56 Feeds and Feeding. 



excreted more than twice as much nitrogen as urea as did the other 

 pig, the nitrogen waste thus depending on the nitrogen intake. 



Experiments show that a pound of carbohydrates has somewhat 

 greater protein-sparing action than a pound of fat, a surprising 

 fact when we remember that, on burning, fat produces over twice 

 as much energy as do carbohydrates. Evidently there is no rela- 

 tion between the fuel values of these nutrients and their protein- 

 sparing power. Landegren 1 explains this superiority of carbohy- 

 drates over fat as follows : For the carrying on of their normal 

 functions, living cells need a certain minimum not only of protein 

 but also of carbohydrates, especially glucose. When carbohydrates 

 are not supplied, the body forms the necessary glucose by decom- 

 posing protein. So long, however, as there is an ample supply of 

 carbohydrates in the food, protein is not used for this purpose. As 

 the body can form carbohydrates from fat only with great diffi- 

 culty, if at all, the fats are less potent than the carbohydrates in 

 checking the protein wastes in the body. 



78. Amids. Some scientists hold that the amids do not serve to 

 form the protein tissues in the body of farm animals. Numerous 

 experiments have shown that asparagin, a pure amid, cannot spare 

 protein or take its place in the bodies of the carnivora and omniv- 

 ora. It is quite generally agreed, however, that the amids fur- 

 nish energy to the body and may in some cases prevent the waste 

 of the protein tissues. Nearly half the nitrogen in corn silage, and 

 not over 15 per ct. of that in dried corn forage, is in amid form. 

 Yet, based on dry matter, corn silage is somewhat more valuable 

 than corn forage as a feed for dairy cows, which require a liberal 

 supply of crude protein. The amids are abundant in grass, roots, 

 and silage, all of which feeding stuffs are especially useful to grow- 

 ing or pregnant animals and to those producing milk and wool. 

 Kellner 2 and Strusiewicz 3 have shown that sheep given protein- 

 poor rations gained in weight when asparagin was added. In view 

 of such facts it is reasonable to hold that at least some of the amids 

 can be built up into the protein tissues of the bodies of farm ani- 

 mals. 



79. Origin of body fat. The exact source of the fat which ani- 

 mals store in their bodies has been the subject of much controversy. 

 Kellner, Armsby, Hagemann, and other modern authorities agree 

 that the body fat of animals may originate either from fat or carbo- 



1 Skand. Archiv. Physiol., 14, 1903, p. 112. 3 Ztschr. Biol., 47, p. 143. 



2 Ztschr. Biol., 39, 1900, p. 313. 



