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U.S. Department of Agriculture, Beltsville, MD 20705
Abstract
The effect of ruminal ammonia concentration on nitrogen utilization was determined in eight rumen-fistulated steers weighing 183 to 226 kilograms. The steers received a basal 8.0% crude protein equivalent (CPE) diet that consisted of a forage-concentrate (30:70) mixture of 10.9% CPE which was fed plus cerelose that was infused. The animals were assigned to one of eight treatments in two 4 x 4 Latin squares. Ruminal ammonia concentration was varied by continuous intra-ruminal infusion of 15.3 liters/day of a tap water solution that contained cerelose, volatile fatty acids (VFA), magnesium sulfate, and between 0 and 140 g of urea. Dry matter digestibility was increased with the lowest level of urea infusion. Higher levels of urea infusion had no further effect on dry matter digestibility. Protein assayed as tungstic acid precipitable nitrogen (TAP-N) increased in ruminal digesta as urea input was increased to where the ammonia nitrogen (NH3-N) concentration was 2.2 mg/100 ml of rumen fluid. Increasing the NH3-N content beyond 2.2 mg/100 ml of rumen fluid had no effect on rumen digesta TAP-N. Increasing the NH3-N content beyond 4.5 mg/100 ml had no significant effect on total ruminal VFA concentration. Nitrogen retention increased as urea infusion increased until the ruminal NH3-N concentration was 4.5 mg/100 ml. Slight but nonsignificant further increases in nitrogen retention were observed as larger amounts of urea were infused, and ruminal NH3-N increased to 22.5 mg/100 ml. Possible explanations for this continued increase in nitrogen retention are discussed. Higher urea inputs increased blood plasma urea, arginine, and citrulline. The results of this study support the view that concentration of 2 to 5 mg NH3-N per 100 ml of rumen fluid is sufficient to allow maximum growth of rumen microbes.
1 SEA, Animal Science Institute, Ruminant Nutrition Laboratory.
2 Department of Dairy Science, University of Wisconsin, Madison 53706.
3 The technical assistance of E. E. Williams. Jr. and M. Poos is gratefully acknowledged.
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