X.] VALUE OF CONCENTRATED FOODS 195 



made for a sudden and excessive output of energy, must 

 have the most concentrated and digestible foods that 

 can be obtained. No increase in the amount of the 

 lower-grade foods will compensate for their lack of 

 concentration, because so much time would be spent by 

 the animal in heaping up the necessary surplus energy. 

 To take another example, cattle or sheep will never 

 grow fat in one season on the grass growing on the 

 majority of fields, however great an area they may be 

 given to graze ; it is only on certain choice fattening 

 pastures that the increase is rapid enough to prepare an 

 animal for market without artificial assistance. On the 

 ordinary grass lands the animal spends so much of the 

 energy obtained from its food in digesting it that the 

 surplus left for production does not permit of rapid 

 growth ; on the fattening fields the grass possesses a 

 smaller proportion of fibre, and therefore less of its heat 

 value is wasted in the digestion processes. 



Similarly, in the last stages of fattening animals in 

 stalls carbohydrates are of much less value than they are 

 at an earlier period, because they call for an expenditure 

 of energy in digestion which is disproportionate to the 

 increase they produce at that stage, when the increase 

 bears a very small ratio to the food consumed, whereas 

 much less of the energy of fats and proteins is wasted 

 in the digestion process. 



Treating food from this point of view as supplying 

 energy to the animal, we may now proceed to consider 

 the animal's requirements under different conditions. 

 The simplest case is, of course, that of the animal at 

 rest on a maintenance diet, so that it is neither increasing 

 nor diminishing in weight. As the temperature of the 

 animal is always higher than the surrounding atmo- 

 sphere (from 100° to 104° F., varying with the animal), 

 a constant loss of heat is going on from the surface of 



