Objectives Sanjiangyuan region is located in the Qinghai-Tibetan Plateau, where is the birthplace of mother river of China, and also is the vital grassland for yaks and other typical livestock. Approaching exploring the suitable fertilization methods and mixed sowing ratio of forage crops was aiming to increase the forage yield, and reduce grazing pressure on natural grassland in the ecologically important area.

Methods A field experiment with split design was carried out on Sanjiangyuan area in 2022, the rye (Triticale) cultivar ‘Qingsimai 1^#’ and forage pea (Pisum sativa L.) cultivar ‘qingjian 1^#’ were used as test materials. Three fertilization methods were setup, as: no fertilization (A₁), application of urea 75 kg/hm² (A₂), combined application of urea 75kg/hm² and Ca(H₂PO₄)₂ 300 kg/hm² (A₃). Under each fertilization treatment, rye and forage pea were respectively mixed sown in ratio of 70∶30 (B₁), 50∶50 (B₂), and 30∶70 (B₃). The plant height, and leaf morphological indicators (area, length, width, perennial length) of rye and forage pea were measured before cutting at full blooming stage of forage pea, and the forage grass was weighed and the nutrition quality was determined.

Results Compared with A₁, fertilization (A₂ and A₃) significantly increased the fresh and dry weights, the protein yield of forage grass, and the plant heights and leaf areas of both crops. A₂ and A₃ did not exhibit significant differences in above indexes, but A₃ increased more than A₂ did in leaf width of two crops (P<0.05). A₂ and A₃ treatments had similar crud protein, soluble sugar, neutral and acid detergent fiber content, but A₂ treatment increased gross fat content. The seeding ratios of rye and forage peas did not significantly affect the fresh and dry weights, protein yields and plant heights of the two crops, however, the decreased rye ratio led to a gradual increase in crude fat and soluble sugar content but a significant decrease in neutral versus acidic detergent fibre content of forage grass. Seeding ratio and fertilization showed significant interactions, A₃B₁ exhibited the highest fresh and dry weight of forage, which were 36.09%−180.08% and 62.27%−281.59% higher than the other treatments, and the highest leaf area and protein, soluble sugar content. Compared with A₁B₁, A₁B₂, and A₁B₃, A₃B₁ increased leaf perimeter of rye by 15.76%, 40.29%, and 54.51%, leaf area of forage pea by 20.31%, 14.06%, and 64.65%, and neutral detergent fibre content of forage grass by 7.15%, 27.14%, 17.89%, respectively.

Conclusions Fertilization, especially nitrogen and phosphorous combined application, could significantly improve forage production performance, leaf morphology and forage quality. And rye and forage peas show the best forage production performance, leaf morphology and quality under mixed seeding ratio ratio of 70∶30. Therefore, this mixing ratio under nitrogen and phosphorus combined fertilization is recommended as suitable cultivation technology in Sanjiangyuan area.