Optimizing Defoliant Application Timing to Improve Boll Opening, Fiber Quality, and Yield in Summer-Sown Short-Season Cotton in Hunan, China

The optimal timing of chemical defoliation is a critical bottleneck in stabilizing yield and fiber quality for short-season cotton, particularly under the intensifying pressure of mechanized global production. Current practices rely heavily on population-level boll opening rates, often overlooking the physiological maturity of late-season bolls. Here, we investigate the trade-offs between late-boll development and defoliation-induced senescence in short-season summer cotton. Our results demonstrate that defoliation timing based on a specific heat-unit or temporal threshold after flowering—rather than simple visual indicators—is essential for maximizing biological potential. We identified a critical physiological window (43 days post-anthesis) that synergistically optimizes boll weight, seed cotton yield, and fiber micronaire. Beyond this window, delayed defoliation leads to excessive fiber coarsening and reduced spinnability, while earlier application terminates dry matter accumulation prematurely, incurring significant yield penalties. These findings provide a mechanistic basis for synchronizing reproductive maturation with mechanical harvesting requirements. By establishing a precision defoliation framework, this study offers a scalable strategy to enhance the economic sustainability and resource-use efficiency of short-season cotton systems in double-cropping regions globally.