Eugenol for dairy cows fed low or high concentrate diets: Effects on digestion, ruminal fermentation characteristics, rumen microbial populations and milk fatty acid profile

Four ruminally cannulated primiparous lactating cows were used in a 4 × 4 Latin square design experiment (28 d periods) with a 2 × 2 factorial arrangement of treatments to examine effects of eugenol supplementation, concentrate proportion of the diet and their interaction on digestion, ruminal fermentation, microbial populations, milk production and milk composition, including milk fatty acid (FA) profile. Cows were fed for ad libitum intake a low (LC) or high (HC) concentrate total mixed ration (TMR) without or with eugenol (50 mg/kg of dry matter (DM) intake. The forage:concentrate ratio was (DM basis) 650:350 and 350:650 for LC and HC, respectively. Adding eugenol to the diets had no effects on DM intake, digestion, ruminal fermentation, rumen microbial populations of bacteria and protozoa and milk performance. Increasing the concentrate proportion of the diet resulted in changes typical of cows fed high starch diets (i.e., lower ruminal pH and acetate:propionate ratio; shift in bacterial populations; lower milk fat and higher milk protein concentrations). The ratio t-11 18:1 to t-10 18:1 was not affected by dietary treatments, indicating no changes in the pathway of biohydrogenation of FA in the rumen. Despite the alteration of rumen function due to increasing the proportion of the concentrate of the diet, adding eugenol to these high or low concentrate diets did not modify digestion, ruminal fermentation and microbial populations, suggesting that the effect (i.e., antimicrobial) of eugenol is neither pH nor diet dependant. The lack of efficacy of eugenol at the dosage rate evaluated under the experimental conditions of this study suggests that this essential oil may have low potential for use as feed additive in dairy cow nutrition.