Title: Nitrogen transactions along the gastrointestinal tract in cattle: a meta-analytical approach Authors
|Fox, D - CORNELL UNIV|
|Murphy, M - UNIV ILLINOIS|
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: June 8, 2007
Publication Date: September 9, 2007
Citation: Marini, J., Fox, D.G., Murphy, M. 2007. Nitrogen transactions along the gastrointestinal tract in cattle: A meta-analytical approach [abstract]. In:2nd International Symposium on Energy and Protein Metabolism and Nutrition, September 9-13, 2007, Vichy, France. 124:583. Technical Abstract: Endogenous nitrogen (EN) secretions occur along the whole gastrointestinal (GI) tract of animals constituting a loss of amino acids for the animal, but a supply of nitrogen (N) for the microbial population of the foregut and hindgut of ruminants. The quantification of these transactions is not only challenging, but also expensive, and consequently very little data is available in cattle. To determine the N transactions along the GI tract of cattle we utilized a statistical approach on data obtained from the literature. A database on N flow along the GI tract in cattle was compiled from published data on intestinally cannulated cattle, by systematically searching two leading animal science journals (J. Anim. Sci. and J. Dairy Sci.) between January 1990 and December 2006. A multilevel analysis was performed on the 455 treatment diets (from 108 studies) included in the database utilizing a mixed model, with study as a random component of the model. Model selection was done by utilizing the corrected Aikake's information criterion. Observed values of the dependent variables (Y'(is)) come from a multidimensional space, and because it is of interest to represent the data solely as function of the main variable of interest (N entering a particular segment of the GI tract), the value of the observations were adjusted (Y'(is)) for the lost dimensions. This was done by adjusting the Y(is) values to a reference diet (32% NDF, carbohydrates of medium fermentability rate, ingested at an intake level of 2% body weight). Y'(is)=mu+beta(N).X(N)(is)+epsilon(is where, Y'is = adjusted value of the dependent variable for the i treatment in the s study, beta(N) = regression coefficient, X(N) = the value of the continuous variable N entering the GI segment in study s, treatment i. The EN entering the foregut, small intestine, and hindgut, as well as metabolic fecal N, were estimated from the intercept of the regressions. These values were consistent with the ones reported previously in the literature. True digestibility of N for each segment, as well as for the whole GI tract, was derived from the slope of the regressions. Utilizing the coefficients reported in Table 1 of this paper, a model for the N transactions along the whole GI tract was constructed for the reference diet containing 24.2 g N/kg OM. The contribution of EN to ruminal microbial production (16%) and to duodenal flow (22%) was similar to the values obtained utilizing isotopic labeling methods. In conclusion, EN is an important contributor to total duodenal N flow. The statistical approach followed can be utilized to determine the N transactions along the GI tract of cattle under a variety of feeding conditions.