Effects Of Niacin On Grouth Performance,Apparent Digestibility Of Nutrients And Lipid Metabolism In Yak Under Heat-Stress

The present study was conducted to investigate the changes in production perfo rmance,apparent digestibility of nutrients and lipid metabolism of niacin in heat-str ess conditions in yak living in Alpine high altitude pastoral areas.This thesis consi st two trials:（1）Evaluation of heat-stress in yak;（2）The effect of dietary niacin o n the performance of yak production and lipid metabolism.Trial 1:Evaluation of heat-stress in yak.Six healthy yaks similar mean body weight（187.58±14.90kg respectively）,and similar age（1.5 years old）were used and their own stage comparison method was used.Each column was raised in single column,and each column was repeated.Feeding experiment was carried out from June to September 2018.The entire trial period consisted of preliminary feeding period（30d）and formal feeding trial（90d）.Following parameters were studied during the research trial,temperature humidity index（THI）,respiratory rate,production performance,apparent digestibility of nutrients and blood indices,to evaluate the impact and extent of heat stress on calves.The results showed that:1)The temperature and humidity index（THI）of the barn is obviously changed with the summer temperature.In June,the THI was lower than 72,and the thermal environment was not reached.The THI in July was increased（P<0.05）,which is greater than 72 but lower than 79.In mild thermal environment,THI increased significantly in August（P<0.05）,which was moderate thermal environment.2)Compared with non-heat stress period,the respiratory rate of yaks in mild and moderate thermal environment were significantly increased（P<0.05）,while in moderate heat stress,the concentration of HSP-70,ACTH and COR in yaks serum were significant increased,growth was decraesd but not significant,DMI significantly decreased,weight loss reached 1.74%to normal body weight,when yaks were under heat stress.Trial 2:Effects of niacin on growth performance and lipid metabolism of calves during heat stressTwelve healthy Maiwa yaks,1.5 years old and body weight（211.92±17.29kg）were selected and randomly divided into two experimental groups（control andexperimental groups）,each group was repeated 6 times,and the control group was tested one by one.During the trial period from July to September 2018,the control group was fed the same diet as in the trial 1.However,the experimental group niacin supplement according to the body weight（0.049 g/kg/d）on the basis of the experimental diet from August to September.The respiratory rate,production performance,apparent digestibility of nutrients and blood parameters were measured to investigate the mitigation effect of niacin on heat stress response of yak.The trial results are as follows:1)Supplimentation of niacin during heat stress period,the respiratory rate of yak did not increased significantly（P>0.05）,the concentration of HSP-70,ACTH and COR in serum decreased significantly compared with the control group,and also decreased（P<0.05）the heat stress to yaks.The weight of yak was not significantly decreased（P<0.05）,DMI and ADG were significantly increased（P<0.05）,and DMI was significantly higher than control group in September month（P<0.05）.2)supplementation of niacin during heat stress,which has an effect on the digestion and utilization of nutrients.With the increase of THI,the rumen pH was significantly lower than that of control group（P<0.05）,and the NH₃-N is significantly higher than the control group（P<0.05）,with the decrease of heat stress intensity,TVFA,AA,PA,BA and NH₃-N were significantly increased（P<0.05）.However,supplementation of niacian significantly increased the apparent digestion of EE and CP rate（P<0.05）during heat stress.3)Supplementation of niacin during the heat stress period alleviate the body’s inflammation and also improve the capacity of blood oxygen supply.Heat stress significantly（P<0.05）increased the White blood cells（WBC）,Monocytes（Mon）,Red blood cell count（RBC）,Mean red blood cell hemoglobin（MCHC）,Red blood cell distribution width（RDW）,and Mean platelet volume（MPV）concentration in yak,while,Red blood cell volume（MCV）and Platelet distribution width（PDW）concentrations were decreased significantly（P<0.05）,in addition,Interleukin-1β（IL-1β）,Tumor necrosis factor alpha（TNF-α）and Interleukin 10（IL-10）concentrations were significantly increased（P<0.05）.While the addition of nicotinic acid significantly（P<0.05）increased the concentration of RDW in the blood and decreased the concentration of MCV,Mean corpuscular hemoglobin（MCH）and PDW.The concentration of WBC was significantly lower than that of the control group（P<0.05）.At the same time,with the decrease of heat stress intensity,nicotinic acid significantly（P<0.05）decreased the concentrations of IL-10,IL-1βand TNF-α.4)Supplimentation of nicotinic acid during heat stress has an inhibitory effect on lipid catabolism.Heat stress increased concentrations of high density lipoprotein（HDL-C）,adiponectin（ADP）,and cyclic adenosine monophosphate（cAMP）in yak serum（P<0.05）,while significantly（P<0.05）,decreased the adenylate-activated protein kinase（AMPK）concentration,The addition of nicotinic acid significantly（P<0.05）inhibited the increase of ADP concentration,meantime inhibiting the decrease of AMPK（P<0.05）,and also significantly decreased the ADP,cAMP and HSL when the heat stress intensity decreased,while the concentration of TG significantly（P<0.05）increased.In summary,the change of THI has different degrees of stress to yak response.As the THI increases,the yak’s respiratory rate also increases,the body weight decreases,and the secretion of hormones and immune function in the body are enhanced.Yak has ability to protect his body from the heat stress,while heat stress reduces the digestion and utilization of nutrients by yak.Yak mobilizes body fat to maintain energy needs.The RBC,MCHC,RDW and MPV concentrations are increased,which affects the blood oxygen supply to yak The addition of niacin has a positive effect on the respiratory rate,production performance and immune function of yak during heat stress.The reduction of HSP-70alleviates the stress caused by the increased ambient temperature.The apparent digestibility of EE and CP increases,providing the energy needed by yak,reduces the mobilization of body fat,and also increases the regulation of red blood cells by MCV and MCH in whole blood,to ensure the adaptation of yak and growth during heat stress...