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 a few specific stages of seed filling, our objective was to gain a greater understanding of the mechanisms for yield reduction with defoliation at various stages across the seed filling period.  Two experiments, one in Kentucky (38^o N Lat) and the other in Louisiana (30^o 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.  Results indicated that defoliation-induced yield losses gradually diminished with each delay in defoliation timing.  The 33% defoliation generally had no effect on yield, whereas the 66% defoliation affected yield prior to R6 but not beyond.  Yield losses were greatest for the 100% defoliation; however the losses were much greater in early seed filling (R5-R5.6, 70-85%) vs. late seed filling (R6.5, 20-30%).  Results indicated that for canopies at or below 95% light interception, yield loss is avoided by  maintaining at least 2/3 leaf area index (LAI) during R5-R6 and 1/3 LAI during R6-R6.8.