Microbial contribution to duodenal purine flow in fattening cattle given concentrate diets, estimated by purine N labelling (15N) of different microbial fractions

The origin of duodenal purine bases (PB) was studied in a digestion experiment with four heifers, cannulated in the rumen and duodenum, which received a basal concentrate (152 g crude protein (CP) per kg dry matter (DM)) together with barley straw (85 : 15 fresh weight basis) or the same concentrate supplemented with soya-bean meal, carbohydrate-treated soyabean meal, maize gluten meal or fish meal to increase its protein content to 192 g/kg DM. Treatments were assigned to the four animals in five experimental periods according to an incomplete Latin-square design. Each 30-day period included 20 days of change-over adaptation and 10 days of experimental measurements. The flow of digesta entering the duodenum was estimated using Yb and acid-detergent insoluble ash as indigestible markers according to a double-marker system and microbial nitrogen (N) and PB were labelled with 15N infused into the rumen. The proportion of duodenal PB of microbial origin estimated from 15N enrichment of PB-N averaged 0.66 (s.e. 0.029) and did not differ between treatments nor when protozoa or bacteria associated with liquid (LAB) and solid (SAB) fractions were used as a reference sample. On average microbial contribution to duodenal non-ammonia N was higher when estimated from the PB/N ratio than from 15N (0.67 v. 0.55 (s.e. 0.015)) although differences were small and not significant when LAB was the reference sample (0.58 v. 0.52 (s.e. 0.018)) reflecting the higher PB/N ratio of this fraction compared with SAB and protozoa (2.04 v. 1.65 and 1.60 (s.e. 0.04) mmol/g). Considering only the duodenal PB of microbial origin resulted in estimates of microbial N synthesis from the PB/N ratio of SAB similar to those derived from 15N enrichment of both bacterial fractions (12.9 v. 13.5 and 13.3 (s.e. 0.83) g/kg of organic matter apparently digested in the rumen OMADR)) but underestimated the values derived from LAB (9.9 g/kg OMADR). Regardless of the estimation method, neither the duodenal flow of microbial N nor the efficiency of microbial synthesis differed between treatments. These results suggest that a significant proportion of duodenal PB have a non-microbial origin which may lead to overestimation of microbial yield when PB are used as a marker. Differences in PB/N ratio between microbial fractions is another important factor to be considered.