Effects of Ecotypes and Reduced N Fertilization on Root Growth and Aboveground Development of Ratooning Sorghum × Sudangrass Hybrids

Reduced N input while maintaining biomass production of sorghum × sudangrass hybrids (Sorghum bicolor L. × Sorghum sudanense; SSG) is essential; however, its effects on root sustainability and photosynthetic capacity during the ratooning period are not well defined in a multiple harvests system. The physiological response and root morphology of SSG were investigated under different N application levels during the ratooning period in a two-year field experiment. Treatments were all combinations of two ecotypes (late-flowering, Greenstar; early-flowering, Honeychew) and four N levels (0, 50, 100, 150 kg N ha−1). The total root length, surface area, volume, tips, and dry matter (DM) were significantly influenced by both ecotype and N level, with Greenstar outperforming Honeychew. Specifically, Greenstar’s root length increased by up to three times with reduced N application (50 kg N ha−1), while Honeychew showed significant root length increases only at higher N levels (100 and 150 kg N ha−1). Our data support the conclusion that a low level of N (50–100 kg N ha−1) was the optimal rate for ratooning root sustainability. The findings highlight the critical role of root development in sustaining biomass production and suggest that the late-flowering ecotype, Greenstar, is more suitable for a multiple harvests system with a robust root system.