This study evaluated the effects of rumen‐protected folic acid (RPFA) supplementation and dietary protein level on growth performance, ruminal fermentation, nutrient digestibility and hepatic gene expression in calves. Forty Holstein male calves (161 ± 5.7 days of age and 192 ± 5.4 kg of body weight) were assigned to one of four groups in a randomized experimental design with a 2 × 2 factorial arrangement. Moderate crude protein (130.1 g CP/kg [MCP] or high crude protein (150.2 g CP/kg [HCP]) diets were fed without (RPFA−) or with 3.6 mg FA (RPFA+) as RPFA per kg dietary dry matter (DM). Calves were fed a total mixed ration with a corn silage to concentrate ratio of 50:50 on a DM basis. The CP×RPFA interaction was not significant for any of the studied variables. The unchanged DM intake, higher average daily gain and lower feed conversion ratio were observed for HCP or RPFA+. Ruminal pH was lower, and total volatile fatty acids (VFA) concentration was higher for HCP or RPFA+. Acetate proportion was higher, and propionate proportion was lower for HCP or RPFA+. As a result, the higher acetate to propionate ratio was observed. Ruminal ammonia N was higher for HCP, but was lower with RPFA supplementation. The higher digestibility of DM, OM, CP and NDF was observed. Blood glucose and insulin were unchanged, but albumin, total protein, GH and IGF‐1 were higher. Similarly, the higher hepatic expression of GH, IGF‐1, GHR, IGF‐1R, PI3K, mTOR and P70S6K was observed for HCP or RPFA+. The results indicated that increasing dietary CP content or supplementation with RPFA promoted growth performance of calves by improving nutrient utilization and up‐regulating hepatic expression of gene related to protein synthesis.

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