The effect of increasing dietary fat concentration on mammary arteriovenous fatty acid difference and milk fat yield in Holstein cows.

Dairy producers invest in fatty acid (FA) supplements to increase energy intake and maximize milk fat yield, but the effect of dietary fat level on mammary FA uptake and metabolism are not well described. Our objective was to characterize mammary arteriovenous (AV) difference and extraction percentage of total plasma FA, triglycerides (TG), and nonesterified FA (NEFA) at 2 levels of dietary fat. The hypothesis was that mammary AV difference of palmitic acid (16:0), stearic acid (18:0), TG, and NEFA would be greater in cows fed a higher fat diet compared with a lower fat diet due to increased arterial concentration and therefore substrate availability of these FA. The secondary hypothesis was that mammary extraction percentage of FA would increase as arterial concentration of FA increased, suggesting a concentration-dependent mechanism of uptake. Nine multiparous Holstein cows were arranged in a crossover design with 14 d periods. Treatments were a lower fat diet (LF; 2.7% FA) or a higher fat diet that contained roasted soybeans, whole cottonseed, and a prilled FA supplement (HF; 4.9% FA). Data were analyzed in a mixed model that included the random effect of cow and period and fixed effect of treatment. For plasma analysis, the fixed effect of time and interaction of treatment and time were added to the model. There was no effect of treatment on milk or milk protein yield. Feeding the HF diet increased milk fat percent 0.26 units and milk fat yield by 88 g/d, with an increase in the yield of > 16 C preformed and 16 C mixed source FA. The HF diet increased the proportion of 16:0, 18:0, and linoleic acid (18:2 n-6) in both arterial and venous plasma FA and decreased the proportion of oleic (18:1 c9) and α-linolenic (18:3 n-3) acid. It also increased total arterial FA concentration by 21%. Approximately half of the increase in arterial FA was due to 18:2 n-6 (+ 232 mg/L), while smaller increases were also observed for 16:0, 18:0, 18:1 c9, and 18:3 n-3. There was a treatment by time interaction for the AV difference of total FA and 18:1 c9 and a tendency for an interaction for 18:0 and 18:2 n-6, where there was a greater AV difference between treatments at 1600 h compared with other time points. The HF diet also increased the AV difference of 16:0, but there was no interaction of treatment and time. There was no treatment or treatment by time interactions for AV difference of TG and NEFA. There was an interaction of treatment and milking time for milk fat percent that aligned with total plasma FA AV data, where the difference between HF and LF was greater at the evening milking compared with the morning. In conclusion, increasing dietary fat increased milk fat yield through an increase in total plasma FA concentration and an increase in AV difference of specific FA with variation in the response across the day.