Study On The Nitrogen Metabolism And Limiting Amino Acids Of Lactating Dairy Cows Fed Diets Differing In Forage To Concentrate Ratios

Four cows fitted with permanent rumen cannulas and simple T-type duodenal cannulas were first used to compare the effects of external digesta markers (Co-EDTA, YbC13 and Cr-mordanted straw) and recombined marker systems on the flow of digesta entering the duodenal canal of lactating dairy cows. Samples of digesta got from the duodenal canal were pooled and separated using filtration and highspeed centrifugation into two or three fractions defined as the liquid phase and solid phase (small particulate and large particulate). Co was primarily associated with the liquid phase; Yb was concentrated in small particulate matter, whilst Cr was associated with large particles. The results showed that the samples collected at duodenum were not representative of "true" digesta. Use of single markers resulted in large variance in dry matter flow estimation compared with the combination of two or three markers. The results showed that the combination of two or three markers could increase the accuracy of estimation of the AIA (Acid Insoluble Ash) flow at duodenum.The second part was mainly conducted to study the effects of four typical diets on rumen nitrogen metabolism, protein digestion and absorption in the small intestine, and dairy cow's milk performance. Four rumen, duodenum and Ileum cannulated cows were used in a 4X4 Latin square experiment design with different forage to concentrate ratios. The growth of rumen bacteria was inhibited due to the shortage of the nitrogen supply in the high forage diet (70% Chinese wildrye and 30% concentrate). There were no significant differences of the conversion of dietary RDOM (Rumen Digestible Organic Matter) into microbial nitrogen among the diets. The efficiency of microbial nitrogen synthesis was 29.08g/kg RDOM. The non-ammonia nitrogen flow at duodenum was increased due to the increment of microbial nitrogen. The ratios of microbial nitrogen to the total nitrogen flow at the duodenum ranged from 52.8% to 62.9% among the four diets. There was no significant difference in the digestibility of the non-ammonia nitrogen (average 67.70+1.91%) entering into the small intestine.The conversion rates of dietary nitrogen into milk nitrogen and deposition nitrogen in the tissue were increased with higher concentrate. Among the total nitrogen intake, about 25.61 % and 48.56% nitrogen were excluded through feces and urine respectively. Milk production and protein content were increased by increasing the quality of the forage and the level of the concentrate in the diet. There was a positive correlation between milk protein content and the non-ammonia flow in the small intestine.The third part was conducetd to compare the effects of four typical diets on the supply of the amino acids in small intestine and utilization of them. Forage to concentrate ratios did not affect the amino acid profiles of rumen bacteria and milk protein. The four diets did not affect the amino acids digestibility (ranged from 46.47% to 82.98%) in small intestine except for Arginine. Based on essential amino acid profile of the milk protein, the small intestinal ideal amino acid model was evaluated in four diets and the results showed that the Lysine and Methionine were the main limited amino acids. BCAA (Branch Chain Amino Acids) were next to them.In the fourth part, four rumen, duodenum and Ileum cannulated cows were used in a 4 × 4 Latin square experiment to compare the effects of duodenum infusing Lysine, Methionine and BCAA on milk...