INCORPORATION OF RECYCLED UREA-N INTO RUMINAL BACTERIA FLOWING TO THESMALL-INTESTINE OF DAIRY-COWS FED A HIGH-GRAIN OR HIGH-FORAGE DIET

Dairy cows fitted with flexible T-type duodenal cannulas were fed eith er high-grain or high-forage diets in a switchover design. Urea labele d 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 durin g 5 d of collection. Also, urine was totally collected for the 5-d per iod. Enrichment of N-15 in digesta and bacteria entering the duodenum and in urine, feces, milk and blood all increased until cessation of i nfusion, but N-15 ratios of urinary N:bacterial N were constant betwee n 40 and 72 h. Nonlinear regression was used to estimate enrichment va lues at the isotopic equilibrium applying the Marquardt method. Incorp oration 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; w hereas, values for the high forage diet were 7.4 and 12.7%, respective ly. Flow of N from the rumen to the small intestine tended to be highe r 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.