MEASUREMENT AND USE OF D-ALANINE AS A BACTERIAL MARKER

Use of D-alanine as a marker of bacterial dry matter (DM) and nitrogen (N) was evaluated and rates of degradation of soluble and insoluble protein in soybean meal (SBM) were determined. Accuracy and precision of the method used to measure D-alanine were high. D-alanine was superior to diaminopimelic acid as an indicator of bacterial N because of its smaller coefficient of variation (15.4 vs. 35.2%, respectively) when expressed as a ratio with bacterial N. During an in vitro incubation, ratios of bacterial N to D-alanine declined and when bacteria harvested from steers and from continuous culture fermentors receiving the same diet were compared, different bacterial N to D-alanine ratios were observed. Therefore, constant ratios should not be assumed and bacterial samples should be taken whenever possible to avoid errors in correcting for bacterial DM and N. Ten SBM samples were analyzed for soluble and insoluble protein content. Soluble, insoluble and total protein from SBM were incubated in vitro to estimate ruminal rates of protein degradation. Rate of degradation of soluble protein was greater (P < 0.05) than that of insoluble protein (10.69 vs. 9.57% h −1 , respectively). Rate of degradation of total protein was 10.61% h −1 . The small differences in rates of ruminal degradation for soluble and insoluble protein indicate that, at passage rates frequently observed for liquids and solids in cattle, a larger proportion of soluble SBM protein will escape bacterial degradation in the rumen than insoluble SBM protein. Key words: Cattle, D-alanine, bacterial nitrogen, degradation rate, soybean meal