Evaluation of Drip Supplemental Irrigation against Rainfall Deficits: Effects on the Phenology and Yield of Cotton (Gossypium hirsutum L.) in the Municipality of Banikoara (Northern Benin)

<jats:p>Background: Seed cotton (Gossypium hirsutum L.) production in the municipality of Banikoara, in northern Benin, is severely constrained by climate variability and increasing rainfall deficits during critical stages of plant development. Aims: This study aimed to evaluate the effects of deficit drip supplemental irrigation, calibrated to different fractions of the rainfall deficit relative to maximum evapotranspiration (ETM), on the phenological development and yield components of cotton over four consecutive cropping seasons. Study Design: A randomised complete block design (RCBD) with five irrigation treatments and four replications was used. Place and Duration of Study: The study was conducted at the cotton experimental perimeter in the municipality of Banikoara, Alibori Department, northern Benin, over four consecutive cropping seasons from June to November 2021 to 2024. Methodology: Twenty elementary plots of 10 m² each were sown on 15 June at a density of 31,250 plants ha⁻¹ (spacing of 0.80 m by 0.40 m). The five treatments corresponded to drip supplemental irrigation supplying 0% (T₀, strict rainfed control), 25% (T₂₅), 50% (T₅₀), 75% (T₇₅) and 100% (T₁₀₀) of the rainfall deficit relative to ETM, calculated according to the FAO 56 Penman-Monteith method. Phenological parameters and yield components were monitored fortnightly on 10 tagged plants per plot and analysed by two-factor ANOVA (Treatment by Year), with means separated by Duncan's Multiple Range Test at the 5% significance level. Results: Supplemental irrigation significantly improved all phenological and agronomic parameters measured. Mean plant height at maturity rose from 60.65 cm (T₀) to 131.61 cm (T₁₀₀). Mean seed cotton yield increased from 750 kg ha⁻¹ (T₀) to 3,725 kg ha⁻¹ (T₇₅) and 3,700 kg ha⁻¹ (T₁₀₀), representing gains of 397% and 393% relative to the rainfed control. Treatments T₇₅ and T₁₀₀ produced statistically equivalent yields, identifying 75% deficit compensation as the threshold for maximum water use efficiency and productivity. Conclusion: These findings provide a robust empirical and technical framework for adapting cotton-growing practices to climate change in the Sudano-Sahelian zone, with 75% deficit compensation identified as the optimal water-saving strategy.</jats:p>