Estimation of flows of organic matter and nitrogen components in postruminal digesta and effects of level of dietary intake and physical form of protein supplement on such estimates.

I . Steers fitted with simple rumen, abomasal and sometimes duodenal cannulas were given isonitrogenous and isoenergetic diets consisting of pellets containing equal amounts of rolled barley and chopped straw together with tapioca alone (B), or with some tapioca replaced by coarse soya-bean meal (M) or finely ground soya-bean flour (F). Diet B had an estimated rumen degradable nitrogen (RDN): metabolizable energy (ME) value of approximately 0.8 g/MJ. The corresponding value for each of the supplemented diets was 1.3-1.4. Diets were given at two levels of intake designed respectively to provide ME intakes to support weight gains of 0.5 kg/d (L) and 1 .O kg/d (H). Chromic oxide and polyethylene glycol were given as indigestible markers. Values were calculated for flows (g/24 h) at the abomasum of organic matter (OM), non-ammonia-N (NAN) and microbial (bacterial)-N (MN). Estimates of MN were based on RNA and a-s-diaminopimelic acid (DAP) measurements. The steers had low rumen protozoal counts throughout the whole experiment with fewer than 8000 ciliate protozoa/ml of rumen fluid. 2. Proportions of OM apparently digested between the mouth and abomasum were significantly lower for H than for L intakes but showed no other significant differences. Mean estimated efficiency of microbial protein synthesis was approximately 17 g N/kg OM apparently digested in the rumen for the LB diet, but was significantly greater at approximately 26 g N/kg OM apparently digested for the HB diet. The protein-supplemented diets gave values of 31-32 which were significantly greater than the value for the LB but not the HB diet. 3. Amounts of feed nitrogenous constituents surviving between the mouth and the abomasum were estimated from NAN minus MN corrected for endogenous N. Values for the proportions of feed N degraded, derived from these estimates, were 0.71, 0.66, 0.72, 0.69, 0.67 and 0.70 for diets LB, HB, LM, HM, LF and HF respectively. There were no significant differences due to level of feeding, supplementary protein or the physical form of that protein. 4. Estimates of MN flows at the abomasum based on RNA determinations in fresh samples did not differ significantly from estimates based on DAP determinations. However, storage of aqueous suspensions of mixed rumen bacteria by deep-freezing or freeze-drying resulted in significant losses of RNA. No such losses were observed for abomasal digesta. This meant that if samples were stored by these means before analysis, the use of RNA as a microbial marker was vitiated. 5. In some experiments digesta samples were taken from the duodenum as well as the abomasum. Estimates of total N flows did not differ significantly between these sites.