Methodology for concurrent determination of urea kinetics and the capture of recycled urea nitrogen by ruminal microbes in cattle

We measured the incorporation of recycled urea-nitrogen (N) by ruminal microbes, using five ruminally and duodenally fistulated steers (237 kg) fed low-quality grass hay (47 g crude protein/kg dry matter (DM)). Three received 1 kg/day of soybean meal (SBM) and two received no supplemental protein (control). The experiment was 15 days long. Background enrichments of N-15 were measured on day 9 and continuous jugular infusion of 0.12 g/day [(NN)-N-15-N-15]urea began on day 10. Daily samples of urine, feces, ruminal bacteria and duodenal digesta from days 10 through 14 were used to determine plateaus in N-15 enrichment Duodenal and bacterial samples collected on day 15 were used to measure duodenal N flows. Bacterial N flow was calculated as duodenal N flow multiplied by duodenal N-15 enrichment divided by bacterial N-15 enrichment Bacterial N from recycled urea-N was calculated as bacterial N flow multiplied by bacterial N-15 enrichment divided by urinary urea N-15 enrichment Urinary enrichment of [(NN)-N-15-N-15]urea plateaued within 24 h, whereas (NN)-N-14-N-15 urea plateaued within 48 h of [(NN)-N-15-N-15]rea infusion. Bacteria reached a plateau in N-15 enrichment within 24 h and duodenal samples within 48 h. Urea production was 17.6 g of urea-N/day for control and 78.0 g/day for SBM. Gut entry was 0.99 g of urea-N/g of urea-N produced for control and 0.87 g/g for SBM. Incorporation of recycled N into microbial N was 9.0 g of Way for control and 23.0 g/day for SBM. Recycled urea-N accounted for 0.33 g of N/g of microbial N at the duodenum for control and 0.27 g/g for SBM. Our methods allowed measurement of incorporation of recycled urea-N into ruminal microbial N.