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Utilization of Nitrogen from Soybean Meal, Casein, Zein, and Urea by Mature Sheep

Department of Animal Husbandry, University of Missouri, Columbia, Missouri 65201^,3

The utilization of nitrogen contained in soybean meal, casein, zein, and urea was studied in 12 mature wethers. Net microbial synthesis rates during the 6-hour period after feeding corresponded to ruminal ammonia concentrations, suggesting that the ruminal ammonia level in these experiments was a limiting factor in microbial protein synthesis. Maximum conversion of dietary nitrogen to microbial nitrogen in the rumen during a 24-hour period was estimated to be 62.5, 85.3, 40.9, and 90.1%, respectively, for soybean meal, casein, zein, and urea. Estimated production rates of acetic, propionic, butyric, isovaleric, and valeric acids during the first 6 hours after feeding suggested that zein supported the lowest microbial activity in the rumen, and that urea supplementation resulted in isovaleric and valeric acids production rates equivalent to or greater than rates when the other nitrogen supplements were present in the diet. Daily endogenous urinary and metabolic fecal nitrogen determined by regression analysis utilizing soybean meal as the only nitrogen supplement were 72 mg/kg body weight^0.75 and 340 mg/100 g of dry matter intake. Biological values of 84.8, 78.3, 84.3, and 76.4 were obtained for soybean meal, casein, zein, and urea, respectively.

KEY WORDS: • net protein value • microbial protein synthesis • soybean meal • casein • urea • zein • sheep

¹ Present address: Department of Animal Science, Louisiana State University, Baton Rouge, La. 70803.

² Present address: Department of Animal Science, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691.

³ Journal Series no. 6389. Approved by the Director of the Missouri Agricultural Experiment Station.

Manuscript received 4 April 1973.