SOYBEAN RESPONSE TO STEM CUTOFF AND DEFOLIATION DURING VEGETATIVE DEVELOPMENT

Damage to soybean [Glycine max (L.) Merr.] caused by hail can significantly affect the profitability of production. Previous studies have documented the response of soybean injured by individual types of plant injury, but less is known about the effects of multiple types of injury that occur simultaneously. Our study evaluated the effects of varying levels of defoliation and stem removal, both alone and in combination, on the agronomic performance of soybean injured during vegetative development. The experiment was conducted at Ames, IA; Lamberton, MN; and Fargo, ND, during 1985 and 1986. Plant injury treatments consisted of a factorial arrangement of 0, 33, and 66% stem cutoff and 0, 33, 66, and 100% defoliation. Plots were treated at stage V3 or V6. No consistent differences in plant response to plant injury were observed across locations or years of the study. For both stages of treatment, defoliation alone did not result in significant seed yield reductions compared with the untreated check. Removing 33 and 66% of the main stem resulted in yield reductions of 7 and 18%, respectively. Defoliation levels of 33 and 66% did not reduce yield of plants injured by stem cutoff, but 100% defoliation resulted in yield reductions of 17 and 33% for plants treated with 33 and 66% stem removal, respectively. The stage of vegetative development at which plant injury was imposed did not affect seed yield.