Mixing cereal with legume crops is an efficient approach for improving forage production and ensuring the sustainable development of agriculture and livestock. However, the knowledge of the relationship between forage production and interspecific competition in the forage oat (Avena sativa L.) and common vetch (Vicia sativa L.) mixed cropping system remains unclear. A 2-year field experiment was conducted in 2020 and 2021 to investigate the effects of different mixed cropping systems (peer mixing (PM), alternate-row mixing (AM), cross mixing (CM), bar mixing (BM), sole forage oat (SO), and sole common vetch (SV)) on the dry matter production, forage quality, land equivalent ratio (LER), and competition parameters. The results showed that the system forage yield increased by 13.4–202.8% when forage oat was mixed with common vetch (p < 0.05). However, no significant difference was obtained between the AM and PM mixed cropping systems. Additionally, the forage oat–common vetch mixed cropping improved crude protein yield by 52.5–150.1% compared with monocultures (p < 0.05). The LER values were greater than the one when forage oat was mixed with common vetch, especially for the AM system (averaged 1.38). In addition, forage oat was the dominant crop and had higher aggressiveness and competitive ratios compared to common vetch, but without a significant difference in the aggressiveness and competitive ratio in mixed cropping systems. It indicated that mixing of cereal with legume crops was helpful in enhancing resource use efficiency without obvious interspecific competition. Consequently, the AM mixed cropping system is recommended for supporting the sustainable development of agriculture and livestock production in the arid region of China when considering both forage production and nutritional quality.

Intensive annual forage cultivation via diversified sowing can increase resource-use efficiency and meet the food requirements of the growing population. However, increasing cultivation time could increase energy demands and production costs with accompanying environmental hazards, such as greenhouse gas (GHG) emissions. A 2-year field experiment including three cropping systems (forage oat monoculture [O], forage oat–common vetch mixed-cropping [O/V], and common vetch monoculture [V]) and two sowing dates (spring and summer) was conducted to comprehensively assess productivity performance, energy utilization, and ecological efficiency under a cereal–legume mixed system over two sowing seasons in Northwestern China. The average system dry matter yield and economic benefit in the mixed system increased by 70.9% and 132.6%, respectively, compared to those of the monoculture system. The application of a mixed-cropping system increased energy efficiency and the ecoefficiency index (EEI) by 61.4% and 132.3%, respectively, with a decline in GHG emission intensity (GHGI) by 49.9%. Spring sowing increased average forage productivity and economic benefit by 9.3% and 18.6%, respectively, with a 23.0% decline in GHGI compared with that engendered by summer sowing. However, there was no correlation between the mixed-cropping system and single sowing date, and the average dry matter yield, economic benefit, and GHGI of the mixed-cropping system were 7.8 t ha⁻¹, 1478.4 USD ha⁻¹, and 0.3 kg CO₂e kg⁻¹, respectively. Furthermore, compared with the monoculture system, the combination of spring and summer seasons under the mixed-cropping system could increase the average annual system forage productivity, economic benefit, and EEI by 37.0%, 49.9%, and 53.1%, respectively, while reducing GHGI by 48.4%. This study demonstrates that the mixed forage oat and common vetch during spring and summer was the sustainable cropping system that achieved high system productivity, good economic returns, high energy utilization, and low environmental risk in Northwestern China.