trenbath. 1971. A revised analysis of plant competition experiments. Biometrics. 27:659-671. Minjas, A. N. and v. C. Runeckles. 1984. Application of monoculture yield/density relationships to plant competition in binary additive series. Ann. of Hot. 53:599-606. Mohammed, E. s. and R. D. Sweet. 1978. Redroot pigweed (Amaranthus retroflexus L.) and tomato (Lygppersicon esculentum L.) competition studies: I. influence of plant densities. Proc. weed Sci. Soc. Amer. p. 29. . 1978. Redroot pigweed (Amaranthus retroflexus L.) and tomato (Lycopersicon esculentum L.) competition studies: II. influence of moisture, nutrients and light. Proc. weed Sci. Soc. Amer. p. 30. Moolani, M. R., E. L. Knake, and F. W. Slife. 1964. Competition of smooth pigweed with corn and soybeans. weeds. 12:126-128. Moore, G. C. 1937. The effect of certain methods of potato cultivation on growth and yield and accompanying soil conditions. Amer. Potato J. 14:175-184. Moursi, M. A. 1954. The effect of weed competition and pruning of roots on the physiological ontogeny of the potato crop. Amer. Potato J. 31:178-182. 44. 450 460 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. S7. 58. 24 . 1955. Effect of intensity and width of inter- row tillage on the yield of the potato crop. Amer. Potato J. 32:211-214. Nelson, D. C. and M. C. Thoreson. 1981. Competition between potatoes (Solanum tubersum) and weeds. Weed Sci. 29:672-677. Norris, R. F. 1985. Weed population dynamics and the concept of zero thresholds. Abstr. Weed Sci. Soc. Amer. p. 58. Nussbaum, E. S., A. F. Wiese, D. E. Crutchfield, E. W. Chenault and D. Lavake. 1985. The effects of temperature and rainfall on emergence and growth of eight weeds. Weed Sci. 33:165-170. 099, A. G. and J. H. Dawson. 1984. Time of emergence of eight weed species. weed Sci. 32:327-335. Oliver, R. L. and J. M. Chandler. 1985. Sphere of influence on individual weeds. Abstr. Weed Sci. Soc. Ozturk, M., Rehder H., and H. Zeigler. 1981. Biomass production of C3- and C4-plant species in pure and mixed culture with different water supply. Oecolgia. 50:73-81. Patterson, D. T. 1985. ”Comparative Ecophysiology of Weeds and Crops.” Chapter 4 in 5.0. Duke, ed. weed Physiology: Reproduction and Ecophysiology. Boca Raton, FL: CRC Press, Inc. Pearcy, R. W., N. Tumosa, and K. Williams. 1981. Relationships between growth, photosynthesis and competitive interactions for a C3 and a C4 plant. Oecologia. 48:371-376. Putnam, A. R. and C. S. Tang. 'Alleopathy: State of the Science”. Chapter 1, pp. 1-19 in A. R. Putnam and C. S. Tang, ed. The Science g£.Allelopathy. New York: John Wiley 8 Sons, Inc., 1986. Raby, B. J. and L. K. Binning. 1985. Weed competition study in 'Russet Burbank' and ‘Superior' potato (Solanum tuberosum) with different management practices. Proc. NorthCent. Weed Cont. Conf. 40:4 Radosevich, S. R. and J. S. Holt. 1984. weed Ecologyi_Implications for vegetative Management. John Wiley 5 Sons, Inc., NY. Pp. 265. Renner, K. A. and J. J. Kells. 1988 Weed Control Guide for Field CrOps. Michigan State University Ext. Bull. E-434. Rioux, R., J. E. Compeau, and H. Genereux. 1979. Effect of cultural practices and herbicides on weed population and competition in potatoes. Can. J. Plant Sci. 59:367-374. Roberts, H. A. and M. E. Potter. 1980. Emergence patterns of weed seedlings in relation to cultivation and rainfall. weed Res. 20:377- 386. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 25 Robinson, E. L. 1976. Effect of weed species and placement on seed cotton yields. Weed Sci. 24:353-355. Roche, B. F. and T. J. Muzik. 1964. Ecological and physiological study of Echinochloa crusgalli (L.) Beauv. and the response of its biotype to sodium 2,2-dichloropropionate. Agron J. 56:155-160. Rodrigues, M. and H. Faiguenbaum. 1985. Competitiveness of beans (Phaseolus vulgaris), sunflower (Helianthus annuus), and potatoes (Solanum tuberosum) against weeds in the critical period of competition. Simiente 55:40. Roush, M. L. and S. R. Radosevich. 1985. Relationships between growth and competitiveness of four annual weeds. J. Appl. Ecol. 22:895-905. Saghir, A. R. and G. Markoullis. 1974. Effects of weed competition and herbicides on yield and quality of potatoes. Proc. Brit. Weed Cont. Conf. Schweizer, E. E. 1981. Broadleaf weed interference in sugarbeets. Selleck, G. W. and S. L. Dallyn. 1978. Herbicide treatments and potato cultivar interactions for weed control. Proc. Northeast Weed Shurtleff, J. L., and H. D. Coble. 1985. Interference of certain broadleaf weed species in soybeans. weed Sci. 33:654-657. Silvertown, J. W. 1982. Introduction 32 Plant Population Ecology. Longman House, NY. Pp. 209. Singh, G. 1969. A review of the soil-moisture relationship in potatoes. Amer. Potato J. 46:398-403. Singh, R. D., R. K. Gupta, K. venugopal, and G. B. Singh. Undated. Evaluation of weedfree maintenance for mustard and potato in Sikkim. Proc. Indian Soc. weed Sci. p. 69. Siriwardana, G. D. and R. L. Zimdahl. 