Effects of dose and application time of combined mepiquat chloride and prohexadione calcium (Medax Top) on grain-yield components in two forage sorghum [Sorghum bicolor (L.) Moench] cultivars

Introduction
The growing global population,diminishing freshwater resources,and climate change have imposed additional pressure on agriculture in arid and semi‑arid regions.One suitable strategy for managing limited water resources is cultivating drought‑tolerant, high water‑use‑efficiency crops such as sorghum. Forage sorghum, due to its high tolerance to drought and heat, efficient water use, and considerable dry‑matter production, is a promising option; however, the low grain yield of open‑pollinated forage sorghum cultivars has limited the expansion of this crop in the country.Open‑pollinated cultivars offer advantages over hybrids—such as greater genetic diversity, adaptability to poor soils, and lower seed‑production costs—but require enhancement of grain‑production potential. Plant growth regulators like mepiquat chloride+prohexadione calcium(Medax Top)have increased grain number, thousand‑grain weight, and grain yield in various cereals including wheat, rice, and maize;nonetheless, very limited information exists on their effects on grain‑production potential in forage sorghum.The present study was conducted to evaluate the response of grain‑yield components in open‑pollinated forage sorghum cultivars to different doses and timings of Medax Top application and to identify the optimal dose–timing combination

Materials and methods
This study was carried out as a factorial split‑plot experiment based on randomized complete block design with sixteen treatments and three replications at the Seed and Plant Improvement Institute, Karaj, Iran, during the 2023 and 2024 growing seasons. Dose and application tim of Medax Top was assigned as factorial in main plots, and cultivars to sub‑plots.  MedaxTop doses were four levels, including zero (control), 0.5, 1, and 2 L.ha^-1; foliar‑spray timings were two levels, including 3–4 and 6–8 leaf stages, and cultivars included two open‑pollinated forage sorghum cultivars, Mansour and Behesht. Evaluated traits were panicle density, No. of grains per panicle, No. of grains per plant, No. of grains per m², 1000‑grain weight, plant height, panicle‑to‑plant weight ratio, grain yield, and grain protein content. Given homogeneity of the experimental error variances across both years, data were subjected to combined analysis of variance using SAS  9.1, and comparison of means were performed by LSD at 5% probability level.

Research findings
The results showed that application of Medax Top as a growth regulator significantly improved grain‑yield components of forage sorghum.Increasing dose enhanced grains per plant, grains per square meter, and thousand‑grain weight. The maximum panicle density(40.9 panicles m-2) was achieved with 1 L.ha-1 of Medax Top applied at the 3–4‑leaf stage. The highest grains per panicle (721) were recorded in cultivar Mansour with 2 L.ha-1 during the first year, whereas the greatest grains per plant (1,134) and per square meter (18,970) occurred in cultivar Beheshte with 2 L ha-1 during the second year. The highest thousand‑grain weight (25.3 g) was obtained in Mansour with 1–2 L ha-1 in the second year. Application of 0.5, 1, and 2 L ha-1 of Medax Top increased the panicle‑to‑plant weight ratio by 71 %, 109 %, and 180 %, respectively. The maximum grain protein content (9.66 %) resulted from 2 L ha-1 applied at the 6–8‑leaf stage. For all yield components, Medax Top application at the 3–4‑leaf stage was more effective, whereas for grain protein content, application at the 6–8‑leaf stage was superior.

Conclusion
This study demonstrated that using Medax Top as a plant growth regulator is an effective and practical method for improving grain‑yield components and quality in forage sorghum. Application of 1–2 L.ha-1 of Medax Top, especially at the 3–4‑leaf stage, enhanced panicle density, grain number, and thousand‑grain weight, thereby significantly boosting grain yield. For grain production of the Mansour cultivar, 1 L.ha-1 is sufficient, whereas cultivar Beheshte responds best to 2 L.ha-1. Early application at the 3–4‑leaf stage strengthens photosynthate allocation to reproductive organs, maximizing yield components, conversely, for elevating grain protein content, application at the 6–8‑leaf stage is preferred. Overall, the application of Medax Top in seed‑production fields of open‑pollinated forage sorghum cultivars can improve yield components and grain quality.