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Effect of Dietary Fiber on Endogenous Nitrogen Flows in Lactating Dairy Cows¹

D. R. Ouellet^*, M. Demers^,2, G. Zuur, G. E. Lobley, J. R. Seoane, J. V. Nolan^|| and H. Lapierre^*

^* Agriculture and Agri-Food Canada, Lennoxville, QC, J1M 1Z3 Canada;
Laval University, Ste-Foy, QC, G1K 7P4 Canada;
Biomathematics and Statistics Scotland, Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK;
Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK;
^|| PLUniversity of New England, Armidale, NSW, Australia

Corresponding author:
D. Ouellet; e-mail:
ouelletd{at}agr.gc.ca.

The effect of dietary fiber on endogenous N secretion was studied using a ¹⁵N isotope dilution technique in four fistulated Holstein cows. Two isonitrogenous diets differing only in fiber (NDF and ADF) content were used in a crossover design. One diet (HF) contained 37.4% NDF, while the other (LF) contained 23.3%. A new model was developed to estimate endogenous N secretions and losses for the preintestinal, intestinal, and the total sections of the gastrointestinal tract. Three precursor pools: TCA-soluble fraction of plasma, intestinal mucosa, and milk were compared. Although endogenous losses estimated with the model were numerically different for each precursor pool selected (TCA-soluble fraction > mucosa > milk), treatment effects were similar. As intestinal mucosa is probably closest to the precursor pool, these data are discussed. Non-urea N endogenous secretions contributed 13% of the duodenal N flow but were not affected by the fiber content of the diet. The nonurea N endogenous flow at the duodenum was comprised of approximately equal inputs from endogenous N direct, and that incorporated into the microbial biomass. Total endogenous N flows at the duodenum exceeded, by nearly twofold, estimated inputs of urea-N to microbial biomass. Metabolic fecal output averaged 17% of fecal N and was not affected by level of dietary fiber, but net losses from secretions occurring in the small intestine were higher with the low fiber diet. Overall, endogenous N secretions represented 30% of total digestive tract protein synthesis.

Key Words: endogenous N • dietary fiber • isotope dilution technique

Abbreviation key: EN = endogenous N, HF = high fiber, LF = low fiber

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