1984. Competition between barnyardgrass (Echinochloa crusgalli) and redroot pigweed (Amaranthus retroflexus). weed Sci. 32:218-222. Sweet, R. D. and J. B. Sieczka. 1973. Comments on ability of potato varieties to compete with weeds. Proc. Northeast weed Sci. Soc. 27:302-304. Taylorson, R. B. 1970. Changes in dormancy and viability of weed seeds in soils. Weed Sci. 18:365-369. Taylorson, R. B. and H. A. Borthwick. 1969. Light filtration by foliar canopies: significance for light-controlled weed seed germination. Weed Sci. 17:48-51. Tenhunen, J. D. 1982. The diurnal course of leaf gas exchange of the 75. 76. 77. 78. 790 80. 81. 82. 83. 84. 85. 86. 87. 26 C4 species Amaranthus retroflexus under field conditions in a 'cool' climate: comparison with the C3 species Glycine max and Chenopodium album. Oecologia. 53:310-316. Thakral, K. K., M. Pandita, and S. Khurana. 1985. Effect of time of weed removal on growth and yield of potato. Proc. Indian Soc. weed Sci. p 16. Thurlow, D. L. and G. A. Buchanan. 1972. Competition of sicklepod with soybeans. weed Sci. 20:379-384. vanHeemst, J. D. J. 1985. The influence of weed competition on crop yield. Agric. System. 18:81-93. vengris, J. 1963. The effect of time of seeding on growth and development of rough pigweed and yellow foxtail. Weeds. 11:48-50. . 1965. Seasonal occurrence of barnyardgrass in potato fields in Massachusetts. Weeds. 13:374-375. vengris, J., A. E. Kacpersk-Palacz and R. B. Livingston. 1966. Growth and development of barnyardgrass in Massachusetts. Weeds. 14:299-301. Vitolo, D.B..'Barnyardgrass (Echinochloa crusgalli) - White Potato (Solanum tuberosum) Competition.” Ph.D. Thesis, Rutgers University, 1985, pp. 77. Watson, D. J. 1956. ”Leaf Growth in Relation to Crop Yield“. Chapter 14 in F. L. Milthrope, ed. The Growth of Leaves. Proc. 3rd Easter Sch. Univ. Nottingham. Butterworth Scientific Publications, London. Weaver, 8. E. and E. L. McWilliams. 1980. The biology of canadian weeds. 44. Amaranthus retroflexus L., A, pgwellii S. Wats. and A. hybridus L.. Can. J. Plant Sci. 60:1215-1234. webb, R. E., D. R. Wilson, J. R. Shumaker, B. Graves, M. R. Henniger, J. Watts, J. A. Frank, and H. J. Murphy. 1978. Atlantic: a new potato variety with high solids, good processing quality, and resistance to pests. Amer. Potato J. 55:141-145. Wiese, A. F. and R. G. Davis. 1967. weed emergence from two soils at various moistures, temperatures, and depths. Weeds 15:118-121. Williams, E. J. 1962. The analysis of competition experiments. Aust. Yip, C. P., R. D. Sweet, and J. B. Sieczka. 1974. Competitive ability of potato cultivars with major weed species. Proc. Northeast Weed 801 0 30¢. 28: 271—281. REDROOT PIGWEED (Amaranthus retroflexus L.) AND BARNYARDGRASS (Eghinochloa crus-galli L. Beauv.) INTERFERENCE IN POTATOES (Solanum tuberosum L., var. 'Atlantic'fr MARK J. VANGESSEL AND KAREN A. RENNER2 Abstract. In field studies of barnyardgrass and redroot pigweed seeded at densities of l, 2, and 4 weeds/m within the 'Atlantic' crop row at potato planting and between the crop row after hilling, barnyardgrass was not more competitive than redroot pigweed. Neither redroot pigwweed nor barnyardgrass seeded between the crop row at time of billing reduced aboveground potato biomass or tuber yield. weed density of l weed/m of either species within the crop row reduced tuber yield both years. Redroot pigweed seeded within the crop row had greater dry weight than barnyardgrass, but barnyardgrass reduced aboveground potato biomass more than redroot pigweed in the row in 1987, yet both weeds were equally competitive in regards to tuber yield. In 1988 redroot pigweed reduced tuber yield 7% more than barnyardgrass. Tuber yield correlated well with weed density/plot and weed biomass/total plant biomass, respectively. Neither specific gravity nor tuber quality were altered by the presence of either weed species at any density. 1Received for publication December xx, 1988, and in revised form January zz, 1989. Michigan Agric. Exp. Stn. J. Former Grad. Res. Asst., and Asst. Prof., respectively, Dep. Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824. 27 28 'Atlantic‘ and 'Russet Burbank' potatoes were equally competitive when aboveground biomass was measured under moist soil conditions in greenhouse replacement series experiments. Barnyardgrass and redroot pigweed were less competitive than either potato variety, and barnyardgrass was more competitive than redroot pigweed. Nomenclature: Potato, Solanum tuberosum L.; redroot pigweed, Amaranthus retroflexus L. #3 AMARE; barnyardgrass, Echinochloa crus-galli (L.) Beauv. 4 ECHCG. Additional index words. Aboveground biomass, canopy closure, interference, photosynthetic active radiation (PAR), tuber quality, AMARE, ECHCG. 