280 NUTRITION OF FARM ANIMALS 



2. THE MAINTENANCE REQUIREMENTS OF FARM ANIMALS 



372. True maintenance and live weight maintenance. - 



The maintenance of an animal in the strict sense signifies the 

 preservation of the store of matter and of potential energy 

 contained in the body, and only a ration which effects this is 

 really a maintenance ration. As will appear in subsequent 

 pages, however, much of the recorded information regarding 

 the maintenance ration is derived from experiments in which 

 the criterion of the sufficiency of the ration was its effect in 

 maintaining the live weight of the animal. In experiments on 

 mature animals and 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 on young animals, the live weight 

 is a notoriously untrustworthy guide. The general reasons 

 for this are familiar, but in young animals another very impor- 

 tant factor enters into consideration. As is well known, the 

 tendency to growth is one of the most marked characteristics 

 of young animals. Waters l has shown that this impulse to 

 increase of tissue is so marked that it may apparently take pre- 

 cedence over the demand for maintenance, and that an animal 

 may even maintain its weight and continue to increase in size 

 of skeleton for a considerable time on a sub-maintenance ration. 



Some 15 immature cattle were fed for considerable periods on 

 rations just sufficient to maintain their live weight. Under these 

 conditions, the animals continued to grow in height, in depth of chest 

 and length of head. At the same time, however, there was an evi- 

 dent 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 analyses 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 

 tissue was produced, which, on account of the relatively low energy 

 value of protein and of the relatively large amount of water accom- 

 panying it, represented a much smaller quantity of energy than did 

 the fat tissue which disappeared. In other words the rations were 

 1 Soc. Prom. Agr. Sci., Proc. 2gth Annual Meeting (1908), p. 71. 



