Mixing Ratio Rather than Higher Legume Proportion Drives Agronomic Performance of Oat–Forage Pea Mixtures in Manure-Enriched Alpine Livestock Corrals

Alpine pastoral areas suffer seasonal forage shortages, and seasonally idle manure-enriched livestock corrals offer potential for forage production, yet the effect of species proportion remains unclear. We tested the hypothesis that mixture ratio, rather than simply increasing forage-pea proportion, determines the system-level performance of oat–forage pea mixtures under high-nutrient corral soil conditions. A two-year field experiment was carried out with monocultures and three oat–forage pea mixed-sowing treatments. The 1:1 mixture yielded the highest dry-matter and crude-protein production among mixed groups. Oat exhibited consistent competitive dominance across all ratios. All mixtures showed land-use advantage with land equivalent ratio >1. Increasing forage-pea proportion improved forage nutritional quality but reduced total biomass yield. The 1:1 treatment achieved the best comprehensive trade-off among forage yield, nutritional quality, land-use efficiency and economic return. Our results suggest that optimizing mixing composition instead of blindly raising legume share is critical for productive utilization of seasonally idle alpine corrals.