The objective of this research was to evaluate the effect of both continuous and sequential defoliation on soybean yield (variety BRS 137). The treatments were evaluated during the vegetative and reproductive stages by using the injury levels recommended for the management of soybean defoliating pests. The study was conducted at the University of Passo Fundo, in Passo Fundo-RS, from December 1999 to May 2000. The experimental design was complete randomized blocks with six treatments and four replications. The following treatments were tested: (i) 33% of continuous defoliation at stages V4 to V9 and of (ii) 17 and (iii) 33% at stages R1 to R4 (FEHR & CAVINES, 1977, stage); sequential defoliation of (iv) 33+17% and (v) 17+33% from stages V4 to V9 and from stages R1 and R4; and (vi) control (no defoliation). The defoliation levels 33% in the vegetative stage and 17% in the reproductive stage did not affect grain yield (kg ha-1). In contrast, the sequential defoliating levels of 33+17% and 17+33% in the vegetative and reproductive stages, respectively, caused an yield reduction for BRS 137 soybean variety. The grain yield, the number of pods, number of grains, and the weight of grains were the most important components in determining the yield decrease with the sequential leaf defoliation of 33+17% and 17+33%. All the leaf losses for decreased the number of grains/plant and the number of pods/plant. Considering the leaf action levels recommended to control of the leaf-feeding insects in soybean, it should be indicated for the BRS 137 soybean variety a leaf loss of 33% in the stages V4 to V9 or 17% in the stages R1 to R4.
artificial defoliation; action level; soybean
