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EE 33333 3 'I 333533? ‘ , 3E'3.3E' “'"E WEE 3E3} ‘. EE""" EH" 3 E33 "" 'I'H'E" ‘33'E‘EE 3M )4, J 2 'E33_f,'E'333I "E" ' )"E'E" '14 . E;,,,.3EE333333, '33333"33333333'33'33E33E3E33333'33EIE333'EE1E-I ' E333 WM' HEEE J l 5—21 Date 0-7639 M..— ?1"3 =13 A my 4':th as? firing, 1"? .{fio'gn --I‘ [*3- “9- Ego-1;“ ““8; :3. II'J'IL'I-‘fl ‘. Sub-EVE)... Va Umfivemfifiy ‘I This is to certify that the thesis entitled EFFECTS OF ROW SPACING, PLANT POPULATION, AND VARIETIES ON BOTH IRRIGATED AND NON IRRIGATED SOYBEAN (GLYCINE MAX (L.) MERRILL) PRODUCTECOchy presen Frank William Pearsall has been accepted towards fulfillment of the requirements for -82 M 8 degree in Crop & $011 Sciences J9 9am professor mlililxllihhhi 3 1293 01072 8453 BETURNING MATERIALS: ‘PV1531_] Place in book drop to [JBRARJES remove this checkout from “ your record. FINES will be charged if book is returned after the date stamped below. f i i ,‘I’ “Welt; 43318 i t , 9 icf9%?8 rem EFFECTS OF ROW SPACING, PLANT POPULATION, AND VARIETIES ON BOTH IRRIGATED AND NON IRRIGATED SOYBEAN [GLYCINE MAX (L.) MERRILL] PRODUCTION BY Frank William Pearsall 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 Science 1982 ABSTRACT EFFECTS OF ROW SPACING, PLANT POPULATION, AND VARIETIES ON BOTH IRRIGATED AND NON IRRIGATED SOYBEAN [GLYCINE MAX (L.) MERRILL] PRODUCTION BY Frank William Pearsall In the sandy soils of southwestern Michigan, supple- mented irrigation is required to produce optimum corn yields. However, little research is available to chronicle ideal soybean production practices for irrigation. The objective of this study was to determine the effects of irrigation, row spacing (25, 51, and 76 cm), plant population, and vari- eties of maturity Groups I and II on soybean production. The irrigation study was conducted in southwestern Michigan during the 1980 and 1981 growing seasons on an Elston series sandy loam soil. The experiment was designed as a split-split plot and analyzed as a factorial arrangement. Irrigation significantly increased lodging both years, but caused increased yields and delayed maturity only in 1981. Plants in narrow rows produced an average of 18% greater yields than those in 76 cm rows both years. Differ- ent plant populations produced few effects. Later maturing varieties were highly correlated with higher yields. TABLE OF CONTENTS Page LIST or TABLES. . . . . . . . . . . . . . . . . . . . . iv LIST or FIGURES . . . . . . . . . . . . . . . . . . . . vii INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . LITERATURE REVIEW . . . . . . . . . . . . . . . . . . . Effects of Water Stress on Soybean Physiology. . . . Effects of Water on Soybean Morphology . . . . . . . Soybean Irrigation . . . . . . . . . . . . . . . . . Cultural Practices . . . . . . . . . . . . . . . . . Row Spacing. . . . . . . . . . . . . . . . . . . . . (DQO‘UIMNNH Plant Population 0 O O O O O O O O O O O O O O O O O H O Soybean Varieties. . . . . . . . . . . . . . . . . . H w “TERIALS AND METHODS O O O O O O O O O O O O O O O O O Tensiometers and Irrigation. . . . . . . . . . . . . +4 H O\£fl Plant Samples. . . . . . . . . . . . . . . . . . . . H 0‘ Light Penetration. . . . . . . . . . . . . . . . . . H \l Leaf Water Potential . . . . . . . . . . . . . . . . H \I Harvest. O O O O O O O O O O O O O O O O O O O O O O H \0 RESULTS AND DISCUSSION 0 I O O O O O O O O O O O O O O 0 Climate O O O O C O O O O O O O O O O O O O O O O O O NH U0 Herbicide Damage . . . . . . . . . . . . . . . . . . N UT Irrigation . . . . . . . . .~. . . . . . . . . . . . 1980. . . . . . . . . . . . . . . . . . . . . . . 1981. . . . . . . . . . . . . . . . . . . . . . . Row Spacing. . . . . . . . . . . . . . . . . . . . . WNNN U1£OQUI Plant Population . . . . . . . . . . . . . . . . . . U \D variety. 0 C O O I O O O O O O O O O O O O O O O O O A H Main Effects 0 O O O O O O O O O O O O O O O O O 0 ii 1980 . 1981 . 2-Year Summary of the Main Varieties. Interaction Effects Yield. Maturity . Height . Lodging. Interaction Effects Yield. Maturity . Height . Lodging. Seed Weight. CONCLUSIONS . APPENDIX. . . BIBLIOGRAPHY. 1980. 1981. iii Effects Page 41 45 47 48 48 51 51 51 51 52 56 57 58 58 61 62 67 LIST OF TABLES Table Page 1. Plant populations at each row width. . . . . . . l4 2. Varieties used in the soybean irriga- tion Study 0 O O O O O O O O O O O O O O O O 0 1 4 3. Monthly precipitation and irrigation means (mm) . . . . . . . . . . . . . . . . . . 19 4. Effects of irrigation on the 1980 harvest data (averaged over row spacing, popu- lation, and variety) . . . . . . . . . . . . . 26 5. Effects of irrigation on the components of yield, 1980 (averaged over row spac- ing, population, and varieties). . . . . . . . 26 6. Effects of irrigation on the 1981 harvest data (averaged over row spacing, popu- lation, and variety) . . . . . . . . . . . . . 27 7. Effects of row spacing on leaf area index and light penetration in 1980 (averaged over water, population and variety). . . 32 8. Effects of row spacing on the 1980 com- ponents of yield (averaged over water, population, and variety) . . . . . . . . . . . 32 9. Effects of row spacing on the 1980 harvest data (averaged over water, population, and variety) . . . . . . . . . . . . . . . . . 33 10. Effects of row spacing on the 1981 harvest data (averaged over water, population, and variety ) O O I O O O O O O O O O O O O O O 3 3 11. Effects of pOpulation on the 1980 compo- nents of yield (averaged over water, row spacing, and variety). . . . . . . . . . . 35 12. Effects of the population x row spacing interaction on the 1980 components of yield (averaged over water and variety). . . . 36 iv Table 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. A1. A2. Effect of population on the 1980 har- vest data (averaged over water, row spacing, and variety). . . . . . . . . . Effects of population on the 1981 har- vest data (averaged over water, row spacing, and variety). . . . . . . . . . Effects of varieties on the 1980 har- vest data (averaged over water, row spacing and populations) . . . . . . . . Effects of variety on the 1980 components of yield (averaged over water, row spac- ing, and population) . . . . . . . . . . Effects of variety on the 1981 harvest data (averaged over water, row spacing, and population). . . . . . . . . . . . . Effects of row spacing x variety inter- action on 1980 yields (averaged over water and population). . . . . . . . . . Effects of the water x variety inter- action on 1980 plant heights (averaged over row spacing and population) . . . . Effects of water x variety interactions on 1981 yields (averaged over row spac- ing and population). . . . . . . . . . . Effects of row spacing x variety inter- actions on the 1981 yields (averaged over water and population) . . . . . . . Effects of water x variety interaction' on maturity in 1981 (averaged over row spacing and population). . . . . . . . . Effects of the water x variety inter- action on lodging in 1981 (averaged over row Spacing and population) . . . . Criteria for Herbicide Injury Rating . . . Significance Table for Herbicide Injury Ratings . . . . O O . . . . . . . . . O . Page 38 38 41 43 47 49 52 54 55 57 58 62 62 Table A3. A4. A5. Components of Yield - 1980 . . . . . . . . . . . 63 Summary of Water and Row Spacing Effects on Variety Performance - 1980. . . . . . . . . 64 Summary of Water and Row Spacing Effects on Variety Performance - 1981. . . . . . . . . 65 vi 10. 11. LIST OF FIGURES Climatic Data for 1980 . . . . . . . . . Climatic Data for 1981 . . . . . . . . . Yield Increase (%) Due to Decreasing Row Width (Averaged over water, popu- lation, and varieties) . . . . . . . . Yield Response Curves for Row Spacing in 1980 and 1981 (Averaged over water, population, and varieties) . . . . . . The Effect of Row Spacing on Yields in 1980 and 1981 (Averaged over water, population and varieties). . . . . . . Population x Row Width Interaction Ef- fects on Yield (Averaged over water and varieties) . . . . . . . . . . . Linear Regression Analysis Comparing Variety Yields to Maturities in 1980 (Averaged over water, row spacings and populations) . . . . . . . . . . . Linear Regression Analysis Comparing Variety Yields to Maturity Dates in 1981 (Averaged over water, row spac- ing, and population) . . . . . . . . . Yield Increase (%) due to Decreasing Row Spacing from 76 cm to 25 cm in Irrigated Treatments in 1980 (Averaged over population) . . . . . . . . . . . Yield Increase (%) of Varieties in Irri- gated Treatments Over Non Irrigated Treatments in 1981 (Averaged over row spacing and population). . . . . . . . Yield Increase (%) Due to Decreasing Row Widths in 1981 (Averaged over water and population). . . . . . . . . vii Page 21 22 30 31 31 37 42 46 50 53 53 Figure Page 12. Lodging Scores by Variety and Irriga- tion Treatment in 1981 (Averaged over row Spacing and population). . . . . . . . .9. 