Significant losses of endogenous nitrogen (EN) occur during the processes of digestion and absorption along the gastrointestinal tract, EN secretion originating from saliva, gastric juices, bile, pancreatic secretions sloughed epithelial cells, mucin, and AA during digestion originating predominantly from various digestive secretions, mucoproteins, desquamed epithelial cells shed from the gut lining and from the intestinal flora. EN can contribute significantly to the ruminal N pool which is necessary for rumen microbial growth. Concerns of manure disposal, nitrates in ground water, and high ammonia loss to the environment have increased a need to utilize feed N more efficiently and minimize N excretion in urine and feces of domestic animals. Correction of these inevitable losses is necessary for the estimation of true digestibility values of N, which allows better adjustment of supply to requirement, estimation of the net availability for maintenance and growth, and reduction of N pollution.

Prof. Tan Zhiliang, Institute of Subtropical Agriculture, CAS and his team developed a series of studies to determine and manipulate EN losses in goats. The results showed that the difference and amino acid profile (AAP) methods underestimate the duodenal flows of EN, the ileal flows of EN were overestimated by the water soluble method compared to the isotope dilution method (Zhou CS, et al. Archives of Animal Nutrition, 2009, 63:1-12). The effect of dietary methionine (Met) levels on EN flows at different part of the digestive tract of growing goats was determined using a 15N isotope dilution technique. The dietary treatments consisted of a total mixed ration containing three levels of Met (0.15%, 0.25% and 0.35%) respectively. It was found that at 0.15% Met level, the lowest flow in EN at the duodenum and ileum occurred. The EN secretion contributed to 26% and 23% of the duodenal and ileal total N flows respectively, and the proportions were not affected by the dietary Met levels, when dietary Met supplementation was at a 0.15% level, the lowest flow of EN occurred at the duodenum and ileum (Zhou CS, et al. Journal of Animal Physiology and Animal Nutrition, 2010, 94:594-604). There were no significant effects of the dietary starch source (mainly from corn (CR), wheat (WR), paddy (PR) and sorghum (SR) treatments) on the intestinal flows of EN. The duodenal flows of EN were 2.40, 2.39, 2.18 and 1.56 g/d for the CR, WR, PR, and SR treatments, respectively, as determined by the difference method, and the flows of EN at the ileum were 1.17, 1.12, 1.01 and 0.70 g/d, respectively, as estimated by the water-soluble method (Zhou CS, et al. British Journal of Nutrition, 2010, 103:1755-1761).