6lO NUTRITION OF FARM ANIMALS 



whole, however, the conclusion seems justified that animals of 

 the same species may differ to some extent in their digestive 

 power but that these individual differences are probably less 

 than appear to be indicated by the results of single digestion 

 trials and are certainly much too small to account in any degree 

 for the economic differences in animals. Even the differences 

 observed in the results of short digestion trials rarely exceed 

 three or four per cent and are usually materially less than this. 



719. Breed. The foregoing facts are sufficient of them- 

 selves to render improbable the existence of any considerable 

 breed differences as regards digestion, and this conclusion has 

 been confirmed by the experiments of Haubner and Hofmeister, 1 

 of Wolff 2 and of Armsby and Fries. 3 The recorded data 

 taken together fail to indicate any material difference in the 

 digestive power of different breeds or between pure-bred and 

 scrub animals. 



There exists a somewhat general impression that animals 

 which show themselves superior as producers of meat, milk, 

 etc., whether as the result of breed, heredity, or of individual 

 variation, owe that superiority, in part at least, to a superior 

 digestive power ; that is, it is supposed that the improved breeds 

 of farm animals and the superior individual animals within a 

 breed are able to extract more nutriment from a given weight 

 of a feed than can inferior animals. The reasons for the un- 

 doubted economic superiority of some individuals over others 

 have been considered in previous chapters. So far as differ- 

 ences in digestive power are concerned, however, the experimen- 

 tal evidence gives little support to the popular impression. 



720. Age. Comparisons of the digestive power of the 

 same animals (lambs) at different ages were made by Wolff 4 

 in 1871-72 which led to the conclusion that between the ages of 

 six and fourteen months the percentage digestibility of the 

 feed remained practically unchanged and this conclusion is 

 confirmed by the results of an experiment by Weiske 5 under- 

 taken primarily for another purpose. 



721. Work. Investigations on the effect of the performance 

 of work upon the digestibility of rations have naturally been 



1 Landw. Vers. Stat., 12 (1869), 8. 2 Landw. Jahrb., 1 (1872), 533. 



3 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 128. 



4 Landw. Jahrb., 2 (1873), 221. 5 Ibid., 9 (1880), 205. 



