饲粮中添加缓释尿素对肉牛瘤胃发酵特性与氮代谢的影响OA

This study aimed to investigate the effects of replacing soybean meal in the diet with slow-release urea(SRU)at an isonitrogenous level on the growth performance,rumen fermentation,nitrogen metabolism,and rumen microbial community structure of beef cattle.A total of 18 healthy Simmental bulls(18-19 months old,weighing(642.90±15.79)kg)were randomly assigned to three groups,with six replicates per group and one bull per replicate,where the control group(CON)was fed a basal diet,while the low-dose SRU group(LSRU)and high-dose SRU group(HSRU)were provided with experimental diets formulated to be isoenergetic and isonitrogenous by replacing soybean meal with 0.36％and 0.72％SRU respectively.A 14-day adaptation period and a 60-day formal trial period were implemented.On the final day of the trial,rumen fluid samples were collected before the morning feeding to determine rumen fermentation parameters and rumen microbial communities.Subsequent to the formal trial,a 5-day digestion and metabolism experiment was conducted,where the total feces and urine collection method was adopted to measure nitrogen metabolism indices of beef cattle.The results showed that:1)Dietary supplementation with two doses of slow-release urea did not significantly alter the productive performance of beef cattle(P＞0.05).but there was an increasing trend in daily weight gain(ADG)in the LSRU group(P=0.058).2)Supplementation with slow-release urea at both doses led to a significant increase in ruminal ammonia nitrogen(NH3-N;P=0.004)and microbial crude protein(MCP;P=0.050)concentrations.3)Both doses improved nitrogen digestibility(P=0.018)and showed a tendency to reduce fecal nitrogen excretion(P=0.075),while the LSRU group can avoid the significant increase in urinary nitrogen caused by the HSRU group(P=0.021).4)Microbiome analysis revealed that the HSRU group had significantly higher ACE and Chao1 indices than the CON and LSRU groups(P=0.020),and the abundance of Bacteroidetes in the HSRU group was significantly reduced(P＜0.001),while the abundance of Firmicutes(P=0.003)was significantly increased.At the genus level,the abundance of Unclassified F082 was substantially lower(P＜0.001),whereas Ruminococcaceae UCG_011 was significantly enriched(P=0.003)in the HSRU group.In conclusion,appropriate supplementation of slow-release urea in beef cattle effectively promotes ruminal microbial protein synthesis,improves dietary nitrogen digestibility,and enhances daily weight gain without disrupting the structure of the rumen microbial community.However,the high-level supplementation increased ruminal ammonia-N concentration,microbial protein yield,and the abundance of fiber-degrading bacteria.The abundance of core bacteria phyla and genera changed,but daily weight gain decreased and urinary nitrogen excretion significantly increased.Therefore,the suitable inclusion level of slow-release urea during the finishing phase of beef cattle is recommended to be 0.36％of the diet.