Influence of Certain Nutritional and Physiological Factors on Urea Toxicity in Sheep

Urea toxicity, as influenced by various nutritional and physiological factors, was investigated with 160 sheep. The variables (energy levels, protein levels, age, period of fasting and urea levels) were studied in a 25factorial design. The two energy levels were a low-energy (15% concentrate) and a high-energy (85% concentrate) ration; and the two digestible protein levels were 3.8 and 8.8%. Lambs, 5 months of age, and ewes, approximately 5 years of age, were the two age groups. The sheep were administered either 44 or 176 g urea per 100 kg live weight at 1 or 24 hr. after feeding. The effect of these various factors was determined by blood NH3-N levels at 0, 30, 60, 90 and 120 min. after treatment and by mortality. The animal survival and blood NH3-N levels were influenced by all factors studied. The highest mortalities and highest blood NH3-N levels occurred with animals fed the high-energy, low-protein diet; thus, the ratio of energy to protein was most important. The lambs were more susceptible to the toxicity of urea than the ewes. The period of fasting, per se, had very little effect on the toxicity of urea. The mortality and blood NH3-N levels were directly related to the urea dose level.The LD50of urea was determined with 50 wether feeder lambs using a “multiple sample up-and-down method” bioassay technique. The mean LD50for urea was 1.45 log dose (g/100 kg body weight).