Abstract
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 mechanisms 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 of 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 MCP concentration(P < 0.05) by 37.50% and 29.17%, respectively, and increased ruminal sulfide (S(2-)) concentration (P < 0.001). Both taurine and Na(2)SO(4) increased neutral detergent fiber digestibility (P < 0.05). Taurine tended to increase (P = 0.087) while Na(2)SO(4) decreased (P = 0.049) plasma urea concentration, while the taurine group exhibiting higher plasma urea concentration than the Na(2)SO(4) group (P = 0.003). Compared with the control group, taurine did not affect urinary urea excretion (P = 0.246) whereas Na(2)SO(4) decreased urinary urea excretion (P = 0.002) and both taurine and Na(2)SO(4) increased urinary allantoin excretion (P < 0.05), total purine derivatives excretion (P < 0.05), and 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 conclusion, 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 treatments.