Nutritional quality of fresh yak milk and its influencing factors across the northern Tibetan Plateau

To explore the relationship between the nutritional quality of fresh yak milk and forage plant species across the northern Tibetan Plateau, 12 sites and 36 adjacent herder families were selected from alpine meadow, alpine steppe, and desert steppe zones to sample fresh yak milk and forage plants in August 2021. The differences in nutritional quality indicators of fresh yak milk samples were compared among different grassland zones using a one-way analysis of variance. The influence of the nutritional quality of forage plant species on fresh yak milk was quantitatively evaluated using structural equation modeling and generalized mixed linear models. The results showed that: 1) The protein content of fresh yak milk from alpine deserts was the highest (4.36%), significantly higher than that from alpine steppes and alpine meadows (4.05%, 3.59%) (P < 0.05). The lactose and mineral content of fresh yak milk from alpine meadows were the highest (5.45%, 0.82%), significantly higher than that from alpine steppes (4.31%, 0.71%) (P < 0.05) and alpine deserts (4.24%, 0.73%) (P < 0.05); 2) The crude protein and crude fat content of forage plant species had significant impacts on the protein content of fresh yak milk, explaining 53.58% and 26.07% of the total variance in protein content, respectively (P < 0.05). The crude protein and neutral detergent fiber content of forage plants had significant impacts on the lactose content of fresh yak milk (P < 0.05), explaining 49.88% and 40.92% of the total variance in lactose content (P < 0.05). The effect of neutral detergent fiber content on the mineral content of fresh yak milk was also significant (P < 0.05), explaining 53.33% of the total variance in mineral content. 3) The structural equation model showed that temperature and precipitation during the plant growth season indirectly or directly affected the crude protein, crude fat, and neutral detergent fiber content of forage by controlling soil total nitrogen and organic carbon content, which in turn affected the lactose, protein, fat, and mineral content in fresh yak milk. In summary, the nutritional quality of fresh yak milk on the northern Tibetan Plateau is regulated by multiple factors, including climatic conditions, soil nutrients, and forage together.