39 



opinion of many prominent investig'ators that, provided a sufficient 

 amount of protein is supplied for ph3\siolo^ical maintenance, i. e., to 

 replace the wear and tear of body tissue, it is immaterial which of the 

 three classes of nutrients (protein, fat, and carbohydrates) furnishes 

 the cnerg3^ necessary for external muscular work performed by horses. 



A view very commonly held to-day is much the same and in accord 

 with the above, viz, that provided an adequate quantity of protein and 

 energy are available for maintenance, it is theoretically immaterial 

 which class of nutrients furnish the energy for muscular work, 

 although carbohydrates and fat are practically better suited for this 

 purpose than protein, since any great excess of the latter is costly and 

 ma}' prove injurious to the health. In this case the term maintenance 

 is not used in its strict ph3^siological sense, but refers to a condition in 

 which no appreciable amount of external muscular work is performed, 

 and in which, the internal muscular work is fairly uniform from day to 

 day and the body weight practically' constant. 



The speed at which the horse travels, the way in which the load is 

 distributed, the external temperature, and other conditions evidently 

 have an effect upon the work performed, increased speed, increased 

 temperature, and faulty distribution of the load increasing the work. 



It is commonly recognized that when work is increased more feed is 

 required. Many experiments have shown that the pace at which work 

 is done also has an effect, and in general the greater the speed the 

 larger the feed requirement. Some of the reasons given for the fact 

 that rapid work is less economical than slow work are the increased 

 action of the heart when the horse is trotting or galloping; the lifting 

 of his own weight at each step only to allow it to fall again, thus devel- 

 oping heat; and the increase of body temperature with exertion and 

 the loss of heat b}' the evaporation of water through the skin and 

 lungs. 



A number of investigators have studied the relation between muscular 

 work and digestibility. Small variations have been observed under 

 the different experimental conditions, the feed being on an average a 

 little less thoroughly digested when severe work was performed. 

 But on the whole it seems fair to say that from a j^ractical stand- 

 point the diminished digestibility due to muscular work is not very 

 important. 



A recent German investigator found that the amount of food required 

 was affected by anything that disturbed the horse. In one experiment 

 a horse confined in a stable was much disturbed by flies and conse- 

 quently restless. The increased work in fighting the flies caused an 

 increase of 10 per cent of the carbon dioxid excreted. This means 

 that more food material was burned in the bod}^ than was the case 

 when the horse was quiet, for the combustion of food in the bod}', it 

 will be remembered, furnishes the carbon dioxid excreted in the 

 breath. 



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