502 ANIMAL BIOCHEMISTRY 



where. The general food requirements of individual animals are de- 

 termined by their overall energy requirements which depend upon the 

 several factors considered below. 



Digestibility 



The tabulated heat contents are based on the assumption that carbo- 

 hydrates are completely digestible except for a small inefficiency fac- 

 tor of about 2 per cent. However, many mammals can digest only 

 starches of the polysaccharide group. Therefore, cellulose, inulin, 

 and pentosans must be excluded from consideration for these ani- 

 mals. Others, including the ruminants especially, digest and utilize a 

 portion of these other polysaccharides. Thus the digestive differences 

 of animals lead to differences in the energies recovered from ingested 

 foods. 



In Table 21-3 the column headed "Crude Fiber" refers to material 



TABLE 21-3. Digestibility of a Mixed Ration of Alfalfa 

 and Cornmeal for Steers 



Nitrogen-free Crude Crude Ether 



Nutrient Distribution Extract Fiber Protein Extract 



(mostly cellulose) not utilized by some species but partly digested by 

 others. Many plant materials, like leaves, stems, stalks, and twigs, 

 supply little energy to fowl, human beings, pigs, rats, dogs, and the 

 like. Since digestion of crude fiber is never complete for any animal, 

 the energy contents of rations for ruminants must be determined for 

 each species. Furthermore, the fraction of the crude fiber digested 

 depends upon the nature of the ration. Hence feeding trials must be 

 conducted with each ration of interest. 



Results from studies of this type are often expressed as total digest- 

 ible nutrients based on the classes of nutrients present and the extent 

 to which they are digested in the experiment. The estimation of 

 digestible nutrients is really a balance study in which the nutrients 

 consumed are compared with the amounts of these nutrients appear- 

 ing in the feces. The amount of each nutrient excreted is subtracted 

 from that present in the feed, and the remainder is recorded as nu- 

 trient digested. The percentage of the nutrient digested is known as 



