ANIMAL FOODS AND FEEDING 303 



though perhaps not as valuable as originally thought, or at 

 least in the sense first given to it, is yet often used. This is 

 known as the nutritive ratio and expresses the ratio, calculated 

 directly from the amounts present, between the proteins and the 

 other two constituents. It will be observed that, approximately, 

 the protein and carbohydrates are equal in food value while 

 fats are 2.25 times as large. If the amount of fat food is multi- 

 plied by 2.25, it is then in equivalent terms to the other two. 

 If we make the following proportion : 



Amount protein : (Amount carbohydrates + amount fats X 2.25) 



then I : x will represent the ratio of the protein constituents to 

 the other two. This is termed the nutritive ratio. In a general 

 way the results of feeding depend upon this ratio, for when x 

 is large, e.g. 8 to 12, the ratio is usually a fattening one and 

 is termed broad, whereas when it is small, e.g. 4 to 7, the ratio 

 is not fattening but still supplies the energy requirements of 

 animals, especially for work, and is termed narrow. In the 

 broad nutritive ratio it will be observed the carbohydrates 

 and fats which produce body fat are in large proportion, while 

 in the narrow ratio they are small. 



Protein Requirement 



In discussing the maintenance requirement we have spoken 

 only of the energy factor. Not only does the animal body 

 require a certain amount of energy to be supplied in its food, but 

 there must also be in it a certain minimum amount of protein. 

 As food protein is absolutely essential to the formation of body 

 protein, the torn-down muscle tissue must be replaced by protein 

 supplied in the food. The energy represented in this protein 

 metabolism is part of the total energy requirement, but aside 

 from this there must be in the food a certain amount of protein 

 considered solely as building material. This may be illustrated 

 by the fact that while the necessary maintenance requirement 

 of 2000 Cal. per day for man, or 12,000 Cal. per day for steers, 



