| Yaks(Bos grunniens)as the key species of the Qinghai-Tibetan Plateau and the only large ruminants living in the extreme cold environment,are highly adaptable to climate and nutritional stress.The nutritional stress caused by the significant difference in quality and quantity between seasonal natural pastures leads to the failure of grazing yaks to break through the production patterns of alive in summer,strong in autumn,thin in winter and tired in spring,which has a fatal impact on the reproduction and production of yaks.In recent years,researchers pay great attention to alleviating weight loss of yak in cold season and improving its production performance through dietary nutrition regulation,and the related research is becoming more and more thorough and meticulous.However,the molecular mechanism of improving the energy use efficiency and the effective response to seasonal nutrient deficiency of the rumen nutrient metabolism and fat transportation in the cold season of yaks remains not clear.This study selected a total of 15 healthy castrated yaks with similar body weight(274.3 ± 3.14 kg)by feeding different energy level diets(LE: low energy with NEg: 5.5 MJ/kg,ME: medium energy with NEg: 6.2 MJ/kg,HE: high energy with NEg: 6.9 MJ/kg)to evaluate the effects of different dietary energy levels on production performance,rumen microbial colony structure,short chain volatile fatty acid(VFA)absorption and transportation as well as lipid metabolism of liver,subcutaneous adipose tissue and longissimus dorsi muscle of yaks in clod season.The objective of this study was to reveal the molecular regulation mechanism of energy feed on nutrient metabolism and adipose transportation and provide a scientific basis for improving the utilization of feed energy and scientific feeding of yak.The main results of this study are as follows: 1 Effects of different dietary energy levels on production performance,serum biochemical parameters and hormone concentration of yakThe production performance of yak was significantly increased with increasing dietary energy level,the ADFI decreased significantly with the increase of dietary energy level(P < 0.05).However,the ADG,feed conversion ratio,carcass weight,dressing percentage and average backfat thickness were significantly increased(P < 0.05)with increasing dietary energy level.When feeding highenergy level diet(NEg: 6.9 MJ/kg),the ADG of yak reached 1013 g/d,and the profit generated by this energy level was also the most.The serum concentrations of GLU,CH,NEFA,TG,HDL and LDL were higher in HE group than LE group(P < 0.05).The contents of serum INS,IGF-1 and IGF-2 increased significantly with increasing dietary energy level(P < 0.05),however,serum concentration of GH was higher in LE group than ME and HE group(P < 0.05).2 Regulation of different dietary energy levels on ruminal fermentation parameters,ruminal microbial diversity and volatile fatty acid transporters in rumen epitheliumThe concentrations of acetic acid,propionic acid,butyric acid,pentanoic acid and total VFA increased significantly with the increase of dietary energy level(P < 0.05).The results showed that the mRNA and protein expression levels of MCT1,AE2,DRA and PAT1 which involved in VFA absorption and transportation increased with the increase of dietary energy level in rumen epithelium(P < 0.05).Ruminal bacteria were mainly composed of Firmicutes and Bacteroidetes of yak,which accounted for about 50% and 40%,respectively.The relative abundance of Lentisphaerae and Spirochaetes in HE group was significantly higher than that in LE and ME group(P < 0.05).The dominant bacterial communities of yak rumen were Erysipelotrichaceae,Succiniclasticum,Prevotella,Treponema and Victivallis in HE group,which involve in degradation of non-structural carbohydrate and VFA production in rumen.3 Effects of different dietary energy levels on lipid metabolism in liver,subcutaneous adipose tissue and longissimus dorsi muscle of yakThe mRNA expression of lipogenic genes(PPARγ,FASN,ACC,SCD,LPL and DGAT-1)significantly increased with increasing dietary energy level(P < 0.05),but the mRNA expression of lipolytic gene ATGL decreased with the increase of dietary energy level(P < 0.05)in liver and subcutaneous adipose tissue.Moreover,the relative expression of lipolytic gene HSL and CPT-1 decreased with increasing dietary energy level(P < 0.05)in subcutaneous adipose tissue.The enzymatic activities of FASN,ACC,SCD,LPL and DGAT-1were increased with the increase of dietary energy level(P < 0.05)in liver and subcutaneous adipose tissue,but enzymatic activity of ATGL decreased(P < 0.05).The enzymatic activities of HSL and CPT-1 decreased with increasing dietary energy level(P < 0.05)in subcutaneous adipose tissue.The gene expression of PPARγ,FASN,ACC and SCD significantly increased with increasing dietary energy level(P < 0.05),the concentrations of IMF,SFA,MUFA and PUFA also increased(P < 0.05).In each group,the content of C16:0,C18:0 and C18:1n9c had a large proportion,which also increased with increasing dietary energy level(P < 0.05).In summary,the increase in dietary energy levels can effectively decrease weight loss and promote the production performance of yaks under traditional grazing system.The feeding of high energy level diet can facilitate the degradation of nonstructural carbohydrates in the rumen and increase the abundance of VFA production related bacteria to promote the production of VFA,as well as increase the expression of VFA transporters in the rumen epithelium,so as to provide the substrate for lipid metabolism.The results further indicated that the genes involved in lipogenesis were upregulated significantly in liver and subcutaneous fat by the increasing of dietary energy level,which contributing to the rate of fat synthesis in the body increased and the fat deposition accelerated.The up-regulation of the lipogenic gene in the longissimus dorsi muscle promoted the biosynthesis of fatty acids in the muscles,increased fatty acid deposition and dietary energy conversion efficiency.This study reveals the molecular mechanism of nutrient metabolism and fat transport in the rumen of yaks under different energy levels for the first time,which laid an important scientific and theoretical basis for improving the precision and production performance of yak nutrition. |
