Study On The Protein Nutrition Requirement,Digestion And Metabolism In Yak During Pregnancy And Lactation

The purpose of this study is to investigate the protein requirements and the effects of different feeding levels on serum biochemical indexes and rumen fermentation indexes of yak during pregnancy and lactation periods through feeding experiments,digestion and metabolism trials and nitrogen balance test.The results of this experiment would enrich the basic database of yak nutrition research and provide the theoretical basis for feeding standard of yak.Paper main research results are as follows:Experiment 1 Study on the Protein Nutrition Requirement,Digestion and Metabolism in Yak During PregnancyFifteen healthy yaks treated for estrus synchronization and natural service,and thenrandomly divided into three feeding levels groups after pregnancy tests（ad libitum group,80%the ad libitum intake group and 60%the ad libitum intake group）,Digestibility and respirometry trial were studied in the periods of the 150th,180th and220th days of pregnancy,and the rumen fluid and blood of experimental animals were collected for determination.The results showed that:1)The rumen ammonia nitrogen concentration increased with the pregnancy progressed（P<0.05）and dietary restriction significantly reduced the content of butyric acid in the total volatile fatty during pregnancy of the yak（P<0.05）.During pregnancy,the nitrogen metabolism of yak was significantly enhanced and the total protein level in yak blood was higher.The alanine aminotransferase in blood of yak decreased when the concentrate feeding level decreased（P<0.05）.With the pregnancy progressed.the glucose in blood of yak increased first and then decreased.Blood glucose levels will be significantly reduced due to low feeding levels in late pregnancy（P<0.05）.Low feeding level would reduce the reproductive hormone levels in yak during pregnancy,the levels of progesterone and prolactin in the blood of yak would be significantly reduced on the 150th day of pregnancy（P<0.05）,the levels of follicle stimulating hormone in the blood of yak would be significantly reduced on the 180th day of pregnancy（P<0.05）,and the levels of estradiol and luteinizing hormone in the blood of yak would be significantly reduced on the 220th day of pregnancy（P<0.05）.2)Apparent nutrient digestibility of feed in yak would be reduced due to low feeding levels during pregnancy.The apparent digestibility of ADF in yak increased with the pregnancy progressed（P<0.05）.The apparent digestibility of DM,OM,NDF,ADF and nitrogen were 59.27%-65.41%,63.02%-69.32%,51.34%-59.70%,29.66%-49.99%and 59.27%-68.80%,respectively.3)The net N requirements of yak increased with the pregnancy progressed.The recommended net protein requirements in this study during middle and late pregnancy were 2.84 g/kg W^0.75?d^-1 and 3.35 g/kg W^0.75?d^-1,respectively.On the premise of sufficient feed energy supply,the intake protein of yak in the middle and late pregnancy should not be less than 4.81 g/kg W^0.75?d^-1and 5.94 g/kg W^0.75?d^-1,respectively,Otherwise,the rumen metabolism,blood biochemical index and reproductive hormone index of yak would be affected.Experiment 2 Study on the Protein Nutrition Requirement,Digestion and Metabolism in Yak During LactationFifteen healthy yaks with close calving dates were randomly divided into threefeeding levels groups（ad libitum group,80%the ad libitum intake group and 60%the ad libitum intake group）,Digestibility and respirometry trial were studied in the periods of the 65th and 120th days of lactation,and the rumen fluid and blood of experimental animals were collected for determination.The results showed that:1)During lactation,the effects of different feeding on average day gain,milk yield and milk protein content were significant（P<0.05）and the effects of different feeding on milk fat was non-significant in yak（P<0.05）,The daily lactation of Yaks in the ad libitum group was2.7 kg-3.75 kg.2)The ammonia nitrogen concentration increased with decreasing feeding levels（P<0.05）and the total volatile fatty acids decreased as the level of feeding decreased during lactation of the yak（P<0.05）.he blood urea nitrogen content in the late lactation period of yaks was higher than that in the early lactation period,and low feeding level would significantly reduce the alanine aminotransferase in blood of yak during early lactation（P<0.05）.The P₄ levels of yaks in the late lactation period was higher than that in the early lactation period（P<0.05）.The contents of E₂ and PRL in yak blood would increase significantly when the food intake at a low level during lactation（P<0.05）.3)As feed levels decrease,the apparent digestibility of yak nutrients tended to increase.During lactation,the apparent digestibility of DM,OM,NDF,ADF and nitrogen were 66.37%-72.95%,68.65%-75.18%,56.78%-65.42%,45.30%-54.94%and 57.43%-68.32%,respectively.4)The net N requirements of yak decreased with the lactation progressed.The recommended net protein requirements in this study during early and late lactation were 2.44 g/kg W^0.75?d^-1 and2.05 g/kg W^0.75?d^-1,respectively.On the premise of sufficient feed energy supply,the intake protein of yak in the early and late pregnancy should not be less than 8.31g/kg W^0.75?d^-1and 9.06 g/kg W^0.75?d^-1,respectively,Otherwise,the rumen metabolism,blood biochemical index and reproductive hormone index of yak would be affected.