This difference appears to be more marked in the earlier years, 

 while as the animals matured the scrub nearly or quite overlook the 

 pure-bred steer in this respect, so that it seems questionable whether 

 the differences in the third year's experiments are of much sig- 

 nificance. 



In explanation of such a difference, little but speculations can be 

 presented. It is not easy to conceive how any metabolic process 

 or set of processes, such, for example, as are involved in the pro- 

 duction of fat from carbohydrates, can be conducted more efficiently 

 in one individual than in another. It would seem that if such a 

 difference between individuals exists as is indicated by the foregoing 

 results, we must seek for its explanation in differences in the 

 character of the body substance gained or katabolized. Upon this 

 point, of course, respiration calorimeter experiments afford but 

 little information. They show the total gain or loss of ash, nitro- 

 genous matter and fat (and glycogen), but reveal nothing as to 

 the part of the body where they are deposited or katabolized, nor 

 as to the exact nature of the material involved. It may not be 

 without significance in this connection that, as shown on previous 

 pages, the growth of the pure-bred steer was more largely iu 

 body girth, while the scrub steer increased relatively more rapid- 

 ly in length and height, or that, as will appear later, the pure-bred 

 steer showed a tendency to fatten while the scrub inclined to gain 

 relatively more nitrogenous material. In other words, the dif- 

 ference in availablity may conceivably be related to the apparent 

 difference in the nature of the gains or losses. 



As regards any influence of age upon the percentage availabilty 

 of the feed, the results appear indecisive. In the case of the bay, ('aside 

 from the questionable results of 1906), the differences are very small 

 and in opposite directions with the two animals. In the case of the 

 mixed grain of 1906 and 1907, there is apparently a tendency to 

 a somewhat greater availability by the older animals. On the 

 whole, the results as regards the influence of age afford little support 

 to the belief in a greater ability of young animals to utilize the 

 metaholi/.ablc energy of their lecd. 



Energy Required for Maintenance. 



From the results of the two periods on hay alone in each 

 year, it is easy to compute on the one hand the amount by which 

 it would have been necessary to increase the supply of metaboli/able 

 energy to prevc'in any loss of potential energy from the body, or, 



(12. 



