GROWTH 



387 



TABLE 86. INFLUENCE OF ENERGY SUPPLY ON PERCENTAGE RETENTION 

 OF NITROGEN 



470. Meaning of utilization. The percentage of the digest- 

 ible protein of the feed which is retained in the body of the 

 growing animal, then, is not in itself a measure of the efficiency 

 of the animal organism in converting feed protein into body 

 protein, since the proportion retained is affected both by the 

 magnitude of the protein supply in the feed and by the energy 

 content of the ration. What then is the correct conception of 

 the utilization of the feed protein ? 



As appeared in the previous section, the amount of protein 

 which a growing animal can store up seems to be a function of 

 its age (463), and the attempt was made to formulate approxi- 

 mately the capacity for growth in this sense at different ages. 

 The percentage utilization of the feed protein in the physio- 

 logical sense, as distinguished from the percentage retention, is 

 the ratio between the body protein thus stored up and the least 

 amount of feed protein in excess of the maintenance require- 

 ment which is necessary to support this growth under the most 

 favorable conditions, especially as to energy supply. Suppose, 

 for example, that an animal three months old actually has the 

 capacity, as computed by the formula on page 378, to store 

 up daily 1.23 pounds protein per 1000 pounds live weight, and 

 that it has been shown that it can just reach this capacity on 

 a ration supplying 2 pounds of digestible protein per day. De- 

 ducting 0.5 pound for maintenance (415), there remains 1.5 

 pounds of protein in the ration out of which is produced 1.23 

 pounds of body protein. The utilization is therefore 1.23 -5- 

 1.5 = 82 per cent. If, on the other hand, it was found that 

 2.5 pounds of protein had to be supplied in the ration in order 



