Urea is a synthetic compound containing 46 per cent, nitrogen; now widely used as a fertiliser. It also has a use in ruminant nutrition as a substitute for feed protein, with a marked advantage of being cheap at 1/- per lb. The cost of feeding 6 grammes daily to sheep is 1/5th. penny.
It has been employed for some time in experimental nutrition work with sheep at the Burdekin Drought Feeding Unit, where the superiority of wheat and urea mixtures have been amply demonstrated; compared with wheat supplements fed on an equivalent energy basis. It has, however, several disadvantages, one of which is toxicity.
When ingested the urea is rapidly broken down by ruminal organisms into ammonia and absorbed as such. High blood ammonia levels are associated with tetany, coma and death.
Current experiments at the Burdekin involve feeding a daily supplement of 4 ounces of wheat combined with 3, 6 and 9 grammes of urea; with the addition of 31 per cent, anhydrous sodium sulphate. This ration is fed as 3 and 4-day feeds on Mondays and Thursdays; allowing a mean intake of 9-36 grammes of urea within the period of eating the ration. This varies from 6 hours up to 4 days in the twice weekly feeding regime. The urea-sulphate mixture is added to cracked wheat with the addition of 1¼ per cent limestone, and fed out in troughs to pens of 12-16 sheep. In addition, an allowance of one pound of rough age is provided from self-feeders.
During the initial period of the experiment 21 sheep were lost from urea poisoning. It was found that losses could occur when as little as 12 grammes of urea were given in the one feed. Factors such as access to water and the addition of sodium sulphate were ruled out as contributing to the toxicity of the urea. The principal cause was the rate of ingestion of the urea mixture. The addition of a small quantity of roughage (3-6 lbs.) was instrumental in slowing down the rate of consumption from 30 minutes to 8 or more hours. By this method up to 36 grammes of urea were fed without any untoward results. After a period of conditioning, the roughage was omitted and the stipulated regimen of wheat and urca alone fed without further losses.
Symptoms occurred within 40 minutes after the feed was put out. Affected sheep tended to isolate themselves; with a stilted gait, rapid respiration and varying degrees of tympany. From this point there was a rapid progression to sternal or lateral recumbency with frothing at the mouth and a degree of tetany. Tympany became more marked as the condition progressed. Milder cases persisted in this state with recovery within the hour; severe cases passed into a condition of coma and death, associated with profuse salivation and stertorous breathing. All deaths occurred within 24 hours after the feed was put out.
Treatment attempted was the oral administration of peanut oil, 6 per cent. acetic acid, ruminal massage and trocarising. The results were disappointing in that ten died out of twelve affected.
Post-mortem examination showed injection of the visible mucous membranes, often leaden in colour. There was a quantity of fluid in the pericardial sac and congestion of peripheral blood vessels. Ruminal contents were dry; in comparison to abundant fluid seen when trocarised before death. No inflammatory changes were seen in the abomasum or small intestine. Extensive myocardial haemorrhages were a consistent feature
Experiments at Glenfield have indicated that 8 grammes of urea administered to a 60 lb. sheep as an aqueous solution can induce toxic symptoms. As against this double the dose may be administered with molasses, without any toxic symptoms.
It would appear, therefore, that when urea is first fed daily intakes exceeding 6 grammes should be viewed as dangerous, and it is preferable to condition sheep by daily allowances of 3 up to 6 grammes over a period of one week, fed daily. Following this, 2-day rations may be put out with a small quantity of roughage; 18-36 lbs. per 100 sheep, gradually reducing the roughage. Full bi-weekly feeding, however, still is attended by some risk and further experiments are necessary to determine the exact dangers involved.
It should be stressed, though, that urea has shown itself to be a valuable adjunct in the utilisation of low quality roughages when added to the cereal supplement. It requires care in feeding and the points made above should be kept in mind until more definite recommendations can be made concerning feeding. Pelleting wheat and urea mixtures have given promising results and this avenue undoubtedly will be pursued in the future.