59 Al. Plot Plan for the Soybean Irrigation StUdY O O . . . . O . . O . . O O O O O O O O O 6 6 viii INTRODUCTION .Michigan is faced with a unique situation. It is sur- rounded by water, yet it has the least precipitation during the growing season of any state east of the Mississippi River. Lack of rainfall plus sandy soils necessitate crOp land irrigation in many areas of the state. However, unlike western states which are rapidly de- pleting their deep underground water reserves, Michigan has vast renewable ground water supplies and a shallow water table. The long term water supply outlook for Michigan looks bright. Until recently, corn has been the main crop to receive irrigation in southwestern Michigan. However, growers are interested in including soybeans in their rotation to im- prove both pest management and marketing flexibility. While research on irrigated soybean production prac- tices have been conducted in the southern and western United States, little research has been done in Michigan's unique climate. This study attempts to uncover the effects of row spacing, plant population, and varieties of different matu- rities and plant types on irrigated and non irrigated soy- bean production in Michigan. LITERATURE REVIEW The effects of water stress on plant growth and de- velopment have been widely reviewed (15,27,34,39,40). A general review of water relations in soybeans is also avail- able (25). The following review looks at the effects of water stress on soybeans from both the physiological and morphol- ogical standpoints. Effects of Water Stress on Soybean Physiology In general agronomic texts, the wilting point from which plants do not recover is -15 bars. However, this varies among plant species. Boyer (16) found that soybeans can recover from leaf water potentials as low as -41 bars. According to Gardner and Neiman, physiological processes within plants have different sensitivities to water stress (14). Boyer (5) showed that leaf enlargement is the most sensitive physiological process in soybeans. The rate of leaf enlargement at -4 bars was only 25 percent of the rate in well watered plants (5). The lowest water potential associated with leaf enlargement was -12 bars (5). ’ The next process to be affected by decreasing water potentials is photosynthesis (6). In soybeans, 2 photosynthesis was relatively unaffected until leaf water potentials dropped below -11 bars. At -16 bars, photosyn- thesis was 60 percent of that of well watered plants (6). Shaw and Laing (34) reported that net photosynthesis de- clined sharply below a relative leaf water content of 90 percent. Boyer (6) concluded diffusive resistance of the stomates to carbon dioxide entry resulting from water stress, appeared to be the primary factor limiting photosynthesis. In soybeans, as in most C3 plants, respiration is di- vided into dark respiration and photorespiration. Boyer (5) found that dark respiration decreased 50 percent when leaf water potential dropped from -8 to -16 bars. Below -16 bars, dark respiration was constant. The effect on photorespiration is unknown. However, it may be related to stomatal closure. Research has Shown that drought stressed plants have higher leaf temperatures than nonstressed plants (30). Furthermore, increasing leaf temperatures have been correlated with decreasing water po- tentials (20,33) and greater stomatal resistance (20). Since respiration increases with higher temperatures, per- haps photorespiration increases as the stomates begin clos- ing at -11 bars. Effects of Water Stress on Soybean Morphology The physiological effects just discussed are reflected in the morphological changes in water stressed plants. The height of water stressed plants is less than that of well watered plants (2,12,13,26). On the other hand, irrigated soybeans attain greater height which results in increased lodging (2,13). The leaf area index of stressed plants is reduced (32) and plant dry weight is reduced at various rates depending on the stage of phenological develOpment in which water stress occurred (1,32). The most important economic effect of water stress is the effect on yield. Many field research studies have shown water stress, due to lack of irrigation in dry years, re- duces soybean seed yields (1,9,12,13,20,23,24,26,34). The phenological timing of drought stress is important. Shaw and Laing (34), and Sionit and Kramer (38), showed that yields suffer the greatest reduction when water stress oc- - curs during the late pod initiation and pod filling periods. Momen, et al (26) indicated that the pod filling period was most critical. Both studies showed the components of yield varied due to drought stress during various phenological periods. Shaw and Laing showed that stress during flowering and early pod development resulted in decreasing numbers of pods per plant, while stress during pod filling resulted in fewer seeds per pod and a reduction in seed size (34). Their study further pointed out the amazing adaptability of inde- terminate soybean varieties. When moisture stress occurred during early flowering, plants set fewer pods on the lower nodes. Stressed in late flowering, plants set fewer pods on the upper nodes. In both cases, with fewer pods per plant, the number of seeds per pod and the seed size increased.- Soybean Irrigation Several studies on soybeans have been conducted to as- certain the optimum timing Of irrigation; Doss, et a1 (12) found that adequate moisture during the pod filling period was critical in obtaining Optimum yields. Several research- ers have shown that yields did not significantly differ be- tween full season irrigation and irrigations starting at the full bloom period (1,12,23). Long-term correlations Of pre- cipitation and soybean yields have shown similar results. After analyzing 48 years of data for Urbana, Illinois, Runge and Odell (31) found above normal precipitation during July and from mideAugust through September to be Optimum for high soybean yields. Summarizing 38 years Of data for Illinois, Indiana, Iowa, Missouri, and Ohio, Thompson (42) found that above normal precipitation coupled with below normal temper- atures during July and August produced Optimum soybean yields. The rate of irrigation water applied also affects soy- bean yields. Doss and Thurlow (13) found an average yield increase Of only 2 percent between irrigating when 40 per- cent of the available soil moisture was depleted versus ir- rigating when 80 percent of the available soil moisture was depleted. Cassel, et a1 (9) concluded that under-irrigation reduces yield and over-irrigation may result in the depletion Of mobile plant nutrients from the solum. Their inference was that optimum irrigation rates would provide the amount of water equal to the amount Of crop evapo- transpiration depletion. Cultural Practices Cultural practices in field grown soybeans include spacial arrangement (row Spacing and plant population) and variety selection. These cultural practices can affect not only the final soybean yields, but also the water uSe of soybeans. Row spacing, population, and varieties are dis- cussed in separate sections which follow. Row Spacing One theory for altering the distance between rows is that light interception is maximized when the Space between rows is minimized, resulting in Optimum yields. Much research on the effects of row spacing on yields has been conducted over the years. In the 19303, Whiggans (47) in New York reported a 36 percent yield increase from narrowing the rows from 81.3 cm to 20.3 cm. In more recent years, researchers in the northern states have shown that yields increased as soybeans were planted in narrower rows (3,11,17,21,45). The effects Of row spacing on lodging are unclear. Narrowing the row width has been shown to both in- crease (17) and decrease (3) lodging. Hicks, et a1 (17) and weber, et a1 (45) have shown the photosynthetic area, or leaf area index (LAI), increases as the space between rows decreases. While soybeans do not have an Optimum LAI for seed yield (36), LAI is somewhat related to the narrow row advantage. Shibles and Weber (37) reported that "for maximum yields to be attained, com- plete (maximum light) interception must be reached prior to the period of production of economic yield (flowering)." Furthermore, they found that soybeans in narrower rows achieved full light interception sooner than those in wider rows. The length Of the vegetative period prior to flowering is also important. In the southern U.S., the growing season and the vegetative periods are longer. Therefore, even in wide rows, soybeans can form a closed canOpy before flower- ing. In the northern U.S., where the growing season is shorter, narrow rows enable plant canopies to close before flowering (19). Costa, et a1 (11) found that earlier maturing Group 0 varieties showed a greater yield response to narrow rows than later Group I and II varieties. They speculated that the taller stature Of the Group_I and II varieties enabled them to fill the rows sooner. Row width also affects water use patterns. Peters and Johnson (28) found that between 25 to 50 percent Of the total water loss in a dry year and greater than 50 percent Of the water loss in a wet year is due to evaporation from the soil surface. They also found 50.8 cm rows had a greater water use efficiency (kg seed produced/kg water used) than 101 cm rows. They suggested this may be due to earlier shading Of the ground between the rows by the narrower rows. Similarly, Doss, et a1 (13) found that during the vegetative period, soybeans in 90 cm rows used more water than those in 60 cm rows. Again, soil shading by the nar- row rows was credited. However, there are dissenting viewpoints. Taylor (41) hypothesized that without irrigation in a dry year, the rapid canopy development in narrow rows would cause an in- crease in transpiration and deplete the existing soil mois- ture. As a result, less water would be available during the critical seed filling period and yields would be reduced. Furthermore, he expected wide rows (100 cm) to yield more than narrow rows under those conditions. The results from his experiment showed a greater seasonal soil moisture de- pletion in 25 cm rows. However, the 25 cm rows yielded slightly, though not significantly, more than the 100 cm rows. .Timmons, et a1 (43) found no difference in evapo- transpiration between soybeans in 20.3, 61.0, and 101.6 cm rows in dry years in west central Minnesota. Plant Population Plant population refers to the number of plants in a given unit area. Given a constant row width, changes in population affect the