64 



MAINTENANCE RATIONS OF FARM ANIMALS. 



ration is derived from experiments in which the sufficiency of the 

 ration was judged of from its effect in maintaining the live weight 

 of the animal. In experiments on mature animals extending over a 

 considerable period of time, it is unlikely that any gross error is 

 involved, especially if determinations of the nitrogen balance show 

 the protein supply to be adequate. In short periods, on the other 

 hand, and especially in experiments upon young animals, the live 

 weight is a notoriously untrustworthy guide. The general reasons 

 for this are familiar, but in young animals another very important 

 factor enters into consideration. As is well known, the tendency to 

 growth is one of the most marked characteristics of young animals. 

 Waters 1 has shown that this impulse to increase of tissue is so marked 

 that it may apparently take precedence over the demand for main- 

 tenance, so that an animal may continue to increase in size of skeleton 

 for a considerable time even on a submaintenance ration. 



Some 15 immature cattle were fed for considerable periods on ra- 

 tions just sufficient to maintain their live weight. Under these condi- 

 tions the animals continued to grow in height, in depth of chest, and 

 length of head. At the same time, however, there was an evident 

 falling off in the amount of fat tissue, both as judged by the eye and 

 as shown by the appearance and by the chemical composition of the 

 carcass. Histological studies, too, showed a reduction in the size of 

 the fat cells, and analysis of the adipose tissue showed a lower fat and 

 higher water and protein content than in check animals. What 

 occurred was evidently a consumption of body fat to supply energy, 

 while at the same time an approximately equal weight of protein tis- 

 sue was produced which, on account of the relatively low energy value 

 of protein and of the relatively large amount of water accompanying 

 it, represented a much smaller quantity of energy than did the fat tis- 

 sue which disappeared. In other words, the rations were not really, 

 but only apparently maintenance rations. It is perhaps hardly correct 

 to say that in these experiments growth was maintained at the ex- 

 pense of the fat of the tissues. A more exact statement of the case 

 would be that the increase of protein tissue and water masked the 

 loss of fat. Presumably this effect would be less marked in more 

 mature animals, in which the true maintenance and live-weight 

 maintenance would doubtless approach each other closely when meas- 

 ured over long periods. 



FACTORS AFFECTING THE ENERGY REQUIREMENT. 



The results of the experiments upon farm animals reported on 

 previous pages render it evident that the actual maintenance require- 

 ment, even when computed to a uniform weight or size, is more or 



1 Society for the Promotion of Agricultural Science, Proceedings of 29th Annual Meet- 

 ing, p.' 71. 



