HOW INDIVIDUAL SELECTION IS MADE 153 



at least 12 years old. Occasionally a cow continues to breed 

 until she is 16 or 18 years old. 



The Per Cent of Fat. Table 7 gives the average per cent 

 of fat by lactation periods. This average is the true average 

 found by dividing the total fat by the total milk produced. 

 It will be noted that there is little change during the first three 

 milking periods; but from that time on there is a slow but 

 slight decline with advancing years. This agrees with the 

 results found by Hills. 1 There are several conditions that 

 may bring about considerable of a variation from the average 

 with an individual, but these variations are naturally much 

 less than is the case with the milk yield. While the daily 

 variations in the per cent of fat in the milk of all cows are 

 constant and striking, the average for the entire milking 

 period varies but little from year to year. While it is not 

 entirely safe to judge the future milk production of a cow 

 from her two-year-old record, it is reasonably safe to judge 

 the richness of her milk. 



The whole subject may be summed up as follows : 



A dairy cow on the average as a two-year-old may be ex- 

 pected to produce about 70 per cent; as a three-year-old 

 around 80 per cent ; and as a four-year-old about 90 per cent 

 of the milk and butter fat she will produce under the same 

 treatment when mature. 



The richness of milk remains practically constant from 

 year to year, except that after the third milking period there is 

 a slow, gradual decline with advancing years. 



1 20th Annual Report, Vermont Exp. Sta. 



