Impact of soybean defoliation on canopy recovery, yield, and seed quality

Soybean [Glycine max (L.) Merr.] yield loss from hailstorms depends on the growth stage when hail occurs and the magnitude of plant damage. We evaluated how soybean canopy recovery, yield, and seed quality were affected by simulated hail damage in Iowa and Indiana from 2016 to 2018. Five levels of hail damage were simulated by defoliating 0%, 25%, 50%, 75%, and 100% leaves at the full-pod (R4) and beginning of seed-fill (R5) stages. Canopy closure was similar for plants with 0%–50% defoliation but significantly reduced for plants with 75% and 100% defoliation. The normalized difference vegetation index (NDVI) and normalized difference red edge index (NDRE) predicted defoliation levels better than canopy closure, with NDRE being more sensitive for detecting canopy variation among defoliation rates. Soybean yield and yield components decreased quadratically with increasing defoliation severity. Yield loss was minimal with 25% defoliation, regardless of growth stage or location. Soybean yield declined more with 100% defoliation at the R5 stage (80%–83%) compared to the R4 stage (67%–79%). The yield loss when plants were defoliated greater than 25% was due to a reduction in seed numbers (up to 54–88 seeds plant⁻¹) and seed weight (up to 0.022–0.052 g seed⁻¹). Defoliation at both stages minimally affected seed protein but decreased oil concentrations when defoliation reached 75%–100%. Soybean yield and seed quality loss should not be an issue of concern for fields with up to 25% hail defoliation damage at the R4–R5 stages. Results will help refine crop insurance guidelines, improving damage assessment for farmers.