Variation in nutritional quality of Vicia sativa across climatic gradients and development of a comprehensive forage quality index.

Vicia sativa is a leguminous forage crop with important ecological and feeding value, and its nutritional quality is closely influenced by its growing environment. Field experiments were conducted over one growing season across three ecological zones: the cold, humid alpine region of the Qinghai-Tibet Plateau (XH), the humid transitional zone between the Qinghai-Tibet and Loess Plateaus (MX), and the semi-arid Loess Plateau (QY). Four V. sativa cultivars were evaluated using a randomized complete block design with three replicates. Nutritional components, including soluble sugars, crude protein, crude fat, and multiple fatty acids, were measured. Correlation analysis and principal component analysis (PCA) were performed to evaluate relationships among nutritional traits. A comprehensive forage nutritional quality index (CFNQI) was developed to assess forage quality quantitatively. Results indicated that both ecological zone and cultivar significantly affected nutritional traits, although ecological effects were generally stronger than cultivar effects for most traits. XH and MX samples showed higher CFNQI values than those from QY.. Specifically, V. sativa grown in XH was characterized by higher soluble sugar and unsaturated fatty acid contents, whereas samples from MX showed relatively greater crude fat and unsaturated fatty acid accumulation. In contrast, forage quality in QY was limited due to warm, dry conditions. This study highlights ecological differences in nutritional traits of V. sativa and supports the optimization of regionally adapted forage cultivation and high-quality forage resources.