Effect Of Supplementary Feeding In Warm Season On Meat Quality Of Yaks And Underlying Mechanisms

The traditional grazing-based yak production system has issues such as long production cycle,low yield,high grazing pressure on grassland and poor meat quality of yaks.This production system needs to transform for better efficiency and improved meat quality.The transformation would also fulfill the urgent requirements of the central government,the local authority and herders for grassland ecological protection,economic development and increase in income and living standard.The climate conditions in warm seasons are suitable for the growth of both forage and animals.Grassland in warm seasons produces quality forage containing functional secondary metabolites and unsaturated fatty acids.Furthermore,yaks have compensation growth in warm seasons.All the advantages together in warm seasons generate great potential for the improvement of both growth performance and meat quality of grazing yaks at a low cost.This study aimed to evaluate the nutrient value of pasture in warm seasons,to determine deficient nutrients limiting the potential to the production of grazing yaks,and to formulate supplementary feed with balanced nutrients for 27-month-old grazing yaks.The study also aimed to investigate the effects of supplementary feeding on meat quality,pasture utilization and carbon footprint of livestock products.Furthermore,the study would explore the response mechanism of meat quality to supplementary feeding in warm seasons using the UHPLC-QE-MS non-target metabolomics technique.The study was targeted to generate theoretical guidelines and practical techniques applied for ecological,quality and efficient yak production.The main results of this study are as follows:（1）Experiment 1.Evaluation of nutrition,digestibility and production potential of grass in warm season of alpine grassland.The results:The ratio of calcium to phosphorus was unbalanced and five elements including phosphorus,sodium,potassium,zinc and selenium were deficient,resulting in the requirements of grazing yaks for nutrients not fully fulfilled during warm seasons.The contents of cobalt in forage in the late regreening and browning stages and the content of manganese in the late regreening stage could not meet the requirements of yaks as well.Dry matter intake（DMI）of grazing yaks in the late regreening,green-grass and browning stages was4.31,4.01 and 3.27 kg/head·d,respectively,dry matter digestibility（DMD）72.5%,65.5%and58.1%,respectively.The percentage of feed intake per unit of body weight was 2.38%to 3.11%.Protein intake（CPI）was 632,510 and 254 g/head·d for these stages,respectively,and protein digestibility（DCP）69.0%,59.9%and 56.0%,respectively.The calculated nutrient output per unit of grassland area was extremely uneven among the three stages of growth with the late regreening stage being the smallest.Grassland nutrient carrying capacity expressed as standard yak units was relatively insufficient in all the three stages of growth.（2）Experiment 2.Study on the effect of grazing yak supplementary feeding on its body weight gain and grassland pasture utilization in warm season.The results:The nutrient supply of forage in warm seasons cannot fully meet the nutritional requirements of yaks for the maximum potential of growth.Supplementary feeding in the late regreening,grass-green and browning stages extremely significantly increased the average daily body gain of grazing yaks（P<0.01）and economic profits.The total feed intake of yaks with supplementary feed increased by 18.8%,29.4%and 41.8%in the three stages of growth,respectively,in comparison with those without supplementary feed.The amounts of pasture consumed decreased by 16%,8.04%and 4.12%,respectively.The conversion efficiency of the diet increased by 23.1%,29.2%and 44.5%,respectively and dietary dry matter digestibility（DMD）increased（P<0.01）by 6.08%,10.56%and8.12%,respectively.These results indicated that supplementary feeding substantially reduced grazing pressure on grassland,improved feed utilization by yaks and improved dietary conversion efficiency.（3）Experiment 3.Effects of grazing yak supplementary feeding on its meat quality and methane emissions in warm season.The results:Yak supplementary feeding in warm seasons extremely significantly increased all the indexes,except bone weight,of slaughter performance and the pH value of carcass 45 min and 24 h after slaughter（P<0.01）,reduced the cooking loss rate,water loss rate and shearing force（P<0.01）,improved meat tenderness（P<0.01）,and had no negative effect on flesh color（P>0.05）.Supplementary feeding extremely significantly increased the intermuscular fat content of the longissimus dorsi muscle（P<0.01）,reduced the cholesterol content of meat（P<0.01）,had no effects on the contents of water,crude protein and crude ash（P>0.05）,increased the content of n-3 polyunsaturated fatty acids in meat and the PUFA/SFA ratio（P<0.01）,reduced the n-6/n-3 ratio and the atherogenic index（AI）（P<0.01）,increased the contents of essential amino acids,the ratios of EAA/NAA and EAA/TAA,and the contents of VB₁ and Vpp in meat（P<0.01）,and had no significant effect on mineral content（P>0.05）.For all yaks either with or without supplementary feed,heavy metal and drug residues in meat were far below the national limits for safety.Supplementary feeding extremely significantly reduced the amount of methane emissions per kg of feed intake and per kg of body weight gain（P<0.01）and improved dietary energy utilization.