Soybean Cultivar Responses to Row Spacing and Seeding Rates in Rainfed and Irrigated Environments

Soybean [Glycine max (L.) Merr.] are grown under both irrigated and rainfed conditions in the western Corn Belt. We wanted to determine whether optimum row spacing and seeding rate were similar for different cultivars in different water regimes. A 3‐yr field study at Clay Center, NE, and a 2‐yr study near Oxford, NE, were conducted. We used a factorial combination of three row spacings (10, 20, and 30 in.), four seeding rates (45 000, 140 000, 230 000, and 330 000 seeds/acre) and five cultivars with four replications planted in adjoining irrigated and rainfed fields at each site. Determinate cultivar yield increased 42% (11 bu/acre) as seeding rate increased from 45 000 to 140 000 seeds/acre and then remained constant with increased seeding rate. Seeding rate generally did not affect indeterminate cultivar yield. The 20‐in. row spacing consistently had the greatest yields or yields equal to the high‐yielding row spacing. Yield of indeterminate cultivars in both water regimes and determinate cultivars with irrigation was greater with 20‐in. rows than either 10‐ or 30‐in. rows. Yield of determinate cultivars in rainfed was similar in 20‐ and 30‐in. rows and less in 10‐in. rows. The 20‐in. row spacing also consistently produced the highest yield at all four seeding rates. This report goes beyond previous research findings by concluding that, with moisture stress, determinate cultivars in 20‐ and 30‐in. rows may produce more than in 10‐in. rows. Research Question Soybean are grown in the U.S. Corn Belt over a wide range of conditions. These include areas with adequate moisture for rainfed production to western areas where irrigation is necessary for optimum soybean production. We conducted this study to determine whether optimum row spacings and seeding rates are the same for irrigated and rainfed water regimes. In addition we wanted to determine how cultivars were affected by changing cultural practices and environments. Literature Summary Most researchers have found that yields increase as row spacings decrease. Some have reported situations with no affect of narrow rows. Narrow rows have also decreased yields in environments with low yield potential. The most typical soybean response to changes in seeding rates is increased yields up to a point with increased seeding rates. Some scientists however, find occasionally that a wide range of seeding rates does not affect yields. Others have shown that increasing seeding rates may reduce yield. Study Description We conducted a 3‐yr field study in Clay County and a 2‐yr study in Harlan County, Nebraska, on silt loam soils. The two sites were chosen in part because they represent a wide range of production environments in south central Nebraska. A combination of three row spacings (10‐, 20‐, and 30‐in. rows), four seeding rates (45 000, 140 000, 235 000 and 330 000 seeds/acre), and five cultivars was planted at each site in adjoining imgated and rainfed fields. Three of the cultivars had indeterminate growth habits: ‘Hack’, Asgrow ‘A3127’, and ‘Williams 82’ the remaining two were determinate: ‘Hoyt’ and ‘Hobbit 87’. Hack and Hoyt were maturity group II and the remaining cultivars were maturity group III. These cultivars represent the normal range of maturity groups and both growth habits grown in the area. Applied Questions Was the optimum row spacing the same in all environments? Yes. Although soybean yield responses to row spacing depended on the site, water regime, and cultivar, the 20‐in. row spacing consistently had the highest yields (31 and 48 bu/acre, rainfed and irrigated, respectively) at all five sites when averaged over the other factors. The ranking of the other two row spacings varied with cultivar type and water regime. Yield of indeterminate cultivars in both water regimes and determinate cultivars with irrigation were greater with 20‐ than either 10‐ or 30‐in. rows. In contrast, determinate cultivars in rainfed regimes yielded the same in 20‐ and 30‐in. rows and less in 10‐in. rows. Canopies of both cultivar types closed sooner with narrower rows than with wider rows. Others have also found that canopy closure occurred sooner with narrow rows than with wide rows and that this resulted in greater evapotranspiration during vegetative growth and increased plant stress with narrow rows. In the extreme water stress situations in our trials, determinate cultivars in either 20‐ or 30‐in. rows produced more than in 10‐in. rows. Were optimum seeding rates the same in all environments? Yes, yields among the three highest seeding rates were greater than those of the lowest seeding rate for all row spaces and both water regimes. However, the yield difference between 45 000 and 140 000 seeds/acre was greater with irigation than with rainfed. There was no row spacing × seeding rate interaction (no change in rank). The 20‐in. row spacing consistently produced the highest yield at all four seeding rates. Were cultivars affected by changing these cultural practices in different environments? Yes, but the optimum row spacing for all cultivars in both water regimes was 20 in. Determinate cultivars in our high‐yield environments had increased yield between the 45 000 and 140 000 seeds/acre and contrary to other reports, similar yield at higher seeding rates (140 000, 235 000, and 330 000 seeds/acre. Indeterminate cultivars were normally not affected by changes in seeding rate. The 140 000 seed/acre seeding rate averaged over other factors either maximized yield or had yields equal to the highest yield for both determinate and indeterminate cultivars.