Long-term effects of feeding monensin on milk fatty acid composition in lactating dairy cows.

The objective of this study was to determine the long-term effects of feeding monensin on milk fatty acid (FA) profile in lactating dairy cows. Twenty-four lactating Holstein dairy cows (1.46 +/- 0.17 parity; 620 +/- 5.9 kg of live weight; 92.5 +/- 2.62 d in milk) housed in a tie-stall facility were used in the study. The study was conducted as paired comparisons in a completely randomized block design with repeated measurements in a color-coded, double blind experiment. The cows were paired by parity and days in milk and allocated to 1 of 2 treatments: 1) the regular milking cow total mixed ration (TMR) with a forage-to-concentrate ratio of 60:40 (control TMR; placebo premix) vs. a medicated TMR [monensin TMR; regular TMR + 24 mg of Rumensin Premix per kg of dry matter (DM)] fed ad libitum. The animals were fed and milked twice daily (feeding at 0830 and 1300 h; milking at 0500 and 1500 h). Milk samples were collected before the introduction of treatments and monthly thereafter for 6 mo and analyzed for FA composition. Monensin reduced the percentage of the short-and medium-chain saturated FA 7:0, 9:0, 15:0, and 16:0 in milk fat by 26, 35, 19, and 6%, respectively, compared with the control group. Monensin increased the percentage of the long-chain saturated FA in milk fat by 9%, total monounsaturated FA by 5%, total n-6 polyunsaturated FA (PUFA) by 19%, total n-3 PUFA by 16%, total cis-18:1 by 7%, and total conjugated linoleic acid (CLA) by 43% compared with the control group. Monensin increased the percentage of docosahexaenoic acid (22:6n-3), docosapentaenoic acid (22:5n-3), and cis-9, trans-11 CLA in milk fat by 19, 13, and 43%, respectively, compared with the control. These results suggest that monensin was at least partly effective in inhibiting the biohydrogenation of unsaturated FA in the rumen and consequently increased the percentage of n-6 and n-3 PUFA and CLA in milk, thus enhancing the nutritional properties of milk with regard to human health.