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 only practical way to incorporate the vast amount of published data into conceptual models of animal metabolism. The ability to accurately predict dietary N requirements of dairy cows depends upon the ability to accurately measure digesta flow and identify the origins of its N. Four lactating Holstein cows were fed a low-protein (9.8% CP of DM) ration of 28.6% timothy silage, 27.2% whole crop barley silage, and 44.2% gram-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 bacteria and protozoa were isolated and assayed for components. Rumen ingesta were manually evacuated to estimate rumen pool sizes, and duodenal digesta were sampled to estimate intestinal flow. Use of diaminopimelic acid (DAPA) was judged to result in rumen bacterial N pool sizes that were quantitatively reasonable and reflective of expected changes due to treatments. Conversely, use of ribonucleic acid (RNA) was judged to underestimate the rumen microbial N pool size and result in biologically implausible differences due to treatments. Use of DAPA was judged to slightly underestimate duodenal flow of bacterial N and suggested changes due to treatments consistent with expectations. In contrast, RNA suggested biologically implausible differences in duodenal flow of bacterial N due to treatments. A novel mathematical procedure utilizing both DAPA and RNA to estimate rumen pool sizes and intestinal flows 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. Key words: Rumen bacteria, protozoa, diaminopimelic acid, ribonucleic acid, phosphatidylcholine