Vegetation diversity can increase the nutrition content of sheep diet in native grasslands dominated by Leymus chinensis and Stipa grandis

Grazed livestock usually face the challenge of obtaining sufficient nutrition due to uneven distribution of plant species and fluctuating vegetation productivity and nutrient levels in the native temperate grasslands. Leymus chinensis and Stipa grandis are the dominate perennial plants of native grasslands in northern China, which provide limited nutrition compared to forbs with higher crude protein (CP) content. The dietary allowance and ingredients from the grassland can affect animal intake, which grazing management is a key issue to meet the requirement of animal dietary and available vegetation. In this study three sheep grazing strategies (lambs grazed alone, mixed grazed of lambs and ewes, ewes grazed alone) at the moderate stocking rate of 0.80 sheep ha^-1year^-1 were used to explore vegetation diversity effects on the diet selection of grazing livestock. We investigated the influence of vegetation characteristic (above-ground biomass production, height, and species diversity) and foraging behavior (forage intake, organic matter digestibility, and daily grazing time) on the dietary selection of animals (taxonomic family richness and composition). Dietary selection was quantified from June through August by analyzing fecal samples with DNA metabarcoding techniques. In addition, daily sheep grazing time was monitored using triaxial accelerometer collars. Forage intake was estimated using an external marker (TiO₂) which dietary organic matter digestibility was calculated from crude protein (CP) concentrations in feces samples. Forage consumption across the grazing strategies revealed that species from Poaceae, Rosaceae, and Cyperaceae families were frequently consumed. Both ewes and lambs in the mixed-grazing preferentially consumed forbs with diverse species composition (Jacob's D > 0), which contained higher CP than those available in the overall vegetation (p < 0.05). In addition, dietary richness was significantly (p < 0.05) influenced by vegetation species diversity except for lambs grazing alone. Compared to lambs in the mixed-grazing, lambs grazed alone had both greater daily grazing time and consumption of grass with lower digestibility (p < 0.05). Our study demonstrate that lambs can develop similar dietary selection and behavioral pattern during grazing with adult ewes, which indicates that the conservation of species diversity in native grasslands is critically beneficial to livestock nutrition.