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S U M M A R YTwo experiments were carried out to determine endogenous losses and the response of urinary purine derivatives to increased duodenal inputs of purine bases. Four ewes each fitted with a re-entrant cannula at the proximal duodenum, and conventionally fed, were subjected to full replacement of duodenal digesta followed by the administration of a solution either free of purines (Expt 1) or enriched with increasing amounts of purines, to supply 0-48-21-27 mmol/animal per day (Expt 2). Basal daily urinary excretions of allantoin, uric acid, hypoxanthine and xanthine were 11-5 + 0-94, 9-9 ±0-67, 6-9 ±0-46 and 1-2 ±016 mg/kg W 075 . Allantoin was the only purine derivative which increased in response to incremental inputs of duodenal purines. The relationship between allantoin excretion and infused purines showed a urinary recovery of 0-8 for purines infused at > 220 umol/kg W 076 . Lower rates of infusion did not alter allantoin excretion. The results show urinary allantoin to be a useful index to estimate duodenal input of purines when animals are fed close to or above their energy maintenance requirements.

An experiment was conducted with dairy cows to study the partitioning of excreted purine derivatives between urine and milk and to quantify the endogenous contribution following the isotopic labeling of microbial purine bases. Three lactating cows in their second lactation that had been cannulated in the rumen and the duodenum were fed a mixed diet (48:52, roughage/concentrate ratio) distributed in equal fractions every 2 h, and duodenal flow of purine bases was determined by the dual-phase marker system. Nitrogen-15 was infused continuously into the rumen to label microbial purine bases, and the endogenous fraction was determined from the isotopic dilution in urinary purine derivatives. Urinary and milk recovery of duodenal purine bases were estimated at early (wk 10) and late (wk 33) lactation by the duodenal infusion of incremental doses (75 and 150 mmol purine bases/d) of RNA from Torula yeast. Each period was 6 d, with RNA being infused during the last 4 d, followed by measurement of the flow of purine bases to the duodenum. The isotope dilution of purine derivatives in urine samples confirmed the presence of an endogenous fraction (512 +/- 36.43 micromol/W0.75 or 56.86 mmol/d) amounting to 26 +/- 3.8% of total renal excretion. Total excretion of purine derivatives in urine plus milk was linearly related to the duodenal input of purine bases, but the slopes differed (P < 0.005) between lactation stages resulting in a lower equimolar recovery in early (y = 58.86 (+/-3.89) +0.56 (+/-0.0164) x; r = 0.90) than late lactation (y = 58.86 (+/-3.89) + 0.70 (+/-0.046) x; r = 0.80). Excretion of purine derivatives through milk represented a minimum fraction of total excretion but responded significantly to the duodenal input of purine bases. No differences between lactation stages were detected, and variations in milk yield did modify significantly the amount of purine derivatives excreted through the milk.

Two dry cows fitted with simple cannula in the rumen and duodenum, and fed with a mixed diet (straw:barley, 50:50), were used to determine endogenous losses and response of urinary purine derivatives (PDs) to duodenally infused yeast-RNA. Duodenal flow of purine bases (PBs) was determined by a dual marker system, and 15N was infused continuously into the rumen to label microbial PBs. The isotope dilution of urinary PDs in relation to duodenal PBs confirmed the presence of an endogenous fraction (236 μmol/kg LW0.75) bigger than in sheep and lower than values estimated in cows with impaired rumen fermentation. Excretion of PDs increased linearly in response to incremental supply of PBs with an equimolar recovery of 0.84 mol/mol. However, net recovery of duodenal PBs was 0.67 for the basal diet and 0.65, 0.90, 0.79 and 0.82 for the four levels of PB infusion. It is suggested that differences in digestibility between yeast-RNA and duodenal PBs might explain differences in recovery estimations.

The aim of this study was to investigate the effect of presence or absence of protozoa on rumen fermentation and efficiency of microbial protein synthesis under different diets. Of 20 twin paired lambs, 1 lamb of each pair was isolated from the ewe within 24 h after birth and reared in a protozoa-free environment (n = 10), whereas their respective twin-siblings remained with the ewe (faunated, n = 10). When lambs reached 6 mo of age, 5 animals of each group were randomly allocated to 1 of 2 experimental diets consisting of either alfalfa hay as the sole diet, or 50:50 mixed with ground barley grain according to a 2 × 2 factorial arrangement of treatments. After 15 d of adaptation to the diet, the animals were euthanized and total rumen and abomasal contents were sampled to estimate rumen microbial synthesis using C(31) alkane as flow marker. Different ((15)N and purine bases) and a novel (recombinant DNA sequences) microbial markers, combined with several microbial reference extracts (rumen protozoa, liquid and solid associated bacteria) were evaluated. Absence of rumen protozoa modified the rumen fermentation pattern and decreased total tract OM and NDF digestibility in 2.0 and 5.1 percentage points, respectively. The effect of defaunation on microbial N flow was weak, however, and was dependent on the microbial marker and microbial reference extract considered. Faunated lambs fed with mixed diet showed the greatest rumen protozoal concentration and the least efficient microbial protein synthesis (29% less than the other treatments), whereas protozoa-free lambs fed with mixed diet presented the smallest ammonia concentration and 34% greater efficiency of N utilization than the other treatments. Although (15)N gave the most precise estimates of microbial synthesis, the use of recombinant DNA sequences represents an alternative that allows separate quantification of the bacteria and protozoa contributions. This marker showed that presence of protozoa decrease the bacterial-N flow through the abomasum by 33%, whereas the protozoa-N contribution to the microbial N flow increased from 1.9 to 14.1% when barley grain was added to the alfalfa hay. Absolute data related to intestinal flow must be treated with caution because the limitations of the sampling and maker system employed.

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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

To study the absorption of microbial lysine in growing rabbits, a labelled diet (supplemented with 15 NH 4 Cl) was administered to six animals (group ISOT); a control group (CTRL, four rabbits) received a similar, but unlabelled, diet. Diets were administered for 30 d. An additional group of six animals were fed the unlabelled diet for 20 d and then the labelled diet for 10 d while wearing a neck collar to avoid caecotrophy (group COLL), in order to discriminate it from direct intestinal absorption. At day 30 animals were slaughtered and caecal bacteria and liver samples taken. The 15 N enrichment in amino acids of caecal bacteria and liver were determined by GC -combustion/isotope ratio MS. Lysine showed a higher enrichment in caecal microflora (0·925 atom% excess, APE) than liver (0·215 APE) in group ISOT animals, confirming the double origin of body lysine: microbial and dietary. The COLL group showed a much lower enrichment in tissue lysine (0·007 (SE 0·0029) APE for liver). Any enrichment in the latter animals was due to direct absorption of microbial lysine along the digestive tract, since recycling of microbial protein (caecotrophy) was avoided. In such conditions liver enrichment was low, indicating a small direct intestinal absorption. From the ratio of [ 15 N]lysine enrichment between liver and bacteria the contribution of microbes to body lysine was estimated at 23 %, with 97 % of this arising through caecotrophy. Absorption of microbial lysine through caecotrophy was 119 (SE 4·0) mg/d, compared with 406 (SE 1·8) mg/d available from the diet. This study confirms the importance of caecotrophy in rabbit nutrition (15 % of total protein intake).