RESUMO
Fat supplementation of diets for dairy cows produces changes in nutrient supply and milk composition. The effect of abomasal infusion of either cis-C18:1 or trans-C18:1 fatty acid isomers on the digestibility of fatty acids and milk composition was determined in lactating dairy cows. Six multiparous midlactation Holstein cows were used and fed a control diet containing 50% forage and 50% concentrate. Treatments were (per day): no infusion, infusion of a 630-g fat mixture high in cis-C18:1 isomers, and infusion of a 623-g fat mixture high in trans-C18:1 isomers using two 3 x 3 Latin squares with 4-wk experimental periods. Fat infusion did not affect total dry matter intake and increased apparent digestibilities of total fatty acids. Apparent digestibilities of C18 fatty acids were directly related to the number of double bonds within isomers, and cis-C18:1 isomers were slightly more digestible than trans-C18:1 isomers. The lower yield of C12:0, C14:0, and C16:0 fatty acids in milk fat and higher milk citrate observed when cows were infused with trans-C18:1 suggests a depressed de novo milk fatty acid synthesis. Effects of trans infusion on milk fat were independent of ruminal fermentation, fatty acid apparent absorption, and fatty acid plasma concentrations. Lower milk protein yield in cows infused with fat may have been caused by a decrease in milk protein synthesis.
