Animal Husbandry Investigations in 1917. 211 



erally classified as solids-not-fat. The proportion of the butter- 

 fat to the solids-not-fat is i to 2.55. This ratio is quite high 

 for cow's milk as found in the other breeds and closely ap- 

 proaches that of human milk. 



The interdependence of butter-fat per cent, amount of milk 

 produced and age at commencement of test, in comparison with 

 the association of solids-not-fat per cent, amount of milk pro- 

 duced and age at commencement of test may be determined by 

 the correlation method. The following conclusions were de- 

 rived from these relations. 



As the amount of milk given by the cows in this test in- 

 creases, the percentage composition of the butter-fat decreases. 

 The amount of this decrease is highly significant measured sta- 

 tistically. Considered practically this fall in butter- fat content 

 could not be easily detected in small samples. No such effect 

 is noted for the solids-not-fat or put in another way, the quan- 

 tity of milk produced for one year is independent of the con- 

 centration of the solids-not-fat. This, from the view of inheri- 

 tance, means that the hereditary units for high or low milk 

 production are separate and distinct from those causing a high 

 or low percentage of solids-not-fat. 



The correlations of the age when the yearly record com- 

 mences with butter-fat and with solids-not-fat brings out the 

 following points. Age of the cow does not effect the percentage 

 of butter-fat in the milk significantly. As the age of the cow 

 advances each lactation brings with it a decrease in the percent- 

 age of the solids-not-fat found in the milk. 



This differential action of amount of milk produced and 

 age gives us the citerion to prove that butter-fat and solids-not- 

 fat cannot have a common mother substance from which they 

 are derived by splitting. 



The correlations between the variables, pounds of milk, 

 butter-fat and solids-not-fat lead to the following conclusion, 

 (a). Some of the factors responsible for high concentration 

 of butter-fat are also responsible for high concentration of some 

 of the solids-not-fat in cow's milk. (b). Practically consid- 

 ered this means that if it is desired to improve either the butter- 

 fat or solids-not-fat concentration of the milk of a given herd,. 





