Relationship between ruminal ammonia and nonprotein nitrogen utilization by ruminants. I. Development of a model for predicting nonprotein nitrogen utilization by cattle.

The influence of ration composition on mean ruminal ammonia concentration was studied by collecting samples of ruminal ingesta from cattle fed rations varying in crude protein and total digestible nutrient content. A minimum of four sampling times distributed throughout the day permitted calculation of mean ruminal ammonia concentrations. Mean ruminal ammonia concentration was positively related to dietary crude protein concentration and negatively related to total digestible nutrient concentration. It is postulated that mean ruminal ammonia concentration may be a useful criterion for predicting efficacy of nonprotein nitrogen supplementation. A quantitative approach for evaluating nonprotein nitrogen supplementation based upon determination of the point at which ruminal ammonia exceeds the requirement (5 mg ammonia nitrogen/100 ml rumen fluid) of the ammonia-utilizing bacteria is proposed. Dietary conditions expected to result in excessive concentrations of ruminal ammonia are defined and recommended upper limits for nonprotein nitrogen supplementation are presented. Theoretical relationships between composition of the unsupplemented ration, amount of nonprotein nitrogen added, and efficiency of nonprotein nitrogen utilization are discussed. The practice of adding nonprotein nitrogen so as to exceed 12 to 13% crude protein in typical dairy or feedlot rations needs to be reevaluated.

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