The Results of Microbial Digestion

There are several ways in which comparisons between the results of microbial digestion have been made based on the quantities of the products of fermentation formed in relation to bodyweight or more simply on the amounts of cellulosic fiber disappearing from the food as it passes through the gut. These comparisons are not valid on their own account, for strictly controlled tests have not yet been made. However, the evidence available sustains the widely held view that microbial digestion plays a more important role in the ruminant than in animals where microbial digestion is confined primarily to the large intestine. 

The studies of Crampton et al on the comparative value of foodstuffs of low fiber to human subjects, rats, guinea pigs, swine, and sheep so far suggest that, of themselves, the anatomical differences in the digestive systems of the five species studied do not result in large differences in digestive capacity with respect to diets of negligible crude fiber content. 

Comparisons may also be made of the quantities of methane produced and the quantities of fatty acid present in the digesta. In comparing the capabilities of different species of digesting cellulose, attention must be paid to the form in which the cellulose is offered, with particular reference to encrusting substances. It is well known that the age of the plant, which determines the degree of lignification of the structural components of the plant, influences markedly the digestibility of the cellulose. 

Pure cellulose or the cellulose of straw after alkali or sulfite treatment is almost completely digested (80% or more) by the ruminant and also by the pig. When the digestibility of the fibers of straw is compared, however, the ruminant digests more than the horse, and the digestibility in the pig is very low and variable.  

Woodman, Biol. Revs. Biol. Proc. Cambridge Phil. Soc. 6, 273 (1930). 

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