within-the-row spacial arrangement Of individual plants. In the 19303, Whiggans (48) found the optimum population for soybeans was six plants per square foot (645,820 plants/ha). He concluded yields were Opti- mized as row spacings narrowed and populations decreaSed to allow an equidistant spacing between plants. In 1945, Probst (29) found the Optimum population in 76.2 cm rows between 172,218 and 258,224 plants per hectare. Bassnet, et a1 (2) and Costa, et a1 (11) found that populations within reasonable rates had no significant ef- fect on yield. Costa, et a1 (11) concluded wide variations in intra row spacing have little effect on soybean yields. Still other researchers have found that the effect of popu- lation on yield varied from year to year (13,17,21,22). Doss and Thurlow (13, and Lueschen and Hicks (22) found higher populations had a significant effect on yields in only one year Of a three-year study. Populations higher than the Optimum rate result in de- layed maturity (29), increased plant height (13,17,45) and increased lodging (2,17,23,29,45). Cooper (10) reported that lodging can decrease yields as much as 23 percent. I Soybean plants can compensate for a wide range Of plant densities by varying the number of branches per plant and the number of pods per plant (22). High levels of popula- tion increase the dry matter production on a per plant basis (45), and the LAI Of individual plants (17,45). But, high levels Of population cause a decrease in the number of the following components Of yield: branches per plant 10 (2,11,21,45), pods per plant (2,17,22), seeds per pod (21), and seeds per plant (2,21,22). It appears as populations are increased, the harvest index decreases. The effect Of population on seed yields due to irriga- tion has also been studied. Doss and Thurlow (13) found no irrigation x population interaction. Bassnet, et al (2) found no significant yield increase due to population in their irrigation study. Timons, et a1 (43) found no signif- icant different in evapotranspiration rates with 226,378, 452,732, and 905,464 plants per hectare. Soybean Varieties There is great diversity among soybean varieties in .maturity, height, lodging, and yield potential. Some of these factors are reflected in the differences Of yield com- ponents among varieties (21,22,26). It is not surprising that varieties respond differently to changes in the macro and micro environments. Water and water availability constitute a part of the macro environment. Several researchers have found that yield response tO different irrigation regimes varies among varieties (1,13,23,24). Timmons, et al (43) found no differ- ence in evapotranspiration between the varieties Chippewa and Merit under water stressed conditions. Mederski and Jeffers (24) found a difference among varieties within the same maturity group for their yield response between high and low moisture stress conditions. 11 Boyer and Johnson (7) discovered a difference in the mid afternoon leaf water potentials between newer and Older varieties. They suggested that plant breeders, while se- lecting for high yields over the years, had also produced varieties with superior shoot water balances. They found that the root density of wayne, a newer variety in their ex- periment, was greater than Richmond, a Older variety. Boyer and Johnson concluded that the difference in the water sta- tus between varieties was due to increased root densities Of newer varieties. Researchers have also found that varieties have differ- ent responses to spacial arrangements in the field (micro environment). Variety x row space interactions have been reported (21,48) as well as variety x pOpulation interac- tions (22,29,47,48). Beaver and Johnson (3) found determi- nate and indeterminate varieties respond similarly to both row spacing and population. In recent years, the difference among plant types (determinate, semi-determinate, and indeterminate) adapted to the north central United States has received much atten- tion. In 1968, Weber (44) suggested that determinate vari- eties may result in increased yields in the north due to less competition between vegetative growth and seed filling as photosynthetic sinks. Several studies using near isolines, varying only in morphological traits, have been conducted to determine the important traits in soybean plants. Hicks, et a1 (17) found 12 that crosses with tall determinate traits produced the best yields. Hartung, et a1 (16) found that crosses with semi- determinate and short internode traits gave best yields. However, Wilcox (49) found nO yield difference between semi- determinate and indeterminate crosses. But, he found that semi-determinate plants had increased resistance to lodging. Beavers and Johnson (4) studied genotype x environment interactions of pre-existing determinate, semi-determinate, and indeterminate varieties. Their study considered vari- eties in maturity Groups II, III, and IV at several loca- tions in Illinois. They found that the yields Of determi- nate varieties were not as stable as yields Of indeterminate varieties among locations and years. This may be due to the compactness Of the flowering period Of the determinate vari- eties. Ashley (1) in Alabama, found that determinate vari- eties responded to irrigation at flowering. MATERIALS AND METHODS A soybean irrigation study was conducted on the Marantette farm near Mendon, Michigan, during the 1980 and 1981 growing seasons. The soil was an Elston sandy loam (coarse loamy, mixed mesic, Typic Argiudoll). In 1980, the study followed a maize crOp, and no additional fertilizer was needed. In 1981, the study followed a wheat crop, af- ter which 336.3 kg/ha of 7-28-28 fertilizer was applied. In both years, the fields were fall plowed. A pre- plant incorporated herbicide mixture containing 0.841 kg/ha profluralin and 2.94 kg/ha vernolate was applied. The field was disced twice before planting. The planting dates were 24 May 1980 and 22 May 1981. The study was designed as a 2 x 3 x 10 x 2 factorial and arranged in a split-split plot design. The main plot was split into irrigated and nonirrigated halves. Due to irrigation equipment limitations, the succeeding replica— tions were not randomized. Each of the four replications Of the water treatments was further split into three row spacings. In each row spacing, the smaller plots consisted Of ten varieties at two plant populations. The three row spacings used were 25, 51, and 76 cm. In each row spacing, varieties were seeded at either 13 14 recommended populations or 75 percent Of recommended pOpu- lations. The plant populations are shown in Table 1. Table 1. Plant populations at each row width. ROW RECOMMENDED 75% OF RECOMMENDED WIDTH POPULATION POPULATION (CM) (PLANTS/HA) (PLANTS/HA) 76 344,438 258,328 51 385,698 289,328 25 466,426 349,819 All plots were 5.5 meters long. The plots with the 51 and 76 cm. row spacings were four rows wide and the plots with the 25 cm. row spacings were eight rows wide. The ten varieties used are shown in Table 2. Table 2. Varieties used in the soybean irrigation study. VARIETY MATURITY GROUP COMMENT Hodgson 78 I Hardin I SEE 150P I Narrow leaf Nebsoy Early II Wells II Early II Corsoy 79 II Harcor II SRF 200 II Narrow leaf Beeson 80 Late II Gnome Late II Determinate 15 These varieties reflect a range in both maturities and plant types. Beeson 80 was not included in the study in 1981 due to errors made when the seed was prepared for planting. Tensiometers and Irrigation Soil tensiometers were placed at 30 cm depths in the varieties Harcor and Hardin and at 46 cm depths in Nebsoy and Gnome at the recommended population levels at each row spacing for the four varieties. In 1980, tensiometers were placed in reps I and II of the irrigated half and rep II of the nonirrigated half on 17 July. In 1981, tensiometers were placed in reps I and IV in both the irrigated and non- irrigated halves on 17 July. Readings were made twice a week both years and daily from 31 July through 18 August in 1980 and 21 July through 26 August in 1981. Irrigations were scheduled when the average soil mois- ture tension exceeded -50 centibars, which corresponded to a 40 percent depletion Of the available soil moisture as calculated from a soil moisture characteristic curve. The irrigation water was pumped from the St. Joseph River and applied with a traveler irrigation system. The traveler system used applies 2270 liters per minute. The amount of irrigation water was determined by the speed of the winch pulling the traveler. In 1980, the only irrigation required was 3.8 cm of water, applied on 26 July. In 1981, three applications were made on the following dates at the 16 following rates: 11 July, 3.8 cm; 24 July, 2.4 cm; and 15 August, 5.1 cm. A fourth irrigation was scheduled on 24 August, but tensiometer readings indicate it was not applied. Plant Samples In order to determine how the production practices af- fected plant morphology, plant samples were taken and ana- lyzed. One third meter of row was harvested at stage R5.0 in 1980 from one rep of each row spacing, population rate, and water treatment for the varieties Hardin, Harcor, Nebsoy, and Gnome. The leaf area was measured for each sample on a Li-cor LI-3000 area meter and the leaf area index was cal- culated. Another one third meter Of row was harvested from one rep Of each of the 120 possible treatment combinations at stage R8.0. The number of nodes, branches, and pods on a per plant basis was determined, as was the average number Of seeds per pod. Light Penetration In 1980, at growth stage R6.5, light penetration through the canopy was calculated for one rep in both water treatments for the following varieties: Harcor, Hardin, Nebsoy and Gnome. A meter long Li-cor light