200 THE FEEDING OF ANIMALS 



must have protein for the formation of the nitrogen com- 

 pounds of the milk, or a steer for the growth of flesh, in a 

 quantity proportional to the production, and food must 

 supply it. There is, therefore, a minimum supply of pro- 

 tein, which, in a particular case, is necessary for the 

 maintenance and for constructive purposes, less than 

 which ultimately diminishes production to the extent of 

 the deficiency, or else requires the use of body tissue. 



288. The different classes of nutrients are to some 

 extent interchangeable in their functions. That is to say, 

 all the organic nutrients may be burned to supply energy. 

 Protein may be so used even to withdrawing it from the 

 purposes to which it is necessary unless the carbohydrates 

 or fats are sufficient to protect it from being consumed as 

 fuel. A proper supply of the non-nitrogenous nutrients 

 is required, therefore, to insure the application of the 

 necessary minimum of food protein to its peculiar uses. 

 The carbohydrates and fats are the physiologically eco- 

 nomical source of the main part of the energy used by 

 farm animals. 