3Letters following this symbol are a WSSA-approved computer code from composite List of weeds, Weed Sci. 32, Suppl. 2. Available from WSSA, 309 West Clark Street, Champaign, IL 61820. The first step in developing an integrated pest management program is determining if a pest reduces crop yield or alters crop quality. Cable (5) developed a competitive index for various weeds in soybeans (Glycine max (L.) Merr.), and determined the infestation level of various weed species where crop yield was reduced. Dawson (7, 8) discussed the 29 concept of a weed fre period prior to weed infestation where soybean yield would not be reduced as well as a time period that the crop and weeds could compete before weed removal without causing a yield reduction. Weed competition thresholds and weed free periods have not been as extensively developed in other crops, including corn (Zea mays L.), cotton (Gossypium hirustum L.), sugar beets (Beta vulgaris L.), and potatoes (3, 4, 14, 15, 24, 31). Research has shown potatoes to be a competitive crop (30). Singh et a1. (26) and Thakral et a1. (29) reported that 45 days or a 4 to 6 week weed free period was required for optimum tuber yield. In other research (23), the greatest reduction in tuber yield occurred after 9 weeks of interference from a natural infestation of weeds, with redroot pigweed one of the predominant weed species. Vitolo et a1. (31) found that a natural stand of grasses, including barnyardgrass, could compete with potatoes ('Superior‘) for 6-8 weeks before yield was reduced. Also, a 2 to 4 week weed free period was sufficient to assure maximum yield (31). Researchers in have primarily evaluated the influence of 'natural infestations' of weeds on tuber yield, with weed pressure primarily occurring within the crop row (15). Nelson et a1. (15) and Saghir et a1. (23), concluded that weeds reduced tuber yield due to a decrease in both the number of tubers and the average size of tubers. A 10% increase in dry weight of weeds, decreased fresh tuber yield 12% (15). However, the presence of weeds did not alter the specific gravity of the potatoes (23). These authors found no literature examining competitive thresholds for individual weed species in potatoes. Billing of potatoes is a common cultural practice which shields tubers from light, assists in harvest, and serves as a means of mechanical weed control. The hilling process, however, disturbs the 3O herbicide treated soil and brings weed seeds to the soil surface where weed seed germination may occur (9, 20). The effect these late emerging weeds have on yield has not been documented. Plants capable of gaining an early growth advantage due to early emergence or greater relative growth rates are capable of capturing limited resources and 'outcompeting' neighbors (6). Plants compete for various resources, including moisture, nutrients, and light (1, 10, 11). Competition occurs for light because the upper plant canopy absorbs a major portion of the photosynthetically active radiation (PAR), which results in the shorter plants being less competitive since they receive a lower percentage of PAR (11). Allen and Scott (2) examined yield and leaf area index for potatoes. A linear relationship existed between both total dry weight and tuber dry weight and the amount of PAR intercepted by the potato canopy. Redroot pigweed and barnyardgrass are two common weed pests in potato production. Moolani et a1. (4) reported redroot pigweed at a 2.5 cm spacing in the row reduced corn yield 39% and soybean 55%. Redroot pigweed in cotton reduced yield linearly as the density increased from 0 to 32 weeds/15 m of row. Both barnyardgrass and redroot pigweed are C4 plants, thus their relative growth rate increases with warmer conditions because C4 plants have a higher optimum temperature for growth (17). Cultivation and billing practices resulted in increased redroot pigweed germination (21), but neither practice influenced barnyardgrass emergence (l6). Barnyardgrass was more competitive than redroot pigweed in both additive and replacement series greenhouse experiments (13, 22). Siriwardana and Zimdahl found redroot pigweed to emerge sooner than 31 barnyardgrass when seeded at equal depths, but concluded that earlier emergence did not lead to greater competitiveness. Barnyardgrass's competitiveness was favored by a lower barnyardgrass to redroot pigweed ratio, higher soil moisture, or deeper seed depth (27). In other research comparing the emergence of barnyardgrass and redroot pigweed (33), barnyardgrass and redroot pigweed emerged from a 0.3 to 8 cm depth in a greenhouse experiment, with optimum emergence at 0.5 cm. Replacement experiments in the greenhouse were initiated to evaluate the influence of barnyardgrass and redroot pigweed on the growth of two potato varieties under