Optimizing Seeding Ratio for Legume Forage to Maximize System Productivity and Resource Use Efficiency in Mixed Cropping Systems

Cereal and legume mixed cropping has been widely adopted to increase forage production in the sustainable development of agriculture and livestock. Among the different mixed cropping combinations, forage sorghum and lablab bean mixed cropping can be adapted globally. However, knowledge regarding the relation between forage production, interspecific competition, and resource utilization efficiency in the forage sorghum and lablab bean mixed cropping system remains unclear. A 3-year field experiment was conducted in 2020, 2021, and 2022 to investigate the effects of different cropping systems (16.5 kg·ha−1 lablab bean mixed cropping with forage sorghum [SD1], 33.0 kg·ha−1 lablab bean mixed cropping with forage sorghum [SD2], 49.5 kg·ha−1 lablab bean mixed cropping with forage sorghum [SD3], 66.0 kg·ha−1 lablab bean mixed cropping with forage sorghum [SD4], sole forage sorghum [SS], and sole lablab bean [DD]) on forage production, forage quality, competition parameters, water use efficiency (WUE), and radiation use efficiency (RUE). The results obtained revealed that mixed cropping practices enhanced forage yield by mitigating soil water depletion and optimizing canopy structures. Specifically, SD3 treatment was an efficient farming practice that increased system dry matter yield by 32.6–67.5%, crude protein yield by 12.5–15.1%, WUE by 9.2–67.4%, and RUE by 39.6–38.2% compared with other treatments. In addition, SD4 treatment increased crude protein content by 11.1% compared with forage sorghum monocropping; however, there were no significant differences in crude protein between SD3 and SD4 mixed cropping systems. The land equivalent ratio values were greater than one when forage sorghum was mixed with lablab bean, especially for the SD3 system (averaged 1.43). In addition, forage sorghum was more dominant and had higher aggressiveness (0.65) and competitive ratios (3.44) than lablab bean. This indicates that mixing cereals with legumes enhances RUE by interspecific competition. Consequently, the SD3-mixed cropping system is recommended for supporting the sustainable development of agriculture and livestock production in the arid region of China when considering forage production and nutritional quality.