|Rain is needed to fill soybean pods.
Soybean yield is determined by the genetic potential and the interaction of a wide range of environmental factors and management systems. Yield is influenced more by changes in growth during early flowering (R1) to physiologically mature (R7) compared with the emergence to R1 period. The early reproductive period (R1) to shortly past beginning seed filling (R5) stages are most sensitive to altered growth to prevent pod abortion and to insure adequate seed numbers.
Lack of strong associations between yield and yield components have been observed in field-grown soybean because seed yield is a function of complex interactions between various traits that usually alter one or more growth characteristics. The number of seeds per unit area and seed size determines seed yield. Seed size is often inversely correlated with seeds per unit area.
The commonly observed compensation between seed number and size suggests that a given yield can be obtained in many ways, such as from a high seed number per plant but small seed size, low seed number per plant but large seed size, or some combination of the two. The size and number of seeds produced depend upon both genetic and environmental factors. Large seeds can either result from a high seed growth rate, a long duration of seed fill, or some combination of the two. The compensation that occurs between seed size and seed number led researchers to conclude that soybean seeds are just receptacles for assimilate and that yield-limiting factors occur somewhere outside the seed.
Overall, yield loss resulting from drought stress depends on both the phenological timing of the stress and the degree of yield component compensation. The negative effects of stress are particularly important during flowering, seed set, and seed filling where it reduces yield by reducing number of pods, number of seeds, and seed mass. We still have approximately 3 to 4 weeks of seed and pod filling left, so it is too early to talk about yield loss from drought conditions in Iowa. However, we must recognize that we have the potential for a really good crop this year and rain is needed to fill those pods.
This article originally appeared on pages 146-147 of the IC-490(20) -- August 18, 2003 issue.