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Maintenance Nitrogen Requirements and Intestinal Microorganisms in Rats¹

Department of Animal Science, Iowa State University of Science and Technology, Ames, IA 50011

Nitrogen (N) balance trials with young growing rats fed diets with and without nonabsorbable antibiotics and with various concentrations of crude protein (CP) were conducted to measure maintenance N requirements. In addition, cecal contents were analyzed for ammonia (NH₃), urea (U) and free amino acids (AA) to study the reduction in maintenance N requirements due to the action of antibiotics. Results of the N-balance trials verified the reduction in maintenance N requirements with antibiotics as described in the literature. The reduction, however, was not as large as previously demonstrated, possibly because of severe diarrhea and greater liquid accumulation in the cecum of rats fed antibiotics. The concentration of free AA and U in the postabsorptive region of the intestine (cecum) of rats consuming antibiotics was greater than when antibiotics were not consumed. Cecal N concentrations supported the concept that the mode of action of antibiotics in reducing maintenance N requirements was via reduced destruction of AA of body origin in the lower intestine. Cecal levels of NH₃ were similar in both antibiotic and nonantibiotic groups. The amount of N needed for body maintenance when conditions favored minimum destruction of body AA by lower intestine microorganisms was small, less than 1% dietary CP. Maintenance N may be limited to the needs for tissues on the exterior of the body, such as hair, which are poorly recycled through the digestive tract.

KEY WORDS: • nitrogen balance • intestinal microorganisms • protein • rats • biological value

¹ Journal Paper No. J-11491 of the Iowa Agriculture and Home Economics Experiment Station, Ames IA. Project No. 2454.

² Department of Dairy Science, Louisiana State University, Baton Rouge, LA 70803.

Manuscript received 30 October 1985. Revision accepted 30 September 1986.