Growth and yield of indeterminate soybeans. 1. Effect of defoliation

Abstract

Indeterminate soybeans (Glycine max, varieties Chaffey and Farrer) were subjected to defoliation treatments in the field over 3 years to determine their ability to recover from leaf damage from foliage feeding pests. Defoliation treatments were imposed in years 1 and 2 by clipping all leaflets in half mechanically (50%) or by removing all leaves leaving the petioles on the plant (1 00%) to simulate 2 severe levels of sudden defoliation. The variety Chaffey was defoliated early in vegetative growth (V2), at the beginning of flowering (Fl), at full flowering (F100) or at the end of flowering (EF100) as single treatments in years 1 and 2. In year 3, only the top 4 leaves of each plant were clipped in half to simulate levels of defoliation experienced in commercial crops. In that year the variety Farrer was treated at stages V3 (early vegetative), F1 or F100 as single treatments or at V3 + F1, F1 + F100 or weekly from V3 to EF100 as repeated treatments. Severe defoliation (100%) at EF100 hastened maturity (95% of pods dry) by 24 days (year 1) and 22 days (year 2), which reduced seed size by 34 and 41%, seed number by 38 and 32% and seed yield by 58 and 60%, respectively. Seed yield was also reduced by a single 100% defoliation at F1 due to reductions in seed number rather than to seed size. The repeated defoliations in year 3 reduced leaf area index, plant height, seed number and .seed yield. Weekly defoliations reduced yield by 20% by reducing seed size by 8% and seed number by 13%. We conclude that, prior to flowering, 50% defoliation is unlikely to reduce yield, but repeated damage will reduce yield significantly. Also, indeterminate soybeans can withstand an initial 50% loss over the top 4 leaves at F1, but repeated defoliations reduce seed yields.