Effects of monensin and essential oil blend supplementation on lactation performance and feeding behavior in dairy cows

The objective of this study was to evaluate the effects of supplementation with an essential oil blend (0.16 g/kg DM of carvacrol, eugenol, thymol, and capsaicin) and monensin (17.6 mg/kg DM TMR) on lactation performance, feeding behavior, and rumen fermentation in high-producing dairy cows. Sixty-four multiparous Holstein cows (89 ± 21 DIM and 729 ± 68 kg of BW at covariate period initiation), including 16 cannulated cows, and 32 gate feeders were enrolled in a study with a completely randomized design and a 2 × 2 factorial arrangement. Cows and gate feeders were randomly assigned to treatments (4 cannulated cows and 12 cows not cannulated, for a total of 16 cows, and 8 gate feeders per treatment). Cows were allowed 1 wk to acclimate to gate feeders followed by a 2-wk covariate period. During the acclimation and covariate periods, all cows were fed a diet containing the essential oil blend and monensin. Following the covariate period, cows were enrolled in a 10-wk treatment period during which cows were randomly assigned to 1 of 4 treatments: (1) a combination of the essential oil blend and monensin (EOB-MON), (2) the essential oil blend alone (EOB), (3) monensin alone (MON), or (4) neither the essential oil blend nor monensin (CON). Data were analyzed using a mixed model with week as a repeated measure and essential oil blend, monensin, week, and all their interactions as fixed effects. Cow (treatment) was included as a random effect. The average covariate period value of each variable was used as a covariate. A 3-way interaction was observed for DMI, where DMI was lowest for CON and MON in wk 1 but then was lowest for EOB in wk 4, 9, and 10. Supplementing the essential oil blend also decreased BW during wk 4 and 10 and tended to decrease BW change but increased the maximum pH in the rumen. Monensin tended to improve milk production and lactose yield but decreased milk fat concentration without affecting milk fat yield. Thus, feeding monensin appears to improve milk yield but maintain or increase component yields. However, feeding dairy cows a combination of carvacrol, eugenol, thymol, and capsaicin decreased DMI for 3 out of 10 wk at the expense of BW gain.