J. Alan Murray 177 



in the food when the pure nutrient is added to an adequate basal ration. 

 In a recent experiment Armsby^ found that 57-3 per cent, of the meta- 

 bolisable energy in excess of maintenance requirements was recovered 

 in the gain made by an unfattened steer and 56-4 per cent, in that made 

 by the same animal in fat condition. Moulton^, quoted by Armsby, 

 "computed that in a fat and a very fat steer 53-39 per cent, and 52-49 

 per cent, respectively of the metabolisable energy supplied in excess of 

 maintenance was recovered in the gain." It appears therefore that the 

 value of y in fattening foods is between 50 and 60 and that, apart from 

 the variation in individuals, it is practically a constant quantity. At all 

 events it is not materially affected by the condition of the animal. 



It is well known, however, that the return for food consumed 

 diminishes rapidly as condition improves. In the same experiment 

 Armsby found that 5-2 lb. of digestible organic matter was consumed 

 for each pound of increase made by the unfattened animal and 9-G lb. 

 — nearly twice as much — after fattening. He attributes this to increased 

 requirements for maintenance. 



This conclusion might have been anticipated — it was in fact an- 

 ticipated by the author — from the fact that, when the rations of a fat 

 beast are reduced to what is adequate for maintenance in store con- 

 dition, it "goes back," i.e. it loses weight and becomes lean again. It 

 is evident also that, in the case of fully grown animals, the return for 

 food consumed must eventually fall to zero. In other words there must 

 be a point, somewhere, beyond which an animal cannot be further 

 fattened, the maximum amount of food it can consume being just 

 sufficient to maintain it in that condition. 



This point has not been exactly determined but available data serve 

 to narrow the field of enquiry. If condition be expressed in terms of 

 percentage of body fat the mathematical limit is 100. The actual limit 

 must be something short of that for it is physiologically impossible for 

 the body to consist entirely of fat. On the other hand it cannot be less 

 than 48-3 per cent, for that amount of body fat has been found in a 

 " very fat "" sheep. Other considerations indicate that the.limit of fatness 

 lies between 50 and 60 per cent, of body fat. The point is probably not 

 well defined as it depends mainly upon the animal's capacity for food. 

 The specific capacity for food — the ratio of dry matter consumed (when 

 given food ad lib.) x 1000 to the fat-free live weight — was, in the case 

 of this particular animal, [9-9116 ; 1000/410-4 =] 24-22. 



• Journ. Agr. Research, 11, No. 10. 

 - Journ. Biol. Chem. 31, No. 2. 

 Journ, of Agric. Sci. rx 12 