sensor attach- ed to a Li-cor LI-185 photometer was used for the measure- ments. The sensor was placed perpendicular to the rows at 17 the base Of the canopy. The irradiance below the canOpy was divided by the irradiance above the canopy and multi- plied by 100 to arrive at the percent light penetration. Two readings were made per plot in one rep of both the ir- rigated and nonirrigated treatments. Leaf Water Potential Random measurements of the leaf water potential were made in 1981 using a portable pressure bomb (PMS Instrument Company, Corvallis, Oregon) within one hour of solar noon. A nitrogen gas source was used. The uppermost fully expand- ed leaf Of the plant to be sampled was excised and secured in the pressure chamber. Gas pressure was increased at a rate Of one bar per minute and readings were taken when gas bubbles were seen coming from the xylem vessels. Each read- ing took six to twelve minutes. Harvest At the end Of each season, the maturity date, lodging score, and plant height for each plot was recorded. Plants were considered mature when 95 percent of the pods had turn- ed tO the mature color and the pods would crack under finger pressure. Lodging scores were recorded on a scale of one to five. A lodging score of one indicates all plants are erect and five indicates all plants are prostrate. Plant heights (cm) were measured from the ground surface to the uppermost node of a representative plant in each plot. The 18~ plots were trimmed to a uniform length. The plots were har- vested with a Hege B125 self-propelled combine. With the exception Of Gnome, all treatments were harvested on 30 September, in 1980. Due to excessive seed moisture, the Gnome plots were hand harvested on 10 October 1980, dried, and threshed with the same Hege combine. In 1981, all non- irrigated plots Of the study were harvested on 3 October and the irrigated plots were harvested on 7 October. The seeds were dried and cleaned. Seed size (g/100 seeds) was measur- ed and yields were reported in kg/ha at 13 percent moisture. RESULTS AND DISCUSSION Climate The growing season precipitation was above normal both years of the study. During 1980 and 1981, total precipita— tion from June through August was 63 percent and 43 percent above the 30 year mean at Three Rivers, Michigan, the closest reporting weather station, some 16 km southeast Of the study location. Between the two years of the study, 1981 was drier. Table 3. Monthly precipitation and irrigation means (mm) 1980 1981 30 YEAR PPT. MEAN MONTH PPT IRR PPT IRR THREE RIVERS, MI JUNE 147 --- 129 --- 107 JULY 107 38 147 64 93 AUGUST 194 --- 113 51 74 TOTAL 448 38 389 115 274 As Table 3 shows, precipitation during June and August, 1980 was higher than in 1981. More important than the total amount Of precipitation is the timing and the rate Of rainfall on a given day. 19 20 Figures 1 and 2 Show the daily rainfall totals as well as the cumulative crop evapotranspiration, cumulative precipi- tation, cumulative precipitation + irrigation, and tensio- meter readings for both the irrigated and nonirrigated treatments. The cumulative precipitation line does not take into consideration the amount Of water lost due to gravitational flow through the sandy loam soil. If this loss were considered, the cumulative precipitation line and the cumulative precipitation + irrigation lines would be lower. A The tensiometer readings are Of particular interest. They chart the difference in soil moisture tension between the irrigated and nonirrigated treatments. In Figure 1, the difference in the tensiometer read- ings between the water treatments following the 26 July irrigation is very obvious. This three week difference in soil moisture status occurred during the pod filling period. The resulting yield increase, probably due to irrigation, is discussed in later sections. ‘ In 1981, three irrigations were applied (Figure 2). Tensiometers were not in place when the first irrigation was made on 11 July. However, differences between the-water treatments following irrigations on 24 July and 15 August are marked. Heavy rains (89mm) on 28 July saturated the soil and equalized the soil moisture between the treatments. The three irrigations correspond to the early pod initia- tion period (11 July) and the pod filling period (24 July (szeqrauao) sSqueax Ianamotsual on 83 1 2 n. e a O m... (”A on B 3 m. oea u .omma no. name usuuefiao A cheese ueowo< haan 0:36 a ”M A e i 3 a a I . tiuuuuu 3H4?! .61....a...... 3...... av... ............ O. O. C I .. .... . m 00 as... o 3......- ooooo00000535000030.0030... luau-.33....33.3.3333...- m u an ..................... I ....m.... . ... .3... - .‘CCCC-I § $3.33... \ .. - . o pouemwuuw : umcaceom Heuoeowmcoa we. . mw coumwuuum coz u awcaooom nouosofimcoh .o. O a. cofiuwwwuuw+ coneuagooum z coauuuaedooum 9,332.50 :3. cowumuoao>m mono III coauowauuw m :Owumuumaooum — OOH cow con coo fifii)uotaafiiiJI + uoxaanrdraaza put on co.— ‘uotasatdtoazd “notaszrdsusznodsag dOJQ aatastnmna 22 (Isqrnuao) Sflurpeau (mm) uorneadtoazd 41180 zaaamflsual am God cm cad a t C. O. C a o c 0.0 ... .. 000000 .0. o 3.3. 33333333.. 3 0"... .Hmad so. some OAOJEAAO m Juneau Ocoh couowwuuu ceaumwwuun m will-limp a pennies: n «9592:. sauce—36:8. c... :02 u emcwoeom scuoEO«acoa.... scuuouuuuH + cowuognwooum 90 .coEauueaooum 2,332.50 2.... coauOHHAmGouuanfi no.5 II :Ouumuwafiooum.- cad 3 0013 aaraetnmna ocm + ‘uornsntdtoaid ‘uoraszrdsuszaoden .om cog on oca 23 and 15 August). The effects on yield and other factors are discussed in later sections. The climatic information presented in Figures 1 and 2 does not depict all the meteorological events. On 12 July 1980 a hail storm caused some damage tO the plants. An in- surance investigator estimated a 15 percent yield reduction in the cooperator's adjacent field. The main stems Of some plants were broken, but the plants responded by increasing the number of branches. Herbicide Damage Herbicide damage was encountered both years Of the study due to different causes each year. In 1980, areas Of the study were damaged by atrazine carry-over. Prior to planting the 1979 corn crop, atrazine applications were ac- cidentally overlapped in the headlands. Although the coop- erator noticed no damage in the corn crop, enough atrazine remained to damage soybeans in 1980. Two Of the four reps of the nonirrigated treatments were severely damaged. The data from these reps were not used in the final analysis. Fortunately, the irrigated treatments were undamaged. The harvest data from 1980 were analyzed in the following three groupings: the remaining two reps of the nonirrigated treatment, all four reps of the irrigated treatment, and a combined analysis containing the two reps Of the nonirri- gated treatment and two adjacent reps Of the irrigated treatment. 24 In 1981, herbicide damage resulted from excessive rates of preplant incorporated herbicides coUpled with cool wet weather and soil crusting. The recommended rates Of vernolate and profluralin for sandy loam soils are 2.26 and 0.56 kg/ha Of active ingredient, respectively (8). They were applied by the cooperator at'rates Of 2.94 and 0.84 kg/ha. Research at Iowa State University has shown that ex- cessive application rates Of profluralin to soybean plants result in thickened hypocotyls and inhibition Of lateral roots. Furthermore, high rates of vernolate result in slowed emergence and leaf malformations (18). Soybean plants in our study exhibited delayed emer- igence, thickened, brittle hypocotyls, and malformed leaves. Soil crusting compounded the situation. The reduced vigor of the seedlings coupled with soil crusting resulted in missing plants within rows. This was especially evident in 25 cm rows. In 51 and 76 cm rows a fissure in the soil crust formed between emerging seedlings which aided their survival. In 25 cm rows, the Space between seedlings was greater and no cracks in the crust developed between seed- lings. Consequently, the seedlings were slower tO emerge and many were killed, resulting in skips in the rows. A subjective rating Of the herbicide-induced stunting and resulting Skips within rows for each plot was made on 20 July 1981. Stunting increased in lower plant popula- tions, narrower row spacings, and varied among soybean 25 varieties. The Appendix contains the rating system used and the resulting analyses Of variance. Because Of the herbicide damage, the skip and stunt scores were used separately and in conjunction as potential covariates. It was found that the stunting score, when used as an adjustment to remove the effects Of the herbi- cide damage, resulted in a Significant and substantial re- duction of the error mean square term in the analysis of variance. While the analysis of covariance reduced the er- ror term, the herbicide damage partially masked the effects Of plant populations and varieties. Overall yields did not seem greatly reduced as soybeans are versatile in responding to catastrophe by altering their morphology. Irrigation Irrigation increased yields in both 1980 and 1981. However, since the precipitation patterns and amounts varied each year, the response to irrigation also varied between years. Consequently, the results from each year will be discussed separately. $280. The irrigated treatments yielded 7 percent more than the nonirrigated treatments. 'However, this difference was not significant at the 5 percent level. Perhaps this was due to the relatively short period Of water stress, or to the experimental design. The use Of a split split plot design results in a lack Of precision between the main plots. 