Urinary excretion of allantoin or total purine derivatives (PD) may constitute a valuable index of rumen microbial production in sheep (Chen et al.1990; Balcells et al. 1991) provided the urinary recovery of absorbed purines is constant. Purine losses through saliva account for less than 10 % of the non-urinary losses as suggested by the low salivary concentrations of allantoin and uric acid (Surra et al. 1993). In addition to salivary secretion, a significant amount of blood PD may be directly excreted into the gut, as suggested by the fate of the uric acid and guanine infused, respectively, into the blood of humans (Sorensen, 1960) and pigs (Simmonds et al.1973). However little is known about these potential losses in ruminants and how they might be affected by variations in intestinal fermentation consequent to the site of digestion promoted by different feeding conditions.

The aim of this experiment was to elucidate wether variations in hindgut fermentation may affect the urinary excretion of purine derivatives.

The experiment was performed on four adult Rasa Aragonesa ewes (35 ±0.4 kg) fitted with permanent catheters in both duodenum and caecum and receiving 700 g DM/d of alfalfa hay. Treatments attempted to induce variations in hindgut fermentation and PD concentration and consisted in a continuous infusion (2 l/d) of saline solution as control or either 3 g/d of RNA, 100 g/d of starch, or 200 g/d of cellulose dissolved in water up to 21. All infusates were given via caecum in a complete randomized cross over design, except cellulose which was infused into the duodenum due to frequent clogging of the caecal catheter.