| The growth and development of calves is directly related to the economics of cattle rearing,especially diarrhea being the most common disease in calf rearing.In the past,antibiotics have been commonly used to prevent and control disease in calves,but this increases the chances of the emergence and spread of antibiotic-resistant bacteria in meat and dairy products.With the use of antibiotics banned,one of the most concerned issues in calf farming at this stage is how to improve the growth performance and survival rate of calves.Therefore,there is an urgent need for alternatives to antibiotics,and probiotics are one of the most potential feed additives.However,due to the variety of probiotics,the different levels of addition,and the influence of feeding and management,the conclusions on the effect of probiotics on calves between different studies are controversial.Therefore,this study combined Meta-analysis and animal feeding experiments to further explore the effect of probiotics on the health of calves.Part 1: Effects of probiotics on growth and immune performance of calves: a metaanalysisThe study used a systematic review method to perform a meta-analysis of the published literature.Using the systematic review and meta-analysis(PRISMA)method as the analysis guide,a total of 4 databases were searched: NCBI,Science Direct,Web of Science,Google Scholar,and a total of 5058 studies were retrieved from the database and 2 studies from other sources.The included studies were published between 1996 and 2021.According to the established screening criteria,a total of 34 studies were finally included in the meta-analysis.The results showed that the addition of probiotics to calf diets could significantly improve the growth performance of calves,including average daily gain(PSMD < 0.001,RMD = 52.602 g/d,I2 = 34.8 %)and body weight(PSMD < 0.001,RMD = 2.390 kg,I2 = 8 %),while significantly reducing the feed-to-gain ratio of calves(PSMD < 0.001,RMD =-0.107,I2 = 37.4 %)and significantly reducing their fecal scores(PSMD = 0.002,RMD =-0.080,I2 = 57.0 %).In addition,feeding probiotics to calves also increased calf immunoglobulin G to a certain level(PSMD = 0.093,RMD = 0.340 g/L,I2 = 21.5 %).In conclusion,supplementation of probiotics in calf diets can effectively improve calf growth performance,promote digestion and metabolism,improve the immune performance of calves,and prevent diarrhea.The results of subgroup analysis showed that the effect of feeding single probiotics was better than that of compound probiotics,and the effect of adding probiotics to whole milk was better than that of milk replacer.In addition,the effects of probiotics of different genera on the growth performance of calves were also different.Compared with Bacillus,Lactobacillus and Saccharomyces,the effect of Bacillus was better.Part 2: Effects of supplementation of probiotics in calf diets on growth performance,digestive metabolism,and immune performance and rumen floraBased on the results of the first part of the Meta analysis,a single strain of Bacillus subtilis was selected for feeding experiments.Eight healthy Simmental calves of about 60 days of age with similar body weight were selected,with 4 in each group.They were randomly divided into B.subtilis group and control group.The B.subtilis group was supplemented with 2×1010 CFU/d of Bacillus subtilis on the basal diet.The pre-feeding period was 10 days,and the experimental period was 63 days.Measurements and sampling were performed every 21 days during the trial period.The pre-feeding period was 10 days,and the experimental period was 63 days.Measurements and sampling were performed every 21 days during the trial period.The results showed that from 1 to 21 days,the average daily weight gain of the B.subtilis group was significantly higher than that of the control group(P < 0.05),and the difference was not significant in other experimental stages(P > 0.05).The rest of the indicators had no significant effect(P > 0.05).At the end of the experiment,the average body weight of the B.subtilis group was 6.5 kg higher than that of the control group,and the average daily gain of the B.subtilis group was 17 % higher than that of the control group.There was no significant difference in body size indicators(P > 0.05).On the 63 rd day of the experimental period,the apparent digestibility of DM,ADF and NDF of calves in the B.subtilis group was significantly increased(P < 0.05),but had no significant effect on Ash,EE,CP,Ca and P(P > 0.05).On the 63 rd day of the experiment,the blood Ig G level of the B.subtilis group was significantly higher than that of the control group(P < 0.05),which was 12 % higher than that of the control group.On the 42 nd day of the experiment,the interleukin-4 in the B.subtilis group was significantly higher than that in the control group(P < 0.05),which was 15 % higher than that in the control group.On the 21 st day of the experiment,the immunoglobulin A of the B.subtilis group was 10% higher than that of the control group.On days 42 and 63 of the experiment,Ig M in the B.subtilis group was 7 % and 8 % higher than that in the control group,respectively.The addition of probiotics to the diet had no significant effect on the microbial diversity of calf fecal flora,but adjusted the structure of fecal flora and rumen flora.There was no significant effect on the microbial diversity of calf fecal flora,but the structure of fecal flora and rumen flora was adjusted.The results showed a significant increase in the relative abundance of beneficial bacteria,such as Bifidobacterium,Bacteroides,Prevotellaceae＿UCG-001,Prevotellaceae＿Ga6A1 and Fibrobacterota.They can promote calf rumen development,increase the digestibility of the diet and the production of volatile fatty acids,and improve calf immunity.It also reduces the abundance of pathogenic bacteria,such as chlamydia,Proteobacteria and Saccharibacteria,which can inhibit the growth of intestinal pathogenic microorganisms,prevent intestinal diseases,and reduce the incidence of diarrhea.Based on the results of meta-analysis and feeding experiments,as well as the sequencing results of rumen fluid and rectal fecal samples,the experiment proved that feeding probiotics to calves can effectively promote the growth and development of calves and improve feed digestion.In addition,it can effectively improve the immunity of calves and reduce diarrhea.The reason may be that the addition of probiotics to the calf diet improved the structure of calf rumen flora and increased the abundance of beneficial bacteria in the calf rumen.This facilitates the digestion of the diet,strengthens the gastrointestinal barrier,reduces the occurrence of inflammation,and inhibits the growth of intestinal pathogenic microorganisms. |
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