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Influence of Certain Nutritional and Physiological Factors on Urea Toxicity in Sheep¹

Shell Development Company, Modesto, California 94720^,4

Abstract

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 2⁵ factorial 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 LD₅₀ of urea was determined with 50 wether feeder lambs using a "multiple sample up-and-down method" bioassay technique. The mean LD₅₀ for urea was 1.45 log dose (g/100 kg body weight).

¹ Journal article MP 105, Biological Science Research Center, Shell Development Company, Modesto, California.

² Present address: Department of Animal Sciences, Washington State Univerity, Pullman.

³ The authors express their appreciation to Dr. L. J. Brown for his assistance in statistical design and analysis and to S. R. Wagoner and Melvina C. Sanguinetti for their valuable technical assistance.

⁴ Health and Nutrition Division.