Quantification of intraruminal recycling of microbial nitrogen using nitrogen-15

Studies were done to derive a technique to quantify intraruminal recycling of microbial nonammonia nitrogen (NAN). After dosing 15NH4+ into the rumen, ruminal NH3N and NAN pools were sampled over time for analyses of 15N enrichment. Compartmental analysis of both pools was used to quantify the percentage of microbial NAN that recycles in the rumen. Microbial NAN does not need to be fractionated from total NAN, assuming that only microbial NAN becomes labeled with 15N and that the ratio of microbial NAN:nonmicrobial NAN remains constant over time. Based on the data obtained from eight nonlactating cattle fed 85% corn silage diets at 1.5 to 1.0% BW, percentage of intraruminal recycling of microbial NAN was related to efficiency of bacterial protein synthesis (grams of CP/100 g of OM truly digested) in a curvilinear manner (y = 100 - .073x2). From this function, recycling was predicted to be approximately 75% for dairy cows consuming 50:50 forage:concentrate diets at 3.5% of BW. However, more data are needed to evaluate intraruminal recycling of microbial NAN using this technique under different dietary conditions.