26 Table 4. Effects Of irrigation on the 1980 harvest data (averaged over row spacing, population, and variety). WATER YIELD MATURITY HEIGHT LODGING TREATMENT (KG/HA) (MONTH-DAY) (CM) SCORE Irrigated 3191 9-20 78.7 1.90 Non-irrigated 2970 9-21 70.7 1.57 LSD (.05) N.S. N.S. N.S. 0.11 It appears that irrigation had little effect on matur- ity in 1980. Lodging is the only factor which proved to be significantly increased by irrigation. Table 4 shows that irrigated plots tended to be taller. This increased height coupled with the increased yields is perhaps the cause Of the greater lodging seen in irrigated plots. Table 5. Effects Of irrigation on the components Of yield, 1980 (averaged over row spacing, population, and varieties). SEEDS PODS SEEDS SEED NODES BRANCHES PER PER PER WT. PER PER POD PLANT PLANT (G/lOO) PLANT PLANT Irrigated 2.20 37.5 82.4 16.1 15.6 1.63 Non-irr. 2.27 33.7 76.5 16.9 15.1 1.34 Table 5 gives the yield components for the irrigated and nonirrigated treatments. Since the yield components 27 were measured on only one rep of each water treatment, a statistical analysis was impossible. Several interesting trends which help explain the con- tribution Of irrigation to yield can be seen in Table 5. Irrigated plants tended to have more pods per plant and smaller seed while nonirrigated plants had fewer pods but larger seed. Soybeans are known to respond to water stress by aborting some pods and devoting their diminished photo- synthetic supply to the development Of the remaining seed. Once the period Of water stress passes, photosynthesis re- sumes to normal rates. Each seed of the previously stressed plant gets a proportionately larger amount of photosynthate compared to nonstressed plants which have more seed. 1981. Irrigation significantly increased yields by 11 percent in 1981. Irrigation also caused delayed maturity, increased plant height, and increased lodging. Table 6. Effects of irrigation on the 1981 harvest data (averaged over row spacing, population, and variety). WATER YIELD MATURITY HEIGHT LODGING SEED TREATMENT (KG/HA) (MONTH-DAY) (CM) SCORE WT Irrigated 3569 9-22 92.0 2.96 16.1 Nonirrigated 3207 9-17 77.6 2.20 16.2 LSD (.05) 359 O- 1 4.0 . 0.25 N.S. 28 Table 6 shows that averages for yield, maturity, height, and lodging were all significantly changed by irrigation, but that seed size based on weight was not. One of the most striking differences illustrated by the data was the delay in maturity induced by irrigation. Ma- turity was five days later in irrigated treatments than in nonirrigated treatments. This was probably due to the in- creased plant height and perhaps due to increased leaf area index and branching. Similar results were not Obtained in 1980 due to the shorter water stress period and the more abundant rainfall in August. Components of yield were not measured in 1981. As mentioned earlier, one of the effects Of the herbicide dam- age in 1981 was skips within rows. Soybeans can readily compensate for population densities by altering their morph- ological development. This made selecting representative plant samples all but impossible. Due to the herbicide dam- age, a representative sample of a plot would not have neces- sarily represented the cultural factors for which the plot was designed. With skips within plots and a general stunt- ing of plants, the results would probably have Shown both the effects of the cultural practices and the herbicide dam- age, with no indication of which effect was being measured. The data in Table 6 Show an increase in both yield and plant height. Perhaps the herbicide damage is also reflect- ed in these results in that irrigation helped plants over- come the deleterious effects Of herbicide injury. However, 29 these results are consistent with the results Of previous soybean irrigation experiments (2,12,13,26). Of considerable importance is the fact that lodging scores increased in response to irrigation in both years Of the study. Lodging continues to be a serious problem with irrigated soybeans. Row Spacing Yields increased as the space between rows decreased in both years of the study. Yields increased 18.1 and 18.3 percent in 1980 and 1981 respectively, when row spacing was decreased from 76 to 25 cm. As shown in Figure 3, most Of the yield increase occurred between 76 and 51 cm rows. Yields were not significantly increased by narrowing the rows from 51 to 25 cm in either year although there was an increase. The increases in yield are graphically presented in Figure 4. In 1981, results of an orthgonal polynomial ana- lysis indicated that the yield response due to narrowing row widths was both linear and quadratic. Yields seemed to increase linearly between 76 and 51 cm, but the response was quadratic between 51 and 25 cm. In 1980, the trend was strictly linear. The results from both years suggest that optimum soybean yields are obtained in southwestern Michigan when soybeans are planted in rows narrower than 51 cm. While both irrigation and narrow rows increased yields separately, there was no interaction between the factors Yield (kg/ha) 30 4000— Z . 3500 - 15.5% ' 18.3% 3000- 18.176 .9.'.....VOI 2500 2000- Row Spacing 25 cm 1500' llllllllllll 51 .. 76 cm 1000— 500- 1980 1981 Figure 3 Yield Increase (2) Due to Decreasing Row Width ' (Averaged over water, population, and varities) Yield (kg/ha) Yield (kg/ha) 31 3800“ 3600- 3400— 3200 '- nul 1980 o. — 1981 3000-- dye \““ O _ “““‘ I O 2800— 72’ I l 7% 51 25 ---------Row Width (cm)--4---- Figure 4 Yield Response Curves for Row Spacing in 1980 and 1981 (Averaged over water, population, and varieties) 3 6 O o ........... $§3§fi9 %&%&% oog”’ 0.900 3200'-' ”335% “5“, 00000 O .......... .......... 9. o. 2 0 00;“. ..... ..... 9.... 99... .00 o .0009 — .‘OO...... 2400 . .o... o 909.. .. 00.9. o 0.9.0 o .0900 .... .O... .o co... ..... .0909 09000 . 0.090 00000 . 000.. 9.900 .099. be... 0.0.. .09.. 09.00 09009 ' ... %&%%% P‘S ’ 09000 . " 90000 . .I’ 000.. ’° .. a333¢ 1200— 1.0000. 800— ...... '.......... l‘-‘ N O O O O O O fihfifi$ .LLDOO 76 51 25 ------—-----Row Width (cm)-—----—----- Figure 5 The effect of Row Spacing 0n Yields in 1980 and 1981 (Averaged over water, population and varieties) 32 either year. We cannot predict whether this independence would hold with greater drought stress. Table 7 presents data from leaf area and light pene- tration measurements made in 1980. Table 7. The effect Of row spacing on leaf area index (at stage R5.0) and light penetration (at stage R6.5), averaged over measurements made on Hardin, Harcor, Nebsoy, and Gnome, 1980 ROW WIDTH LIGHT PENETRATION LEAF AREA INDEX (CM) (%) (LEAF AREA/SOIL AREA) 25 5.47 4.9 51 6.34 3.3 76 10.38 2.9 As Table 7 shows, light penetration was reduced as row width decreased and leaf area increased. Hence, light intercep- tion by plants increased as row widths were narrowed. The 1980 components of yield are presented in Table 8. Table 8. The effects of row spacing on the components Of yield, 1980 ROW SEEDS PODS SEEDS SEED NODES BRANCHES WIDTH PER PER PER WT. PER PER (CM) POD PLANT PLANT (G/100) PLANT PLANT 25 2.24 32.8 74.4 16.7 14.7 1.46 51 2.23 36.0 80.2 16.6 15.8 1.57 76 2.25 38.1 85.7 16.3 15.4 1.43 33 As Table 8 shows, the number of pods per plant and seeds per plant tended to increase as the Space between rows increased. This reflects the decrease in plant popu- lation associated with wider rows. The average of normal and 75 percent of normal plant populations for each row width was as follows: 408,123 plts/ha in 25 cm rows, 337,513 plts/ha in 51 cm rows, and 301,384 plts/ha in 76 cm rows. Since plant populations were lowest in 76 cm rows, plants responded by increasing the number of seeds and pods per plant. Consequently, the changes in seeds per plant reflect the difference in plant population, not row spacing. The harvest data changes due to row Spacing are shown in Tables 9 and 10. Table 9. Effects Of row spacing on the 1980 harvest data (averaged over water, pOpulation, and variety). ROW WIDTH YIELD MATURITY HEIGHT LODGING (CM) (KG/HA) (MONTH-DAY) (CM) SCORE 25 3294 9-22 75.1 1.67 51 3157 9-21 74.8 1.67 76 2790 9-20 74.2 1.88 LSD (.05) 327 0- 1 N.S. 0.11 Similar increases in yields due to row spacing were found both years. 34 Table 10. Effects Of row spacing on the 1981 harvest data (averaged over water, population, and variety). ROW WIDTH YIELD MATURITY HEIGHT LODGING SEED (CM) (KG/HA) (MONTH-DAY) (CM) SCORE WT. 25 3602 9-20 83.2 2.51 16.3 51 3518 9-20 85.4 2.51 16.1 76 3046 9-20 85.4 2.71 15.9 LSD (.05) 142 N.S. 1.4 0.15 N.S. Differences existed in other factors, however. In 1980, a slight delay in maturity in narrower row spacings was recorded. This is believed to be due to the restriction of air movement within the plant canopy in narrower rows. A similar, but not significant trend was noticed in 1981. In 1980, plant heights increased as row widths decreas- ed. This can be explained by the basic physiological prin- ciple that as light becomes limiting within a dense plant canopy, the auxin produced in the stems is not degraded. The result is taller plants. In 1981, plant height decreased as row width decreased. This is attributed to two factors. The herbicide damage produced skips in the rows, resulting in Open areas in the plant canopy and allowing more light penetration and more auxin degradation. This resulted in shorter plants. One of the herbicides, vernolate, inhibits cell division and elonga- tion and may alter gibberellic acid distribution in plants 35 (18). Soybeans in 25 cm rows were slower to emerge and may have absorbed more chemical. As a result, long term stunt- ing would occur. However, the decrease in height between row spacings was slight. In both years lodging increased slightly as row width increased. This may have been caused by the weight Of the extra seeds per plant. Plant Population The effects of plant population on yield are unclear. In 1980, low plant populations resulted in a small