Testing the Purity of Milk. 155 



The per cent, of total solids may go above or below the yearly 

 average by more than 14 per cent. 



The per cent, of solids not fat may go above or below the 

 yearly average by more than 10 per cent. 



To illustrate, if the average test of a herd during a whole 

 period of lactation is 4.0 per cent., the test on a single day may 

 exceed 4.0 -f ^ X 4.0=5.2, or may go below 2.8 per cent, (viz., 

 4.0_ ^j). X4.0) ; if the average specific gravity is 1.031 (lacto- 

 meter degrees, 31 )* the specific gravity of the milk on a single 

 day may vary between 1.0279 and 1.0341 (31+-^ X 31=34.1; 

 31 i 1 oX31=27.9). 



174. Influence of heavy grain-feeding on the qual- 

 ity of milk. If cows are not half-starved or underfed, 

 an increase in the feeding ration will not materially 

 change the richness of the milk produced; this has been 

 shown by numerous careful feeding experiments con- 

 ducted under a great variety of conditions and in many 

 countries. Good dairy cows will almost invariably give 

 more milk when their rations are increased, so long as 

 they are not overfed, but the milk will remain of about 

 the same quality after the first few days are passed as 

 before this time, provided the cows are in good health 

 and under normal conditions. Any change in the feed 

 of cows will usually bring about an immediate change 

 in the fat content of the milk, as a rule increasing it to 

 some extent, but in the course of a few days, when the 

 cows have become accustomed to their new feed, the fat 

 content will again return to its normal amount. 



175. The records of the cows included in the feeding 

 experiment at the Illinois station, to which reference 

 has been made on p. 144, furnish illustrations as to the 

 effect of heavy feeding on the quality of milk. The 



1 See page 103. 



