THE SCIENCE OF FEEDIXG AXIMALS. 2^ 



and supply the nutrients which contain carbon as a most essen- 

 tial ingredient. The mineral constituents of plants and animals 

 are identical; but if the plant has the requisite mineral foods 

 the carbon takes care of itself, while the animal secures all of 

 its mineral nutrients as an incident along with the more impor- 

 tant organic substances. 



DIGESTIBILITY. 



The value and nutritive effect of any given fodder is not deter- 

 mined by its composition alone or the percentages of protein, 

 carbohydrates and fat it contains ; but rather by that part only 

 which the animal digests. In considering the usefulness of any 

 feeding substance, therefore, the digestibility deserves careful 

 notice. It is interesting to observe how the digestibility of foods 

 is determined, for it promotes confidence in scientific results to 

 understand the processes by which these results have been 

 obtained. In the first place we must know- that digestibility can- 

 not be ascertained in the laboratory by chemical reagents, acids, 

 alkalies and such. But in order to learn to what extent an 

 animal can digest the nutrients of a fodder we must put the 

 question to the animal directly. Laboratory reagents and con- 

 ditions cannot be made exactly like those of the digestive tract 

 of the animal, so that if we relied upon the former, errors would 

 be sure to be considerable. In studying digestibility with the 

 animal we first prepare the system by a period of preliminary 

 feeding for about a week in order to get the system accustomed 

 to the fodder in question, and allow the previous fodder to pass 

 off. Then for a stated period all the nutrients in the fodder 

 consumed are carefully determined chemically and the same 

 processes followed with the dung or solid excrement. The 

 differences between the protein, fat, extract matter and fibre 

 consumed in the food and that voided in the dung represent the 

 portions digested. The percentage digestibility of each nutrient 

 is determined by dividing the portion digested by the amount of 

 that nutrient eaten. The percentage thus obtained is called the 

 co-efficient. The digestion co-efficients for protein vary between 

 40 and 80 ordinarily, and may be said to approximate 70 on the 

 average. It will be readily seen that it is of vast importance 

 whether a fodder has a high or a low digestibility. One fodder 

 might easily be w-orth nearly double another of the same material 

 if the digestibility was nearly twice as great. 



