28 



more thoroughly digested — it must actually be more valuable than the 

 other material. The bulk of the substance of almost all feeding stuffs 

 is insoluble when eaten. Only material in solution can pass through 

 the walls of the stomach and intestines into the circulation and be 

 utilized by the body, therefore digestibility consists chiefly in render- 

 ing insoluble materials soluble. This is effected by the aid of digest- 

 ive ferments and also by bacteria. 



Digestion experiments are frequently made to learn how thoroughly 

 a given feeding stuff or ration is assimilated. The usual method is to 

 feed the material under consideration for a longer or shorter time, the 

 amount and composition being determined. From the total nutrients 

 consumed, the amounts excreted undigested in the feces are deducted, 

 showing the amount of each retained in the body. It is the usual cus- 

 tom to express the amounts digested in percentages, the results thus 

 obtained being termed coefficients of digestibility. 



The digestibility^ of a number of different feeding stuffs has been 

 tested with horses in this country and in Europe, although the number 

 of such experiments is much smaller than in the case of cattle and 

 sheep. The most extended series of American experiments with 

 horses was carried on at the Maryland Experiment Station. 



The average results of the available digestion experiments with 

 horses were used to compute the digestible nutrients furnished per 100 

 pounds b}^ the different feeding stuffs included in the table on page 10. 



It has been found that in the majority of the feeding stuffs tested 

 the percentage of protein digested is fairly high, being greater in 

 grains and seeds than in hay and grasses, and least in the case of timothy 

 hay and spelt straw. 



Generally speaking, the values obtained for the digestibility of fat 

 are rather low, the fat of oats being most digestible and that of peas 

 least digestible. There are reasons connected with the analj^tical 

 methods commonly followed which render the results obtained with 

 fat not altogether satisfactory. 



Nitrogen-free extract is quite thoroughl}^ digested by horses, the 

 values ranging, in a number of experiments, from 100 per cent in the 

 case of molasses to 17.9 per cent in the case of spelt straw. The prin- 

 cipal sources of nitrogen-free extract in the ration are the cereal grains 

 and their by-products, and it is interesting to note that the coefficients 

 of digestibility of nitrogen-free extract of these materials is high. In 

 the majority of feeding stuff's the crude fiber is not ver}^ thoroughl}" 

 digested, the coefficients of digestibility being on an average less than 

 50 per cent. 



COMPARATIVE DIGESTIBILITY BY HOBSES AND RUMINANTS. 



In computing the digestible nutrients furnished by different feeding 

 stuffs, it has been a common custom to use availa])le data ol)tained from 

 digestion experiments with farm atiimals without distinguishing 



170 



