Principles of Feeding 57 
- enters largely into the composition of the bones, muscle, lean 
meat, white of egg, and the curd of milk. Animals can procure 
this protein only from the protein of food. 
The amount of digestible protein in various foods varies within 
rather wide limits, corn containing only 8 per cent, while cotton- 
seed meal runs as high as 32 per cent, but the larger number of 
food materials contain rather small amounts of protein. There 
are comparatively few foods that are relatively rich in protein. 
The value of the food depends very largely on the protein that 
it contains. Protein is not, however, as digestible as some of the 
other nutrients. 
107. Carbohydrates. — There are two sub-groups of carbohy- 
drates — nitrogen-free extract and crude fiber. The carbohy- 
drate group contains carbon, hydrogen, and oxygen, but no nitro- 
gen. The nitrogen-free extract is composed largely of starches 
and sugars, and constitutes the larger part of the dry-matter of 
most plants. Starch alone forms as much as 75 per cent of the 
dry-matter of corn, wheat, pota- 
toes, and some other foods. It is 
easily digested, more so than any of 
the other three constituents, and 
therefore foods rich in nitrogen-free 
extract are ordinarily classed as 
most readily digestible. Fiber or 
crude fiber constitutes the tough 
woody part of plants. The stems 
of all plants contain more fiber than 
does the leafy part. Variation in Fic. 32. — Clydesdale Mare. 
the digestibility of foods depends 
largely on the amount of crude fiber, which is mostly insoluble and 
hence largely indigestible. Crude fiber is the least valuable of 
the four nutrients, because of its indigestibility. Young plants 
contain less crude fiber than plants that have matured and formed 
seeds, and hence are more digestible. 
