The present study compares estimates of rumen microbial-N production derived from duodenal flow measurements ("N and purine bases) with those from measurements of the urinary excretion of purine derivatives. Four Rasa Aragonesa ewes fitted with simple cannulas in the rumen and proximal duodenum were used. Four diets consisting of 550 g lucerne (Medicago safiva) hay/d as sole feed or supplemented with 220, 400 and 550 g rolled barley grain/d were given in a 4 x 4 random factorial arrangement. Yield of microbial protein from the rumen is the largest element of uncertainty associated with the estimation of protein supply in the new Feed Evaluation Systems for ruminants (Webster, 1992). Protein reaching the duodenum is predicted from fermentable metabolizable energy (FME) intake, but recognizing that the efficiency of microbial synthesis varies with the level of feeding. Additional sources of variation may be incorporated into this approach as long as they are identified and their effects properly quantified. Current measurements of rumen microbial production, based on duodenal flow estimates, are of limited value in this respect due to the need for fistulated animals and assumptions involved in the flow of digesta and in the microbial markers used.Recently, non-invasive methods based on the urinary excretion of purine derivatives (PD) have been developed to estimate microbial protein supplied to the duodenum (Chen available at https://www.cambridge.org/core/terms. https://doi

The present study compares estimates of caecotrophes production from urinary purine derivatives (PD) excretion with that from preventing caecotrophy by using a neck collar. A total of 64 New Zealand growing male rabbits were used to study the effect of diet composition on caecotrophes production. Diets were formulated using two sources of structural carbohydrates (fibre): alfalfa hay (AH) and sugar-beet pulp (SBP), mixed at two constant proportions, (0·75: 0·25) AH diets and (0·25: 0·75) SBP diets. Both diets included either barley or maize grain at two fibre: grain ratios (F/G, 80: 20 and 45: 55). Diets were given ad libitum. Growth rate, dry matter intake and digestibility were not modified by the grain source, although high F/G diets resulted in a lower growth rate (19·8 v. 26·4 g/day; P < 0·001). Between fibre sources, dry-matter intake and growth were higher in AH than in SBP diets (122·5 and 25·6 v. 101·6 and 20·4 g/day, respectively, P < 0·001 and P < 0·01). Rabbits given high F/G ratio and AH diets excreted more caecotrophes than those given low F/G ratio and SBP diets (19·5 and 20·9 v. 16·3 and 14·85 g/day, respectively). Microbial-N recycling through the caecotrophy process was higher when considering data from PD excretion (1·33 g/d) than when estimated by preventing caecotrophy (0·72 g/day).

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 soya-bean meal, maize gluten meal or fish meal to increase its protein content to 192 g/kg DM. Tr eatments 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 with15N infused into the rumen. The proportion of duodenal PB of microbial origin estimated from15N 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 from15N (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 from15N 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.

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Developed models to estimate rumen microbial yield from the urinary excretion of purine derivatives fail to account for a complete absorbed purines (Chen et al., 1990a, Balcells et al., 1991). Ruminal degradation of purine derivatives recycled via saliva has been suggested as a possible explanation after detecting the presence of allantoin and uric acid in the sheep saliva (Chen et al. 1990b). However, it is not known to what extent these losses may be affected by dietary induced variations in saliva flow (Kay, 1966).This paper reports preliminary results of an experiment designed to study the effect of diet processing and blood allantoin concentration on the rate of purine derivatives losses through salivary secretion.