Acetate dose-dependently stimulates milk fat synthesis in lactating dairy cows

Background: Acetate is a short-chain fatty acid (FA) that is especially important to cows because it is the major substrate for de novo FA synthesis. However, the effect of acetate supply on mammary lipid synthesis is not clear. Objective: The objective of this experiment was to determine the effect of increasing acetate supply on milk fat synthesis in lactating dairy cows. Methods: Six multiparous lactating Holstein cows were randomly assigned to treatments in a replicated design to investigate the effect of acetate supply on milk fat synthesis. Treatments were 0 (control), 5, 10, and 15 mol acetate/d continuously infused into the rumen for 4 d. Rumen short-chain FAs, plasma hormones and metabolites, milk fat concentration, and milk FA profile were analyzed on day 4 of each treatment. Polynomial contrasts were used to test the linear and quadratic effects of increasing acetate supply. Results: Acetate increased milk fat yield quadratically (P < 0.01) by 7%, 16%, and 14% and increased milk fat concentration linearly (P < 0.001) by 6%, 9%, and 11% for 5, 10, and 15 mol acetate/d, respectively, compared with the control treatment. Increased milk fat yield predominantly was due to a linear increase in 16-carbon FAs (P < 0.001) and a quadratic increase in de novo synthesized FAs ( < 16-carbon FAs; P < 0.01), indicating that there was stimulation of de novo synthesis pathways. Apparent transfer of acetate to milk fat was 33.4%, 36.2%, and 20.6% for 5, 10, and 15 mol/d, respectively. Acetate infusion linearly increased the relative concentration of rumen acetate (P < 0.001) before feeding, but not after feeding. Acetate linearly increased plasma β-hydroxybutyric acid by 29%, 50%, and 78%, respectively, after feeding compared with the control treatment (P < 0.01). Conclusions: Increasing acetate supply to lactating cows increases milk fat synthesis, suggesting that nutritional strategies that increase ruminal acetate absorption would be expected to increase milk fat by increasing de novo FA synthesis.