The objective was to evaluate interactions between starter protein (180 vs 200 g kg-1, DM basis) and a mixture of essential oils (EO; containing thymol, eugenol, vanillin, limonene and guaiacol) on growth, metabolic and ruminal functions of Holstein dairy calves. In a completely randomized 2 × 2 factorial design, 48 calves, 3 d-old (averaging BW 42.7 ± 1.9 kg) were allocated into groups fed the following diets: 1) 180 g kg-1 CP with no EO (180P-NEO); 2) 180 g kg-1 CP with EO (180P-EO); 3) 200 g kg-1 CP with no EO (200P-NEO); and 4) 200 g kg-1 CP with EO (200P-EO). The EO was supplemented as 1 g/kg of starter DM. Calves were fed ad libitum starter diet and were weaned at d 59 of age, but diets continued until d 80. There was no interactive effects of CP and EO on intake and growth. Pre-weaning feed efficiency tended to be increased for 200P-EO (P = 0.09). Average daily gain and feed efficiency during pre-weaning period as well as weaning weight were increased (P < 0.05) by EO, whereas wither height was increased by EO (P = 0.03) and tended to be increased for 200P vs 180P (P = 0.06). Post-weaning blood urea nitrogen concentration tended to be lower in 180P vs 200P (P = 0.08). Ruminal short chain fatty acids concentration was greatest in 200P-EO. The EO increased both butyrate (P = 0.02) and propionate proportions (P = 0.01) and reduced acetate proportional ratio (P < 0.01). Ruminal ammonia-N was tended to be lower in calves fed EO (P = 0.05) and was lower in those fed 180P vs 200P (P < 0.01). In conclusion, supplementation the starter diet with essential oil improved weight gain, growth and feed efficiency of dairy calves, irrespective of dietary protein content.
