Mechanism Of Galacto-oligosaccharides Regulating Rectal Microbial Community Structure Of Holstein Dairy Calves

The organs and tissues of newborn calves are not fully developed,and the gastrointestinal microecosystem is not established.Therefore,they are easy to be infected by pathogenic bacteria,with high levels of disease,such as diarrhea,which will result in serious economic losses.Studies have shown that early intervention by prebiotics can improve the intestinal microecological environment,immunity,and growth performance of animals.In this study,we explored the distribution of rectal microbial community of preweaning calves,and then,the effects of supplementation with galacto-oligosaccharides(GOS)on the growth performance and rectal community structure were investigated,which will provide a theoretical basis for the rational application of prebiotics in dairy calf rearing.This dissertation is mainly composed of the following five experiments.EXP.1: The distribution of rectal microbiota in Holstein dairy calvesEight female newborn Holstein dairy calves with similar birth date and body weight were randomly selected.The rectal content samples were collected at 12-time points,which included d 1,3,7,14,21,28,35,42,56,and 70.Their community structures and diversities were detected using 16 S r RNA gene sequencing.The results showed that the colonization of the bacterial genera from birth to preweaning period changed significantly.The bacterium before d 3 was the phylum Proteobacteria,mainly including the genus Shigella.The bacterium from d 7 to d 28 was the phylum Firmicutes,mainly including the genus Fecalibacterium,Lactobacillus and Blautia.The bacteria Bacteroides,Prevotella,and Fecalibacterium were mainly included in d 35 to d 70.EXP.2: The effects of GOS on the growth performance and serum immunoglobulin levels of calvesForty-eight newborn female Holstein dairy calves were randomly allocated to 4 groups of 12,which were the control group(CON),the GOS1 group(10 g/d),the GOS2 group(20 g/d),and the GOS3 group(40 g/d).The study lasted 70 d and was composed of 2 phases.The calves in the experimental groups were fed the diet containing GOS for 28 d,whereas the diet for the control group did not contain GOS.The 2nd phase of the study lasted for 42 d,during which the calves in the experimental group were randomly allocated to 2 subgroups of 6 calves each.The calves in one of these subgroups continued to consume GOS,whereas the calves in the other subgroup did not consume any more GOS.The growth performance and incidence of diarrhea were recorded and calculated.The serum in d 7,14,21,28,42,56,70 was collected to detect immunoglobulin levels.The results showed that the supplementation with 10g/d GOS for d 28 significantly improved body weight and reduced the incidence of diarrhea of calves on d 28 and d 70(P < 0.05).It also increased the serum Ig A level on d 28,and increased the serum Ig G level of calves on d 56 and d 70(P < 0.05).EXP.3: The effects of GOS on the community structure and diversity of Holstein dairy calvesThe rectal content samples in the CON and GOS1 groups in the EXP.2 on d 28 and d 70 were collected and detected using 16 S r RNA gene sequencing.The results showed that supplementation with10 g/d GOS significantly increased the diversity of rectal bacteria and the relative abundances of Actinobacteria,Lachnospiraceae,Lactobacillus,and Ruminococcus2 in d 28 of calves,but reduced the relative abundance of Clostridium＿XVIII(P < 0.05).Supplementation with 10 g/d GOS for 28 d significantly increased the relative abundances of Prevotella and Coprococcus,whereas feeding GOS for either 28 d or 70 d both significantly decreased the relative abundance of Clostridium＿Xl Vb(P < 0.05).EXP.4: The effects of GOS on the metabolites in the rectal content and plasma of Holstein dairy calvesThe plasma and rectal content samples of calves in the CON and GOS1 groups on d 70 were collected,and metabolites were detected by liquid chromatography-mass spectrometry.The results showed that the differential metabolites in rectal content were mainly enriched in linoleic acid metabolism and melanogenesis.The representative differential metabolites in rectal content were butyric acid,hippuric acid,D-glucuronic acid,propionylcarnitine,6-ketoprostaglandin E1,and 20-carboxy-Leukotriene B4.The plasma differential metabolites were mainly enriched in GABAergic synapse,and D-glutamine and D-glutamate metabolism.The representative differential metabolites in plasma were L-valine,oxoglutaric acid,urocanic acid,L-isoleucine and nicotinamide ribotide.EXP.5: Correlation analysis of rectal microbiota,metabolites and plasma metabolites of calves on d70Spearman correlation analysis was conducted between the differential bacteria in the rectal content of claves on d 70 screened out in EXP.3 and the differential metabolites in the rectal content and the plasma of claves on d 70 screened out in EXP.4.The results showed that supplementation with GOS for28 d could promote the proliferation of Coprococcus and Prevorella in rectal content,and regulate the metabolism of L-isoleucine in plasma,and then enhance the absorption and utilization of amino acids.Feeding GOS for 70 d could up-regulat the concentrations of butyric acid in rectal content and fatty acids in the plasma,such as oxyglutaric acid,12-Hydroxydodecanoic acid,and oleic acid,and then regulate glucose,lipid and amino acid metabolism of calves through beta oxidation pathway and tricarboxylic acid cycle pathway.In summary,supplementation with 10 g/d GOS has been chosen based on the distribution of rectal microbiota in preweaning calves.It can improve the growth performance and serum immunoglobulin concentration of calves,with improving the community structure of rectal microbiota,and regulating glucolipid metabolism and amino acid metabolism.Then,feeding GOS for 28 d was recommended,which is more beneficial to the healthy growth for calves.