344 NUTRITION OF FARM ANIMALS 



to say, since there is at present no way to determine how much 

 of any particular ash ingredient found in the feces consists of 

 undigested material and how much is to be regarded as an ex- 

 cretory product. This being the case, a table like that of the 

 Appendix can give only a general and approximate idea of the 

 total quantity of ash or of the balance of its basic and acid 

 elements or of the alkalies in the materials actually resorbed 

 and entering into the metabolism. 



Influence of supply on excretion. Moreover, it is necessary 

 to take into consideration the influence discussed in previous 

 paragraphs (429-433) of the nature and proportions of the ash 

 ingredients actually resorbed upon their excretion. For ex- 

 ample, potassium may lead to a loss of sodium and this in turn 

 to losses of calcium and phosphoric acid, thus possibly affecting 

 to a considerable extent the ratio of acid to basic elements in 

 the excreta. Adding to this the facts that more or less of the 

 acids produced in the body may be neutralized by ammonia 

 instead of by fixed bases (427 b), and that the kidneys have the 

 power, in some species at least, to separate acids from bases 

 (427 c), leading especially to excretion of acid phosphates, it 

 is evident that the data of the table may fall considerably short 

 of representing the actual value of the feed as regards main- 

 tenance of the ash balance. 



435. The ash balance. These considerations render it evi- 

 dent that the value of conclusions as to the balance of income 

 and outgo of ash elements drawn from the composition of the 

 feeding stuffs concerned must be more or less problematical, 

 particularly as regards farm animals. Such conclusions are 

 more or less probable deductions from the facts outlined in 

 previous paragraphs regarding the functions of ash in the body 

 and need to be checked by direct experiments. The actual 

 effect of a feeding stuff or ration on the ash maintenance of 

 herbivora can be determined with certainty only by means of 

 direct comparisons of the income and outgo of all the ash ele- 

 ments, i.e., by determinations of the amounts contained in feed, 

 feces and urine (metabolism experiments) or by comparative 

 analyses of carefully selected test and control animals (com- 

 parative slaughter tests). 



Data of this sort for mature animals are very scanty but 

 some tentative conclusions may be drawn from experiments by 



