Effects of Dietary Active Dry Yeast Supplementation on Lactation Performance and Rumen Fermentation in Holstein Cows

Active dry yeast (ADY) as a feed additive has been proven to have functions such as enhancing feed digestion and utilization rates, strengthening the body's immunity, alleviating stress and improving rumen fermentation. This experiment mainly explores the effects of a new type of ADY on the production performance, rumen fermentation and nutrient digestibility of dairy cows. A total of 100 Holstein cows with similar milk yield (46.9 ± 8.5 kg/d), days in milk (66.7 ± 5.4 d), and parity (2.5 ± 1.0) were selected and randomly allocated into four groups (25 cows per group). The treatments were as follows: (1) Control group (CON), cows fed the basal diet only; (2) ADY I group (ADY I), basal diet supplemented with 5 g/d of common ADY (RF7, 1 × 10¹⁰ CFU/g) per cow; (3) Low ADY II group (LADY II), basal diet supplemented with 5 g/d of Actisaf Sc48 (viable yeast cell concentration is 1 × 10¹⁰ CFU/g) per cow; (4) High ADY II group (HADY II), basal diet supplemented with 10 g/d of Actisaf Sc48 per cow. The 10-week experimental period consisted of a 1-week adaptation period followed by a 9-week formal experimental period. The results showed that, compared with the CON group, mean dry matter intake (DMI) increased by 3.48%, 5.35%, and 6.96% in the ADY I, LADY II, and HADY II groups, respectively (p < 0.05), with a significant treatment × time interaction (p < 0.001). Compared with the CON and ADY I groups, the HADY II group showed increases in milk yield of 5.76% and 4.76% and in apparent crude protein (CP) digestibility of 5.22% and 3.17%, respectively (p < 0.05). Milk lactose concentration was 3.43% higher in the HADY II group than in the ADY I group (p < 0.05). Additionally, milk linolelaidic acid concentration was lower in LADY II and HADY II than in CON (p < 0.05). Compared with the CON group, the LADY II and HADY II groups showed increases in ruminal total volatile fatty acid (TVFA) concentrations of 42.11% and 44.72% and in acetate concentrations of 45.76% and 46.71%, respectively (p < 0.05). In this trial, the group receiving ADY II at 10 g/cow/day showed the best overall performance. However, this finding does not establish 10 g/cow/day as the optimal inclusion level for this product. Future studies should compare the two products at matched doses and integrate multi-omics analyses with functional validation and economic evaluation to clarify their mechanisms of action and guide practical supplementation.