SELECTION AND IMPROVEMENT OF DAIRY CATTLE 69 



The richness of milk is less affected by age than is the quan- 

 tity. The average fat content remains practically constant 

 from year to year except that after the cow is eight or nine 

 years old the percentage of fat always declines slowly and 

 gradually with advancing years. A Jersey cow, for example, 

 thai averages 5.0 per cent fat when in her prime will decline 

 to ii bout 4.5 per cent when 12 to 15 years of age. The fol- 



5.00 



20 40 60 80 100 120 140 160 180 200 220 240 

 DAYS IN MILK 



.00 



FIG. 25. Influence of the advance in lactation period upon the milk 

 yield and fat content, averages for ten mature cows and ten two-year-old 

 heifers. 



lowing gives the average percentages of fat by lactation 

 periods for four Jerseys the records of which are complete 

 for 9 years. 



Lactation period . 1 

 Per cent fat . 4.8 



23456789 

 4.82 4.96 4.64 4.62 4.62 4.57 4.49 4.39 



SELECTION OF A BULL 



54. The Selection of the Bull. The successful develop- 

 ment of a dairy herd depends more upon the selection of the 

 bull than on any other one thing, unless it be the proper 

 culling out of inferior cows. One-half the inheritance of each 

 young animal in the herd comes from the bull, and for this 

 reason his influence on the herd is far greater than is that of 