but not significant yield increase. In 1981 however, it was the normal pOpulations which produced a significant increase in yield. This may have been due to the different precipita- tion patterns for the two years. Table 11. Effects of population on the 1980 components Of yield (averaged over water, row spacing, and variety). POPULATION SEEDS PODS SEED GRAMS NODES BRANCHES PER PER WT. PER PER PER LEVEL poo PLANT (0/100) PLANT* PLANT PLANT Normal 2.24 32.9 16.6 12.2 14.7 1.09 Low 2.25 37.8 16.4 13.9 16.2 1.80 * As a function seeds/pod x pods/plant x (seed wt./100). Table 11 shows that at low populations, each plant tended to have more pods per plant, nodes per plant, and 36 branches per plant, while seed weight was slightly lower. Since the low population was 75 percent Of the rate of nor- mal population, the product of the yield components (grams per plant) would have to increase 133 percent before the yields Of the two populations would be equal. While plant population by itself did not have a signif- icant effect on yield in 1980, the population x row spacing interaction proved significant (Figure 6). Plants in nor- mal populations produced higher yields in 51 and 76 cm rows in 1980. However, plants in low populations produced higher yields in 25 cm rows in 1980 than did normal populations. In 1981, yields were higher for normal populations in all row spacings. Table 12. Effects of the population x row spacing interac- tion on the 1980 components Of yield (averaged over water and variety). ROW SEEDS PODS NODES BRANCHES SPACING POPULATION PER PER. PER . PER (CM) LEVEL POD PLANT PLANT PLANT 25 Normal 2.23 29.8 14.1 1.09 Low 2.16 35.3 15.9 1.83 51 Normal 2.19 30.8 15.4 1.28 Low 2.22 41.2 16.3 1.86 76 Normal 2.22 33.3 14.6 1.15 Low 2.27 43.0 16.3 1.71 Data from Table 12 Show that the yield components for plants in low populations were higher than those for plants Yield (ks/ha) 3700 3600 3500 3400 3300 3200 3100 3000 2900 2800 37 3700 3600 3500 3400 3300 3200 """"IIIIIIIIOIII||. 3100 3000 2900 0000000000..... 2300 ‘1I:RII4IIIIIIIIIIIIIII+IIIIII NOrmal Low Normal Low POPULATION 1980 1981 YEAR Figure 6 Population x Row Width Interaction Effects on Yield (Averaged over water and varieties) Row Width III-IIZS cm Illllllll 51 cm .O....76 cm ‘ earl! a... hflnJi II...‘ 1!. ll if? .a Hall Inilibilufi 38 in normal populations in all row spacings. The difference in yield components between population levels is no greater in 25 cm rows than in other row spacings. Hence, there is no evidence to explain the yield increase in 25 cm rovs caused by low populations shown in Figure 6. Table 13. Effect of population on the 1980 harvest data (averaged over water, row spacing, and variety). POPULATION YIELD MATURITY HEIGHT LODGING LEVEL (KG/HA) (MONTH-DAY) (CM) SCORE Normal 3048 9-20.4 74.5 1.77 Low 3112 9-21.4 74.9 1.70 LSD (.05) N.S. 0.2 N.S. N.S. Table 14. Effects of population on the 1981 harvest data (averaged over water, row spacing, and variety). POPULATION YIELD MATURITY HEIGHT LODGING SEED LEVEL (KG/HA) (MONTH-DAY) (CM) SCORE WT. Normal 3442 9-20 85.0 2.65 16.2 Low 3335 9-20 84.6 2.50 16.1 LSD (.05) 63 N.S. N.S. 0.07 0.1 The effects of population were not limited to yield (Tables 13 and 14). In 1980, low populations resulted in a slight but significant delay in maturity. No maturity delay 39 was noticed in 1981. Population had no significant effect on plant height in either year. However, there was a population x water interaction on plant height in both years. In 1980, irrigated plants were taller in low populations compared to normal populations. Nonirrigated plants were taller at normal populations. In 1981, the trends were reversed. Seed weights were higher at normal populations in 1981 (Table 14). Lodging decreased at low populations both years. In 1981, lodging was significantly less at low populations. Variety Yields were affected more by varieties than by any other factor, including water and row spacing. Varieties responded differently in each year of the study. As mentioned in the climate section, 1981 was a rel- atively dry year. However, the results of the 1981 season were obscured by herbicide damage. Hopefully, most of the error and the effects of the herbicide damage were removed with the analysis of covariance. The two herbicides used in 1981 were vernolate and profluralin. While both were used at excessive rates for the particular soil texture, it is interesting to note their longevity and possible differential effects on varieties. Vernolate has a half life of 1.5 weeks in a loam soil at 21-27 c. While it is absorbed by, and translocated through soybean plants, it is quickly metabolized (46). Profluralin, 40 on the other hand, is longer lived with a half life between 80-128 days (46). According to the herbicide handbook of the Weed Science Society of America, there is evidence that profluralin may interfere with both photosynthesis and respiration in plants and that the degree of selectivity may be at least partly related to the lipid content of the plant or its seeds (46). The subjective rating of the herbicide damage, con- ducted on 15 July 1981, was itself statistically analyzed. It showed a differential varietal damage (skips and stunts) due to herbicide injury. The degree of skips, due to seed- ling mortality, may have reflected seed quality. However, the stunting was undoubtedly a result of altered plant physiology from herbicide injury. The harvest data were analyzed with the stunt and skip scores as possible covari- ates. Using the stunt score as a covariate, the mean square error of the resulting analysis was found to be significant— ly and substantually reduced. However, some inconsistencies were seen, especially regarding the row spacing x variety response. In light of this, the major emphasis of the variety section will be on the 1980 results. Varieties were subplots of both water and row spacing treatments and were coupled with populations. Therefore, many interactions occurred. The variety section is divided into two parts. The main effects subsection discusses the yield, maturity, height, lodging and seed weight response of varieties when averaged over the other factors (water, 41 row spacing and population). The interaction subsection discusses varietal response as affected by the other fac- tors. Each of the subsections is further subdivided into years. Main Effects 1980. When results were averaged over water, row spacing, and population treatments, the yields of individ- ual varieties in 1980 varied by as much as 1026 kg/ha (Table 15). As expected, linear regression analysis indicated a strong relationship between yield and the maturity date of the varieties (Figure 7). Later maturing varieties yielded more than earlier maturing varieties. Since all varieties were planted on the same date, the later maturing varieties had more time for photosynthetic accumulation and perhaps longer reproductive periods as well. Table 15. Effects of varieties on the 1980 harvest data (averaged over water, row spacing and population). YIELD MATURITY HEIGHT LODGING VARIETY (KG/HA) (MONTH-DAY) (CM) SCORE Gnome 3498 9-28 52.4 1.00 Harcor 3404 9-23 81.2 2.53 Corsoy 79 3262 9-22 80.5 2.18 Beeson 80 3253 9-23 79.1 1.86 SRF 200 3142 9-23 85.8 2.24 SRF 150P 3024 9-19 74.6 1.51 Hardin 3011 9-19 75.8 1.99 Wells II 2936 9-19 74.8 1.21 Nebsoy 2802 9-19 72.2 1.43 Hodgson 78 2472 9-12 70.6 1.40 x 3080 9-21 74.7 1.74 LSD (.05) 132 0- 1 2.3 0.14 Yield (kg/ha) 3600 3400 3200 3000 2800 2600 12 14 Figure 7 42 Harcor o Corsoy 79 . Beeson 80 o SRF 200 odgson 78 SRF 150P Hardin ’ Wells II r2 - .888 o Nebsoy l6 18 20 22 24 26 28 Maturity (date in September) Linear Regression Analysis Comparing Var- iety Yields to Maturities in 1980 (Aver- aged over water, row spacings and populations) * 43 The regression line in Figure 7 shows two main clusters of varieties and two outlying varieties. Gnome, a late maturity Group II variety, was the latest maturing variety and had the highest yield. Farther down the regression line is a cluster of varieties containing Harcor, Corsoy 79, Beeson 80, and SRF 200, all Group II varieties. The second cluster contains SRF 150P and Hardin, both Group I varieties, and Wells II and Nebsoy, both early Group II varieties. At the bottom of the line is Hodgson 78, the earliest maturing and lowest yielding variety. Table 16. Effects of variety on the 1980 components of yield (averaged over water, row spacing, and population). SEEDS PODS SEED GRAMS NODES BRANCHES PER PER WT. PER PER PER VARIETY POD PLANT (G/100) PLANT* PLANT PLANT** Gnome 2.26 41.7 15.8 14.9 14.2 3.10 Harcor 2.05 41.8 16.2 13.9 15.4 2.10 Corsoy 79 1.98 43.3 16.3 14.0 15.5 1.55 Beeson 80 2.09 33.8 19.3 13.6 16.0 1.49 SRF 200 2.80 29.2 16.3 13.3 16.7 1.54 SRF 150P 2.48 32.4 15.6 12.6 16.6 1.29 Hardin 2.02 39.4 14.9 11.9 15.6 1.16 Wells II 2.27 32.1 17.2 12.5 15.1 0.98 Nebsoy 2.26 32.8 17.4 12.9 15.4 0.53 Hodgson 78 2.08 32.4 16.6 11.2 15.2 1.18 * Calculated weight of seeds per plant. ** Average from plants without hail damage. The components of yield for each variety (Table 16) help explain some of the differences within and among the clusters. Orthogonal comparisons showed that varieties with 44 above mean numbers of pods per plant and branches per plant had higher yields. However, these factors seemed to be linked in the varieties tested. Linear regression analysis showed seed weight and the number of seeds per pod were not correlated with yield in the varieties tested. Plant height varied among varieties. With the excep- tion of Gnome, plant height increased with increasing matur- ity. A linear regression analysis comparing height and maturity resulted in a correlation of 0.839 between the fac- tors. The yield components showed a somewhat constant num- ber of nodes per plant among varieties, so the difference in height must be due to differences in internode length. Lodging also differed among varieties. Interestingly, linear regression analyses excluding Gnome, indicated that increased lodging is associated with increased yield (corre- lation of 0.773) and increased plant height (correlation of 0.447). A multiple linear regression analysis with yield and height as independent variables and lodging as the de- pendent variable, resulted in a correlation of 0.888. Ob- viously, tall indeterminate varieties laden with seed are prone to lodging. Gnome, the only determinate variety in the study, was the exception to generalizations on both height and lodging. While Gnome was the latest maturing and highest yielding variety, it was also the shortest and lodged the least. Gnome's short stature is a unique part of its genetic make- up and contributes to its resistance to lodging. 