Nitrogen transactions along the gastrointestinal tract of cattle: A meta-analytical approach

¹ U.S. Department of Agriculture/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston 77030
² Department of Animal Science, Cornell University, Ithaca, NY 14853
³ Department of Animal Science, University of Illinois, Urbana 61801

^* To whom correspondence should be addressed. E-mail: marini{at}bcm.edu.

	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 semi-synthetic 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 N/kg 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: foregut 10.54, small intestine 3.10, and hindgut 5.0 g/kg 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 different 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.

Key Words: cattle, endogenous, meta-analysis, nitrogen, requirement