Yield sensitivities of short season soybeans to irrigation management

Summary Field experiments were conducted for three seasons on a loam textured soil in East-central North Dakota. Soybean seed yield responses were measured for three cultivars (maturity groups 00 and 0) as a function of water management treatments. Water stress development per treatment was generally limited to one of three growth periods, planting to first bloom (R1), R1 to full seed (R6), or R6 to physiologic maturity (R8). Stress levels were defined as low (L), moderate (M) or severe (S) and were based on the degree of root zone water depletion and/or an allowable depression of leaf water potential in midafternoon before stress relief by irrigation. Yield vs seasonal ET relationships were linear. Mean slope of the regression based functions (per three cultivars) was 1.01 kg/m³. Maximum seasonal seed yields were generally produced with an irrigation regime that maintained low stress levels in each growth period. Results when normalized as relative yield (Y/Y_m) vs relative seasonal evapotranspiration (ET/ET_m) indicated an average slope of 1.26% yield loss per 1% decline in seasonal ET from the ET_m level. Average observed Y/Y_m ratios were 0.57 for dryland/non-irrigated treatments, 0.88 for MLL (i.e., a moderate stress in planting to R1 growth period followed by the maintenance of low stress levels in the R1 to R6 and R6 to physiologic maturity growth periods), 0.87 for LML, 0.80 for LSL, 0.91 for LLM and 0.97 for LLL stress sequences. Stress effects were most detrimental to yield if imposed in the R4 (full pod) to R6 (full seed) period. The average Y_m yields were 3,266, 2,937 and 3,248 kg/ha for McCall, Maple Amber and Ozzie cultivars, respectively. Yield reductions of less than 5% from Y_m levels appear likely in this climatic setting if remaining available water levels are maintained above 40–50%. Depressions of midafternoon leaf water potentials to about-1.20 MP_a were estimated as the threshold at which seed yield declines from potential levels. Relative yields vs relative irrigation (IR/IR_m) relationships suggest that irrigation amounts could be reduced by 20% from IR_m levels with MLL and LLM regimes for less than a 5% yield reduction in average rainfall seasons. In very dry seasons this yield reduction could result with about a 10% reduction in seasonal irrigation.This work was supported by ND Agr. Exp. Sta. Proj. 1442 and by funds provided by the U.S. Dept. of Interior, USBR