GROWTH 



385 



storage at different ages, but the percentage of the feed protein 

 which is retained will depend upon the relation between this 

 capacity and the amount of protein actually supplied. For 

 example, suppose a calf weighing 100 pounds to be capable of 

 storing up per day 0.25 pound of protein and to require 0.05 

 pound for maintenance. If it receives 0.35 pound digestible 

 protein in its feed and is able to store up the maximum amount 

 of 0.25 pound on this ration, 71.4 per cent of the digestible 

 protein would be retained, while 28.6 per cent would katab- 

 olize and its nitrogen excrete in the urine. But if the feed 

 of the animal supplied 0.45 pound digestible protein, the gain 

 would still be 0.25 pound, since this is the maximum possible 

 for the animal, but the percentage of the feed protein retained 

 would be only 55.6, while 44.6 per cent of it would be katab- 

 olized. The organism is unable to use the added one-tenth 

 pound for constructive purposes and therefore it is katabolized 

 as shown in Chapter IX (402-404) and serves simply as a source 

 of energy. In other words, the greater the excess of protein 

 supplied in the ration over the minimum required by the de- 

 mands of growth and maintenance, the lower will be the per- 

 centage retained in the body. On the other hand, with rations 

 deficient in protein the percentage retention will increase with 

 the protein supply up to the minimum amount necessary to 

 utilize the growth capacity of the animal. 



Fingerling's experiments afford striking confirmation of the truth of 

 the foregoing deductions from the general laws of protein katabolism. 



A calf received daily in one period 8 kgs. of whole milk with an 

 addition of butter fat and lactose, while in the succeeding period 

 whole milk alone was fed in amounts proportional to the age of the 

 calf, averaging 11.875 kgs. per day. The results as regards protein, 

 expressed in terms of nitrogen, were as follows : 



TABLE 84. INFLUENCE OF PROTEIN SUPPLY ON PERCENTAGE RETENTION 



OF NITROGEN 



2 C 



