Optimizing Cotton (Gossypium hirsutum L.) Defoliation with Thidiazuron in Suboptimal Conditions

Cotton defoliation plays a vital role in ensuring clean harvests and maintaining fiber quality in mechanized systems. Thidiazuron (TDZ) is a commonly used chemical defoliant that stimulates ethylene production and induces leaf abscission. However, its performance tends to decline under cooler conditions, particularly when nighttime temperatures fall below the optimal threshold of 18°C. This inefficiency often leads to incomplete leaf drop, complicating the harvest and lowering cotton fiber quality. In response to this challenge, the present study explores whether applying BSURE foliar fertilizer alongside TDZ can improve defoliation outcomes when temperature conditions are suboptimal. The research was conducted through multi-season field trials from 2021 to 2024 in Crosby County, Texas. Different combinations of TDZ and BSURE, applied at varying rates, were tested under temperature conditions ranging from a minimum of 33°F (1°C) to a maximum of 100°F (38°C).. The study employed ground-based measurements and drone imagery phenotyping approaches to evaluate cotton defoliation progress over time. Statistical analyses, including two-way ANOVA, were used to determine whether differences in defoliation efficiency were significant across treatments and environmental conditions. Particular attention was given to the interaction between temperature patterns and chemical combinations. Findings revealed that in years 2023 and 2024, with nighttime minimums dropping to around 38°F (3°C) to 55°F (13°C), significant variation existed among treatments. Overall, TDZ alone underperformed, resulting in defoliation efficiency up to 18% lower than the control across the trial years, whereas moderate rates of BSURE (8–16 oz/acre) enhanced defoliation efficiency. The treatments combining 32 oz Ethephon with 6 oz Folex and 2 oz TDZ, as well as combinations of 32 oz Ethephon with 4 oz TDZ and moderate BSURE rates (4–8 oz), demonstrated the strongest defoliation efficiency under cooler post-application temperature conditions. In contrast, results from the 2022 trial characterized by maximum temperatures ranging from 54°F (12°C) to 91°F (33°C) and minimum temperatures between 43°F (6°C) and 66°F (19°C) showed slight variation among treatments, with differences in green leaf retention averaging less than 10% and confirmed statistically insignificant (F = 0.50, p = 0.683). This suggests that conditions with maximum temperatures between 54°F (12°C) and 91°F (33°C) and minimum temperatures from 43°F (6°C) to 66°F (19°C) allowed TDZ to achieve efficient defoliation (final green leaf counts as low as 199.9 at 12 DAT) without the need for additional surfactants or micronutrient additives such as BSURE.. These results confirm that temperature exerts a dominant and significant influence on defoliant performance efficiency. Specifically, low post-application temperatures ranging from approximately 33°F (1°C) to 50°F (10°C), can potentially lead to late-season cold stress in cotton. However, carefully chosen combinations—such as TDZ with 6 oz Folex and 2 oz TDZ plus moderate BSURE rates—demonstrated enhanced defoliation efficacy even under these cold stress conditions, as evidenced by consistently lower green pixel percentages in 2023 and 2024 trials. This study offers practical insight for cotton growers facing unpredictable weather late in the season. Moderate BSURE applications, paired with TDZ and ethylene-enhancing products, can improve defoliation in cold environments without compromising fiber quality. The findings support more adaptive defoliation strategies and provide a foundation for optimizing cotton harvest efficiency. As climate variability becomes more common, the ability to adjust defoliant protocols based on environmental data will be increasingly essential for sustainable cotton production.