Optimizing Ethephon Concentrations for Male Plant Feminization and Enhanced Seed Yield in Dioecious Thai Hemp (Cannabis sativa L. cv. RPF3)

Dioecious hemp cultivation faces significant productivity challenges, as approximately 50% of plants (males) produce no seeds whatsoever, functioning exclusively as pollen donors, creating substantial resource inefficiency in commercial production. This study evaluated ethephon (2-chloroethylphosphonic acid) concentrations from 250 to 1000 ppm for inducing feminization in male plants of the Thai hemp cultivar RPF3 under controlled greenhouse conditions. Complete morphological sex conversion (100%) was achieved across all tested concentrations, successfully transforming male plants into seed-producing individuals. Male plants exhibited a linear, concentration-dependent increase in seed production (R2 = 0.76–0.91), with the highest concentration (1000 ppm) producing seven-fold more seeds than the lowest effective concentration (250 ppm). Female plants showed optimal yields at intermediate concentrations (500–750 ppm), with a significant decline in yield at 1000 ppm, indicating hormone toxicity. For commercial optimization, concentration selection depends on production objectives. 500 ppm provides optimal cost-effectiveness for mixed dioecious systems, delivering a 2.2-fold increase in total yield, while 1000 ppm maximizes productivity for male-only cultivation systems. Seeds from feminized males remained consistently 62–73% lighter than those from natural females across all treatments, indicating genetic control over seed morphology, despite hormonal sex conversion. This ethephon-based approach preserves the genetic diversity advantages of dioecious systems while doubling land-use efficiency, providing a sustainable intensification strategy for commercial hemp seed production.