6 BULLETIN 1352, U. S. DEPARTMENT OF AGRICULTURE 
2 years of age, and two cows in this class, constituting the first group, 
wnose initial records average 295 pounds of butterfat, were retested 
when 2^ to 3 years old and made retest records averaging 393 pounds 
of butterfat. Fourteen cows with initial records averaging 396 
pounds of butterfat were retested when 3 to 3J^ years old and made 
retest records averaging 511 pounds of butterfat. 
The difference between the average reentry and average initial 
records is divided into two parts, one of which represents the incre- 
ment due to age and the other the portion of increase due to the 
development brought about by reason of the initial record. 
The column which succeeds the series of reentry record columns 
shows the portion of the reentry record which is due to the increase 
in age since the initial record was made. The next double column 
in the table shows the portion of the reentry record which is due to 
development. This is given in pounds of butterfat and percentage 
of the reentry record. The last column shows the average per cent 
of increase due to development for the entire group. 
To complete the illustration given above, the average reentry 
record of the two cows in the first group was 98 pounds greater than 
their initial record; 45 pounds of this increase was due to the year's 
increase in age, and 53 pounds, or 13.5 per cent, was due to the 
development the heifers underwent because of the feeding and 
handling during the previous test. 
The first group in Table 13 includes 51 heifers with average initial 
records, made under 2 years of age, of 340 pounds of butterfat. 
These same 51 heifers made reentry records at 23^ to 3 years of 
age which average 412 pounds of butterfat. The total increase is 
72 pounds of butterfat, of which 43 pounds is due to the year's 
increase in age, and 28 pounds, or 7 per cent, is due to development. 
A word of explanation should be given as to how these results are 
obtained. The 51 Jerseys under 2 years of age in the first group of 
Table 13 average 340 pounds of butterfat on their initial tests. The 
average initial record of 1,237 Jerseys as shown in Table 2 is 343 
pounds of butterfat. Therefore this group of 51 produced 99.1 per 
cent of the average of all animals under 2 years. The average 
production of 1,613 Jersevs at the age of 2)4, to 3 years is 386 pounds 
of butterfat (Table 2), and 99.1 per cent of 386 is 383 pounds of 
butterfat, which should be the estimated production of this group 
when 214 to 3 years old, allowing for the normal increase due to 
age. The difference between 412 pounds, the actual reentry record, 
and 383 pounds, the estimated production for this group at 23^ to 
3 years of age, is 28 pounds, and this may be said to represent that 
portion of the increase which is due to development. 
In this study there are 209 groups of Jerseys, and all but 23 of 
the groups show a positive development factor. There are 3,722 
individuals in these 209 groups, and the 23 negative groups number 
only 39 individuals. These negative results are frequently based on 
groups numbering only one individual. 
The Guernseys with two records number 1,215, listed in 148 groups. 
Only 14 individuals in 9 groups fail to show a positive development 
factor. 
