564 NUTRITION OF FARM ANIMALS 



katabolized body substance containing 167 -5- J = 502 Cals. 

 of energy and it would be anticipated that one pound of timothy 

 hay would have been sufficient to replace this energy in the body. 

 Similarly, the performance of 1000 Cals. of external work by a 

 horse would cause the mobilization of about 3000 Cals. of body 

 energy, and the feed necessary to support this work would have 

 to supply about 3000 Cals. of net available energy. 



In brief, the net energy values for maintenance, determined 

 in the manner described in Chapter XVII and tabulated in the 

 Appendix, may be regarded as also net energy values for work 

 production, and the energy requirements of the work animal 

 may be expressed in terms of these net energy values. 



672. Net energy requirements. It is plain, in the light of 

 the foregoing discussion, that the amount of net energy required 

 by an animal for work production may be regarded as equal to 

 the body energy metabolized in the performance of the work. 



From the data contained in 2, it is possible to estimate ap- 

 proximately how much energy in excess of its maintenance re- 

 quirement must be mobilized in the body of a horse, e.g., to 

 perform a known amount of mechanical work of a specific kind. 

 Thus a horse in hauling a load having a draft of 100 pounds 

 20 miles on a level road would do 10,560 foot tons of mechanical 

 work, equivalent to 3421 Calories. Table 155 (651) shows 

 the net efficiency of the horse in draft to be about 31.3 per cent. 

 Accordingly, the animal would have to mobilize in his body for 

 the performance of this work 4321 -f- 0.313 = 10,929 Calories, 

 and his feed must therefore supply this amount of net energy 

 in addition to the requirements for locomotion, maintenance or 

 other purposes. 



The total expenditure of body energy during the performance 

 of work by the horse, as appears from 2, includes substantially 

 four factors in varying proportions, viz., the expenditure for 

 maintenance, for horizontal locomotion, for ascent (or descent) 

 and for draft. A fairly accurate estimate of the net energy 

 required to do a certain piece of work may therefore be obtained 

 by computing the requirement for each of these factors sepa- 

 rately from the data for net efficiency already recorded and 

 adding the results. 



For example, let it be supposed that a horse weighing noo 

 pounds hauls a load of 2000 pounds, having a horizontal draft 



