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Four Suffolk x Dorset sheep were allocated in a 4 x 4 Latin square design and received a hay‐barley‐molasses diet in one of four different physical structures: (1) pelleted; (2) pelleted:chopped (60:40); (3) chopped:pelleted (60:40); or (4) chopped. The animals were penned individually and the diet was restricted to provide 20 g of dry matter (DM) per kilogram live weight daily. The effects of the diets on rumen fluid kinetics, fermentation pattern and microbial nitrogen (MN) supply were examined. Saliva secretion was estimated using an indirect method based on water balance in the rumen. When the animals were fed the chopped diet, the salivation rate, rumen fluid volume and rumen liquid outflow were all increased significantly (P < 0.05 to P < 0.01). No consistent effect of dietary structure on rumen fluid dilution rate, purine derivative (PD) excretion or MN supply was observed. Dietary structure had no effect on the rumen fermentation pattern, digestibility of DM or nitrogen.

Growing lambs were fed diets either low or adequate in phosphorus and their effects on Ca and P retention, bone formation and resorption and on ruminal microbial protein synthesis were measured. Plasma osteocalcin was used as an indicator of bone formation rate. Levels in plasma were lower in lambs fed the low P diet and were associated with a fall in plasma P concentration and reduced Ca and P retention. Low P intake had no effect on microbial protein synthesis or on bone resorption determined using urinary pyridinoline and deoxypyridinoline as markers. These results suggest that lack of mineral, rather than reduced ruminal microbial protein synthesis, is the main factor contributing to the reduction in bone formation rate seen in lambs fed low P diets.

The recovery in urine of an intrajugular infusion of physiological amounts of allantoin was measured in four sheep nourished by an intragastric infusion of volatile fatty acids and casein (to eliminate rumen fermentation). The recovery was 72% (S.E.M. 7) and the remainder was presumed to have been lost by diffusion into the gut and degradation by gut microflora. Measured in two sheep, allantoin was removed from the blood at a fractional rate of 0.30 h‐1, and excreted in urine at 0.23 h‐1. Calculation based on creatinine excretion showed glomerular filtration rate and tubular reabsorption of allantoin to be unchanged by the intravenous infusion. Maximal tubular reabsorption at 1.28 mmol day‐1 was saturated by the load of endogenous allantoin alone. In a second experiment with seven normally fed sheep (28‐50 kg live weight, all given 1 kg feed), urinary excretion and plasma concentration of allantoin were linearly related. However, the errors were such that plasma allantoin concentration would be of little value as a predictor of urinary excretion. There was a nearly twofold range in allantoin excretion (the larger animals excreting less), which implied that the supply of microbial biomass to the host animal per unit of feed ingested could be profoundly affected by feeding level.

Four sheep were maintained by infusion of volatile fatty acid (VFA), mineral and buffer solutions to the rumen and casein to the abomasum. After a 3 week period in which control measurements were made, glucose was introduced gradually to the rumen infusion mixture until glucose replaced about 65% of VFA energy. Further measurements were made in the fourth week of glucose additions after which the control VFA infusions were re‐established and measurements repeated. Urea kinetics were measured with [14C]urea. Rumen ATP concentrations were used as an index of microbial growth and increased from 1.5 nmol ml‐1 on control treatments to 46 nmol ml‐1 when glucose was given. The presence of a rumen fermentation resulted in a 40% decrease in plasma urea concentration but was without effect on rumen ammonia concentration. Urea irreversible loss rate decreased from 18.0 to 12.8 g urea day‐1 when glucose was given. This, however, was matched by a decrease in urinary urea excretion, with the result that urea degradation, obtained by difference, remained constant throughout the three treatment periods. Daily nitrogen (N) retention increased from 1.0 to 2.7 g when a rumen microbial population was present. A model of urea and NH3 transactions for the two dietary situations is presented. Calculations indicated that 0.40 of the casein N supply was reutilized as urea by the rumen micro‐organisms. It is suggested, however, that the improvement in N retention resulted from a change in protein:energy ratio of the infused nutrients rather than from an enhanced supply of protein to the intestine. The use of endogenous urea clearance as an index of epithelial permeability to urea is questioned.