Factors for Computing Requirements of Protein for Maintenance of Cattle

Abstract

Factors for daily losses of endogenous urinary N, skin N and metabolic fecal N were derived from published data. Endogenous urinary N from 82 experiments with cattle ranging from 28-625 kg normal body weight averaged .44 g per kg.5. Mean daily skin N losses from 61 experiments were .035 g per kg.6 body wt. From healthy calves fed whole milk, practically all fecal N is metabolic. Under such conditions metabolic fecal N averages .2% of dry matter [DM] intake. Metabolic fecal N for ruminating cattle was estimated from 70 natural and semi-synthetic low protein diets by subtracting from fecal N 10% of feed N. Mean metabolic fecal N was 10.9 g/kg fecal DM or 4.7 g/kg DM intake. The sum of factorial N 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, DM intake and DM digestibility.