238 Elementary Principles of Agriculture 



1. Building material for muscle, bones, skin, etc., 

 and repair the waste. 



2. Heat to keep the body warm, and to supply 

 energy for work. 



The several classes of nutrients act in different ways 

 in fulfilling these functions. The proteids from the 

 muscles, tendons, gristle, hair and hoofs supply the pro- 

 teids of blood, milk and other fluids, as well as the whites 

 and yellow in eggs. The chief fuel or heat-giving ingre- 

 dients are the carbohydrates and fats. These are con- 

 sumed in the body or stored as fat to be used as occa- 

 sion demands. The proteids may supply energy, though 

 it is not supposed that they do so in the presence of 

 sufficient fats and carbohydrates. 



330. Fuel Value of Feeds. Starches, sugars, and fats, 

 are burned (oxidized) in the body and yield heat and 

 power, just as the same substances would if burned 

 in the stove to heat the house or under the boiler to 

 make the steam for the engine. The heat or energy is 

 developed gradually as the needs of the body demand. 

 Scientists have ways of determining the fuel value of 

 substances, and for the purpose of comparison use as 

 the unit of measurement the calorie (equal to the heat 

 required to raise one kilogram of water one degree Centi- 

 grade, or one pound of water four degrees Fahrenheit). 



331. Digestibility of Feeds. The value of a substance 

 as a feed depends not only upon the quantity of the 

 different kinds of nutrients contained, but also upon 

 how much of the nutrients are in a form that they can 

 be digested and used for the support of the animal 

 body. The usefulness of a substance for feeding depends, 

 then, not on its gross weight, but upon the amount of 

 building material and heat energy which the animal 



