Does defoliation affect late planted soybean yield?

Abstract

Double cropping soybean with winter wheat is a common agronomic practice in the southern US. Double cropping is where farmers harvest winter wheat planted the previous fall and then plant soybeans in June or July. This is late planting compared with full-season soybean production where soybeans are planted early in April and May. Usually, soybean planted late have lower yields than early planted soybean due to a shorter period of vegetative growth. Given the short growing period, biotic (e.g. insects) and abiotic (e.g. heat) stressors may also reduce yields relative to early planted soybeans. Insect defoliation may cause yield loss, but this has not been extensively explored. We hypothesized that defoliation during R5 (filling seed) would cause more yield loss compared to defoliation during R2 (full bloom) in late planted soybean. We planted soybean in 2023 and 2024 during July and manually defoliated plants by removing leaflets at R2 and R5. We compared defoliation levels at 0, 17, 33, 66 and 100 %. We evaluated seed yield (kg ha⁻¹), yield components, and the number of trifoliates that regrew after the R2 defoliation. We observed no difference in yield loss between defoliation at R5 and R2 between both field seasons. In both growth stage treatments, overall yield decreased as defoliation levels increased. Yield components varied between years, but in general, the number of pods with two and three seeds and weight of seeds was affected differently across growth stages and defoliation levels.