Nitrogen Supply Regulates Productivity, Forage Quality, and Interspecific Interactions in a Silage Maize–Sweet Sorghum Intercropping System in an Arid Irrigated Region

To clarify the effects of nitrogen application rates on production performance, nutritional quality, and nitrogen use efficiency in a silage maize/sweet sorghum intercropping system, this study established a 3:3 intercropping treatment (B), a silage maize monoculture (S), sweet sorghum monoculture (T)—as well as six nitrogen application rates: 0 (N0), 120 (N1), 180 (N2), 240 (N3), 300 (N4), and 360 kg ha−1 (N5). The results showed that both the cropping system and nitrogen application rate significantly influenced crop growth and yield formation. The N4 (300 kg ha−1) treatment yielded the best results, promoting plant height and dry matter accumulation in both crops. Under intercropping conditions, the biomass of silage maize at the milky stage was significantly higher than that under the N0 treatment. Excessive nitrogen application (N5) did not further promote crop growth and, instead, weakened the complementary benefits of intercropping. The 3:3 intercropping combination with the N4 nitrogen application rate yielded higher forage yields and Feed Value; hay yield and crude protein yield increased by 37.86% and 52.04%, respectively, compared to N0, while crude protein content increased by 10.70%, resulting in a comprehensive production advantage over monoculture. At the same time, the N4 treatment offered a higher return on nitrogen fertilizer investment and helped maintain total nitrogen levels in the 0–40 cm soil layer. In summary, a 3:3 intercropping system of silage maize and sweet sorghum, combined with the application of 300 kg ha−1 of nitrogen fertilizer, is the optimal cultivation model under the conditions of this experiment and can serve as a preliminary recommendation for optimizing nitrogen fertilizer management in grass forage intercropping systems in arid irrigation areas.