| In recent years,the studies on immune function and nutrition regulation of young ruminants have received more and more attention.The early nutrition strategy can be used to regulate the production performance and immune function of young ruminants before and after weaning.In this paper,the effect of milk replacer feeding allowance and weaning age on production performance,nutrient digestibility and rumen function of lambs were investigated by the continuous observation of lambs' performance and nutrient digestibility,as well as,the function of rumen fermentation and the changes of digestive enzymatic activity and microbiota.At the same time,the transcriptome sequencing technology was used to investigate the effect of early nutrient strategy on lambs' gut gene expression,and the possible mechanism of milk replacer feeding allowance affecting the growth of lambs and establishing the growth advantage after weaning was expounded.Moreover,the effect of weaning stress on immune function of lambs were evaluated by the continuous measurement of hormone levels,changes in blood cell counts,concentrations of acute phase protein and the gene expression of cytokines in post-weaning lambs.Furthermore,the possible mechanism of milk replacer feeding allowance regulating the physiological and immune functions of weaned lambs were explained by exploring the intestinal histomorphology,gene expression of cytokines,changes of microbiota in mucosa and chyme,and the relationship between host and bacteria.The main results of current study as follows:1.The effect of milk replacer feeding allowance and weaning age on production performance and rumen function of lambsIn this study,32 male Hu lambs with same birth age and similar birth weight(mean ± SE: 3.29±0.13 kg)were selected and randomly divided into 4 groups,and 4 treatments were accepted respectively: feeding 2% average body weight milk replacer and weaning at 21 d(21dC,n=8);feeding 4% average body weight milk replacer and weaning at 21 d(21dH,n=8);feeding 2% average body weight milk replacer and weaning at 35 d(35dC,n=8);feeding 4% average body weight milk replacer and weaning at 35 d(35dH,n=8).The results showed that the high plane of milk replacer feeding reduced the digestibility of neutral detergent fiber(P = 0.001)at 50 d,and there was no significant difference in other nutrient digestibility(P > 0.05).Lambs weaning delayed to 35 d had an advantage of rapid growth,and had greater average daily gain(P = 0.001),final body weight(P = 0.013)and carcass weight(P = 0.008).The high plane of milk replacer feeding could promote rapid growth of lambs,significantly increase average daily gain(P = 0.013)and weight(P = 0.036),and maintain this advantage to 50 d of age.In this study,the milk replacer feeding and duration had negligible influence on the ruminal fermentation parameters and microbiota.It could speculate that intestinal function plays an important role in early nutrient regulation of lambs.2.The effect of milk replacer feeding allowance on morphology and function of small intestineIn this study,two groups of lambs weaned at 35 d and fed 2% or 4% average body weight were selected(C,n=8;H,n=8),which showed the obvious advantage of growth performance.RNA-Seq technique was used to investigate the differentially expressed genes in lambs' jejunum.The changes of jejunal histomorphology and digestive enzyme activities were combined to investigate the mechanism and regulatory pathway of early nutrition affecting lambs' performance and nutrient digestion.The results showed that high plane of milk replacer feeding could significantly increase the activity of lactase(P < 0.001),amylase(P < 0.001),lipase(P = 0.001),trypsin(P = 0.002),chymotrypsin(P < 0.001)in the jejunum of lambs,as well as intestinal villus length(P = 0.001)and muscle thickness(P = 0.007).A total of 1179 differentially expressed genes were obtained in this study,of which 882 genes were significantly up-regulated(P < 0.05)and 297 genes were significantly down-regulated(P < 0.05).GO analysis showed that differentially expressed genes were mainly enriched in nutrient absorption and metabolic pathways.The high plane of milk replacer feeding could regulate the expression levels of HSG,ACAT1,BCKDHB,FADS1,INSIG1,PCK1 and other genes in the lambs' jejunum to promote the absorption,transportion and metabolism of nutrients,thereby improving the utilization and deposition of nutrients.3.Meta-analysis of weaning stress affecting immune function in young ruminantsIn this study,a total of 11 articles were chosen after screening.The regression analysis was used to explain the source of heterogeneity,sensitivity analysis was used to assess the stability of the results,and funnel plot was used to evaluate publication bias.The results showed that the plasma cortisol concentration significantly increased from 1 to 3 d(P = 0.046)and 11 to 14 d(P = 0.001)after weaning;the plasma concentrations of haptoglobin and fibrinogen were significantly increased from 1 to 14 d(P < 0.01)after weaning.The sensitivity analysis showed that all results were stable and reliable,and funnel plot analysis showed little publication bias in the concentrations of cortisol,haptoglobin and fibrinogen.The cortisol,haptoglobin and fibrinogen can be considered as potential biomarkers for future research on weaning stress in ruminants.4.The effect of milk replacer feeding allowance on immune function of lambs after weaningIn this study,the experimental animal and feeding management were same as experiment 1.The physiological and immune indexes of 4 groups of lambs were tested and analyzed at 0,1,2,3,7,14 and 21 d after weaning.which fed with 2% or 4% average body weight milk replacer and weaned at the age of 21 or 35 d.The results showed that the concentrations of cortisol and TNF? in the plasma recovered within 3 d,and weaning stress could induce a long-term inflammatory response,which was evidenced by sustained increase of the expression of pro-inflammatory cytokines gene,such as CXCL8(P < 0.01),IL-1?(P < 0.01)and GR?(P < 0.001)were still higher than pre-weaning levels at 21 d after weaning.The early nutrient regulation leaded to a completely different immune state of lambs after weaning.The milk replacer feeding allowance was increased from 2% of average body weight to 4% of average body weight could reduce the immune and inflammatory response induced by weaning stress,and protectd the health and animal welfare of the lambs.5.The effect of milk replacer feeding allowance on lambs' inflammatory response and microbiota in jejunumIn this study,two groups of lambs weaned at 21 d and fed 2% or 4% average body weight were selected(C,n=8;H,n=8),which showed the obvious difference of immune response after weaning.The jejunal histomorphology,gene expression of pro-inflammatory cytokines,and microbiota in jejunal mucosa and chyme were investigated to explore the mechanism of early nutrition regulating physiological and immune functions of lambs.The results showed that the high plane of milk replacer feeding could reduce the expression of TLR4 and TNF? in jejunum tissue(P < 0.01),and reduce the intestine inflammatory response induced by weaning stress and protect the structural integrity and barrier function of intestinal tissue.The high plane of milk replacer feeding could reduce the relative abundance of pathogenic microorganisms in the intestine,such as Plesiomonas(0.75% vs.0.18%,P = 0.017),and regulate the internal environment of the intestine to affect the microbiota community composition and stable colonization of the dominant bacterial community,thus,it could provide a long-term protection for the intestinal health of lambs,which is beneficial to the physiological state and immune function. |
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