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Essential oils (EOs) from plant extracts have been reported to have an antibacterial activity against gram-negative and gram-positive bacteria. Several of the gram-positive bacteria are involved in ruminal biohydrogenation of fatty acids (FAs), thus suggesting that feeding EOs could lower biohydrogenation of FA because of a decrease in the number of bacteria involved in that process. As a result, milk FA profiles are expected to be modified. In addition, monensin was approved as an antibiotic to be fed in dairy cattle, and it was reported that dairy cows supplemented with monensin produced milk containing higher concentration of 18:1 t10 and 18:1 t11. The objective of this study was to evaluate the effects of two EOs (garlic and juniper berry oils) and monensin on FA profiles of milk fat. Four ruminally fistulated Holstein dairy cows were used in a 4 × 4 Latin square experiment. Cows were fed for ad libitum intake a total mixed ration without supplementation (control), or supplemented with monensin (330 mg/head per day), garlic oil (5 g/head per day), or juniper berry oil (2 g/head per day). The FA composition of saturated, monounsaturated, or polyunsaturated was not affected by supplementation of EO and monensin. However, proportion of conjugated linoleic acid trans 10, cis 12 (CLA t10, c12) was higher (P < 0.05) for cows fed EO or monensin than for control cows. Supplementation of monensin increased (P < 0.05) the proportion of total trans FA compared with the control. These results indicate that supplementation of the dairy cow diet with garlic or juniper berry EO or monensin had the potential to increase the proportion of CLA t10, c12 in milk fat with minimal overall effects on FA of milk fat. The results also confirm the increase of 18:1 t10 in milk fat by feeding monensin to dairy cows.
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