Investigation on the action mechanisms of taurine on rumen microbial protein synthesis and nitrogen metabolism in beef steers using sodium sulfate as a contrast

Taurine is a sulfur-containing nonproteinogenic amino acid. Recent studies have shown that taurine can improve rumen microbial crude protein (MCP) synthesis. This experiment aimed to investigate the action mechanism of taurine on rumen MCP synthesis and nitrogen (N) metabolism in beef steers using sodium sulfate (Na 2 SO 4 ) as a contrast. Six steers (bodyweight 506 ± 17 kg) were assigned to three experimental groups including a basal diet (control), a basal diet supplemented with 45 g taurine/d or 50 g Na 2 SO 4 /d, and were allocated in a replicated 3 × 3 Latin square design. The amounts of sulfur from taurine and Na 2 SO 4 were equal (11.38 g/d). The results showed that, compared with the control group, both taurine and Na 2 SO 4 increased ruminal production of MCP ( P < 0.05) by 37.50% and 29.17%, respectively, and increased the concentration of sulfide (S 2- ) ( P < 0.001). Both taurine and Na 2 SO 4 increased the digestibility of neutral detergent fiber ( P < 0.05). Taurine tended to increase ( P = 0.087) while Na 2 SO 4 decreased plasma urea concentration ( P = 0.049), with the taurine group exhibiting higher plasma urea concentrations than the Na 2 SO 4 group ( P = 0.003). Taurine did not affect urinary urea excretion ( P = 0.246) whereas Na 2 SO 4 decreased urinary urea excretion compared with the control group ( P = 0.002). Compared with the control group, both taurine and Na 2 SO 4 increased urinary allantoin excretion ( P < 0.05), total purine derivatives excretion ( P < 0.05), and the estimated rumen microbial N flow ( P < 0.05). The urinary urea excretion of the taurine group was higher than the Na 2 SO 4 group ( P = 0.019). Compared with the control group, taurine did not affect N excretion, N retention (NR) or N utilization efficiency (NUE) ( P > 0.10), but Na 2 SO 4 decreased urinary N excretion ( P = 0.018) and total N excretion ( P = 0.024), and increased NR ( P = 0.024) and NUE ( P = 0.022). No differences were found in NR and NUE between the taurine and Na 2 SO 4 groups ( P > 0.10). Taurine improved ruminal MCP synthesis by enriching the pathways associated with sulfur and amino acid metabolism while Na 2 SO 4 improved ruminal MCP synthesis by enriching pathways related to nucleotide and purine metabolism. In summary, both taurine and Na 2 SO 4 improved ruminal MCP synthesis by modulating different pathways. Taurine was less effective in decreasing total N excretion than Na 2 SO 4 but no differences in NR and NUE were found between the two groups of beef steers.