| Based on the present condition of the beef cattle rearing in our country, the beef cattle rears in the peasant household mainly with the inferior diet which composes of the straw chiefly in which protein content is very low and the non- protein nitrogen (NPN) proportion is high. So a great quantity of the protein that the beef cattle require must be synthesized in the rumen. Basing on the nutriology, as each condition is optimal, the largest yield of rumen microbe protein will be obtained only. Level of sulfur in diet becomes the factor that restricts synthesis of microbe protein if the available carbohydrate in the diet satisfies the requirement of beef cattle. Basing on present rearing condition above-mentioned and nutritive theory, we studied that the sulfur requirement with different NPN concentrate and the metabolic rule of nutrient substance in beef cattle with different sulfur content.3 healthy cross-breed(Limousin×LuXi)beef cattle were selected in the study, which were surgically fitted with permanence cannulate in rumen, the beginning of duodenum and the end of ileum. The experiment was designed according 3 × 3 Latins square. The progress of it was divided into 3 stages. Each stage included 3 phases that composed of 15 days for primary test and 5 days for formal test.. The concentration of urea in dietary was nil, 1%, 2% three level respectively. Each urea level corresponded with three content of sulfur that was 0.150%, 0.225% and 0.3000%. The content of protein, the concentrate of energy and the content of other nutrition ingredient were given according to NRC(1996) criterion. Cr2O3 mark and gathering all feces and urine was adopted to measure the digestive tract chyme outflow and to measure the nitrogen and sulfur equilibrium in body in the experiment. Digestive tract chyme and rumen fluid was collected to measure protein, fibre substance, amino acid etc. and to measure PH value, the concentration of NH3-N and Acetate and Propionate in rumen respectively. The vein blood was got through puncturing the neck vein to measure the content of urea nitrogen in plasma.The result showed that: The difference among the groups with different content of sulfur in same level of NPN was not remarkable (P<0.05). The PH value of rumen in each group was in the normal range (5.0-7.8). The raising of sulfur content in diet cut down the content of NH3-N in rumen. As the content of sulfur in diet rose, the maximal value and average value of NH3-N in rumen cut down in the same diet with the different content of sulfur. But the content of NH3-N in rumen did not cut down as the content of sulfur rising, but it did not rise when it attained sure concentrate. When urea was added in diet, the NH3-N concentration in rumen attains maximum peak 1-2 hour and the time of attaining the maximal content of NH3-N sulfur in rumen was postponed. The rising on concentration of sulfur in diet raise accelerated fermenting of rumen microbe, cut down rumen Acetate/Propionate ratio and increased Propionate yield in the rumen Sulfur concentration affected the protein metabolic and rumen amino acid synthesis obviously. In nil urea diet, the amount of true protein and AA flow had no remarkable difference with different sulfur level (P>0.05); in 1% urea diet, the amount of rumen AA-S outflow, true protein outflow of duodenum and the proportion of AA-S seize general amino acid in low sulfur group was smaller than middle sulfur group and high sulfur group (P<0.05), the amount of true protein of outflow of duodenum had no remarkable difference between the middle sulfur group and high sulfur group (P>0.05). In 2% urea diet, the amount of true protein, crude protein and general amino acid outflow of rumen and the duodenum was the largest with high sulfur level (P<0.05), and between low sulfur and middle sulfur had no remarkable difference (P>0.05). The digestibility of true protein and crude protein in small intestine in high sulfur group was higher significantly than middle sulfur and low sulfur group (P<0.05). Adding sulfur in diet wa...
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