Assessment of soybean yield in plants subjected to artificial defoliation simulating the defoliation caused by insects at different crop development stages in a cloudy and low-latitude environment

The damage caused by defoliating insects is one of the main factors limiting soybean productivity. It is crucial to define a level of defoliation that does not cause significant yield reduction, defined as the economic threshold. Therefore, in this study, we aimed to evaluate the effects of artificial defoliations, simulating insect damage, on soybean leaf area index (LAI), total aboveground dry biomass (TAGB), and yield in low-latitude regions. The experiment was performed in a 4 × 3 factorial design with 12 treatments, including four levels of artificial defoliation (0, 17, 33, and 67%) at the 8th-trifoliolate (V8), full flowering (R2), and beginning of seed filling (R5) stages. The LAI and TAGB were evaluated immediately after defoliation and at the fully formed seed stage (R6), along with an evaluation of yield components at the full maturity stage (R8). Linear models were fitted to determine the effects of the defoliation treatments and their interactions with the different phenological stages. The LAI and TAGB declined as defoliation levels increased to 17, 33, and 67%, with a higher impact when defoliation occurred at the R5 stage compared to the other stages. Defoliation at the V8 stage did not affect yield (3,837 kg ha-1) as the crop recovered its growth and yield. However, a positive effect was observed with defoliation levels of 27 and 16% at the R2 and R5 stages, increasing the yield to 4,314 and 3,959 kg ha-1, respectively. This increase was related to a higher grain number and mass because of LAI recovery after defoliation, which was associated with leaf expansion and better solar radiation interception through the canopy under cloudy ambient conditions. The maximum accepted defoliation levels in the R2 and R5 stages were 46 and 35%, respectively, to achieve at least 95% of the maximum yield. The relationships between yield and LAI after defoliation and between yield and LAI were significant at R5 and R6, respectively. Thus, variations in the stages and levels of defoliation differentially affect soybean crop yield and can be used to optimize defoliator management for maximum yield and sustainability.