Mammary uptake of fatty acids supplied by intravenous triacylglycerol infusion to lactating dairy cows.

Supplementing dairy cows with n-3 fatty acid-rich feeds does not easily increase quantities in milk fat. Previous results demonstrated very long-chain n-3 fatty acids are primarily transported in the PL fraction of blood, making them largely unavailable to the mammary gland for enrichment of milk fat. Our objective was to compare mammary uptake of fatty acids of increasing chain length and unsaturation delivered intravenously as TAG emulsions. Late lactation dairy cows were assigned to a completely randomized block design. Treatments were intravenous TAG emulsions enriched with oleic acid (OLA), linoleic acid (LNA), alpha-linolenic acid (ALA), or docosahexaenoic acid (DHA) and were delivered continuously at 16 mL/h for 72 h. Each treatment supplied 30 g/day of the target fatty acid. Treatment did not affect feed intake, milk yield, or milk composition, but all treatments reduced intake and yield. The proportion of DHA increased in plasma FFA, TAG, and PL with infusion. Increases of n-3 fatty acids, ALA, EPA, and DHA, were evident in the plasma PL fraction, suggesting re-esterification in the liver. Transfer efficiencies were 37.8 ± 4.1, 27.6 ± 5.4, and 10.9 ± 4.1 %, and day 3 total milk fatty acyl yields were 37.0 ± 3.4, 10.8 ± 0.4, and 3.3 ± 0.3 g for LNA, ALA, and DHA. Variation in oleic acyl yield prevented calculation of OLA transfer efficiency. Mammary uptake of fatty acids was reduced with increased chain length and unsaturation. Both liver and mammary mechanisms may regulate transfer of long-chain polyunsaturates.