Effect of dietary fiber on endogenous nitrogen flows in lactating dairy cows.
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.
Ouellet
D R
DR
Agriculture and Agri-Food Canada, Lennoxville, QC, J1M 1Z3 Canada. ouelletd@agr.gc.ca
Demers
M
M
Zuur
G
G
Lobley
G E
GE
Seoane
J R
JR
Nolan
J V
JV
Lapierre
H
H
D000818:Animals; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D018592:Cross-Over Studies; D004043:Dietary Fiber / Q000008:administration & dosage / Q000494:pharmacology*; D004063:Digestion; D004386:Duodenum / Q000378:metabolism / Q000382:microbiology; D005260:Female; D005766:Gastrointestinal Contents / Q000737:chemistry; D005772:Gastrointestinal Transit; D007082:Ileum / Q000378:metabolism; D007421:Intestine, Small / Q000378:metabolism*; D007774:Lactation / Q000378:metabolism; D008892:Milk / Q000737:chemistry; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes; D012417:Rumen / Q000378:metabolism*
D004043:Dietary Fiber; D009587:Nitrogen Isotopes; D009584:Nitrogen