MEASURING THE USEFULNESS OF FEEDS 43 



amounts and furthermore ether dissolves not only true fat but also such plant 

 compounds as chlorophyll and waxes, and such products in the feces as the 

 bile residues. The true fats are highly digestible, but the waxes, etc. are of 

 rather low digestibility. Fraps and Rather 3 on studying the ether extract ob- 

 tained from 18 different forage plants, found that only 42 per ct. was true fat. 

 Of this, 66.4 per ct. was digested, while only 29.1 per ct. of the remainder (not 

 true fat) was digestible. 



The ordinary digestion trials give little information concerning the extent to 

 which the mineral matter of feeds is actually digested and absorbed, for the com- 

 pounds of calcium, magnesium, phosphorus, and iron are chiefly or wholly 

 excreted from the body in the feces. Therefore, in a digestion trial these com- 

 pounds would be reported as largely undigested, tho they may really have been 

 digested and absorbed, and later excreted in the feces after being used in 

 the body. 



II. THE ENERGY OP FOOD 



Tables of digestible nutrients tell what part of the food may be 

 digested and absorbed, and thus really enter the body of the animal, 

 but they throw no light on the uses made of the nutrients when once they 

 are within the body. To obtain such information the respiration appa- 

 ratus and the respiration calorimeter have been devised. Thru their 

 use scientists have been able to study the uses of food in the body for 

 the formation of flesh and fat, or for the production of work or milk. 



73. The respiration apparatus. This is an air-tight chamber, arranged 

 with such devices that all that enters and comes from the body of the 

 animal placed within it can be accurately measured and studied. In 

 some cases mechanical work is performed, while in others the animal is 

 at rest. Everything which passes into the animal air, food, and water 

 is carefully measured and analyzed so that the exact intake of the 

 body is known. The air is in turn drawn from the chamber and 

 analyzed, and the feces and urine voided by the animal are likewise 

 weighed and analyzed. If the intake is larger than the outgo, the animal 

 has increased in body substance ; if less, it has lost. 



74. Intake and outgo during trial. The following data, secured in an 

 actual respiration study by Henneberg 4 with a full-grown steer weigh- 

 ing 1,570 Ibs. will show how information is gained in such a trial. 



During one day of the trial this steer ate 11.1 Ibs. clover hay, 13.2 Ibs. oat 

 straw, 8.2 Ibs. bean meal, and 2.13 oz. salt, and drank 123.7 Ibs. water. Accurate 

 determinations were made of the carbon, nitrogen, and mineral matter in the 

 feed, the feces, and the urine, and of the carbon dioxid given off into the air by 

 the steer. 



The feed consumed by the steer during the 24 hours was found to contain 12.84 

 Ibs. of carbon. This was the total intake of carbon, as, of course, none was 

 secured from the water which was drunk. Carbon was lost from the body of the 

 steer as follows: In the undigested material in the feces, 5.69 Ibs.; in the waste 

 products in the urine, 0.49 Ib. ; in the carbon dioxid exhaled from lungs and skin, 

 5.89 Ibs.; and in the methan gas produced thru fermentations in the digestive 

 tract, 0.04 Ib. Adding these losses together we have a total of 12.11 Ibs. Sub- 

 tracting this total loss from the total intake of carbon, 12.84 Ibs., it was found 

 that during this time the steer's body gained 0.73 Ib. in carbon content. As yet, 

 however, there was no information regarding the form in which this carbon was 



Tex. Bui. 150. 4 Neue Beitrage, Gottingen, I, 1870, p. XIX. 



