Eight Hereford steers, fistulated at the rumen and offered a low quality grass hay (Axonopus affinus comprised 75 per cent of the pasture) were studied in pens for 100 days to evaluate urea and urea-molasses supplements in terms of their effect on hay intake, rumen fermentation and liveweight changes. The hay contained 0.58 per cent nitrogen and had an apparent digestibility of 49.8 per cent. Supplements were made up in a liquid form and infused directly and continuously into the rumen so that on a daily basis 56 g urea with and without 500 g molasses, or 112 g urea with 500 g or 1000 g molasses was delivered. When urea alone was infused, rumen ammonia and plasma urea concentrations were increased (P < 0.05) but the urea had no significant effect on liveweight change, on digestible organic matter intake (DOMI) or on the pattern of rumen fermentation. When infusions of urea and molasses were given, total DOMI was increased (P < 0.08) and the pattern of rumen fermentation changed: total rumen volatile fatty acid (VFA) concentrations were increased (P < 0.05), with an increased (P < 0.05) molar proportion of propionate in one of the treatments. Neither urea nor urea and molasses infusions significantly increased the animal's intake of hay. Liveweight change was related (r= 0.60) to DOMI and it was estimated that steers in the experiment required 41 g DOM kg.-0.75 day-1 for maintenance of liveweight. This level of intake occurred only when supplements containing molasses were given. Overall, the results indicated that little improvement in growth of cattle or of their utilization of low quality pastures could be obtained from a urea supplement. However, there may be potential for molasses as a concentrate supplement for cattle grazing native pastures.

Urea metabolism was studied in Brahman cross, buffalo, banteng and Shorthorn cattle offered a low quality hay. Intravenous injections of [14C]urea and 51Cr-EDTA were used to determine the irreversible loss of urea from the plasma, the degradation of urea in the rumen and lower digestive tract, and the glomerular filtration rate. When species were compared at equal liveweights and nitrogen intakes, buffaloes had significantly higher (P < 0.05) plasma urea concentrations and rates of irreversible loss of urea carbon from plasma than the other species. There were no significant differences between species in urinary urea excretion. Urea degradation in the digestive tract was linearly related to the irreversible loss of urea, and the proportion of irreversible urea loss degraded was significantly (P < 0.05) lower in Shorthorn cattle (48%) than in the other species (73-91%). Shorthorn cattle reabsorbed urea from the glomerular filtrate with a lower efficiency (60%) than did the other species (85-94%). In Brahman cross, buffalo and banteng, plasma urea recycled to the rumen was a relatively constant amount (4.3 g nitrogen/d) and represented on average 39% of the urea degraded in all parts of the digestive tract. Urea degraded in the digestive tract increased linearly with increasing irreversible loss of urea from plasma. It was concluded that, despite significant differences between species in urea synthesis and degradation, there was little indication that these differences constituted a significant nitrogen conservation mechanism in any one species.

1. Metabolism of urea in non-pregnant and pregnant sheep (1–25 d from term) has been examined. Injections of (14C) urea were used to estimate urea entry rate, urea pool size and urea space in sheep given 1000g of a diet of equal parts of crushed oats and chaffed lucerne hay (day a) and in the same sheep 4 d after the ration had been reduced to 250 g (day b). 2. On both experimental days (a and b), mean pool size was greater (14% on day a, 29% on day b) and urea space was greater (54% on day a, 24% on day b) in pregnant animals than in non-pregnant animals; mean plasma urea concentrations were lower (35%) in the pregnant animals on day a but were not significantly different on day b. 3. The entry rate of urea was similar in all the animals on day a, but was significantly higher (34%) in pregnant than in non-pregnant animals on day b. There was a significant decrease in urea entry rate in both pregnant (33%) and non-pregnant (86%) animals on day b. 4. The rate of excretion of urea was lower (26% on day a, 35% on day b) in pregnant animals, indicating a higher (31% on day a, 40% on day b) rate of degradation of urea in the digestive tract of pregnant as compared with non-pregnant sheep. 5. Measurements of urea entry rate have been used to calculate the upper limit of amino acid deamination in pregnant and non-pregnant sheep, and this has been used as an indication of the potential availability of amino acid carbon for glucose synthesis. It is suggested that, at a maximum, amino acids may have contributed the carbon required for 63 g/d and 52 g/d of glucose on days a and b respectively.

1. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with [15N]ammonium sulphate and [15N]urea and [14C]urea.2.Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates. 3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 g/d. 4. It was estimated that 59% of the dietary N was digested in the reticulo-rumen; 29% of the digested N was utilized as amino acids by the micro-organisms, and 71 % was degraded to ammonia. 5.Of the 14.2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4.3 g N/d represented recycling,largely within the rumen itself (through the pathways : ruminal ammonia > microbial protein > amino acids > ammonia). 6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through the rumen wall and 16.4 g N/d apparently arising from deaniination of amino acids and ammonia absorbed from the lower digestive tract. 7. In the 24 h after intraruminal injection of [15N]ammonium salt, 40-50 % of the N entering the plasma urea pool arose from ruminal ammonia; 26% of the 15N injected was excreted in urinary N. 8. Although 5-1 g N/d as urea was degraded apparently in the digestive tract, only 1.2 g N/d appeared in ruminal ammonia; it is suggested that the remainder may have been degraded in the lower digestive tract. 9. A large proportion of the urea N entering the digestive tract is apparently degraded and absorbed and the ammonia incorporated in the pools of nitrogenous compounds that turn over only slowly. This may be a mechanism for the continuous supply to the liver of ammonia for these syntheses. 10. There was incorporation of 15N into bacterial fractions isolated from rumen contents after intraruminal and intravenous administration of [15N]ammonium salts and [l5N]urea respectively. 11. A model for N pathways in sheep is proposed and, for this diet, many of the pool sizes and turn-over rates have been either deduced or estimated directly.