high moisture conditions. Field research was initiated to determine at what density redroot pigweed and barnyardgrass reduce tuberquality and/or yield when seeded in the crop row at planting or between the crop row after hilling. Since potato size and quality are critical for maximum economic return, sizing, internal defects, and specific gravity were compared among the various treatments. MATERIALS AND METHODS Greenhouse. The competitiveness of redroot pigweed, barnyardgrass, and two potato varieties, 'Russet Burbank' and 'Atlantic‘, was determined in greenhouse replacement experiments. The soil was a sandy loam soil complex of Montcalm (sandy, mixed, frigid, Alfie, Haplorthod) and McBride (coarse-loamy, mixed, frigid Alfic Fragiorthod) soil series with an 32 organic matter content of 1.7% and 1.5% and soil pH of 5.8 and 5.2, for the first and second experiments, respectively. Each 20 cm pot utilized one indicator weed species or potato variety. This indicator plant was grown with one of the other three plants in ratios of 0:4, 1:3, 2:2, 3:1, or 4:0. All 4 plants in a pot were arranged in a square design, spaced 4 cm apart. Barnyardgrass and redroot pigweed seeds were placed at a 0.5 cm depth. The potato sprout sets were extracted with a fruit baller and planted 7.5 cm deep. Seeds and sprout sets were planted the same day and later thinned to one weed or stem per pot location. There were four replications in the first experiment, and three replications when the experiment was repeated. The plants were surface watered initially, and then subirrigated with water or a dilute fertilizer solution of 20-10-20 (N-P-K) to maintain moist soil conditions. The greenhouse temperature ranged from 18°C to 29°C. The natural lighting in the greenhouse was supplemented with sodium lights which were on a 16 hr daylength. The range of light intensity was 350.uE cm.2 sec-1. Forty-three days after planting, plant biomass above the soil line was removed from each pot, dried to a constant weight, and dry weight of each plant was recorded. Dry weights were then averaged for the species and/or varieties in each pot. Field studies. The interference of barnyardgrass and redroot pigweed on irrigated potatoes was determined in field experiments in 1987 and 1988 at the Montcalm Potato research Farm, in Entrican, MI. Research plots were established on a sandy loam soil complex of Montcalm (sandy, mixed, frigid Alfic Haplorthod) and McBride (coarse-loamy, mixed, frigid Alfic Fragiorthod) soil series with an organic matter content of 1.9%, and a soil pH of 5.7 and 5.2, in 1987 and 1988, respectively. 33 Field preparation and fertilization utilized standard cultural practices and Michigan State University (MSU) soil test recommendations. Muriate of potash (0-0-60) was applied at 224 kg/ha in 1987, prior to spring plowing. Potatoes (variety 'Atlantic') were planted on April 30, 1987 and April 27, 1988. An 18 cm band application of 560 kg/ha of 20- 10-10 (N-P-K) fertilizer and 2.4 kg/ha of aldicarb 15G ( 2-methyl-2- (methylthio) propionaldehyde 0-(methylcarbamoyl) oxume ) was applied at planting each year. Subsequent applications of nitrogen (28% liquid ammonium nitrate at 84 kg/ha) were applied through the irrigation system 48, 67, and 89 days after planting in 1987. In 1988, 26 kg/ha of 28% liquid ammonium nitrate was applied through the irrigation system 62 and 86 days after planting. Solid set irrigation was utilized after billing both years using the MSU irrigation scheduling program for potatoes. Plots were scouted for insects and disease and were treated accordingly. Plots consisted of three potato rows, 6.1 m in length, on an 86 cm spacing. Potato seed pieces were planted 21 cm apart in the row. Plots were billed once, on June 9, 1987 and June 16, 1988, when the potatoes were 32 cm tall. The experiment consisted of 13 treatments with six replications arranged in a randomized complete block. The design was a three factor factorial plus a weed free control. The three factors were: 1) weed species, either redroot pigweed or barnyardgrass; 2) weed location, with weeds seeded within the crop row at the time of potato planting or between the potato row after hilling; and 3) weed density of either 1, 2, or 4 weeds/meter of row (100, 50, and 25 cm between weeds, respectively). Redroot pigweed and barnyardgrass (var. frumentacea (Robx)) were seeded in the crop row within one day of potato planting or hilling (dependent upon treatment), and later thinned to the desired density. 