188 THE FEEDING OF ANIMALS 



undoubtedly be withdrawn from the daily food without 

 causing any drain upon body protein to meet the demands 

 of the organism. As stated, however, such a substitution 

 cannot be carried beyond certain limits without depress- 

 ing the protein-supply below the body needs for main- 

 tenance. Fats are not as efficient protein-sparers as are 

 carbohydrates. To be more explicit, fats and carbo- 

 hydrates do not replace protein in proportion to their 

 energy equivalents, carbohydrates being the more effi- 

 cient. In brief, then, experience and science both indicate 

 that carbohydrates are the most healthful, and physiologi- 

 cally the most economical, source of a large proportion 

 of the food energy. There is every justification for the 

 relative abundance of starch foods in the rations of 

 farm animals. 



272. Nutritive value of the gums. The question has 

 been raised as to whether the gums (pentosans) which 

 exist so abundantly in many coarse foods and in some 

 grain products like wheat bran are not inferior as sources 

 of energy to the other more soluble carbohydrates. It 

 has been observed that the sugars which result from the 

 action of ferments on these bodies have, in some in- 

 stances, not been oxidized, but have passed off in the 

 urine as such. It appears that under normal and usual 

 conditions this does not occur to any extent with herbiv- 

 ora. Pentosans are present in all rations for farm 

 animals, and we have no reason for believing that the 

 pentose sugars are constant ingredients of their urine. 

 Muccollum and Brannon studied extensively the fate 

 of various pentosans in the digestive tract of bovines. 

 They found that these compounds are not equally diges- 

 tible from all sources. Those from the corn, oat, and wheat 

 plants were studied and the range of digestibility was 46 



