Urinary excretion of purine derivatives (PD) was used to estimate the microbial N (MN) supply to sheep in three experiments designed to examine the effects of DMI and BW on the efficiency of microbial N supply (EMNS) to the host animal. In Exp. 1, four sheep of about 45 kg BW were given 328, 656, 984, and 1313 g of DM/d of a hay/concentrate diet in a Latin square design. Excretion of PD per kilogram of digestible organic matter intake (DOMI) increased with intake, and EMNS increased from 12.0 to 28.3 g of MN/kg of OM digested in the rumen (DOMR). In Exp. 2, 19 sheep ranging from 22 to 73 kg BW were all offered 820 g of DM/d of the same diet as that fed in Exp. 1. Although DM digestibility was relatively constant, PD excretion varied from 4.5 to 13.5 mmol/d and EMNS from 8 to 36 g of MN/kg of DOMR, both inversely related to animal BW. In Exp. 3, five sheep of 48 to 57 kg BW were given a different diet at 702, 966, or 1,237 g of DM/d. Purine derivative excretion per kilogram of DOMI increased with the DMI:BW ratio. Calculated EMNS ranged from 23 to 35 g of MN/kg of DOMR. Pooled data from all experiments showed EMNS to be related to the DMI:BW ratio. It is suggested that the DMI:BW ratio defines the ruminal digesta passage rate and hence outflow of microbial protein. The results imply that the EMNS for a given diet is not constant, but changes with intake.

The saliva of sheep was shown to contain significant concentrations of uric acid (16 (SD 4.5) pmol/l) and allantoin (120 (SD 16.4) pmol/l), sufficient to recycle purine derivatives equivalent to about 0.10 of the normal urinary excretion. When allantoin was incubated in vitro in rumen fluid, it was degraded a t a rate sufficient to ensure complete destruction of recycled allantoin. In a series of experiments in which allantoin was infused into the rumen of sheep fed normally, or into the rumen or abomasum of sheep and the rumen of cattle completely nourished by intragastric infusion of volatile fatty acids and casein, no additional allantoin was recovered in the urine. These losses were probably due to the degradation of allantoin by micro-organisms associated with the digestive tract. It is concluded that all allantoin and uric acid recycled to the rumen via saliva will be similarly degraded. Therefore, the use of urinary excretion of purine derivatives as an estimator of the rumen microbial biomass available to ruminants will need to be corrected for such losses.The urinary excretion of purine derivatives by ruminants has been proposed as an estimator of the rumen microbial protein supplied to the host animal (e.g. Topps & Elliott, 1965). This is because the nucleic acids flowing to the small intestines are essentially of rumen microbial origin (McAllan & Smith, 1973). Absorbed purines are degraded to hypoxanthine, xanthine, uric acid and allantoin. These are excreted in urine, and should relate quantitatively to the amount of microbial purines, and hence microbial protein, absorbed.The quantitative relationship between purine supply and derivative excretion has been examined previously following an abomasal infusion of a nucleic acid concentrate in sheep. Recovery as derivatives of absorbed nucleic acid purines was 0.84 (mmol/mmol) after correction for possible utilization by the animal (Chen et al. 1990a), with 0.1 6 of the purines unaccounted. A similar observation was made in steers (recovery of 0.85) (Verbic et al. 1990). Since there are no reports of a mammalian allantoinase (EC 3.5.2. 5), the most likely route of loss would appear to involve microbial degradation within the digestive tract and this hypothesis was tested in the current study.Three aspects were examined : secretion of purine derivatives into the rumen via saliva; the rate of degradation of allantoin in rumen digestd by rumen micro-organisms; and whether allantoin escaping degradation within the rumen would then be excreted unchanged. A provisional report of recycling of purine derivatives via the saliva has already been made (Chen et ul. 1989).