Critical Stages of Development for Defoliation of Soybean1

One objective of our research was to define the most critical stages for defoliation of indeterminate and determinate soybean Glycine max (L.) Merr. strains. The second objective was to determine if yield loss from defoliation at R4.5 could be predicted by the average of loss at R4 and R5 and if loss at R5.5 could be predicted by the average of loss at R5 and R6. Response to 100% defoliation for an indeterminate cultivar, ‘Beeson,’ and a determinate experimental strain, A72‐407R, was evaluated at Ames, Iowa during 3 years. The strains were defoliated at R4 (full pod), R4.5, R5 (beginning seed), R5.5, and R6 (full seed). The indeterminate strain had an average yield loss across stages of 59% compared with 80% for the determinate strain, but the strain × stage interaction was not significant. The most critical stage for defoliation varied among environments, but was either R5 or R5.5 for both strains. The average yield loss at R4.5 of 72% was slightly greater than the average of loss at R4 and R5 of 68%. The average yield loss at R5.5 of 80% was significantly greater than the average of loss at R5 and R6 of 69%. Yield response to defoliation at R4.5 and R5.5 should be considered in estimating yield reduction from factors that cause defoliation of soybeans.