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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

University of Illinois

Abstract

1. The factorial method for the estimation of the protein requirements of animals is briefly outlined, and the divergence between such estimates for cattle and sheep and accepted feeding standards is discussed.
   
2. The metabolic fecal N which can be regarded as an inevitable loss to the body has been incorporated as a term in the general factorial procedure, and it is shown that the requirement of the animal for digestible protein cannot be divorced from the character of the ration which is fed, because the latter determines the output of metabolic fecal nitrogen.
   
3. The term "available protein," represented by the equation
   is suggested, to obviate the difficulties of multiple ration standards implied under a above. Available protein represents the metabolizable protein of a feed, taxed for the loss of body N which its ingestion entails and thus available for covering endogenous losses and the demands of other anabolic processes.
   
4. A method of calculation of the true digestibility coefficients of feeds and classes of feeds is presented, and it is shown that the same equation may be used to predict accurately the metabolic fecal N.
   
5. A table of feed composition, illustrating the computation of metabolizable, apparently digestible, and available protein is given, based on the above methods.
   
6. The importance of the metabolic fecal N in determining the minimum requirement of protein of the ruminant is shown by the explanation thus afforded of the anomalous results obtained when rations high in fiber are fed to cattle.
   
7. Two examples of the factorization procedure are given: for growing and fattening ewe lambs and for Holstein heifers. In both cases the effect of high quality and poor quality rations (as judged by the ratio of TDN to DM they contain) is illustrated.
   
8. It is shown that the metabolic fecal loss of N may account for some 20–70 percent of the requirement of the ruminant for metabolizable protein, depending on age of the animal. The effect of this factor on the protein requirement of monogastric animals is briefly discussed.
   
9. The metabolic fecal loss of N, entailing its replacement by truly digestible N, accounts for a part of the discrepancy between practical feeding standards and the factorized requirement. Nevertheless, it appears that current standards still tend to underfeed young animals and to overfeed more mature animals as far as their protein requirements are concerned.
   
10. The metabolic fecal energy is presumably in the same category with reference to energy requirements of animals as the metabolic fecal nitrogen with reference to protein requirements. But before net energy requirements, or the net (or metabolizable) energy value of feeds can be corrected, information must be obtained on the magnitude of the metabolic fecal energy and its determining factors.
    

¹ Commonwealth Fund Fellow, on leave of absence from the Ministry of Agriculture and Fisheries, Great Britain.

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