FEEDING DAIRY CATTLE 231 



butter fat; a very good dairy cow, on the other hand, will yield 

 over twice this amount, and exceptional producers will give more 

 than quadruple the figures stated. But the average quality of 

 milk of cows of the same breeds also differs greatly. The variations 

 are probably larger within the Jersey and Guernsey breeds than in 

 any other breed ; there are thus cows or families within these breeds 

 that produce milk of an average fat content above 6 per cent 

 during the entire lactation period, while individuals of other 

 families will generally not go over 4 per cent. In the same way, 

 we find some Holstein cows producing milk with over 4 per 

 cent fat, and others whose milk will contain less than 2.8 per cent, 

 on the average, for a considerable period of time. 



Age of Cows. This is not, on the whole, an important factor 

 so far as the quality of the milk is concerned. As a general rule, 

 heifers and young cows will give milk of a somewhat higher fat 

 content than older cows, but the differences in the average quality 

 of the milk, for a long period of time, say one year, are only within 

 a few tenths of one per cent in the fat content, and there are some 

 individuals whose milk changes with advancing age in the opposite 

 direction from that stated. 



The production of milk and butter fat increases with advancing 

 age of cows till they are past maturity. A dairy cow ordinarily 

 produces about 30 per cent more milk or butter fat at full age 

 than she did as a heifer with her first calf. The maximum pro- 

 duction, as a rule, comes at an age of six to eight years, differing 

 somewhat with the different breeds and with individual cows, 

 according to the method of feeding and management, and the con- 

 stitutional vigor of the cows. Good dairy cows will continue to be 

 profitable producers till ten or twelve years old and, in exceptional 

 cases, considerably longer. 



Stage of Lactation Period. This is of greater importance 

 than the age of the cow. The quality of the milk is, in general, 

 higher during the first few weeks after parturition than later on, 

 and remains fairly constant from this time on until toward the 

 close of the lactation, when the milk becomes richer in solids and 

 fat as the yield decreases (Fig. 44). The following compilation 

 of 300 cows in the Wisconsin Dairy Cow Competition, 1909-1911, 4 

 will illustrate the changes in production of dairy cows during the 

 progress of the lactation period: 



4 Wis. Res. Bui. 26; Ohio Mo. Bui. 2, No. 12; Gue. Br. Jr. 4, 35. 



