INTRODUCTORY 5 



the animal to do work. But the various compounds of which 

 the food is composed have very different values in this respect. 

 Some of them give out much more heat or energy than others. 

 A pound of fat, for example, gives out between two and three 

 times as much heat as an equal quantity of protein or carbo- 

 hydrate. 



Any substance will not serve to repair the waste due to 

 oxidation, to increase the mass of the animal when growing 

 or fattening, or for the production of milk, wool, etc. The 

 tissues, milk, wool, etc., are composed of certain chemical 

 compounds. Some of the compounds of the food can be 

 transformed into these; others cannot. Non-nitrogenous 

 compounds, for example, cannot be transformed into nitro- 

 genous compounds. The food of animals must be not only 

 sufficient in quantity, but it must also be of the right kind. In 

 short, it must contain certain minimum quantities of the 

 particular compounds required for each specific purpose. 



This has long been recognised by farmers and other 

 practical men. Store cattle and horses doing no work are fed 

 on hay, straw, roots, etc. ; but when they are to be fattened, 

 or do work, they are given oil cake, oats, and other rich and 

 expensive foods. Practical men are faced with several impor- 

 tant problems in this connection, and if they do not exercise 

 a right judgment in regard to them their operations will result 

 in financial loss or, at least, in diminished profits. They 

 know by experience which kinds of food are more or less 

 suitable for particular purposes and approximately what 

 quantities of each are requisite and advisable in particular 

 circumstances. Indeed, it is from this point the accumulated 

 experience of many generations of farmers in this and other 

 countries that all scientific investigation of the problems 

 must begin. 



Uncontrolled experience, however, does not tell us why 

 one food is more suitable than another or how new foods, 

 which are introduced from time to time, should be used. It 

 does not show which of the suitable foods will be the most 

 economical, financially, for any specific purpose, or what 

 quantities of any one kind of food should be used to replace 



