Abstract

Acetate is naturally produced in the rumen through feed degradation and fermentation. It serves as a primary energy source for ruminants and as a key substrate for de novo fatty acid synthesis in the mammary gland. The interaction of exogenous acetate with different animal and dietary factors is an area of growing interest, as it may have significant implications for milk fat synthesis. This study aimed to assess the effect of two diet fermentability levels on the short-term response of lactation to acetate supplementation in dairy cows. Eight ruminally cannulated multiparous European Holstein cows were randomly assigned to treatments in a crossover design that tested the effect of diet fermentability, acetate supply, and their interaction. Using corn silage as the only forage source and a constant forage-to-concentrate ratio, high-fermentability (HF) and low-fermentability (LF) diets were formulated. Acetate supply was investigated by infusing ruminally 10 moles of sodium acetate/d (ACE) or an equimolar infusion of control (CON). Therefore, the treatments were as follows: LF + CON; LF + ACE; HF + CON; and HF + ACE. No interactions between acetate and diet fermentability were found on performance variables. Acetate infusion decreased dry matter intake (DMI), milk yield, and milk protein yield and content but did not affect milk fat yield; however, it increased milk fat concentration, and this response tended to be more pronounced in the HF diet. Acetate infusions increased plasma beta-hydroxybutyrate in the HF diet, but not in the LF diet, and increased plasma non-esterified fatty acid, which was likely a lipolysis response to reduced DMI and decreased energy balance. This study demonstrates that acetate availability can be a constraint on mammary lipogenesis, even with adequate dietary fiber.