Oat–Rapeseed Intercropping Enhances Forage Yield and Quality in the Southern Foot of the Greater Khingan Mountains

To address the seasonal forage shortage in the southern foothills of the Greater Khingan Mountains in Inner Mongolia, this study investigated the effects of intercropping forage oat with rapeseed on forage yield, nutritional quality, and resource utilization. The experiment was conducted in Arun Banner, Hulunbuir City during 2023–2024, where a second crop was established after a first cut of forage oat. Three planting patterns were compared: monoculture oat (AO), monoculture rapeseed (BR), and oat–rapeseed intercropping (CO‖CR). The results showed that the yield of the intercropping system was higher than monoculture oat but lower than monoculture rapeseed. However, the system demonstrated an advantage in land use efficiency, with a land equivalent ratio (LER) of 1.161. Compared to their respective monocultures, intercropping significantly increased the dry matter yield of rapeseed by 38.4%, whereas the effect on oat yield was limited. Intercropping significantly increased the crude protein and crude fat content in both crops compared to their monocultures but had no significant effect on soluble sugar content. Furthermore, intercropping significantly reduced the neutral detergent fiber and acid detergent fiber content in both crops, resulting in higher relative feed value. Regarding water and nitrogen utilization, the water use efficiency of monoculture rapeseed was significantly higher than that of the intercropping system and monoculture oat by 20.7–21.5% and 90.2–113.2%. The total nitrogen accumulation in the intercropping system was significantly higher than in monoculture oat by 97.7–117.3% but showed no significant difference from monoculture rapeseed. In conclusion, adopting the oat–rapeseed intercropping pattern can significantly increase forage yield, improve nutritional quality, enhance water and nitrogen uptake and utilization efficiency, and achieve a coordinated improvement in high yield, quality, and efficiency for forage production in the southern foothills of the Greater Khingan Mountains.