Artificial defoliation effects on full season and double crop soybean yields

Double-cropping soybean behind winter wheat harvested in June or July is a common production practice in the Southeast USA. This contrasts full-season soybean production where soybean are planted earlier in April and May. Usually, double-cropped soybean does not yield as high as full-season soybean due to a shorter period of vegetative growth. Hence, photosynthetic tissue loss by defoliating insects might have a great negative impact on yield loss in double-cropped relative to full-season soybean, but this has not been extensively explored. We hypothesized that defoliation during R5 (seed filling) would cause more yield loss in doubled-cropped compared to full-season soybean. To explore this, we planted soybean either in May or July, simulating full-season and double-cropped cropping systems, and manually defoliated plants by removing leaflets at R5. We compared 0, 16.7, 33.3, 66.6 and 100 % defoliation levels in a factorial design. We evaluated yield, yield components (number of pods per plant, number of seeds per pod, and weight of 200 seeds) and canopy coverage. Full-season soybean had a higher overall yield compared to double-cropped soybean. Across both cropping systems, yield decreased as defoliation levels increased. Across defoliation levels, some yield components were reduced, but only at high defoliation levels (100 % defoliation). Across cropping systems, seeds were heavier in full-season compared to double-cropped soybean. Overall, the impacts of defoliation did not change between these two cropping systems. These findings need to be supported by further research to refine thresholds for defoliating insects in southeastern US soybean.