Abstract: This study investigated the effects of different combinations of dietary concentrate∶forage ratio and small peptide supplementation on in vitro gas production, nutrient digestibility and ruminal fermentation parameters of yaks. The experiment comprised a factorial combination of three different concentrate∶forage ratios (40∶60, 50∶50, 60∶40; CFRs) and five different small peptide supplementation levels (0, 0.75%, 1.50%, 2.25%, 3.00%), making a total of 15 treatments, with 3 replicates per treatment. Fermentation samples were collected after 48 hours of fermentation. It was found that: 1) CFR had no significant effect on the degradation rate of neutral detergent fibre and acid detergent fibre (P>0.05). Across the range of small peptide supplementation levels, the degradation rate of neutral detergent fibre and acid detergent fibre displayed a quadratic pattern (P<0.01). The optimal small peptide supplementation treatments were 1.50% and 2.25%. There were significant interactions between CFR and the level of small peptides for gas production and the degradation rates of dry matter and crude protein (P<0.01). 2) With increase in CFR from 40∶60 to 60∶40, rumen pH decreased (P<0.01), while microbial protein increased (P>0.05). However, no significant changes in digestive parameters between CFR 50∶50 and 60∶40 were observed. A quadratic response pattern (P<0.01) was also observed for pH and microbial protein across the range of small peptide supplementation rates tested, with the optimal supplementation rate being 2.25%. A significant interaction was observed between CFR and the level of small peptide supplementation, for the concentration of ammonia nitrogen (P<0.05). 3) There was a significant interaction between CFR and the level of small peptide supplementation for rumen volatile fatty acid concentrations (P<0.01). Across the three tested CFRs, acetate∶propionate and the proportion of acetate, isobutyrate and isovalerate presented quadratic variation (P<0.05). The lowest value of acetate∶propionate ratio and lowest proportion of acetate occurred with 1.50% small peptide supplementation, and the highest proportion of isoacids occurred at 2.25%. These results showed that CFR and the level of small peptides affected gas production, nutrient degradation and ruminal fermentation parameters. In this research, the better CFRs were 50∶50 and 60∶40, and the optimal peptide levels were 1.50% and 2.25%.

Key words: small peptides, concentrate-forage ratios, yaks, in vitro fermentation