Energy And Nitrogen Utilization And Ruminal Bacterial Community On Yaks In Response To Dietary Varying With Energy Levels

The yaks（Bos grunniens）,indigenous to the Qinghai-Tibetan Plateau（QTP）,are raised at an altitude between 3,000 and 6,000 m above sea level（a.s.l.）.Yaks are well adapted anatomically,physiologically and behaviorally to the harsh QTP.They serve as the symbol of the Tibetan culture and provide pastoralists with meat and milk for food,hair and hides for textiles,leather goods and clothing,dung for fuel and capital.Under the traditional management,the yaks lose weight when the forage availability,especially energy and crude protein（CP）,are insufficient for grazing herbivores during the long,cold season.As a result,it brings a huge ecology and economy loss and would prevent to achieve the“Two Mountains Theory”and“Rural Revitalization”.Ruminants rely on anaerobic microbiota,namely,bacteria,archaea,protozoa,and fungi for the degradation of structural carbohydrates,such as plant cellulose,hemi-cellulose,and pectin.The end product is converted to volatile fatty acids（VFAs）,which provide approximately 70 to 80%of the energy requirement for ruminants.In addition,rumen microbial protein accounts for 60 to 85%of the total amino acids absorbed by the small intestine.Our groups have been demonstrated that the yaks have a greater utilization of fiber andurea kinetic efficiency than cattle when consuming a dietary with differenet nitrogen levels.In order to comprehensively understand the dietary energy and nitrogen utilization and the ruminal bacteria composition on yaks,this study systematically analyzed the response mechanism of energy and nitrogen utilization efficiency of yaks when consuming a lower protein dietary varying different energy levels.Yak（Tianzhu white yak）and local cattle（Bos taurus,Qaidam cattle）,all castrated and 2.5 years old,were selected.All animals were offered diets differing in metabolizable energy（ME）levels（6.62,8.02,9.42 and 10.80 MJ/kg）,but with the same CP concentration（similar to the average CP level in the forage of the QTP during the cold season）.In vitro:yaks（n=4;Body weight,BW:148±5.0 kg）and cattle（n=4;BW:151±6.9 kg）rumen fluid inoculated with different energy levels of substrates on gas production and methane production,fermentation parameters and microbiota.The experimental design was a randomized block design.In vivo:yaks（n=6;BW:155±5.8 kg）and cattle（n=6;BW:154±8.0 kg）on nutrients utilization,urea cycle,rumimal fermentation parameters and bacterial composition.The experimental design was a two concurrent Latin square design.1.The fiber utilization was greater,whereas CH₄ production was lesser in yaks than cattleThe neutral detergent fiber（NDF）and acid detergent fiber（ADF）digestibilities were greater（P<0.05）in yaks than in cattle,and both of them increased linearly（P<0.05）with increasing energy level.Methane production was lesser（P<0.05）in yaks than cattle,and decreased linearly（P<0.05）with increasing energy level.Concentrations of total volatile fatty acids（VFAs）and acetate,the ratio of acetate to propionate,and ammonia-N（except at 48 h）were greater（P<0.05）in yaks than cattle;whereas concentrations of propionate（except at 6 h）,butyrate,and iso-VFAs were lesser（P<0.05）in yaks than cattle.In addition,total VFAs concentration,propionate,butyrate,and ammonia-N increased linearly（P<0.05）,whereas,acetate and the ratio of acetate to propionate decreased linearly（P<0.05）with increasing energy level.The relative abundance（RA）of Bacteroidetes was greater（P<0.05）,whereas of Firmicutes and the ratio of Firmicutes to Bacteroidetes were lesser（P<0.01）in yaks than cattle,and Bacteroidetes decreased linearly（P<0.001）,whereas Firmicutes and the ratio of Firmicutes to Bacteroidetes increased linearly（P<0.01）with increasing energy levels.The RA of fibrolytic bacteria（e.g.Rikenellaceae＿RC9＿gut＿group and Butyrivibrio）and of[2H]incorporating bacteria（e.g.Quinella,）were greater（P<0.05）,whereas,amylolytic bacteria（e.g.Selenomonas and Succiniclasticum）and of[2H]producing bacteria（e.g.Lachnospiraceae＿NK3A20＿group and