Digestibility of Food. 51 



the same, but is regulated, amongst other circumstances, by the 

 purpose for which the animals are kept oil the farm. 



The fitness of the same kind of food thus varies with the age, 

 natural disposition, and kind of the animals to which it is given, 

 as well as with the purposes for which they are kept on the farm ; 

 and it is therefore quite impossible to classify the various articles 

 of food in an order which will indicate their relative feeding 

 values in all cases. 



But supposing the composition of food to be known, and the 

 wants of the animals are well considered, it is still impossible in 

 all cases to estimate beforehand what practical effect a feeding 

 substance will produce, for it may be rich in flesh-forming sub- 

 stances, and contain fat and heat producing compounds, as well 

 as saline and earthy matters ; and yet it may be, comparatively- 

 speaking, poor food, inasmuch as its constituents are not assimi- 

 lated by the animal organism. The digestibility of food conse- 

 quently is a point which ought to be well kept in view in esti- 

 mating its nutritive value. Our knowledge of this process 

 of digestion, unfortunately, is so limited, that we cannot speak 

 definitely of all the conditions which regulate the digestibility 

 of food. Still, however, a few circumstances may be pointed 

 out, by way of example, which influence the assimilation of food 

 by the animal system. 



Amongst other conditions the digestibility of food depends 



1. On the Kinds of Animals. The same description of food 

 which is assimilated in a great measure by one kind of animal 

 remains almost wholly undigested when given to another. Thus 

 it has been proved by direct experiments that cows will extract a 

 great deal of nourishment from cut straw, whilst horses do not 

 possess the power in the same degree of appropriating nourish- 

 ment from cut straw, and sheep likewise do not appear to digest 

 chaff so readily as cattle. 



2. On the amount and character of the Woody Fibre contained 

 in Food. Feeding materials, containing but a small amount of 

 woody fibre, are generally more readily digested than those 

 articles of food which, like straw, principally consist of woody 

 fibre. Hence barley-meal, oats, and grain in general, substances 

 rich in starchy compounds, are so well adapted to the rapid fatten- 

 ing of animals. 



The condition of the woody fibre further affects the nutritive 

 value of food in no mean degree. Whilst the woody fibre in 

 roots left too long in a growing state on the land, or the fibre of 

 grass and clover left standing until it become dead ripe, is not 

 readily digested, there can be no doubt that the soft fibre of 

 young grass, clover, and roots, is readily assimilated in the 

 animal organism and transformed into starch, sugar, and finally 



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