Uptake and transport of nonprotein nitrogen by the ruminant gut.

Ruminants can use dietary or endogenous nonprotein nitrogen (N) to meet protein requirements largely because of the symbiotic relationship between the ruminant and its gut microbes. Because of gut fermentation, a substantial portion (16-80%) of N is absorbed as ammonia N (NH3N). Net uptake of NH3N by portal-drained viscera ranges from 0.4 to 6.5 times net uptake of alpha-amino N, with proportionally greater net uptake of NH3N with forage diets than with high-energy diets. Uptake of NH3N appears to be by diffusion; therefore, rates of absorption are controlled by factors regulating NH3N concentrations in chyme. Urea N is transferred directly to the lumen of the gut from blood and indirectly from blood as a constituent of saliva. Therefore, rate of urea transfer is controlled in part by blood concentrations of urea. However, other less clearly defined mechanisms relating to type of diet, ruminal fermentation patterns, and intraruminal concentrations of metabolites affect urea transfer to the rumen. Urea N transfer to the lumen of the gut ranges from 10 to 42% of N intake. Nucleic acid N is absorbed from the small intestine as part of purines and pyrimidines, some of which ruminants may incorporate directly in nucleotides. Estimated nucleic acid N absorption is 7-8% of N intake.