Effects of dose and application timing of Medax Top on grain yield components of 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 of the appropriate solutions for managing limited water resources is the cultivation of drought‑tolerant crops with high water‑use efficiency such as sorghum. Forage sorghum is a promising option due to its high tolerance to drought and heat, efficient water use, and considerable dry‑matter production; however, the low grain yield of open‑pollinated 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 they require increased grain‑production potential. Plant growth regulators like mepiquat chloride + prohexadione calcium (Medax Top 35% SC) have been shown to increase grain number, thousand‑grain weight, and grain yield in various cereals such as wheat, rice, and maize; nonetheless, very limited information exists on their effects on grain‑production potential in forage sorghum. The present study aimed to evaluate the effect of dose and application timing of the growth regulator Medax Top on grain yield components of two open‑pollinated forage sorghum cultivars and to identify the optimal combination of dose and application timing.

Materials and methods
This research was carried out as a factorial split‑plot experiment in a 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. The factorial combination of Medax Top dose and application timing was assigned to main plots, and the sorghum cultivars to sub‑plots. Medax Top dose was evaluated at four levels: zero (control), 0.5, 1, and 2 L.ha^-1; foliar‑spray timing was at two levels: the 3-4 leaf stage and the 6-8 leaf stage; and cultivars included two open‑pollinated forage sorghum cultivars, Mansour and Behesht. The evaluated traits were panicle density, grains per panicle, grains per plant, grains per square meter, thousand‑grain weight, plant height, panicle‑to‑plant weight ratio, grain yield, and grain protein content. Given homogeneity of experimental error variances across both years, data were subjected to combined analysis of variance using SAS 9.1, and mean comparisons were performed by LSD at the 5 % significance 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 1000‑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 1000‑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 ratio of panicle to plant weight 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. Overall, the results of this experiment showed that Medax Top application at 3-4 leaf stage had a greater positive effect on all grain yield components of forage sorghum, while for grain protein content, application of Medax Top at 6-8 leaf stage was more effective.

Conclusion
The results of this study indicated that foliar application of the growth regulator Medax Top is an effective and practical method for improving yield components and grain quality of open-pollinated forage sorghum cultivars. 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.