Factors for Computing Requirements of Protein for Maintenance of Cattle

Factors for daily losses of endogenous urinary nitrogen, skin nitrogen, and metabolic fecal nitrogen were derived from published data. Endogenous urinary nitrogen from 82 experiments with cattle ranging from 28 to 625 kg normal body weight averaged .44 g per kg.S. Mean daily skin nitrogen losses from 61 experiments were .035 g per kg.6 body weight. From healthy calves fed whole milk, practically all fecal nitrogen is metabolic. Under such conditions metabolic fecal nitrogen averages .2% of dry-matter intake. Metabolic fecal nitrogen for ruminating cattle was estimated from 70 natural and semi-synthetic low protein diets by subtracting from fecal nitrogen 10% of feed nitrogen. Mean metabolic fecal nitrogen was 10.9 g per kg fecal dry matter or 4.7 g per kg dry-matter intake. The sum of factorial nitrogen losses multiplied by 6.25 is the net protein requirement for maintenance. Crude protein of feed is obtained by dividing the net protein for maintenance by the product of percentage of feed protein absorbed and percentage of absorbed protein utilized. Net protein efficiency varies from .7 for milk-fed calves to .45 for mature ruminant diets. Protein for maintenance can be calculated from body weight, type of diet, dry-matter intake, and dry-matter digestibility.