Objective: Growth and health indices in calves are critical factors for longevity, performance, health and economic efficiency of dairy herds. The aim of this study was to investigate the effects of coated methionine and guanidine acetic acid on liver enzyme levels, antioxidant enzymes, and nutrient digestibility in milk-fed Holstein calves.

Method: 48 Holstein calves with an average weight of 40.39 ± 0.67 kg were used in a completely randomized design with 4 experimental treatments including 1) basal diet without additives, 2) basal diet with 0.73% coated methionine (MET, based on dry matter intake), 3) basal diet with 1000 mg/d guanidine acetic acid (GAA), and 4) basal diet containing 0.73% MET and 1000 mg/d GAA, with 12 replicates in each treatment. The experiment was carried out from 5 to 84 days of age. Blood samples were taken at the beginning of the experiment, and on day 56, 70 (weaning), and 84 (trial end) for liver enzyme and antioxidant activity evaluation. The apparent nutrient digestibility was determined in 84 days of age using internal digestibility marker.

Results: The results of the study showed that coated methionine, GAA, and their combination had no significant effect on aspartate amino transferase concentrations on day 5, 56, and 70, however, on day 84, a significant increase in the level of this enzyme was observed in calves receiving MET, either alone or in combination with GAA. Alanine aminotransferase levels decreased on 5 days of age in calves fed GAA, MET and MET+ GAA (P<0.05), however, on day 84, a significant increase in level of this enzyme was observed in calves receiving GAA or GAA+MET compaired to the control group. At weaning, total antioxidant capacity level was lower with MET and GAA than control group nonsignificantly but malondialdehyde level in calves fed this supplements decreased significantly at weaning age (P<0.05). Digestibility of dry matter, organic matter and protein was not affected by the experimental treatments

Conclusions: The findings of this study showed that the combined supplementation of Met and GAA in milk-fed Holstein calves was associated with a decrease in the level of malondialdehyde enzyme and an increase in the level of aspartate aminotransferase and alanine aminotransferase indicating the impact of these compounds in reducing the oxidative stress during weaning time and increasing liver metabolic activity. Overall, the results of this study showed that GAA and its combination with MET can have a positive effect on some blood parameters such as aspartate aminotransferase and malondialdehyde in milk-fed calves, respectively. However, these additives did not affect the digestibility of nutrients. The findings of this study confirm that the combined consumption of MET and GAA can have positive effects on liver health indices and have positive effects on final body weight and daily weight gain in milk-fed calves.