Ruminococcus）were lesser（P<0.05）in the rumen inoculum from yaks than cattle.2.The average daily gain（ADG）and energy utilization efficiency were greater in yaks than cattleThe NDF and ADFdigestibilities were greater（P<0.05）in yaks than in cattle,and both of them decreased linearly（P<0.05）with increasing dietary energy level;whereas,digestibilities of dry matter,organic matter,crude protein and ether extract increased（P<0.05）linearly with increasing energy level.The ADG was greater（P<0.001）in yaks than in cattle,and increased（P<0.05）linearly with increasing energy levels.From the regressions of ADG on ME intake,the estimated ME requirement for maintenance was lesser（P<0.05）in yaks than in cattle(0.43 vs.0.57 MJ kg^-1BW^-0.75d^-1).The ratios of digestible energy（DE）:gross energy and ME:DE were higher（P<0.05）in yaks than in cattle,and increased（P<0.05）linearly with increasing dietary energy level.Ruminal p H decreased（P<0.05）,whereas concentrations of total VFAs and ammonia increased（P<0.01）with increasing dietary energy level,and all were greater（P<0.05）in yaks than in cattle.Concentrations of ruminal acetate and iso-VFAs were greater（P<0.05）,whereas propionate was lesser（P<0.05）in yaks than in cattle;acetate decreased（P<0.001）,whereas butyrate and propionate increased（P<0.001）linearly with increasing dietary energy level.3.The nitrogen rentention was greater in yaks than cattleThe fecal N and urinary urea-N was not affected（P>0.10）;whereas urinary N and urinary ammonia-N were lesser（P<0.017）in yaks than in cattle,and both of them were decreased linearly（P<0.01）with increasing dietary energy level.N-retention was greater in yaks than in cattle（P=0.045）and increased（P<0.001）with increasing dietary energy level.Urinary ammonia-N:total N was greater（P=0.048）,whereas urinary urea-N:total N was no differ（P>0.10）in yaks than in cattle and both of them decreased linearly（P<0.001）with increasing energy level.4.The urea cycle efficiency was greater in yaks than cattleThe urea entry rate（UER）,gastrintestinal tract entry rate（GER）,ornithine cycle（ROC）,and the gut wall urea nitrogen were greater（P<0.05）,whereas the UFE was lesser（P<0.05）in yaks than in cattle,both of them decreased linearly with increasing energy levels（P<0.05）.The ratio of GER to UER was not affected by animal species,dietary energy levels and their interaction（P≥0.10）.The ratio of ROC to GER was greater（P<0.05）,whereas the ratio of UFE to GER and UUA to GER were lesser（P<0.05）in yaks than in cattle.5.The microbial protein production was greater in yaks than cattleThe allantoin,uric acid,and total purine derivatives（PD）increased linearly（P<0.05）with increasing energy levels,the allantoin tended to be greater（0.05<P≤0.10）and total PD was greater（P<0.05）in yaks than in cattle.Microbial N（MN）and EMPS were greater（P<0.05）in yak than in cattle and both of them increased linearly（P<0.01）with increasing energy level.The MN from recycled urea,microbial capture of recycled urea N,as a proportion of UER and GER were greater in yaks than in cattle,and both of them increased linearly with increasing energy levels.6.in vivo ruminal bacterial communityThe RA of Firmicutes increased linearly（P<0.01）,whereas,of Bacteroidetes decreased linearly（P<0.001）with increasing energy level in both bovine species.The RA of Prevotella and Rikenellaceae＿RC9＿gut＿group decreased linearly（P<0.05）with increasing energy level in both bovine species.The RA of fibrolytic（e.g.Rikenellaceae＿RC9＿gut＿group）,and[2H]incorporating bacteria（e.g.Quinella）were greater（P<0.05）in yaks than in cattle.The difference on fiber digestibility,energy utilization,urea kinetic,ruminal bacterial community,and the microbial protein between yaks and cattle,which could confer an advantage to yaks over cattle in consuming low energy diets during the long winter on the QTP.These findings provided a new insight to understand the mechanism of yaks to adapt the harsh environment on the QTP.In addition,it also providings a theoretical foundation and technical guidance for precision nutrition in yaks and to promote the yak industry upgrade and transformation.