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Dose response effect of acetate on milk fat synthesis in lactating dairy cows
Acetate is the main source of energy and substrate for milk fat synthesis in the dairy cow, however the effect of acetate supply on milk fat synthesis has not been investigated in high producing cows. The objective was to investigate the dose dependent effect of acetate on milk fat synthesis. Six ruminally cannulated multiparous lactating Holstein cows were randomly assigned to treatments in a 4x4 Latin square design. Treatments were 0 (control), 5, 10, and 15 moles/d of acetate. Acetate was neutralized to pH 6.1 with sodium hydroxide and continuously infused into the rumen in 5 L/d for 4 d with 7 d washout periods. Milk samples were collected daily. Blood and rumen samples were collected twice (AM and PM) on the last day of treatment. Time course data was analyzed as repeated measures in SAS and all other data was analyzed using JMP Pro. The model included the random effect of cow and period and the fixed effect of treatment. Rumen concentration of acetate (P < 0.01) and acetate:propionate ratio (P < 0.001) increased linearly before feeding (AM) and rumen concentration of acetate (P < 0.001), butyrate (P < 0.05) and total VFA (P < 0.01) increased linearly after feeding (PM) as acetate dose increased. Acetate infusions linearly increased rumen pH before feeding (P < 0.005). Dry matter intake, milk yield and protein yield and concentration were not affected by treatments. Acetate dose had a quadratic effect on milk fat yield (P < 0.001) and a linear effect on milk fat concentration (P < 0.001). Fat yield increased 7, 16 and 14% and fat concentration increased 6, 9 and 11% at 5, 10, and 15 moles/d compared to control, respectively. Acetate linearly increased yield and concentration of palmitic acid and yield of denovo synthetized fatty acids (both P < 0.001). Acetate infusions had no effect on plasma NEFA, glucose, glucagon, or insulin, but increased plasma βHBA linearly after feeding (P < 0.01). These results demonstrate that acetate supply has an impact on milk fat synthesis under normal dietary conditions and suggest that milk fat yield and concentration may be improved though dietary strategies that increase rumen acetate production.
Keywords: Acetate, milk fat synthesis, rumen infusion