Are yields impacted differently with defoliation in full-season or double-cropped soybean?

Approximately 20% of the soybeans grown in North Carolina are double-cropped. Double-cropping is where farmers harvest winter wheat, planted the previous fall, and then plant soybeans in June or July. This contrasts to full-season soybean that is planted earlier in April and May. Usually, soybeans planted double-cropped do not yield as high as full-season soybeans due to a shorter period of vegetative growth. Further loss of photosynthetic tissue by defoliating insects might differently affect yield loss in double-cropped relative to full-season soybeans, but this has not been extensively explored. We hypothesized that defoliation during R5 (filling pods) would more negatively impact yield in doubled cropped soybeans compared to full season soybean. To explore this, we planted soybean in May and July, simulating full season and double cropped systems, and manually defoliated plants by removing leaflets at R5. We compared defoliation levels at 0, 17, 33, 66 and 100% in a factorial design. We evaluated yield (bu/ac), yield components (number of pods per plant, number of grain per pod and weight of 200 seeds). Full-season soybean had a higher overall yield compared to double-cropped soybean. Across both planting dates, yield decreased as defoliation levels increased. Doubled cropped soybean had higher total number of pods compared to full-season soybeans; however, full-season soybean had significantly heavier seed compared to double-cropped soybean. Overall, the impacts of defoliation were equal between these two planting dates. These findings will be useful to refine thresholds for defoliating insects in North Carolina soybeans.