34 Undesirable weeds were controlled by hoeing and hand-weeding. The weed density was accurate prior to canopy closure both years. Potato height, PAR, and potato leaf area were measured within one day of billing (40 days after planting (DAP)), at canopy closure ((leaves from adjoining rows began to touch) (54 DAP)), and as the potato plants began to senesce (109 DAP) in 1987. In 1988, potato and weed height, PAR, and leaf area were measured at the time of hilling (49 DAP), and at canopy closure (67 DAP). Three samples/plot in four of the replications were measured, and an average from each plot used in data analysis. PAR was measured with a photometer4 which provided total quantum flux density between 400 and 700 nm. Measurements were taken above the canopy and at the soil surface both within and between the crop row at 91, 213, and 457 cm from the edge of the plot. values are reported as percent absorption = (Sa-Sb)/Sa, where Sa is the reading above the canopy and Sb is the reading below the canopy. Leaf area was measured using a portable leaf area meters. The same three plants in selected plots were measured each time. Samples of the three plant species were harvested on August 12 of both years (105 DAP in 1987 and 106 DAP in 1988), and fresh weight, dry weight, plant height and leaf area measured. Plant height, fresh weight, and dry weight were regressed on leaf area. The aboveground portion from 4 plants of each species was harvested at the time of potato senescence (109 and 106 DAP, in 1987 and 1988, ILi-cor LI-18SB Quantum radiometer/photometer, Lincoln, NE 68504. Li-cor LI 3000 Portable leaf area meter, Lincoln, NE 68504. 35 respectively). Two potato plants were selected from each border row of each plot. Two weeds seeded within the crop row were selected from each border row, and two weeds were chosen from 2 locations in plots where weeds were seeded between the crop row after hilling. Sample plants were dried to a constant weight and averaged for data analysis. Immediately prior to vine kill, (September 14, 1987 and September 6, 1988) four random weed samples were collected in plots where weeds were seeded after hilling. Plants were dried to a constant weight and an average used for data analysis to determine if weed weight increased from time of senescence to harvest time. Plots were desiccated on September 14, 1987 and September 8, 1988 with diquat (6,7-dihydrodipyrido l,2-alpha:2',l'-c pyrazinediium ion) at 0.28 kg/ha and non-ionic surfactant6 at 0.5% (v/v). weeds were mowed 14 days later and plots beaten with a mechanical beater for ease in harvesting. The center row of each plot was harvested. The tubers were graded as follows: less than 5 cm in diameter; 5 to 8 cm in diameter; over 8 cm in diameter; and off types. Graded tubers were weighed, and the weight of all tubers over 5 cm was added to determine the weight (metric tons/ha) of marketable tubers. Specific gravity was calculated (ratio of weight in air to weight in water), and 15 tubers (5 to 8 cm in diameter) from each plot were cut from stem to distal end and examined for internal defects. Data analysis. All data from both field and greenhouse studies were ax—77 Spreader (alkylarylpolyoxyethylene, glycols, free fatty acids, and isopropanol). Chevron Chem. Co., San Francisco, CA 94119. 36 subjected to analyses of variance, and main effects and interactions tested for significance. Treatment means were compared using a least significant difference (LSD) test at P: 0.05 if significant main effects and/or interactions occurred. Redroot pigweed, barnyardgrass, and the two potato varieties, 'Atlantic' and 'Russet Burbank' were ranked by competitive indices (CI) and relative competitive abilities (RCA), as developed by Krohl and Stephenson (12). CI's were calculated based on plant dry weight. CI = mean plant weight of the species (variety) in a treatment/mean plant weight of the same species (variety) grown alone. If the CI was greater than one, intraspecific competition was greater than interspecific competition, and if CI was less than one, interspecific competition predominated. RCA's were determined by summing the CI's of each species or variety, with a greater RCA indicating a more competitive plant. Field weed density data were subjected to regression analyses. The resulting equations were compared using a homogeneity of beta variance test (28). Data were not combined over years because of significant year by treatment interactions. RESULTS AND DISCUSSION Under moist soil conditions, barnyardgrass was a superior competitor to redroot pigweed, yet both potato varieties were 37 more competitive than either weed species (Table l). Barnyardgrass has been reported to be more competitive than redroot pigweed in previous research (27). The CI (competitive index) for pigweed was less than 1 for both potato varieties and barnyardgrass, showing pigweed competed more with itself than other barnyardgrass or the potato varieties. Barnyardgrass competed more with itself than potatoes, but more interspecific competition occurred with redroot pigweed. Intraspecific competition occurred for 'Atlantic' when grown with all other plants. Intraspecific competition occurred for 'Russet Burbank' in combination with both weed species, but interspecific competition developed when grown with 'Atlantic'. The 'Atlantic' and 'Russet Burbank' potato varieties had the same ranking for competitive abilities. Previous studies ranking the competitiveness of potato varieties found 'Russet Burbank' to be less competitive than 'Katahdin' and 'Hudson', both late season varieties (25). Raby et al. found 'Russet Burbank‘ to be more competitive than 'Superior' (18). Ranking of the same varieties by different researchers can be inconsistent (25), but generally the longer season varieties are more competitive than early season potato varieties. 