The present study evaluated the interaction of grain source (corn vs. barley) and ruminal undegradable protein: ruminal degradable protein ratios (RUP: RDP; 27:73 and 36:64) on growth performance and blood metabolites of Holstein calves. Forty three-day-old male calves with a body weight of 42.6 ± 2.9 kg were used in a completely randomized design with a 2 × 2 factorial arrangement of treatments. Treatments were: (1) corn grain with low RUP: RDP; (2) corn grain with high RUP: RDP; (3) barley grain with low RUP: RDP; (4) barley grain with high RUP: RDP. Calves were weaned on d 53 of age but remained in the study until d 63. The results showed that starter intake was similar among treatments. The average daily gain and feed efficiency were improved in calves fed barley grain (P = 0.04). Wither height was improved in high RUP: RDP fed calves. Hip height was improved in barely fed calves (P = 0.06). Blood concentration of beta hydroxyl butyric acid was 0.33 mmol and greater in barley fed calves (P = 0.02). The total protein level was greater in calves fed higher RUP: RDP. Barley fed calves had lower blood concentration of blood urea nitrogen (P = 0.01). The blood insulin concentration was increased in barley fed calves along with increase in RUP: RDP (P = 0.02). In conclusion, results indicated that feeding starter contained barley grain and high ratio RUP: RDP and also starter contained corn grain with low RUP: RDP is recommendable for starter diet of dairy calves.
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