Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/21/2007
Publication Date: 3/1/2008
Citation: Marini, J.C., Fox, D.G., Murphy, M.R. 2008. Nitrogen transactions along the gastrointestinal tract of cattle: A meta-analytical approach. Journal of Animal Science. 86(3):660-679. Interpretive Summary: Nitrogen metabolism and digestion involves the absorption and secretion of nitrogenous compounds along the gastrointestinal tract. Limited knowledge is available on the extent of these processes due to the rather invasive and expensive techniques employed for their measurement. A large body of literature is available, however, only on the measurement of nitrogen compounds entering or exiting the different compartments of the gastrointestinal tract. We have used a quantitative metaanalytical approach utilizing these published data to account for different sources of variation, including publication effects. Once coefficients were obtained, we then built a dynamic model that allows for the estimation of the nitrogen transaction along the gastrointestinal tract under different feeding conditions. This is an example in which post hoc analysis of the literature is able to extract data that were not available in the original publications. A better understanding of the endogenous losses of nitrogen will allow for better estimation of nitrogen requirements, a reduction in the amount of nitrogen fed, and ultimately in a reduction in the nitrogen excreted into the environment.
Technical Abstract: In ruminant animals, endogenous N (EN) secretions contribute to meeting the N requirement of the ruminal microflora. The EN also constitutes a sizable portion of the duodenal N flow, which might be available to the host animal. Most measures of EN have been accomplished with highly invasive techniques or unusual semisynthetic diets. By utilizing a statistical approach and data obtained from studies reporting duodenal, ileal, and fecal N flows in cattle, the EN losses and true digestibility of N were estimated for different segments of the gastrointestinal tract of cattle. A simulation for a reference diet (24.2 g of N/kg of OM, 32% NDF and carbohydrates of medium fermentation rate) consumed at 2% of BW daily estimated that the minimal contribution of EN to the N available in the rumen was 39%. The free EN represented 13% of the duodenal N flow, and when bacterial N of EN origin was considered, EN contributed 35% of the total N flow. The minimal entry of EN into various segments of the gastrointestinal tract was also estimated as: foregut, 10.54; small intestine, 3.10; and hindgut, 5.0 g/kg of OMI. Rumen dietary N degradability was 0.68, and true N digestibilities in the small intestine and hindgut were 0.75 and 0.49, respectively. A better understanding of the factors involved in EN losses will allow for a more accurate estimation of both N supply and N requirements. This will translate into improved accuracy of diet formulation and less N excreted into the environment.