低蛋白质饲粮中添加缓释尿素对肉牛生长性能、血清生化指标、瘤胃发酵参数及微生物区系的影响OA

This experiment aimed to investigate the effects of low-protein diet supplemented with slow-release urea on the growth performance,serum biochemical indices,rumen fermentation parameters and microflora of beef cattle.Eighteen Simmental crossbred bulls with similar body weight[(521.86±22.86)kg]and in good health were selected and randomly divided into 3 groups,with 6 replicates in each group and 1 bull per repli-cate.The control group(CON group)was fed a basal diet with a protein level of 12.50%,the low-protein group(LP group)received a low-protein diet with the protein level reduced to 9.50%,and the slow-release u-rea group(SRU group)was fed a diet formulated based on the LP group diet,where 0.6%slow-release urea was used to iso-nitrogenously replace part of the feed ingredients,restoring the protein level to 12.50%,which was consistent with the CON group diet.The experiment included a 15-day adaptation period followed by a 35-day experimental period.The results showed as follows:1)there were no significant difference in the average daily gain,dry matter intake,and feed-to-gain ratio of beef cattle among the groups(P＞0.05).2)The con-tent of serum urea nitrogen in the LP group was significantly lower than that in the CON and SRU groups(P＜0.05).3)Regarding rumen fermentation parameters,the ammonia nitrogen(NH3-N)content in the SRU group was significantly higher than that in the CON and LP groups(P＜0.05);the proportion of butyrate in the SRU group was significantly higher than that in the LP group(P＜0.05);the microbial protein(MCP)content and the proportion of isobutyrate in the LP group were significantly lower than those in the CON and SRU groups(P＜0.05).4)There were no significant differences in rumen microbial diversity among the groups(P＞0.05).At the phylum level,Bacillota and Bacteroidetes were the dominant phyla across all groups,the relative abundance of Patescibacteria was significantly higher in the LP group than in the CON and SRU groups(P＜0.05).At the genus level,dominant genera of all groups were Xylanobacter,Rikenellaceae_RC9_gut_group and norank_f__F082,the relative abundance of NK4A214_group in the LP group was significantly low-er than that in the CON and SRU groups(P＜0.05),and the relative abundance of Lachnospiraceae_NK3A20_group in the CON group was significantly higher than that in the LP and SRU groups(P＜0.05).Correlation analysis revealed that the relative abundance of the Lachnospiraceae_NK3A20_group in the rumen showed an extremely significant positive correlation with MCP content(r=0.611,P=0.007),and showed a significant positive correlation with the proportion of isobutyrate(r=0.520,P=0.027).In conclusion,under the conditions of this experiment,reducing dietary protein level altered the relative abundance of certain rumen bacteria in beef cattle,leading to decreased serum UN content and reduced rumen NH3-N and MCP contents,as well as changes in the proportions of butyrate and isobutyrate.Supplementation with slow-release urea can a-meliorate these effects without negatively impacting the health and growth performance of beef cattle.[Chinese Journal of Animal Nutrition,2026,38(3):2042-2055]