Effects of Feeding Grapevine Branch–Leaf Silage on Growth Performance, Serum Biochemical Parameters, Rumen Microbial Diversity, and Metabolism in Kazakh Rams

Grapevine branch and leaf silage (GBLS), a polyphenol-rich unconventional forage, exhibits antimicrobial and antioxidant properties that can benefit animal health and productivity. A total of 60 healthy six-month-old Kazakh rams (43.29 ± 4.55 kg, p > 0.05 for initial body weight among groups) were randomly assigned to three dietary groups, each consisting of four replicates with five rams per replicate. The control group (CK) was fed a basal diet based on whole-plant corn silage, whereas the experimental groups received diets in which 50% (GBLS50%) or 100% (GBLS100%) of the corn silage was replaced with GBLS. A 10-day adaptation period preceded the 90-day formal feeding trial. Results showed a significant quadratic response for average daily gain (ADG) and average daily feed intake (ADFI) across GBLS substitution rates (p < 0.05), with the 50% level yielding the highest values. Specifically, ADFI at the 50% replacement level was significantly higher than that of the control (p < 0.05), confirming an inverted U-shaped response with 50% as the optimal substitution rate. However, in-depth analysis of serum biochemical parameters revealed that GBLS supplementation significantly reduced serum concentrations of total cholesterol, triglycerides, urea nitrogen, interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and malondialdehyde (MDA), while significantly increasing levels of immunoglobulins (IgA, IgM, IgG), superoxide dismutase (SOD), and catalase (CAT) (p < 0.05). Rumen fermentation analysis showed that the GBLS50% group had significantly lower concentrations of acetate, butyrate, and total volatile fatty acids (VFA) (p < 0.05). In the rumen microbiota study, no significant differences were observed in alpha or beta diversity or at the phylum level between groups (p > 0.05); however, the abundance of Lactobacillus gasseri was significantly reduced in the GBLS50% group (p < 0.05). Metabolomic profiling identified 43 significantly altered metabolites—27 upregulated (e.g., PE (18:1(9Z)/0:0) and 12,14-pentacosadiynoic acid) and 16 downregulated (e.g., deoxyadenosine). Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis highlighted purine metabolism as a significantly altered pathway (p < 0.05), providing insight into the potential metabolic mechanisms underlying the physiological effects of GBLS in rams. In conclusion, replacing 50% of whole-plant corn silage with grapevine branch and leaf silage improves growth performance trends and significantly enhances immunity and antioxidant capacity in Kazakh rams.