Incorporation of recycled urea-N into ruminal bacteria flowing to the small intestine of dairy cows fed a high-grain or high-forage diet

Dairy cows fitted with flexible T-type duodenal cannulas were fed either high-grain or high-forage diets in a switchover design. Urea labeled with N-15 was infused continuously into the jugular vein of each cow for 72 h, following 23 d of adjustment. Duodenal digesta, feces, and urine were sampled every 3 or 4 h, and milk and blood every 12 h during 5 d of collection. Also, urine was totally collected for the 5-d period. Enrichment of N-15 in digesta and bacteria entering the duodenum and in urine, feces, milk and blood all increased until cessation of infusion, but N-15 ratios of urinary N:bacterial N were constant between 40 and 72 h. Nonlinear regression was used to estimate enrichment values at the isotopic equilibrium applying the Marquardt method. Incorporation of endogenous urea into duodenal digesta and ruminal bacteria present in duodenal digesta also was calculated by isotopic equilibria. Endogenous urea contributed 19.1 and 37.5% of N in duodenal digesta and in duodenal bacteria for lactating cows fed the high-grain diet; whereas, values for the high forage diet were 7.4 and 12.7%, respectively. Flow of N from the rumen to the small intestine tended to be higher for cows fed the high-grain than high-forage diet (122 vs. 103% of N intake), a finding consistent with more recycled N on high-grain. (C) 1997 Elsevier Science B.V.