2 BULLETIN 1352, U. S. DEPARTMENT OF AGRICULTURE 
would leave too few for positive conclusions, and to compare records 
of animals of different ages we must use some means of adjusting 
those of immature animals. 
It is also a matter of interest to every breeder to know what rela- 
tionship exists between the record of a 2-year animal and that of 
the same animal at 5 years of age, in order to be able to form some 
fairly accurate estimate of the ultimate ability of his heifers. 
MATERIAL USED FOR THIS STUDY 
The sources of material for this study are the published results of 
official tests of Jersey and Guernsey cattle. The study embraces all 
class A and class AA records up to and including the 1921 volume of 
the Register of Merit of Jersey Cattle ; and all single-letter advanced- 
register records up to and including Volume 34, No. 6 (April, 1922) 
of the Herd Register of the American Guernsey Cattle Club. A 
total of 14,571 Jersey records and 8,602 Guernsey records are used 
in the study of the effect of age on the production of butterfat. 
These sources offer an abundance of authentic material for studies 
of this character. The records are made under careful supervision of 
the State experiment stations. The objections to using them are (1) 
that they are made by a selected group of individual animals, as the 
requirements for entry into the Advanced Register and Register of 
Merit exclude certain animals which fail to meet their requirements; 
(2) that cows on official test are usually those which the breeder 
thinks in advance will qualify; (3) and that before and during the 
test they receive better care and feed than ordinary herd cows. 
It should be remembered, however, that a table of relationship of 
records of different ages evolved from this material would be used 
almost solely in connection with official records, because most studies 
of inheritance and prepotency of sires are based on records made 
under advanced-register conditions. 
Though official records represent a selected group of animals, these 
animals were tested on hundreds of farms scattered across the country 
from coast to coast and from the Canadian line to the Gulf of Mexico. 
All conditions of climate are encountered in such a wide territory, 
systems of herd management range from everyday farm care to highly 
specialized treatment, and a great variety of feeds are used. The 
environmental effect of any particular locality or system of handling 
test cows is neutralized when all records are blended in an average 
drawn from so many animals scattered over a wide area. 
INCREASE IN PRODUCTION WITH AGE 
The objects of this study were to determine (1) the increase in 
production that came with increasing age; (2) whether this increase 
in production with age was the same for younger as for older animals; 
and (3) at what age the maximum production was reached. 
It was first planned to use only the records of animals that had 
two or more records, but the factor of development was found to 
enter into the second record in addition to the factor of increasing 
age, hence to determine the influence of age a study was made of 
initial records only. It may be assumed, therefore, that the differ- 
ence between the average records for the various age groups shown 
in the tables is due to the influence of age onlv. 
