NUTRIENT REQUIREMENTS 149 



the secretory function of the milk glands (cottonseed meal for cows), 

 etc. 



The term concentrates is appUed to feeding stuffs rich in easily diges- 

 tible nutrients and containing comparatively little indigestible material or 

 water, like the grains, oil cake, malt sprouts, dried brewers' grains, bran, 

 feed meals, etc. Feeding stuffs comparatively poor in nutrients but con- 

 taining large quantities of indigestible matter like crude fiber are referred 

 to as roughages, like straw chaff, hulls, low-grade hay, etc. Succulent 

 and fluid feeding stuffs include green feed (soiling), silage, root crops 

 and tubers, distillery slops, milk and creamery by-products. 



The animal body requires water, salts, a part of the albumen and of the fat (or 

 of the carbohydrates) for tissue building and for tissue repair. These substances 

 are therefore referred to as organic substances. The substances which, in the course 

 of metabolism, constitute sources of heat and energy, are called combustibles. The 

 chief and most important of these are the carbohydrates, most of the assimilated 

 fat and no inconsiderable portion of the albumen. The physiological action of 

 nutrients is in proportion to the amount of heat produced in the process of their 

 combustion,! minus the heat units that are lost in the undigested or unassimilated 

 products of excretion (feces and urine). The action of the organic nutrients may 

 therefore be looked upon as a calorimetric effect. Numerous determinations have 

 demonstrated that 1 gram of albumen develops 4.1 calories in combustion, 1 gram 

 of fat 9.3 calories, and 1 gram of carbohydrates 4.1 calories. The different com- 

 bustibles may replace one another, or be substituted for one another in proportion 

 to the amount of heat they produce in combustion. They are isodynamic in the 

 same proportion (Rubner). One part of fat corresponds to 2.27 parts of carbo- 

 hydrates or albumen. These values, expressed in terms of calories are referred to 

 as fuel value or energy value. This unit of measurement may also be used to express 

 the requirements of animals for available energy. For example, an ox weighing 

 1,200 pounds requires for its maintenance a daily supply of 12,800 calories. 



If an animal is supplied with just sufficient nutrient elements in its 

 feed so that it will neither gain nor lose weight, the ration is referred to 

 as a maintenance ration. On the other hand, if in addition to this an 

 excess of nutrients is supplied which goes to form muscle, fat, milk, 

 wool, or useful work, the entire amount of feed supplied is referred to 

 as a productive ration. 



In feeding for production we might compare an animal with an engine 

 or a machine which converts coarse feed products of low value (grass, 

 hay, beets, etc.) into refined food products of high value (meat, fat, 

 milk, etc.), or into work, or valuable raw material (wool, leather, horn, 

 etc) Successful feeding operations must therefore be governed and 

 guided by the principle of producing the greatest possible results with 

 the least possible expense. 



In order to determine with scientific exactness the amount and chemical composi- 

 tion of feed required for an animal under different conditions it is necessary to 

 determine what part of the nutrient elements are converted into tissue and energy. 

 This is computed on the basis of the amount and character of the feed consumed 

 and of the products of excretion found in the feces and urine or thrown off by the 

 lungs and the skin. As a rule these calculations are based on the amount of nitrogen 



IThe fuel value of a given feed is determined by burning a weighed amount of the same in a 

 bomb-like apparatus (calorimeter) filled with condensed oxygen. The calorimeter is submersed in 

 a measured quantity of water and the gain in temperature of the latter, measured with a ther- 

 mometer, gives the result in terms of calories. For more detailed information see Berthelot, 

 Praktische Anleitung zur Ausfuehrung thermochemischer Messungen, 1893; O. Kellner, Landw, 

 Versuchsstationen 1896, vol. 217, p. 293, etc. 