'Russet Burbank' and 'Atlantic' are both considered late season varieties. Potatoes were more competitive than either weed species evaluated, and barnyardgrass was more competitive than redroot pigweed. FIELD STUDIES: weed height. Barnyardgrass was significantly taller than redroot pigweed at hilling time (49 DAP) and canopy closure (67 DAP) when seeded within the crop row when measured in 1988 (Table 2). Barnyardgrass emerged prior to redroot pigweed in 1987 and 1988. The earlier emergence of barnyardgrass compared to pigweed under field conditions was reported previously by 099 and Dawson (16). Early 38 Table 1. Competitive indexes (CI) and relative competitive ability (RCA) for redroot pigweed, barnyardgrass, and two potato varieties ('Atlantic' and 'Russet Burbank'). Determined by greenhouse replacement studies, 1988. CIa Plants evaluated AMARE ECHCG 'ATL' 'RB' RCAb AMARE 1.0 0.7 0.9 0.8 3.4 ECHCG 1.4 1.0 1.0 0.9 4.2 'ATLANTIC' 1.6 1.3 1.0 1.2 5.0 'RUSSET BURBANK' 1.3 1.9 0.8 1.0 5.0 aCI=Competitive index_mean plant weight of species (variety) in a treatment mean plant weight of species (variety) in a pure stand bRCA=Relative competitive ability = sum of CI's for each species (variety). 39 Table 2. Height of weeds seeded within the crop row, measured at hilling time and canopy closure in 1988. Data was combined over weed densities. Height Hilling Canopy weed species time closure (cm/plant) AMARE 33 69 ECHCG 51 92 Significancea * * Comparisons of numbers between columns is not valid. a*‘r-designates significant difference between means of the main effect. 4O emergence may increase the competitiveness of a species (19), although other research has not supported this hypothesis (27). ‘Weed weight. The dry weight of either weed species seeded between the crop row was less than the weight of weeds planted within the crop row in both 1987 and 1988. The dry weight of redroot pigweed seeded in the crop row was greater than that of barnyardgrass in the crop row in 1987 only (Table 3). There was no difference in the dry weight of the weed species when seeded between the rows after billing in 1987 or 1988. There was no change in the dry weight of weeds seeded between the crop row from August 17 to September 14 in 1987, at 28 g/plant. However in 1988, plant dry weight for weeds seeded between the crop row when measured on September 6, had increased 295% compared to weed dry weight on August 12, 22 g/plant to 65 g/plant. The average height of both weeds seeded at the time of potato planting was equal to or greater than the average height of the potato when measured at hilling time and canopy closure. After canopy closure, weeds continued to grow taller while the potato became more prostrate in growth habit and less able to compete for light. weeds which emerged between the crop row after hilling did not reach potato canopy height prior to canopy closure and were unable to absorb adequate PAR for growth. weeds remained stunted in 1988 until canopy senescence at which time increased radiation increased weed growth. The dry weight of the individual weeds did not decrease as density increased. Redroot pigweed and barnyardgrass seeded at 4 plants/m within the crop row was not great enough to reduce intraspecific weed dry weight. Similarly, only 20% of the change in dry weed biomass of individual plants was correlated to the change in weed density in both 41 Table 3. Dry weight of individual weeds measured at potato senescence in 1987 and 1988. Data was combined over weed densities. Dry weights weed species x location 1987 1988 (g/plant) AMARE within row 236 139 AMARE between row 25 26 ECHCG within row 125 152 ECHCG between row 32 _ 18 LSD (0.05)a 45 35 aComparison of numbers between columns is not valid. 