Abstract: [Objective] To investigate the effects of compound probiotics fermented feed on growth performance, rumen fermentation parameters and serum biochemical indices of beef cattle. [Methods] Sixty healthy Simmental crossbred beef cattle with body weight of （320±15） kg were selected and randomly divided into four groups with 15 cattle in each group. The control group （group Ⅰ） was fed a basal diet, and the experimental groups Ⅱ,Ⅲ and Ⅳ were fed the basal diet supplemented with 3%, 6% and 9% compound probiotics fermented feed, respectively. The pre-experimental period lasted 14 days and the formal experimental period lasted 60 days. During the experiment, the growth performance, rumen fermentation parameters and serum biochemical indices of beef cattle were measured, and the suitable supplementation ratio of compound probiotics fermented feed was explored. [Results] Regarding growth performance, compared with group Ⅰ （control group）, the average daily gain （ADG） of group Ⅱ （3%） and group Ⅳ（9%） increased slightly （P>0.05）, and the average daily gain of group Ⅲ（6%） increased significantly （P<0.05）. The feed-to-gain ratio （F/G） of group Ⅱ（3%）, group Ⅲ （6%） and group Ⅳ （9%） decreased by 5.14%, 12.39% and 7.76%, respectively. Among them, the F/G of group Ⅲ decreased significantly （P<0.05）, and there was no significant difference in the other two groups compared with the control group （P>0.05）. There was no significant difference in average daily feed intake （ADFI） among groups （P>0.05）. Regarding rumen fermentation parameters, the pH value of rumen fluid in each group was maintained at 6.35-6.48; compared with the control group, the ammonia nitrogen （NH₃-N） level in group Ⅲ significantly （P<0.05） decreased by 18.50%, the total volatile fatty acid （TVFA） level significantly （P<0.05） increased by 25.62%, and the acetic acid/propionic acid （A/P） value significantly （P<0.05） decreased by 27.56%. There was no significant difference in NH₃-N, TVFA, propionic acid （PA）, butyric acid （BA） content and acetic acid/propionic acid between group Ⅱ （3%） or group Ⅳ （9%） and the control group （P>0.05）. Regarding serum biochemical indices, compared with the control group, the serum total protein （TP） and albumin （ALB） contents in group Ⅲ significantly increased by 12.06% and 14.27%, respectively （P<0.05）, while the serum total protein （TP） and albumin （ALB） contents in groups Ⅱ （3%） and Ⅳ （9%） showed no significant differences compared with the control group （P>0.05）; the blood urea nitrogen （BUN） and total cholesterol （TC） contents in group Ⅲ significantly decreased by 19.69% and 16.01%, respectively （P<0.05）, while the BUN and TC contents in groups Ⅱ （3%） and Ⅳ （9%） showed no significant differences compared with the control group （P>0.05）; alanine aminotransferase （ALT） and aspartate aminotransferase （AST） activities showed no significant differences among groups （P>0.05）. [Conclusion] In summary, supplementing 6% compound probiotics fermented feed to the basic diet can effectively improve the growth performance of beef cattle, optimize the rumen fermentation environment, improve the protein metabolism level of the body, and have no adverse effect on liver function, representing a suitable supplementation ratio in beef cattle production.

Key words: compound probiotics fermented feed, beef cattle, growth performance, rumen fermentation parameters, serum biochemical indices