Yeast Additive Effects on Dry Matter Intake, Milk Production, Milk Composition, and Ruminal Metabolism in Lactating Dairy Cattle.

Saccharomyces cerevisiae yeast products are used to stabilize dairy cattle rumen environments. In this study, multiparous lactating Holstein-Jersey cross cows in late lactation (n = 12; ±150 DIM) were used in a replicated 4 × 4 Latin square design over four 21 d periods to evaluate yeast supplementation on dry matter intake (DMI), milk production, milk components, rumen pH, and redox potential (Eh). A subset of four ruminally cannulated cows were selected for rumen measurements. Treatments included no additive (CON), 14 g yeast culture additive (YCA), 5 g active dry yeast (ADY), and 5 g ADY + 5 g yeast extract additive (YEA) top-dressed once daily. Data were analyzed using GLIMMIX in SAS 9.4, with significance at p ≤ 0.05 and tendencies at p ≤ 0.15. Dry matter intake did not differ among treatments (p > 0.15) but decreased over time (p < 0.05). Milk production, efficiency, lactose, and total solids were not affected (p > 0.15). YCA tended to increase milk fat and fatty acid fractions compared with CON (p < 0.15). Protein in milk was greater in CON (p < 0.05; p < 0.05) than YCA and YEA. Active dry yeast treatments increased the frequency of a buffered rumen (p < 0.05) and promoted a more reducing ruminal environment, characterized by lower redox potential and conditions favorable for anaerobic fermentation (p < 0.05). Overall, supplementing active dry yeasts to dairy cows in late lactation did not affect DMI, production efficiency, or milk quality, but may maintain rumen pH stability.