CHAPTER IV 



DETERMINATION OF THE NUTRITIVE VALUE OF 

 FEEDING STUFFS 



The nutritive value of different feeding stuffs may be determined 

 by two different methods : First, by chemical analysis and digestion 

 trials with farm animals ; second, by trials with animals in a respira- 

 tion apparatus or respiration calorimeter. The former method 

 shows the proportions of the feeds that are dissolved in the digestive 

 processes, while the latteri method furnishes direct information as 

 to the energy values of the feeds or rations and shows the uses which 

 the animal makes of the feed eaten. Both these methods will be 

 discussed in the following pages. 



A. THE DIGESTIBILITY OF FEEDING STUFFS 



Digestion Trials. — The digestibility of feeding stuffs is deter- 

 mined in so-called digestion trials with animals. Numerous such 

 trials have been conducted with ruminants during the past half- 

 oentury in this country and abroad, and a number of trials have 

 also been conducted with horses, pigs, and poultry. In these trials 

 the animals experimented with are fed the feeding stuff whose 

 digestibility is to be determined, for a period of about a week, and 

 the solid excrements voided by the animal are then collected for 

 another week. Samples of both the feed eaten and of the feces are 

 taken for chemical analysis, and by a comparison of the total 

 amounts of feed components in each the proportion of each com- 

 ponent retained or digested by the animal may be determined and 

 calculated on a basis of percentage digestibility. 



An example will readily "explain the method of calculation. 



In an experiment by the author, in which the digestibility of corn 

 silage was to be determined, a cow was fed, on the average, 55.0 pounds of 

 silage per day; a small amount, 0.71 pound, was refused. She voided 58.8 

 pounds of dung daily during the trial. Chemical analyses were made of both 

 the silage fed and that refused, as well as of the dung voided. The digestion 

 coefficients for the silage were then calculated as shown below: 



34 



