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Nitrogen Metabolism and Metabolic Fecal Nitrogen as Related to Caloric Intake and Digestibility¹

Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061

ABSTRACT

Six Holstein bull calves were used in each of two trials to evaluate nitrogen metabolism of the young ruminant. Two replicas of a 3 x 3 Latin square experiment were in each trial. In Trial 1, three isonitrogenous rations were formulated to differ in dry matter digestibility by substitution of 10, 20, and 30% powdered polyethylene plastic for corn starch. All rations were fed at 2.7 kg/100 kg body weight. Digestible energy intake was below the National Research Council's requirements for calves receiving the 20 and 30% polyethylene rations. For Trial 2, three similar rations were formulated to contain 0, 11, and 22% plastic. Crude protein and digestible energy content of these rations decreased with added plastic, but feeding rates were increased accordingly to provide equal protein and adequate digestible energy. Five- or seven-day nitrogen balances were measured for all calves with allowance of a 2-week adjustment during ration changes.

Fecal nitrogen was not altered by fecal dry matter changes in Trial 1. A linear increase in urinary nitrogen indicated that calves in Trial 1 used absorbed protein as an energy source. In Trial 2, fecal nitrogen increased linearly with added plastic to reflect a component of the metabolic fecal nitrogen increment which was 4.58 g per kilogram of fecal dry matter output. Metabolic fecal nitrogen is affected by energy balance of the animal as well as by digestibility of the ration.

¹ Manuscript published with approval of Director of the Division of Animal and Veterinary Science, Virginia Polytechnic Institute and State University.