FOOD REQUIREMENTS 869 



or milk production, or for purposes that require less protein, 

 e.g. work, fat formation or maintenance without production. 

 As protein is usually the most expensive energy yielding 

 constituent of the food, a knowledge of the nutritive ratio is 

 of importance to feeders. 



1. Methods of determining Requirements. 



1. Live Weight Test. — In these, the value of the food in 

 meeting the requirements of the animal is determined by 

 alterations in live weight. When the weight increases, the 

 food is providing more than the maintenance requirements, 

 and the excess is being stored in proportion to the increase 

 in weight. When the weight decreases the food is deficient, 

 and the animal is using its own tissues as a source of energy. 

 In the case of dairy cows, of course, the milk secreted must 

 be taken into account. 



Provided a large number of animals be considered and 

 the weighing be continued over a long enough period, these 

 experiments give results of practical value. Normally, how- 

 ever, the weight fluctuates from day to day, especially in the 

 case of ruminants, owing to the great weight of the contents 

 of the intestinal tract. A bullock may vary from day to 

 day as much as 20 kilos, the change being in the contents 

 of the gut, and not in the living tissue of the animal. 



2. Slaughter Tests- — In these experiments a group of 

 animals as nearly identical as possible in age and condition 

 are taken. Some are killed and the carcases analysed. The 

 others are fed for varying periods before being killed and 

 analysed. The difference in those analysed before feeding 

 and those after feeding is taken as an indication of the value 

 of the food. 



In these experiments, which are very laborious and 

 require a long time to carry out, it is assumed that the 

 animals killed at the beginning of the experiment are 

 identical in percentage composition with what the fed group 

 were at the beginning. 



3. The "Balance Sheet" Experiment. — An exact determina- 

 tion of the food requirements of an animal and of the value 

 of any food-stuff in meeting these requirements can only be 



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