42 1987 and 1988. Thus the dry weight of either weed species on an individual plant basis did not demonstrate plasticity either year. Tbtal weed bio-ass. Total weed biomass varied with weed species, location, and density in 1987 (Table 4). Redroot pigweed seeded at 4 plants/m in the crop row had the greatest biomass/plot. Barnyardgrass seeded within the crop row at 2 and 4 plants/m and redroot pigweed at 2 plants/m within the crop row produced similar dry weed biomass. In 1988, both weed species when seeded within the crop row at 4 plants/m produced the greatest total biomass, followed by either weed species seeded within the row at 2 plants/m. Both weed species at all densities when seeded between the crop row in 1987 and 1988 produced less weed dry weight than if seeded within the crop row. The greater dry weed biomass of redroot pigweed at 4 plants/m of row compared to barnyardgrass at 4 plants/m of row in 1987 was-due to an increase in dry matter of individual pigweed plants (Table 3). For the first 45 days after planting in 1987, the plots received 6.5 cm of moisture compared to 3.1 cm in the same time period in 1988. Lack of early moisture may have hindered the early growth of redroot pigweed in 1988, whereas barnyardgrass germinated earlier and growth decreased compared to growth in 1987. As weed density increased, total weed biomass increased both years for weeds seeded in the row (Table 5). In 1987, weed density predicted 69% and 74% of the variability of redroot pigweed and barnyardgrass total biomass when seeded in the row, respectively. Weed density was a predictor of at least 72% of the variability in total weed biomass for both redroot pigweed and barnyardgrass when seeded in or between the rows in 1988. Potatolheight. Potato heights in 1988 averaged 32 cm and 54 cm at the time of billing, and canopy closure, respectively, and did not vary 43 Table 4. Total weed dry weight/plot when measured at potato senescence in 1987 and 1988. Dry weight/plot Density x weed species x location 1987 1988 (kg) --------------- l AMARE/m within row 1.5 1.1 2 AMARE/m within row 2.7 2.4 4 AMARE/m within row 6.1 4.7 l ECHCG/m within row 0.6 0.8 2 ECHCG/m within row 1.8 2.3 4 ECHCG/m within row 3.0 3.3 l AMARE/m between row 0.2 0.2 2 AMARE/m between row 0.4 0.3 4 AMARE/m between row 0.2 0.6 1 ECHCG/m between row 0.1 0.1 2 ECHCG/m between row 0.3 0.2 4 ECHCG/m between row 1.4 0.5 LSD (0.05)“ 1.4 1.0 aComparison of numbers between columns is not valid. 44 Table 5. Weed density regressed on total weed biomass/plot in 1987 and 1988. 1987 1988 2 Equation 2 Equation Weed species x location r line r line AMARE within crop row 0.69 y a -87.4 + 247.7x 0.72 y = -18.7 + 193.6x AMARE between crop row NS y = 105.7 + 9.6x 0.78 y = 10.2 + 23.6x ECHCG within crop row 0.74 y a 22.4 + 125.6x 0.73 y = 109.4 + 141.1x ECHCG between crop row 0.25 y = -l78.6 + 58.0x 0.84 y = -7.4 + 19.0x Least significant difference of a slope line (0.05)a 79.3 53.4 Least significant differences of the slope line was determined by tests to test independent regression lines for homogeneity (28). aComparisons between years are not valid. individual T- 45 between treatments (Table 6). However, in 1987 potato height at hilling time varied with weed species and weed density, possibly due to uneven potato emergence (Table 6). By canopy closure, potato height did not vary significantly between treatments in either year, averaging 57 cm in 1987 and 54 cm in 1988. Aboveground potato biomass. Weed species and location influenced aboveground potato biomass in 1987, while only weed location influenced aboveground biomass in 1988. There was a greater reduction of potato biomass by barnyardgrass compared to redroot pigweed seeded in the row in 1987 46 g/plant and 53 g/plant, respectively. Potato aboveground biomass in both years was greatest in plots seeded with weeds between the crop row, averaging 55 g/plant when weeds were seeded between the row and 39' g/plant when weeds were seeded in the crop row. Potato biomass in weed free plots did not differ significantly from plots with weeds seeded between the crop row except in 1987 with barnyardgrass at 1 and 2 plants/m between the row. Weed free plots had greater aboveground potato biomass than plots with barnyardgrass or redroot pigweed seeded in the row in 1987, and all pigweed plots in 1988. Barnyardgrass in the crop row in 1988 at 2 and 4 plants/m did not reduce aboveground potato biomass. In previous greenhouse research, barnyardgrass was more competitive than redroot pigweed on a fresh weight basis, yet reduction in soil moisture increased the competitiveness of redroot pigweed (27). In 1988, only 3.1 cm of moisture was recorded for 45 DAP, compared to 6.5 cm for the same period in 1987. Therefore, redroot pigweed appeared to be more competitive under the lower soil moisture conditions found in 1988. Field observations of redroot pigweed indicate that the architecture 46 Table 6. Potato height measured at hilling timea and canopy closureb in 1987 and 1988. Potato height 1987 1988 Density x Hilling Canopy Hilling Canopy weed species time closure time closure — (cm/plant) l AMARE/m 31 55 30 52 2 AMARE/m 36 '57 30 54 4 AMARE/m 31 56 33 57 1 ECHCG/m 33 57 31 55 2 ECHCG/m 32 58 34 53 4 ECHCG/m 36 57 34 53 LSD (0.05)c 3 as us as aMeasurements at hilling time only include treatments with weeds seeded in the crop row. bMeasurements at canopy closure are combined over hilling time. cComparisons between columns are not valid. 