45 1281. In 1981, yields, heights, and lodging scores were generally greater than in 1980. These differences may have been due to more clear days in 1981 than in 1980. The maturity was, on the average, one day earlier, but the field was planted two days earlier. When results were averaged over water, row spacing, and population treatments, the yields in 1981 varied by 499 kg/ha among varieties (Table 17). As in 1980, much of the difference was related to differences in maturity among var- ieties. Linear regression analysis of mean variety yields and maturities resulted in a correlation of only 0.729. However, when Gnome was deleted from the analysis, the cor- relation rose to 0.885 (Figure 8). Gnome matured later than any other variety, but did not yield proportionately higher as it did in 1980. Perhaps this is due to a genotype x environment interaction. On the other hand, the yields of Gnome may have been artificially inflated in 1980, since, unlike the other varieties, it was hand harvested. In 1981, varieties were not clustered on the regression line (Figure 8) as they were in 1980. Maturities were more spread out in 1981 due to either the difference in the grow- ing season environment or to the herbicide damage. This would help explain the uncharacteristically low yield of SRF 150P. As in 1980, the 1981 plant heights differed among var- ieties and were strongly correlated (.735) with maturity. Likewise, lodging reacted as it did the previous year, being Yield (kg/ha) 3600 3400 3200 3000 2800 2600 46 I Harcor O . SRF 208‘““.“““‘ Corsoy 79 'fl.‘..» 7 G Hardin . ...“fl|‘ none 0 «nfl“flfl «Nebsoy We 1 l S I IN“.‘.‘ D , fl“ fl“”“”“’ 0 SRF 150P I Hodgson 78 I nu Analysis with Gnome r2 - .531 - Analysis without Gnone r2 - .784 12 14 16 18 20 22 24 26 28 Haturity (date in September) Figure 8 Linear Regression Analysis Comparing Variety Yields to Maturity Dates in 1981 (Averaged over water, row spacing, and population) 47 correlated to plant height (.814 and yield (.735). Again, Gnome did not fit the generalizations. It was the latest maturing variety, but was the shortest as well. Table 17. Effects of variety on the 1981 harvest data (averaged over water, row spacing, and population) YIELD MATURITY HEIGHT LODGING SEED VARIETY (KG/HA) (MONTH-DAY) (CM) SCORE WT. Harcor 3615 9-22 97.6 3.34 16.0 SRF 200 3604 9-23 94.9 3.15 15.8 Corsoy 79 3528 9-21 96.4 3.11 16.2 Gnome 3492 9-29 56.1 1.58 17.2 Hardin 3451 9-18 88.6 2.85 15.6 Nebsoy 3360 9-19 81.1 1.90 17.2 Wells II 3207 9-15 85.8 2.11 15.2 Hodgson 78 3122 9-14 82.7 2.43 17.4 SRF 150P 3116 9-18 80.1 2.72 14.2 3; 3288 9-20 84.8 2.58 16.1 LSD (.05) 130 0- 1 1.8 0.15 0.3 Seed weights were measured on all plots in 1981 and were included in the analysis of variance. While seed weights differed by variety, there seemed to be no direct correlation to yield. The differences in seed weight mostly reflected the different genetic makeup of the varieties. Two-year summary on the main effects of varieties. Yields were influenced by maturity in that late maturing varieties out-yielded earlier varieties. Furthermore, ma- turity also influenced plant height of the indeterminate varieties. Plant height and yield both had effects on 48 lodging. In 1981, yield was associated with lodging more than height. The reverse was true in 1980. Seed weights varied among varieties, but were not directly correlated to yield. Interaction Effects--1980 Nineteen eighty was a relatively wet year. Consequent- ly, few interactions involving water occurred. Only one significant interaction occurred in each of the measured ef- fects (yield, maturity, height, and lodging). These inter- actions are covered in the effects sections listed below. Yield. While no interactions involving yield were found in the 1980 combined analysis, a row spacing x variety inter- _ action was found in the analysis of variance conducted on the four reps of the irrigated treatment. Under irrigated con- ditions, the varieties showed different yield responses to changes in row width (Table 18). Hardin and Corsoy 79 both showed a significant yield advantage for both 25 and 51 cm rows over 76 cm rows, but no significant difference between 25 and 51 cm rows. Hodgson 78 showed a significant advantage for 25 cm rows over both 51 and 76 cm rows, but no difference between 51 and 76 cm rows. SRF 150P, SRF 200, Harcor, and Wells II all showed significant yield advantages of 25 cm rows over 76 cm rows, but no differences between either 25 and 51 or 51 and 76 cm rows widths. Beeson 80 produced highest yields in 51 cm rows, which were significantly high- er than yields in 76 cm but not 25 cm rows. Surprisingly, 49 neither Gnome nor Nebsoy showed any significant yield dif- ferences among row spacings. Table 18. Effects of row spacing x variety interaction on 1980 yields (kg/ha). (Averaged over water and population.) ROW WIDTH (CM) VARIETY 25 51 76 Gnome 3885 3750 3371 Harcor 3874 3556 3201 Corsoy 79 3672 3631 3065 Beeson 80 3152 3577 2938 SRF 200 3745 3304 2829 SRF 150P 3472 3212 2860 Hardin 3555 3177 2464 Wells II 3299 3124 2670 Nebsoy 2010 2808 2643 Hodgson 78 3046 2528 2088 LSD (.05) - 475 for comparisons within columns. LSD (.05) a 515 for comparisons within rows. The differential varietal yield response can be seen in Figure 9, where the varieties are ranked by maturity. The maturity Group I varieties Hodgson 78 and Hardin showed the greatest yield increase of all varieties between 25 and 76 cm rows. With the exception of SRF 200, a trend of in- creasing response to narrow rows exists as the maturity dates decrease. This reflects the plant stature differ- ences between the shorter Group I and taller Group II vari- eties. As pointed out in the literature review, optimum yield of a particular variety is obtained when it is planted in a row width which allows it to attain full canopy before Yield Increase (Z) 50 40 30 20 10 Figure 9 50 c: 0,. cu U. m: 1n SRF15OP Cotsoy 79 Yield Increase (%) Due to Decreasing Row Spacing from 76 cm to 25 cm in Irrigated Treatments in 1980 (Averaged over popula- tion) 51 the full flowering period (37). In the northern United States, earlier maturing varieties respond more to narrower rows than later varieties. Maturity. Maturity was affected by a row spacing x variety interaction in the combined analysis in 1980. The maturities of Hardin and Hodgson 78 were significantly de- layed in 25 cm rows as compared to 76 cm rows. None of the other varieties, including SRF 150P, also a Group I variety, showed any significant maturity differences among the three row spacings. Height. A combined interaction effect between water and varieties on height was found to be significant at the 5 percent level in the analysis of variance. However, the in- dividual differences between water treatments within any one variety were not significant (Table 19). Many differences among varieties within the same water treatment were signif- icant. Lodging. A water x variety interaction affected lodg- ing in 1980. The lodging scores of only three varieties, SRF 150P, SRF 200, and Beeson 80, increased significantly due to irrigation. No significant lodging changes occurred in the other varieties. Interaction Effects--l981 The growing season in 1981 was drier than in 1980. Consequently, water x variety interactions occurred in all the harvest data. POpulation x variety interactions were 52 also found in all the harvest data except lodging. However, these interactions may reflect the effect of herbicide dam— age on the lower population, rather than a true population x variety interaction. A row spacing x variety interaction proved to have significant effects on yields, and a water x row spacing x variety interaction was found to affect both yields and seed weights. Table 19. Effects of the water x variety interaction on 1980 plant heights (averaged over row spacing and population). WATER TREATMENT VARIETY IRRIGATED NON- IRRIGATED A X Gnome . 53.8 51.1 2.7 Harcor ‘ 85.3 77.0 8.3 Corsoy 79 87.2 73.9 13.3 Beeson 80 83.1 75.1 8.0 SRF 200 90.9 80.7 10.2 SRF 150P 79.1 70.2 8.9 Hardin 78.1 73.4 4.7 Wells II 81.3 68.3 13.0 Nebsoy 75.8 68.7 7.1 Hodgson 78 72.9 68.3 4.6 LSD (.05) a 6.5 for comparisons within colums LSD (.05) = 16.4 for comparisons within rows Yiglg. Irrigation resulted in significant yield in- creases in all varieties. However, the degree of response differed among varieties, resulting in a water x variety in- teraction (Figure 10). As shown in Table 20, the two narrow-leafed varieties, SRF 150P and SRF 200, showed the greatest yield response to irrigation. However, these two varieties were also shown Yield Increase (%) Yield Increase (Z) 53 Figure 10 Yield Increase (%) of Varieties in Irrigated Treatments Over Non Irri- gated Treatments in 1981 (Averaged over row spacing and population) 30 T Row Width 25- mm" 25 cm '51cm i nun H“ ' « lllHI ‘ . -' . z o 15" . ' . . ,- i . 