666 NUTRITION OF FARM ANIMALS 



tendency seems to be in favor of the hypothesis that the proportion 

 of the metabolizable energy utilized is substantially independent of 

 the quantity of feed, provided that the changes in the latter are not 

 so great as to materially modify the course of the fermentations in 

 the digestive tract. Armsby and Fries' results on the same species 1 

 tend to confirm these conclusions, since they afforded no distinct evi- 

 dence of an increase in the heat production per unit of feed as the 

 amount of the latter was increased. 



On the whole the probabilities seem to be that the limit to 

 the most efficient use of feed energy, in herbivora at least, is 

 set by the capacity of the alimentary canal to digest and as- 

 similate feed rather than by the capacity of the organism to 

 utilize the material transmitted to it by the organs of resorb- 

 tion. If this proves to be the case, the net energy values may be 

 regarded as being, if not strictly constant, at least nearly so 

 over a wide range of feeding. 



765. Influence of age, breed and individuality. That dif- 

 ferences due to age, breed or " individuality " may exist between 

 animals as regards the efficiency with which they utilize the 

 energy of their feed and consequently as regards the net energy 

 values of the latter does not appear particularly probable a 

 priori. Such data on these points as are available have been 

 referred to in Chapters XI and XII, the general conclusion being 

 reached that the evidence is insufficient to establish the existence 

 of any marked differences of this sort except, perhaps, in the 

 growth of very young animals. 



766. Influence of kind of production. It will not have es- 

 caped notice that the foregoing data regarding net energy values 

 relate entirely to the production of body tissue, whether directly 

 in growth or fattening or indirectly in maintenance or in work 

 production. While it is perhaps unlikely that the values for 

 these various purposes are stricly identical, the discussions in 

 Chapters VIII, X, XI, XIV and in the present chapter seem 

 to render it probable that the differences are not of sufficient 

 magnitude to interfere seriously with the use of these net en- 

 ergy values for the computation of rations in practice. 



As regards an important branch of animal husbandry, how- 

 ever, viz., milk production, as was shown in Chapter XIII, 

 scarcely any accurate data regarding net energy values are yet 



1 Jour. Agr. Research, 3 (1915), 472-476 and Fig. 2. 



