摘要Recent research has demonstrated that rumen-protected taurine supplementation improves body protein turnover,apparent nitrogen retention(ANR)and nitrogen(N)utilization efficiency(NUE)in beef steers.To further elucidate taurine's role in N metabolism,it is essential to examine whether taurine depletion adversely affects ANR and NUE.Six beef steers(bodyweight 391±10 kg)were allocated in a replicated 3×3 Latin square design.Each experimental period was 20 d,including 15 d for adaptation and 5 d for sampling.Three levels of rumen-protected β-alanine(RPβA,a taurine inhibitor),i.e.,0,17.5,and 35 g/d,were added to the basal diet as dietary treatments.The results showed that RPβA supple-mentations at 17.5 and 35 g/d linearly decreased the plasma taurine concentrationby 12.54％and 22.54％(P=0.026),and the urinary taurine excretion by 15.78％and 21.05％(P＜0.001),respectively,while linearly increased ANR(P＜0.001)and NUE(P＜0.001)in steers.Rumen-protected β-alanine supple-mentation linearly increased the plasma concentrations of methionine(Met,P＜0.001),lysine(Lys,P=0.018),threonine(Thr,P=0.011),leucine(Leu,P=0.042)and histidine(His,P=0.061),as well as growth hormone(P＜0.001),insulin-like growth factor-1(P＜0.001),and the total antioxidant capacity(P＜0.001).Rumen-protected β-alanine supplementation tended to decrease the skeletal muscle protein degradation rate(P=0.055).Specifically,supplementation with 35 g/d RPβA upregulated the plasma amino acid derivatives and oligopeptides,including N-linoleoyl-histidine(P＜0.001),L-Met(P＜0.001),L-4-chlorotryptophan(P=0.006),L-Thr(P=0.022),L-Lys(P=0.026),L-carnitine(P=0.038),suberic acid(P=0.036),formyllysine(P=0.036),N-acetyltyrosine(P=0.042),histidylglycine(P=0.045),and N-formyl-L-glutamic acid(P=0.047).Supplementation with 35 g/d RPβA also altered the muscle cell mRNA expression,upregulated hub genes(GADPH,PFKM,TPII,PGK1,and PKM)and modified arginine-proline metabolism and the AMPK signaling pathway in beef steers.In conclusion,RPβA supplemen-tation effectively reduced the plasma taurine concentrations and improved the ANR and NUE in steers.These effects were mediated by modulation of plasma amino acid profiles and metabolomic pathways,which appear to counteract the negative impacts of taurine depletion on N metabolism.