Interaction of sodium acetate supplementation and dietary unsaturated fatty acid level on total tract digestibility, milk synthesis, and plasma metabolites in lactating dairy cows.

Milk fat is a major source of dairy farm income and methods to increase milk fat yield are of strong interest to producers and nutritionists. Sodium acetate supplementation increases milk fat in lactating cows, but dietary factors that impact ruminal acetate synthesis may modify the response to acetate supplementation. Our objective was to test the interaction between acetate supplementation and dietary UFA concentration. The hypothesis was that acetate would increase milk fat yield more when supplemented in diets containing high levels of UFA compared with low levels of UFA. Twelve multiparous Holstein cows were randomly assigned to treatment sequence in a replicated 4 × 4 Latin square design with 21 d periods and 2 × 2 factorial arrangement of treatments testing the effect of acetate (AC; 600 g/d of acetate as sodium acetate) and increasing UFA by the addition of soybean oil to the diet (1.5% diet DM). There was no effect of UFA, AC, or their interaction on DMI or total-tract DM or NDF digestibility. Increasing UFA increased total-tract fatty acid (FA) digestibility by 8 percentage units, but there was no effect of AC. There was no interaction between UFA and AC for plasma concentration of NEFA, BHB, insulin, or individual 16 and 18 C FA of interest, although there was a tendency for an interaction for plasma glucose. Overall, acetate tended to decrease plasma glucose and increased plasma BHB. Increasing UFA also increased plasma BHB and NEFA as well as plasma 16:0, 18:0, cis-9 18:1, 18:2 n-6, 18:3 n-3, and trans-18:1 FA. There was no interaction of AC and UFA for milk yield, milk fat concentration, or milk fat yield. Both AC and UFA increased milk yield. Acetate increased milk fat yield 140 g/d and milk fat percent 0.16 units, mainly due to an increase in mixed source 16 C FA and a smaller increase in de novo and preformed FA. Increasing UFA had no effect on milk fat concentration or yield. Transfer efficiency of both consumed and absorbed 18 C FA were decreased with UFA while AC increased transfer efficiency of consumed 18 C FA. The lack of interactions between UFA and AC throughout the experiment may have been due to the failure of UFA to decrease fiber digestibility. However, increased FA availability with UFA did decrease milk de novo FA yield and thus mammary total acetate needed for milk fat synthesis. Additionally, the lack of interaction between UFA and AC for milk fat production suggests that both the high and low UFA diets may have been limited in acetate production or that the mechanism of increased de novo FA synthesis when providing additional acetate as substrate is independent of the decrease in de novo synthesis that commonly occurs when increasing dietary FA.