Huntington G B
Fed Proc. 1986 Jul;45(8):2272-6.
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.
反刍动物能够利用日粮中的或内源性的非蛋白氮来满足蛋白质需求，这主要是因为反刍动物与其肠道微生物之间存在共生关系。由于肠道发酵，相当一部分（16% - 80%）的氮以氨氮（NH₃N）的形式被吸收。门静脉引流内脏对氨氮的净摄取量是α - 氨基氮净摄取量的0.4至6.5倍，与高能日粮相比，采食牧草日粮时氨氮的净摄取量相对更高。氨氮的摄取似乎是通过扩散进行的；因此，吸收速率受调节食糜中氨氮浓度的因素控制。尿素氮直接从血液转移至肠道腔，也作为唾液的成分间接从血液转移至肠道腔。因此，尿素转移速率部分受血液中尿素浓度的控制。然而，其他与日粮类型、瘤胃发酵模式以及瘤胃内代谢物浓度相关的不太明确的机制也会影响尿素向瘤胃的转移。尿素氮向肠道腔的转移量占氮摄入量的10%至42%。核酸氮作为嘌呤和嘧啶的一部分从小肠吸收，反刍动物可能会将其中一些直接整合到核苷酸中。估计核酸氮的吸收量占氮摄入量的7% - 8%。
