Metabolic Regulation Mechanism Of Taurine On Improving Laying Rate Of Hens In The Late Laying Period

With the increase of elimination age of laying hens from 72 weeks to 80 weeks,the late laying period occupies nearly half of the whole laying period,and the laying performance directly affects economic benefit of hens industry.The main problem of hens in the late laying period is that with the increase of weeks of age,laying rate decreases and egg quality deteriorates due to ovarian decline,metabolic diseases and weakened ability to resist pathogen infection and environmental stress.Developing green feed additive is an important solution.Taurine is a world-recognized nutritional fortifier with many physiological functions such as regulating lipid metabolism,energy metabolism and endocrine,as well as antioxidation and anti-inflammation,it has been widely used in medicine,food,beverage and so on.In recent years,the demand of taurine in the world is increasing year by year,but the application of taurine in livestock and poultry breeding is still in its infancy.After the Chinese government implemented the feed prohibition and resistance policy in July 2020,the growth performance,feed conversion efficiency and health of livestock and poultry raised on a large scale have been affected to a certain extent,taurine is highly recommended by some scholars for its many beneficial functions in the large-scale livestock and poultry breeding in the post-antibiotic era.Current research reports mainly focus on the improvement of livestock and poultry production performance by taurine,but the mechanism of its action needs to be further clarified.Based on the research group’s previous experiment of taurine on improving laying rate of hens,three experimental groups of hens in peak laying period,hens in late laying period and hens treated with taurine in late laying period were selected to further reveale the metabolic mechanism of taurine improving laying rate of hens in late laying period by serum reproductive hormones levels analysis,serum metabolomics analysis,liver lipidomics analysis and gene expression level analysis of yolk precursors synthesis,so as to provide theoretical basis for the application of taurine in in the antibiotic era of poultry nutrition.The main results are as follows:(1)The ovarian and follicular tissue section observation and follicular number analysis showed that,0.02% taurine made the smooth muscle of ovarian stroma of hens in the late laying period more compact and regular,and also made the thickness and structure of membrane thcea and granular thcea of small white follicle more compact,the number of total follicles and the number of graded follicles in the ovary were significantly increased(P < 0.05).Dietary 0.02% taurine also significantly increased levels of reproductive hormones FSH,E2 and P4 in serun of hens in the late laying period(P < 0.05),and the m RNA expression levels of FSHR and ESR1 of small yellow follicle(P < 0.05).(2)Serum metabolomics analysis screened six core metabolites which affected laying rate of hens in late laying period,including linoleic acid,γ-linolenic acid,pantothenate,Lmethionine,L-carnitine and 2-methylbutanylcarnitine.Further metabolic pathway analysis of aboved 6 metabolites showed taurine mainly disturbed linoleic acid metabolism,pantothenic acid and Co A biosynthesis,cysteine and methionine metabolism and carnitine metabolism,which were mainly related to lipid metabolism and energy metabolism.(3)Liver lipidomics analysis showed that,compared to hens in the peak laying period,the concentration of 121 lipids in the liver of laying hens in the late layng period were significantly increased(P < 0.05),including 34 storage lipids(glycerolipids)and 45 structural lipids(glycerophospholipids),and 0.02% taurine reduced the concentration of111 lipids in the liver of hens in the laying period,all of which were glycerolipids.The analysis of liver lipid droplet section and triglyceride and cholesterol content showed that there was a serious accumulation of lipid droplets in the liver of hens in the late laying period compared to hens in the peak laying period,and the triglyceride content also increased significantly(P < 0.05),and 0.02% taurine significantly reduced the accumulation of lipid droplets,the contents of triglyceride(P < 0.01)and total cholesterol(P < 0.05)in the liver of hens in the late laying period.(4)The mRNA expression level analysis of liver lipid metabolism related genes and tricarboxylic acid cycle key genes showed that the m RNA expression levels of lipolysis and fatty acid oxidation related genes LPL,FABP-1,CPT-1A,ACOX1,PPARα(P < 0.05)and tricarboxylic acid cycle key genes CS,IDH3 A and OGDH in the liver of hens in the late laying period were significantly decreased compared to hens in the peaking laying period(P < 0.05),and 0.02% taurine significantly promoted the expression of above genes except IDH3 A in the liver of hens in the laying period(P < 0.05).(5)The analysis of liver synthesis,blood transport and follicular deposition of yolk precursor showed that,compared to hens in the peak laying period,the contents of very low density lipoprotein(VLDL)and vitellogenin(VTG),and the m RNA expression levels of ESR2,Apo B,Apo VLDL-Ⅱ and VTG-Ⅱ in liver of hens in the late laying period were significantly decreased(P < 0.001).The contents of VLDL and VTG(P < 0.05),and the m RNA expression levels of yolk precursor receptor VLDLR in follicles also significantly decreased(P < 0.001).Dietary 0.02% taurine significantly increased the contents of VLDL and VTG(P < 0.05),the m RNA expression levels of syntheses related genes in the liver of hens in the late laying period(P < 0.05),the content of VTG in serum(P < 0.001)and as well as the VLDLR m RNA expression level of in follicles.The main conclusions of this study are as follows:(1)The possible metabolic mechanism of the declined laying rate of hens in late laying period may be that the reduction of fatty acid oxidation level and tricarboxylic acid cycle level in liver leads to lipid deposition and the decrease of energy supply,which leads to the decrease of the synthesis of yolk precursors very low-density lipoprotein and vitellogenin necessary for follicular development in the liver,and finally declines the laying rate of hens.(2)Dietary 0.02% taurine promotes fatty acids oxidation and TCA to reduce lipid deposition and improve the energy supply in the liver,and also enhances the ovarian function to secrete estradiol,promoting the hepatic synthesis,blood transport,and follicular deposition of yolk precursors necessary for follicular development,so as to promote the development of follicles and ovulation of mature follicles,and finally to increase the laying rate of hens in the late laying period.(3)The laying rate of hens in the late laying period may be mainly affected by concentrations of six core metabolites,including linoleic acid,γ-linolenic acid,pantothenate,L-methionine,L-carnitine and 2-methylbutanylcarnitine.Taurine mainly disturbes linoleic acid metabolism,pantothenic acid and coenzyme A biosynthesis,cysteine and methionine metabolism and carnitine metabolism to increase the concentration of above six metabolites,and then improved the laying rate of hens in the late laying period.