SO POWER AND THE PLOW 



utilizing a larger proportion of the total heat value of the food. 

 In comparing the horse and the mechanical motor, we 

 encounter different standards of estimating efficiency. The 

 engineer compares the total heat units in fuel supplied with the 

 equivalent of heat units delivered by the motor as useful work, 

 calling the ratio thermal efficiency. The physiologist determines 

 a fuel value for a given feed by ascertaining the percentage 

 of digestible nutrients, and on this basis, or one even more 

 favorable, the animal's efficiency is frequently calculated. 



Of the fuel value, or digestible nutrients of the food, about 

 30 per cent, is lost in the energy required to chew and digest 

 the food; consequently there is a further value of the food, 

 which is termed by students the maintenance value. This is 

 the proportion of the food which can be utilized by the horse 

 in maintaining his body weight while performing no work, 

 and from this value can be calculated the ration which must 

 be fed an animal in order to sustain it at a given weight during 

 idleness. If less digestible food is supplied to the animal during 

 such idle period than is necessary for its maintenance, the 

 animal may draw on its store of muscular tissue or fat and con- 

 tinue to live at the expense of weight. 



If work is required of the animal in addition to keeping the 

 body at its normal size and condition, an excess of digestible 

 food must be supplied, and experiments go to show that only 

 about one third of this excess is actually recovered in the shape 

 of external motion. This leaves, then, a value for the food which 

 is known as the production value. In other words, out of the 

 total heat value of the original food, we have now a value repre- 

 senting the amount of external work that can be recovered from 

 a given amount of feed. But even of the external work per- 

 formed by the horse, a certain amount is used in moving his 

 own body forward, since he is able to deliver work only when 

 moving, and under the best conditions his efficiency is low. In- 

 stead of a thermal efficiency (or ratio of work delivered to food 

 consumed) of 35 to 40 per cent., that has been proclaimed by 



