Effects of commercial direct-fed microbials and zinc-methionine supplements on growth performance in preweaning Holstein calves under commercial rearing conditions.

This study was conducted to evaluate the effects of direct-fed microbials (DFM) and zinc-methionine (ZM) as well as their potential interaction and dose response of ZM as nutritional strategies to improve growth performance, blood metabolites, rumen fermentation characteristics, and apparent nutrient digestibility in preweaning Holstein calves. Seventy-two newborn Holstein calves (4 d of age, BW = 38.6 ± 6.27 kg) were assigned to a 2 × 3 factorial arrangement of treatments with DFM at 2 levels (0 or 5 g/calf/d) and ZM at 3 levels (0, 422, and 632 mg/calf/d). Main effects of DFM and ZM increased body weight, final body weight and dry matter intake. As ZM dose increased, body weight increased both linearly and quadratically, while final body weight increased quadratically and dry matter intake increased linearly. Main effect of DFM increased average daily gain. Withers height increased with main effects of DFM and ZM, whereas heart girth increased with main effects of DFM and ZM and also DFM × ZM interaction. Hip width increased by ZM main effect. Zinc-methionine level increased heart girth both linearly and quadratically but increased hip width quadratically and withers height linearly. In plasma, DFM main effect increased glucose, total protein, and cholesterol, whereas ZM main effect increased albumin. Plasma albumin increased linearly in response to ZM level. According to our results, DFM and ZM can be considered as effective additives to improve growth performance in preweaning dairy calves. No consistent evidence was found that combined DFM and zinc supplementation might enhance calf performance.