47 of the individual plant species could be a competitive factor. Weeds with similar dry weights, one growing erect and the other growing more prostrate due to injury or insect damage, may differ in competitiveness in potatoes, where the crop is not erect. Potatoes adjacent to both types of weed architecture, showed poorer growth when weeds were prostrate. The canopy temperature was measured August 12, 1988 at 3:30 pm in the weed free plot and in plots containing barnyardgrass and redroot pigweed seeded within the row at l and 4 weeds/m. Canopy temperature was significantly higher in the weed free plot compared to the pigweed and barnyardgrass plots, and no difference was noted between weed species (data not presented). Weeds in the crop row appeared to absorb radiation due to their height advantage, thus reducing the potato canopy temperature. PAR. As the potato plants emerged and developed foliage, the plants were erect and maximum shading occurred in the crop row. The potato plants assumed a more prostrate growth habit shortly after canopy closure with fewer leaves in the crop row. In addition, the older leaves of the potato plant began to senesce, resulting in further reduction of PAR absorption in the crop row. In 1987, the potato canopy in the row with or without weeds absorbed at least 56% of the PAR at the time of billing (40 DAP) and PAR absorption did not vary among treatments. By canopy closure (54 DAP), the canopy absorbed 96% of the PAR in the row with no difference between treatments, and as the plants began to senesce (109 DAP), the potato canopy alone absorbed 46% of the available PAR and potato plus weed canopy absorbed 47% to 62% of PAR (Table 7). At senescence when the weed free check was included in the analysis, there was no difference between treatments. However there were differences 48 Table 7. PAR absorption by the potato canopy and potato plus weed canopy measured at crop senescence in 1987. PAR absorption data was combined over weed species and densities. weed location PAR absorption ...... (g)----__ weeds between rows 47 weeds within rows 62 Significanta * b weed free 41 a -* designates significant differences between means. b-weed free mean not used in analysis of variance. 49 among the weed treatments, with weeds seeded in the row absorbing 62% of PAR, while weeds between the crop row absorbed only 47%. PAR absorption measured between the crop row was 50% at canopy closure and 45% at senescence for potatoes alone, and 60% and 76% at canopy closure and senescence, respectively, for potatoes plus weed plot. There was no correlation between PAR absorption and weed biomass, potato height, or aboveground potato biomass. Light is a resource required for growth (1, 10, 11), and PAR was available for weeds to grow from planting until sometime past the time of billing when PAR absorption reached 95%. Weeds between the row had little time for growth with PAR below 90%, and thus could only increase growth after senescence. It was noted that weeds were mature at the time Of senescence when seeded in the row, but not for those between the row. Thus growth for these weeds could occur after senescence, as in 1988. Leaf area. Selected plants of potato and weed species were harvested in August to correlate plant size with total leaf area. Height, fresh weight, and dry weight of each species were regressed on leaf area to determine which parameter had the greater correlation with leaf area (Table 8). Plant fresh weight had the highest correlation with plant leaf area for barnyardgrass, redroot pigweed, and potatoes in 1987 and 1988. The height of redroot pigweed in 1987 and the dry weight of redroot pigweed in 1988 also explained 96% of 98% of the variability in pigweed leaf area. Tuber yield. Weeds germinating between the crop row after hilling (40-49 DAP) had no impact upon total or marketable yield when compared to the weed free plot in 1987 or 1988 (Table 9). Yield of oversized tubers (greater than 8 cm in diameter) doubled when weeds were seeded between 50 Table 8. Measured growth parameter for each species that had the greatest correlation with leaf area in 1987 and 1988. 1987 1988 Plant Parameter r Equation line Parameter r Equation line Potato fresh weight 0.71a y=129.1 + 0.1x fresh weight 0.98 y=32.3 + 0.1x AMARE height 0.96 y=l7.5 + 0.0x dry weight 0.98 y=-12.4 + 0.0x ECHCG fresh weight 0.99 y=-46.3 + 0.2x fresh weight 0.99 y=4l.0 + 0.1x Comparisons between years are not valid. aSignificant at alpha = 0.10 level. 51 emowo m. mwmwc cm madame ncomnm one «so omnnmsn nonconwo: cm amnxnnmowm wwowc ooaowsmo o