（4）Experiment 4.Response mechanism of grazing yak supplementary feeding on improving meat quality in warm season.The results:Yak supplementary feeding in warm seasons significantly affected metabolites in muscle（P<0.01）with 955（POS）and 596（NEG）identified,from which 177（POS）and 198（NEG）differential metabolites were found.The further examination of differential metabolites and annotation of KEGG metabolic pathway function found that 38 differential metabolites could be annotated to the KEGG metabolic pathway,of which 31 were up-regulated,7 down-regulated.The up-regulated differential metabolites（81.6%）were involved in the functional metabolic pathways of protein and amino acid,fatty acid,carbohydrate,vitamin,mineral and immunological system,whereas the down-regulated differential metabolites（18.4%）were health-related metabolites.These findings indicated that yak supplementary feeding in warm seasons could promote animal growth and development,energy metabolism and fatty acid metabolism and synthesis,and improve immunity for better health conditions of yaks.Identified key metabolites involved in protein and amino acid metabolic pathways included pyruvic acid,methylmalonic acid,L-tyrosine,L-isoleucine,L-tryptophan,3-iodo-L-tyrosine,L-glutamine,D-phenylalanine,D-arginine,N-FL-aspartic acid,D-pantothenic acid,guanidine glycerol phosphate,jaundice,hypoxanthine,inosine,taurine.The key metabolites in the pathways of fatty acid metabolism were eicosapentaenoic acid,arachidonic acid,and cis-8,11,14-eicosatrienoic acid.The key metabolites of the carbohydrate（energy）metabolic pathway were pyruvate,D-glucose-6-phosphate,oxaloacetate,cis-aconitic acid,L-fumaric acid1-phosphate and D-arabitol.The key metabolites of vitamin and mineral metabolic pathways containedpyruvicacid,L-tyrosine,D-pantothenicacid,pyridoxalphosphate,2-amino-4-hydroxy-6-（D-erythr-1,2,3-trihydroxypropyl）-7,8-Dihydropteridine,selenomethyl L-selenocysteine??and methylselenocysteine??Se-oxide.The key metabolites of the immunometabolic pathway were inosine,taurine,twenty carbaenoic acid,arachidonic acid,cis-8,11,14-twenty triacylate,L-tyrosine,D-pantothenic acid,pyridoxal phosphate.The expression levels of these key metabolites were significantly up-regulated（P<0.01）and the differential metabolites impairing the health of the body,such as sulfites L-citrulline,L-carnosine,L-glutamyl5-phosphate and creatine,were all significantly down-regulated（P<0.01）.These findings indicated that the supplementary feeding to be nutritionally balanced for grazing yaks in warm seasons reduced damage to the body,enhanced immunity,promoted growth and improved meat quality of grazing yaks.（5）A series of experiments in the study establish a pilot yak production system which is characterized with simple feeding management,significant weight gain and improved quality of carcass,food consumption,meat nutrition and environment.The environmental aspect includes reduced grazing pressure on grassland and the carbon footprint of livestock products.The pilot production system may be named as“the 1122 production mode”as the system is composed of“a warm season,a feed,two-year-old yak,feeding twice in the morning and evening".The mode would transform the traditional pastoral yak production in the Qinghai-Tibetan plateau,would have significant social,economic and ecological benefits,and would enhance the functions of the alpine grassland ecosystem in terms of ecology,production and life.In summary,the Qinghai-Tibetan alpine grassland in warm seasons produces better forage and has a greater carrying capacity than other seasons.Supplementary feeding with key nutrients（energy,minerals,etc.）supplemented balances dietary nutrition for grazing yaks.The supplementary feed up-regulates the key metabolites involved in metabolic pathways related to growth and meat quality and facilitates the positive synergistic effects of various nutrients.As a result,supplementary feeding increases animal and slaughter performance,improves the tenderness and nutritional quality of meat,does not negatively affect the technical indexes of meat safety and hygiene,reduces the carbon footprint of animal products,lightens grazing pressure on grassland and increases the overall economic profit.This study implicates that with supplementary feeding for balanced nutrition in warm seasons,short-term fattening grazing yaks is feasible,which allows a high-quality,ecological and efficient yak production system to be developed.