16 BULLETIN 1464, U. S. DEPARTMENT OF AGRICULTURE 
and rounds. As a general thing the overmuscular development of 
these parts is responsible for marked unevenness in development and 
in the thickness of flesh of the different parts. 
They do not as a rule fatten as readily or carry so much finish as 
other classes of slaughter cattle but the fat covering is usually fairly 
smooth, especially in younger animals. As a class, bulls have heavy, 
thick hides, coarse hair, heavy bones, large, heavy heads, and a 
marked degree of coarseness or roughness at joints of the legs. 
AGE-SELECTION GROUPS OF SLAUGHTER CATTLE 
At most markets cattle falling within any one of the several 
classes just described are aften sorted into a number of different 
groups or lots according to their age. In this bulletin these lots or 
groups are termed age selections. 
An age selection of slaughter cattle may be defined as an individual 
or number of individuals of any given class, in which the age of each 
of the individuals composing the group falls within certain specified 
limits. 
It is a well-known fact that an increase in age brings about certain 
marked changes in an animal. These changes affect both its outward 
appearance and the physical and chemical composition of the body 
tissues. As an animal increases in age its build or shape gradu- 
ally changes and any effect which sex condition has on its build 
or shape becomes more marked and pronounced. Its muscular de- 
velopment increases with age up to maturity, although the rate of 
gain in muscular development gradually decreases with increasing 
age. In younger animals the larger percentage of any gain in weight 
is usually attributable to muscular development or growth as opposed 
to a gain in its quantity of fat, whereas in older animals the converse 
is true. Although an animal's percentage of fat, as a rule, gradually 
increases with increasing age, the distribution of the animal's fat 
covering tends to become more or less uneven with increasing age, 
being especially noticeable in the fatter individuals of the older age- 
selection groups of the various classes. 
As a rule the ratio of carcass weight to live weight increases 
with increasing age. This is true of nearly all animals which have 
attained sufficient age to be termed cattle. The percentages of bone 
and hide usually decrease as the animal increases in age and fatness. 
The character or chemical composition of the bone changes, the bones 
becoming harder with increasing age with a like change in the 
character of the connective tissue. 
These changes which occur in an animal as it increases in age are 
directly reflected in the quantity and grade of the products obtained 
from the animal and herein lies the significance of age selections of 
cattle. Hence when trading, buyers and sellers always consider the 
age of the cattle. In this manner certain age-selection groups have 
arisen and are in rather general use at most large central livestock 
markets. The age-selection groups of the classes which are most 
generally used and which are considered necessary are shown in the 
Schedule of Market Classes and Grades of Slaughter Cattle, on pages 
6 to 7, inclusive. 