8 l , l V. 29:11.: LL #3 I?“ .E g, T“ 7 5 10- I, 2 g E J '; . 2;; I .3 3— ' 9, : $3 ,3 :2 I E a; £9 a. W' .0 'U . 5- O . , m. ', . . 8. ' I ‘U . o ;r, Gnome Figure 11 Yield Increase (%) Due to Decreasing Row Widths in 1981 (Averaged over water and population) 54 to have the greatest amount of herbicide damage. So the ir- rigation water may have helped them overcome the root prun- ing effects of profluralin. Harcor, Nebsoy, and Wells II showed a medium to high response to irrigation. Hardin, Corsoy 79, and Hodgson 78 showed a low to medium response. Surprisingly, Gnome, the only determinate variety in the study, showed only a 3.6 percent yield increase between irri- gated and nonirrigated treatments. Rainfall must have been adequate during the flowering period, since water stress during the flowering period will usually reduce the yields of determinate varieties (l3). Table 20. Effects of water x variety interactions on 1981 yields (kg/ha) (averaged over row spacing and population). WATER TREATMENT VARIETY IRRIGATED NON-IRRIGATED A X A % Harcor 3822 3407 414 12.2 SRF 200 3927 3281 646 19.7 Corsoy 79 3662 3394 268 7.9 Gnome 3552 3430 124 3.6 Hardin 3603 3299 304 9.2 Nebsoy 3538 3182 356 11.2 Wells II 3395 3020 375 12.4 Hodgson 78 3248 2996 252 8.4 SRF 150P 3927 3281 646 19.7 LSD (.05) = 121 LSD (.05) 3 for comparing within columns 93 for comparing within rows However, the Gnome did respond differently to water stress than did Nebsoy. Leaf water potentials of both Gnome and Nebsoy were measured in 51 cm row widths at normal 55 populations in both irrigated and nonirrigated treatments on 24 August 1981. Averaging over water treatments, Gnome had a leaf water potential of -4.4 bars compared to -12.3 bars for Nebsoy. It is unlikely that a difference of this magnitude could be explained by different osmotic or pres- sure potentials between the varieties. The higher leaf water potential of Gnome may have been due to root distribu- tion or greater rooting depth as compared to Nebsoy. Un- doubtedly this difference was reflected in the yield dif- ference between the two varieties and the difference in yield response to irrigation. Table 21. Effects of row spacing x variety interactions on the 1981 yields (kg/ha) (averaged over water and population). ROW WIDTH (CM) VARIETY 25 51 76 Harcor 3869 3773 3202 SRF 200 3805 3914 3093 Corsoy 79 3769 3671 3144 Gnome 3812 3426 3237 Hardin 3596 3666 3093 Nebsoy 3612 3411 3057 Wells II 3407 3372 2843 Hodgson 78 3230 3194 2942 SRF 150P 3317 3231 2798 LSD (.05) = 121 for comparing within columns. LSD (.05) = 117 for comparing within rows. A row spacing x variety interaction also had an effect on yields (Table 21). All varieties except Nebsoy and Gnome had significantly greater yields in both 25 and 51 cm rows 56 than in 76 cm rows, but no significant difference occurred between 25 and 51 cm rows. Gnome and Nebsoy had signifi- cant yield differences among all three row spacings. As in 1980, the degree of response to narrow row spac- ing differed among varieties (Figure 11). Unlike the 1980 results, the Group I varieties did not exhibit the greatest responses. Furthermore, both Harcor and SRF 200 had great- est yields in 51 cm rows, a totally unexpected result. Again, these unusual results may be a reflection of the herbicide injury. A population x variety interaction had an effect on yields in 1981. The yields of Hodgson 78, SRF 200, Gnome, Hardin, and Wells II were significantly higher at normal populations. Corsoy 79, Harcor, and Nebsoy showed no sig- nificant yield differences between population levels but, SRF 150P had higher yields at lower populations. A three-way interaction of water x row spacing x var- iety also affected yields. Basically, the response of varieties among row spacings varied due to water treat- ments (Table A3, Appendix). Maturity. Irrigation dramatically delayed the maturity of all varieties. On the average, irrigation delayed matur- ity five days. However, each variety reacted somewhat dif- ferently, resulting in a water x variety interaction on maturity (Table 22). Also, the maturity ranking of varie- ties changed between water treatments. 57 Table 22. Effects of water x variety interaction on matur- ity (date in September) in 1981 (averaged over row spacing and population). WATER TREATMENT VARIETY IRRIGATED NON- IRRI GATED A X Harcor 23.6 19.9 3.7 SRF 200 27.7 19.0 8.7 Corsoy 79 23.4 18.1 5.3 Gnome 31.2* 26.6 4.6 Hardin 20.4 15.3 5.1 Nebsoy 20.8 17.0 3.8 Wells II 16.0 13.2 2.8 Hodgson 78 15.5 12.5 3.0 SRF 150P 21.5 13.7 7.7 LSD (.05) = 0.6 for comparing within columns LSD (.05) = 0.6 for comparing within rows * 10-01.2 A population x variety interaction resulted in slight differences in maturity dates. Population levels had no ef— fect on the maturity of Hodgson 78, Corsoy 79, Harcor, Nebsoy or Gnome. The low level of population caused delayed maturity in SRF 150P, SRF 200, and Wells II. The maturity of Hardin was later in normal compared to low populations. Height. Irrigation significantly increased the height of all varieties, except Gnome, by an average of 20.2 per- cent. Gnome showed no significant change in height between water treatments. The population x variety interaction resulted in dif- ferential heights among varieties between population levels. Normal populations resulted in increased height of SRF 200, Gnome, and Wells II. Low population levels resulted in 58 height decreases in Corsoy 79 and Nebsoy. No significant differences in height were found for Hodgson 78, SRF 150P, Harcor, or Hardin. Lodging. Irrigation significantly increased the lodg- ing of all varieties except Gnome. The water x variety in- teraction was most evident in the degree of lodging response among varieties in different water treatments (Figure 12). Table 23. Effects of the water x variety interaction on lodging score in 1981 (averaged over row spacing and population). WATER TREATMENT VARIETY IRRIGATED NON-IRRIGATED A X A % Harcor 3.62 3.07 .55 17.9 SRF 200 3.72 2.58 1.14 44.2 Corsoy 79 3.45 2.77 .68 24.6 Gnome 1.62 1.54 .08 5.3 Hardin 3.24 2.46 .78 31.7 Nebsoy 2.02 1.77 .25 14.1 Wells II 2.50 1.72 .78 45.4 Hodgson 78 2.91 1.94 .97 50.0 SRF 150P 3.51 1.93 1.58 81.9 LSD (.05) = 0.12 for comparing within columns LSD (.05) = 0.11 for comparing within rows As Table 23 shows, SRF 150? and SRF 200 had the great- est differences in lodging scores between water treatments. Seed Weight. Seed weights changed due to the water x variety interaction. Seed weights of SRF 150P, SRF 200, Harcor, and Wells II increased under irrigation. The seed weight of Gnome decreased due to irrigation. No 59 Acofiuoaom0Q one mcwoomm 3ou uw>o oommuo>¢. Hmma :H ucmEumwuB nodummwuuH one >umflun> an mmuoom mcfiuooq NH ousmwm oo~ cam 85 zomtoo seven: Homeamz cacao mound: zombwz mama mam n :omwooz I _ SJ I I _ I A I oobmmfiu: I peanut: c025? OJ» axons Surfipoq 60 significant difference was seen in the seed weights of the other varieties. A population x variety interaction also affected seed weight in 1981. Seed weights were higher in normal popula- tion levels for Hodgson 78, SRF 200, Corsoy 79, and Hardin. Nebsoy's seed weight was higher at low population levels. No significant differences were found in seed weights of SRF 150P, Harcor, Gnome, or Wells II. Like yield, seed weights were affected by a water x row spacing x variety (Table A5, Appendix). CONCLUS IONS In dry years, irrigation causes increases in soybean yields, heights, and lodging, as well as delaying the maturity. The most important factor in increasing soybean yields is variety selection. Full season maturity Group II varieties had higher yields in southwestern Michigan than earlier maturity Group I varieties. Yields are increased by an average of 14 percent when soybeans are planted in row widths of 51 cm or narrower. Group I varieties, especially, should be planted in narrow IOWS . Lodging continues to be a problem in irrigated soybean production. While decreasing plant population by 25 percent reduces lodging, effects on yield are unclear. More research needs to be conducted to clarify the ef- fects of the population x variety interaction on both yield and lodging. 61 APPENDIX Table A1. Criteria for herbicide injury rating. SKIPS 1” 510 percent skips within plots 2. ‘20 percent skips within plots 3. $30 percent skips within plots 4. 540 percent skips within plots 5. >40 percent skips within plots 'STUNTING 1. No apparent stunting 2. 520 percent moderate or 10 percent severe stunting 3. 540 percent moderate or 20 percent severe stunting 4. £60 percent moderate or 30 percent severe stunting 5. 580 percent moderate or 40 percent severe stunting Table A2. Significance table for herbicide injury ratings. 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EU Hm EU mN odd .2: x x x x x x x x x x x x emumam :OwummfiuuH um~m>mue no numm. » x x x x x x x x x x ...:-nuunnuns-”nav-IWQW-unucannon-QI-I-II-consign-unuunm-v-W-u ”Mum—.....F-nonh-unnn—u-nu owe an. cvw o >..tc cmq a am am. omm om¢ so mN . adv ‘ o- odw How cam xx Sm ..uwsz. .32 2: uuuuuuuuuuuuuuuuuuuuu Incas-OOQOQQILWL-Ion Ihmmwmr-Oul...-I..-I...I.I.-aonion-0....-...-OIOIII-Inl-IIQI Ham ova and SH ... a... 5 c2 . a... so 3 H3 h EU am :10.— :5 mm c~m so an . Ham c- . so m~ Add canal: anuaom caaal. .Iuaca T . a L “10°82 BIBLIOGRAPHY 10. BIBLIOGRAPHY Ashley, D. A. and W. J. Ethridge. 1978. Irrigation effects on vegetative and reproductive development of three soybean cultivars. Agron. J. 70:467-471. Bassnet, B., E. L. Mader, and C. D. Nickell. 1974. In- fluence of between and within row spacing on agro- nomic characteristics of irrigated soybeans. Agron. J. 66:657-659. Beaver, J. S., and R. R. Johnson. 1981a. 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