The Factorization of the Protein Requirements of Ruminants and of the Protein Values of Feeds, with Particular Reference to the Significance of the Metabolic Fecal Nitrogen

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Abstract
The metabolic N of the feces, consisting of body constituents excreted with indigestible constituents of the feed in the feces, has heretofore been used merely in correcting apparent digestion coeffs. to true digestibility. However, the materials containing this fraction of the fecal N must be replaced in the animal body if its integrity is to be preserved. Furthermore, its replacement from the food supply cannot be presumed to be 100% efficient. Hency, the metabolic fecal N becomes an item in the N requirement of the animal, an item that has not previously been considered. Since the metabolic fecal N varies with the dry matter consumed, and hence is greater the greater the fiber content of the ration for a given net energy requirement, this item in the total protein requirement is largest for rations high in fiber, such as those derived from poor quality rough-age. In the immature ruminant in which the growth of new tissue is at its highest rate, some 20% of the requirement of truly digestible (metabolizable) protein is used in the replacement of the metabolic fecal loss, while at maturity over 60% of the total requirement is determined by this factor. This revision of the estimate of the protein requirement of ruminants by the factorization method brings the estimate into close agreement with exptl. results obtained by feeding trials designed to determine absolute protein requirements for maximum growth. However, such estimates still diverge considerably from some current feeding standards, being higher for the young animal and lower for the animal approaching maturity. As an alternative to including the metabolic fecal N with the other items of protein requirements, it may be charged against the feed to give an "available protein content. ".