1Department of Animal Sciences, College of Agriculture, Ilam University, Ilam, Iran
2Department of Animal Sciences, College of Agriculture, Shiraz University, Shiraz, Iran
3Graduated from College of Agriculture, Ilam University, Ilam, Iran
4Department of Animal Sciences, College of Agriculture, Gilan University, Rasht, Iran
The objective of this study was to determine the effects of feeding different levels of monensin on feed intake, milk production and composition, and milk fattyacid profile in lactating Holstein cows. Four multiparous cows averaging 517 ± 47 (SD) kg in body weight and 101 ± 19.8 (SD) days in milk were housed individually in tie-stalls. The study was conducted as a 4 × 4 Latin square design for four periods (14-d for adaptation and 7-d for sampling). Cows were offered four dietary treatments (0, 10, 20, or 30 mg of monensin/kg of DM) as total mixed ration, twice daily. Dry matter (DM) intake was similar among treatments. Monensin supplementation significantly increased (P<0.05) milk yield and 4% fat corrected milk (FCM). Milk fat and protein percentages were not affected by monensin supplementation, but fat yield was increased. Monensin reduced the percentage of the short-chain and saturated fatty acids in milk fat, but had no effect on the percentages of medium- and long-chain fatty acids. Monensin supplementation increased (P<0.05) unsaturated fatty acids concentrations in milk fat. Based on the results of this study, feeding monensin was effective in inhibiting the biohydrogenation of unsaturated fatty acids in the rumen, and consequently increased the percentageof unsaturated fatty acids in milk fat, which improves the health characteristics of milk for human consumption.