ESSAYS 321 



For equal amounts of starch equivalent, meadow hay gives out 50 per cent, 

 more heat than pure starch, and wheat straw more than three times as much. In 

 other words, 12^ lb. of hay and 17I lb. of straw respectively would furnish as much 

 heat to the animal as 6 lb. of starch. If the function of the food were merely to 

 maintain the body temperature, these quantities would suffice for that purpose ; 

 but in that case the starch equivalent system would be destroyed, for, clearly, 6 lb. 

 of starch, or any other definite quantity, could not be accepted as the standard for 

 maintenance. It would be necessary to have a different standard for each kind of 

 food. 



It is apparently for this reason that Wood l has attempted to draw a distinction 

 between the starch equivalent for production and that for maintenance. Using 

 the same notation as before, the difference between the two may be concisely 

 indicated by the formulae — 



P.S.E. m (2-2F+ C + O-04P) x vjioo 

 M.S.E. =(2"3F+ C+ V2$P + o-6A) 



It will be seen that P.S.E. is Kellner's unit, and that M.S.E. differs from it 

 mainly in that the value number is not used, and no deduction is made on account 

 of the fibre. It is, in effect, the discredited "total digestible nutrients," and not 

 starch equivalent at all. The inference that 17! lb. of wheat straw is a sufficient 

 maintenance ration for oxen of 1000 lb. live weight is in conflict with experi- 

 mental evidence. 



The chief function of maintenance rations is not to produce heat, but to furnish 

 the energy required for internal work. This energy differs from that expended on 

 external work only in that it is transformed into heat within the animal's body. 

 It is, therefore, available to maintain the body temperature, and it is, normally, 

 sufficient for that purpose. The amount of heat emitted by an animal when it 

 consumes no food at all, called the basal katabolism, is, therefore, the true measure 

 of its maintenance requirements. For ruminants this cannot be determined 

 directly, but Armsby 2 has shown that it maybe inferred from the difference in heat- 

 production due to large and small amounts of the same ration. Ingestion of 

 solids increases heat-production. The energy so transformed is not available for 

 internal or external work, or for production of milk or meat, and, normally, little 

 if any of it is required to maintain the body temperature. It is, in fact, the non- 

 productive energy observed by Kellner, and ascribed by him to expenditure on 

 mastication and digestion of fibre. For most of the common feeding-stuffs it varies 

 from about i"o to i"2 kt (500 Cal.) per lb. of dry matter ingested, and this amount 

 must be deducted from the metabolisable (thermic) energy. It is only the 

 balance, called by Armsby net available energy, that can be utilised for production 

 or for maintenance. 



The metabolisable energy of feeding-stuffs may be calculated from the per- 

 centages of digestible nutrients by means of the usual factors or, according to 

 Armsby, 3 from the percentage of digestible organic matter ; the factors (kt) to be 

 used are — for hay and straw 35, for grains 39, and for oil-cakes 4*4 to 4' 8. The 

 net energy is then found by deducting the heat-production (kt per 100 lb.) due to 

 ingestion of the food. 



In recent investigations * on the net energy of starch, Armsby has obtained 

 results substantially different from those of Kellner, as follows : 



1 Composition and Nutritive Value of Feeding-Stuffs (Cambridge University 

 Press). 2 Jour. Agric. Resources, vol. xi, No. 10. 



3 Bulletin 142, Pa. State Colony Agricultural Experimental Station. 



4 Jour. Agric. Resources, vol. xv, No. 5, 1918. 



