Abstract
The objectives of the experiment were to compare the effects of rumen-protected taurine (RPT) and rumen-protected methionine (RPM) on the nitrogen (N) metabolism, plasma biochemical parameters, and metabolomics in beef steers and to clarify whether taurine plays similar roles as methionine (Met) in the regulation of N metabolism in beef steers. Six Simmental steers aged 12 months (liveweight 325 ± 7 kg) were used as experimental animals. The experimental treatments included a basal diet, the basal diet + 70.0 g/d RPT and the basal diet + 74.2 g/d RPM. The treatments were assigned in a replicated 3 × 3 Latin square design. Each experimental period included 15 d for adaptation and 5 d for sampling. The results showed that supplementing the diet with RPT or RPM did not affect the apparent nutrient digestibility (P > 0.05). Supplementing the diet with RPT or RPM increased the N retention (P < 0.05) and the N utilization efficiency (NUE) (P < 0.05) and decreased the urinary excretion of 3-methylhistidine (P < 0.05) and the estimated skeletal protein degradation rate (P < 0.05). Supplementing the diet with RPT increased the plasma concentrations of taurine (P < 0.001), cysteine (P = 0.010), valine (P = 0.013) and total non-essential amino acids (NEAA) (P = 0.047) and tended to increase the plasma concentrations of essential amino acids (EAA) + NEAA (P = 0.087), but it did not affect the plasma concentrations of total EAA (P > 0.05). Supplementing the diet with RPM increased the plasma concentrations of methionine (P = 0.033), lysine (P = 0.047), cysteine (P = 0.007), leucine (P = 0.046), isoleucine (P = 0.046), valine (P = 0.034), total EAA (P = 0.028), total NEAA (P = 0.004) and EAA + NEAA (P = 0.004). The plasma metabolomics profiling revealed that supplementing the diet with RPT upregulated the plasma concentrations of taurine (P < 0.001), L-cysteine (P = 0.004) and some amino acid (AA) analogues (P < 0.05) and RPM upregulated the plasma concentrations of Met (P = 0.021), L-isoleucine (P = 0.036), L-tryptophan (P = 0.006) and some AA analogues (P < 0.05). In conclusion, taurine has similar impacts to Met in improving the N retention and the NUE in beef steers. Taurine deficiency negatively affects the NUE of beef steers. Supplementation of the diet with taurine is beneficial to the N utilization in beef steers.