Yield Response of Soybean to Partial and Total Defoliation during the Seed-Filling Period

Understanding how defoliation affects soybean [Glycine max (L.) Merr.] yield during the seed-filling period will aid in making management recommendations for control of stresses that reduce yield through defoliation. Because previous research has studied defoliation effects at only one or two specific stages of seed filling, our objective was to gain a greater understanding of the mechanisms for yield reduction with defoliation at weekly intervals across the seed-filling period. Defoliations were conducted from the bottom of the canopy upward to mimic the progress of soybean rust. Two experiments, one in Kentucky (38° N lat) and the other in Louisiana (30° N lat), were conducted in randomized complete block designs in split-split plot arrangements with four replications. Main plots were two cultivars, split plots were defoliation timings at weekly intervals during seed filling, and split-split plots were defoliation levels of 0, 33, 66, and 100% leaf removal. Data were obtained on yield and several growth dynamic and yield component factors. Defoliation-induced yield losses corresponded more closely with percentage of light interception reductions than percentage of leaf area reductions. During the R5 to R6 period, defoliation levels sufficient to reduce light interception by 18 to 23% were required to cause yield loss. Yield losses from total defoliation were greatest at early seed filling (78%) but gradually diminished as seed filling progressed. Each 0.1 delay in developmental stage resulted in a 5% decline in yield loss.