Dose–response to eugenol supplementation to dairy cow diets: Methane production, N excretion, ruminal fermentation, nutrient digestibility, milk production, and milk fatty acid profile

• Increasing dietary concentrations of eugenol had no effect on enteric methane production.
• Nitrogen excretion (feces and urine) was unaffected by increasing dietary levels of eugenol.
• Milk production was not changed by feeding cows incremental amounts of eugenol.
• Milk fatty acid profile was not modified by including increasing doses of eugenol in the diet.

The objective of this study was to evaluate the effects of increasing dietary concentrations of eugenol on enteric CH4 production, N excretion, ruminal fermentation characteristics, nutrient digestibility, milk production, and milk fatty acid (FA) composition. For this purpose, eight ruminally cannulated multiparous lactating cows were used in a replicated 4 × 4 Latin square design (28-d periods). Cows were fed a total mixed ration without (0 mg/kg dry matter [DM]) or with (25, 50, 75 mg/kg DM) eugenol supplementation. Enteric methane production was measured using the sulfur hexafluoride (SF6) tracer gas technique. Dry matter intake, N excretion, and apparent total-tract digestibility of DM, organic matter, acid detergent fiber, neutral detergent fiber, and gross energy (GE) were not changed by increasing dietary doses of eugenol. Likewise, total volatile fatty acid (VFA) concentration was unaffected while of the individual VFA, only the molar proportion of branched-chain VFA decreased linearly with increasing eugenol levels. Diet supplementation with increasing doses of eugenol had no effect on ruminal ammonia concentration, but tended (P = 0.08) to increase linearly total protozoa number. Milk production and milk concentrations of protein, lactose, and urea nitrogen were unchanged, whereas milk fat yield was quadratically affected by increasing dietary levels of eugenol (1.38, 1.32, 1.31, and 1.33 kg/d for 0, 25, 50 and 75 mg/kg eugenol, respectively). Diet supplementation with eugenol had minor effects on milk FA profile. Enteric CH4 emissions (g/d) and CH4 energy losses (as a proportion of GE intake) were not changed by including incremental levels of eugenol in the diet. Results from this study show no effects of supplementing dairy cow diets with eugenol (up to 75 mg/kg) on enteric CH4 emissions, N excretion, and milk performance. We concluded that at the dosage levels assessed in the current study (up to 75 mg/kg), eugenol cannot be promoted as an effective dietary approach to mitigate enteric CH4 emissions, decrease N excretion, and enhance feed efficiency in dairy cows.