CRITICAL LIGHT INTERCEPTION DURING SEED FILLING FOR INSECTICIDE APPLICATION AND OPTIMUM SOYBEAN GRAIN-YIELD

Relationships between defoliation, light interception, and grain yield for soybean [Glycine max (L.) Merr.] during the last half of the seed filling period are not fully understood. Because defoliating insect p ests in the southeastern USA frequently attack soybean during this per iod, knowledge of such relationships would aid development of criteria for economic insecticide application. Our first objective was to dete rmine the efficacy of using light interception and leaf area index as criteria for economic insecticide application to manage defoliating in sect pests. The second objective was to determine the effects of parti al and total defoliations during seed filling on grain yield and grain yield formation. 'Centennial' soybean (Maturity Group VI) was planted in 1993 and 1994 near St. Gabriel, LA. Experimental design was a rand omized complete block in a split-plot arrangement. Main plots were the temporal midpoint and 3/4-point seed-filling stages. Split plots were four defoliation treatments: 0% leaf removal (control), 37 to 41% lea f removal, 56 to 61% leaf removal, and 100% leaf removal. At the tempo ral midpoint, grain yield declined linearly as light interception fell from 93 to 50%. Thus, for a crop achieving near-maximal light interce ption, insecticide application would be recommended whenever light int erception at the midpoint seed-filling stage fell below 93%. Yield and light interception were not closely related at the 3/4-point seed-fil ling stage. Maintenance of maximal grain yield (yield at 0% defoliatio n) required retention of at Least 39% of tile leaf area at this time. In conclusion, light interception potentially could be used as an inse cticide application criterion at midpoint seed-filling stage, but not the 3/4-point. Partial defoliation at the midpoint will affect grain y ield if it reduces light interception below 93%, whereas at the 3/4-po int total defoliation is required to affect grain yield.