132 THE CHEMISTRY OF CATTLE FEEDING 



cent. The available energy of the digested protein is only 

 about 85 per cent, of the total fuel value (4*93 kt. per Ib.) ; 

 and that of the digested carbohydrates and fibre about 90 

 per cent. (376 kt. per Ib.). 



The total fuel values and available energy of various foods 

 will be found, with other data, in the tables at the end of the 

 next chapter. 



The number of kilo-pound-units of available energy, per 

 unit of mass (i Ib.), may be called the " static" or "thermic 

 value " of the food. This energy is available only to maintain 

 the animal in stationary condition, i.e. without gain or loss of 

 weight. It is not all available for other purposes. The reason 

 for insisting upon this will be made plain in later chapters. 



Protein. The static values form a useful basis for com- 

 parison of the foods used for the maintenance of animals, but 

 they afford no information regarding the amount of protein 

 required. It was shown (p. in) that under all conditions 

 animals require certain amounts of that constituent to repair 

 the waste of the nitrogenous tissues ; and if they do not get 

 enough, they die of nitrogen starvation. 



Theoretically the amount of digestible protein required 

 for the maintenance of an animal is equal to the amount 

 oxidised 1 in a given time. This amount may be estimated, 

 as previously shown, from the amount of nitrogen in the urine 

 which contains all the nitrogenous products of metabolism. 



The amount of protein oxidised, however, varies under 

 different circumstances. It depends partly upon the amount 

 of digestible protein and other nitrogenous compounds (p. 112) 

 in the food, and partly upon the degree of protection from 

 oxidation afforded by the presence of larger or smaller 

 quantities of non-nitrogenous nutrients. In order to ascertain 

 the minimum amount of protein required, it is necessary to 

 investigate how much is oxidised when the animal is fasting. 

 The following example will make this clear. 



1 This is not always a true measure of the amount of protein which 

 should be given in the food. In some cases, if the protein were limited 

 to this amount, the digestibility of the other constituents would be 

 diminished. 



