Irrigation of Soybean Genotypes During Reproductive Ontogeny. I. Agronomic Responses1

Abstract

Successful irrigation management for soybeans [Glycine max (L.) Merr.] requires a knowledge of the effect of irrigations applied during specific stages of reproductive ontogeny. Eight soybean cultivars, varying in stem growth habit and maturity, were subjected to a factorial set of eight irrigation treatments involving either no irrigation or one irrigation applied at three reproductive stages: R1 to R2 flowering (F), R3 to R5 pod elongation (P), or R5 to R6 seed enlargement (S). The experiment was conducted for 3 years on a Sharpsburg silty clay loam (Typic Argiudoll). An F irrigation had little effect on ultimate seed yield (+20 kg/ha) and for some cultivars actually depressed yield below that of the nonirrigated check. A P irrigation consistently increased seed yield (+379 kg/ha), irrespective of cultivar or year. An S irrigation also enhanced seed yield (+ 384 kg/ha), but the degree of enhancement was inconsistent across years, being influenced by the amount of natural rainfall occurring during the seed enlargement stage. Although a dual F‐P irrigation treatment enhanced seed yield above that for both the nonirrigated check and single F irrigation treatment, the seed yield increase was significantly smaller (+166 kg/ha) than that achieved with a single P irrigation treatment. Soybean maturity was delayed 2 to 6 days with irrigation but the delay was greatest when irrigation was applied late in reproductive ontogeny. This senescence delay lengthened the seed‐fill period and was responsible in part for the seed yield enhancement effected by irrigation. Increases in soybean plant height and lodging were greatest with an F irrigation, intermediate with a P irrigation, and minimal with an S irrigation. Although irrigation effects on seed quality were variable depending on the cnltivar, there was an overall tendency for a P irrigation to improve and an S irrigation to worsen seed quality. An F irrigation had no effect on seed quality. Of the eight cultlvars evaluated, the determinate ‘Elf’ was the most yield responsive to irrigation and was relatively resistant to changes induced by irrigation in plant height, lodging, and seed quality. Based on these data, optimal yield enhancement in irrigated soybean culture can be achieved with the use of short‐stature or lodging‐resistant cultivars irrigated during pod elongation (R3 to R4) and seed enlargement (R5 to R6).