Abstract: 【Objective】 This experiment aimed to investigate the effects of adding different levels of mixed microbial silage of king grass (Pennisetum purpureum×Pennisetum americanum cv.Reyan No.4) and mulberry (Morus alba L.) to the diet on the growth performance,serum parameters and rumen environment of beef cattle,in order to provide scientific support for the promotion and application in beef cattle production. 【Method】 Twenty-four Simmental crossbred beef cattle,aged 22 to 24 months and weighted 494.79 kg±11.90 kg,were randomly divided into 4 groups (CON,KM1,KM2,and KM3 groups),and the diet was supplemented with 0,15%,30% and 45% of mixed microbial silage of king grass and mulberry,respectively.Each group had 6 replicates,with one beef cattle per replicate.The pre-feeding period lasted for 10 days,followed by a formal trial period of 60 days.During the trial period,the initial body weight (IBW) and final body weight (FBW) of each beef cattle were measured,and the average daily gain (ADG),average daily feed intake (ADFI),and feed-to-gain ratio (F/G) were calculated.After the trial,blood samples were collected from the jugular vein of each to assess serum biochemical and antioxidant parameters.Fresh rumen fluid was collected using a stomach tube rumen fluid sampler for the determination of rumen fermentation characteristics and 16S rDNA analysis of rumen microorganisms in beef cattle. 【Result】 Compared with CON group,①The FBW,ADG,and ADFI of beef cattle in KM2 group were significant increased (P＜0.05),and the F/G was significantly decreased (P＜0.05).②The serum albumin content of beef cattle in KM2 group was significantly increased (P＜0.05),while the contents of alanine aminotransferase,aspartate aminotransferase,urea nitrogen,triglycerides,and total cholesterol were significantly decreased (P＜0.05).The malondialdehyde content was significantly decreased (P＜0.05),and the total antioxidant capacity,glutathione peroxidase,and catalase activities were significantly increased (P＜0.05).③The contents of ammonia nitrogen,microbial protein,acetate,propionate,and total volatile fatty acids in ruminal fluid of beef cattle in KM2 group were significantly increased (P＜0.05),while the butyrate content was significantly decreased (P＜0.05),and the ACE index,Chao1 index,Faith_pd index,and Shannon index of ruminal fluid were significantly increased (P＜0.05).The ruminal ammonia nitrogen contents of beef cattle in all KM groups were significantly increased (P＜0.05).④The relative abundance of Bacteroidota in ruminal fluid of beef cattle in KM2 group was significantly increased (P＜0.05),while the relative abundances of Firmicutes and Fibrobacterota were significantly decreased (P＜0.05).Additionally,the relative abundances of Rikenellaceae_RC9_gut_group, Ruminococcus,NK4A214_group and Muribaculaceae in ruminal fluid of beef cattle in KM2 group were significantly increased (P＜0.05),whereas the relative abundance of Christensenellaceae_R-7_group was significantly decreased (P＜0.05).The relative abundances of Bacteroidales_RF16_group in ruminal fluid of beef cattle in all KM groups were significantly increased (P＜0.05). 【Conclusion】 Under the experimental conditions,the supplementation of 30% mixed microbial silage of king grass and mulberry in the diet of beef cattle could enhance their growth performance,regulate lipid metabolism and improve antioxidant capacity in the blood,optimize rumen fermentation characteristics,and modulate the Alpha diversity and microbial community structure in rumen.It was recommended that the addition level of mixed microbial silage of king grass and mulberry in beef cattle diets should be 30%.

Key words: king grass; mulberry; beef cattle; growth performance; serum parameters; rumen environment