CRITICAL-EVALUATION OF DIAMINOPIMELIC ACID AND RIBONUCLEIC-ACID AS MARKERS TO ESTIMATE RUMEN POOLS AND DUODENAL FLOWS OF BACTERIAL AND PROTOZOAL NITROGEN

Whole animal mathematical models have become widely accepted as the on ly practical way to incorporate the vast amount of published data into conceptual models of animal metabolism. The ability to accurately pre dict dietary N requirements of dairy cows depends upon the ability to accurately measure digesta flow and identify the origins of its N. Fou r lactating Holstein cows were fed a low-protein (9.8% CP of DM) ratio n of 28.6% timothy silage, 27.2% whole crop barley silage, and 44.2% g rain-based concentrate (DM basis). This was supplemented with 1.3 kg d (-1) of ground barley mixed with either no protein supplement, 1.11 kg of soybean meal, 0.67 kg of blood meal or 0.20 kg of urea. Rumen bact eria and protozoa were isolated and assayed for components. Rumen inge sta were manually evacuated to estimate rumen pool sizes, and duodenal digesta were sampled to estimate intestinal flow. Use of diaminopimel ic acid (DAPA) was judged to result in rumen bacterial N pool sizes th at were quantitatively reasonable and reflective of expected changes d ue to treatments. Conversely, use of ribonucleic acid (RNA) was judged to underestimate the rumen microbial N pool size and result in biolog ically implausible differences due to treatments. Use of DAPA was judg ed to slightly underestimate duodenal flow of bacterial N and suggeste d changes due to treatments consistent with expectations. In contrast, RNA suggested biologically implausible differences in duodenal flow o f bacterial N due to treatments. A novel mathematical procedure utiliz ing both DAPA and RNA to estimate rumen pool sizes and intestinal flow s of bacterial and protozoal N did not provide biologically plausible estimates thereby demonstrating that at least some of the assumptions relative to use of DAPA and RNA are not correct.