PRINCIPLES OF LIVESTOCK BREEDING. O 
early maturity in growth that has been one of the characteristics for 
which the domestic animals have been most carefully selected. 
Under favorable conditions hogs, sheep, cattle, and horses mature 
at remarkably early ages. Females usually become mature a little 
earlier than males under the same conditions, but the difference is 
not great. Boars and sows have been known to breed as early as 
3 months, bulls and heifers at 4 months, rams and ewes at 5 months, 
and stallions and mares at 12 months. It is not, however, considered 
advisable to breed animals until some time after sexual maturity 
has begun, in order to avoid interference with their development. 
Moreover, the young will not obtain so good a start as they should 
unless the dam has nearly finished her own growth. It has also been 
asserted that the offspring of a very young sire are apt to be unthrifty, 
but there seems to be little evidence for this belief. Sows may usually 
be bred without harm at about 8 months, permitting their offspring 
to be born just a year from their own birth. Ewes are often bred 
at 7 months to lamb at a year, but under ordinary conditions a thrifty 
flock can not be maintained unless breeding is deferred until a year 
later. Heifers may usually be bred at 15 months, which means 
calving at 2 years of age. Most fillies can be bred at 2 years and 
practically all by 3 years. Limited use of males may be begun at 
about these same ages. 
FREQUENCY OF SERVICE. 
The number of females which can be served by a mature male 
varies greatly under different circumstances. Most care seems to be 
necessary with the stallion, in which fertility rapidly declines after a 
number of daily services. Eighty mares is about the limit of the 
number which should be served by one stallion in a season. With 
careful handling a single bull may be used with 60 or 70 cows, a single 
ram with even more than 100 ewes, and a single boar with 30 or 40 
sows. Under range conditions the numbers are much less. One 
bull may run with 20 to 30 cows and one ram with 40 to 70 ewes. 
The principal effect of too frequent service on a mature male seems 
to be temporary sterility. Daily service by a vigorous stallion was 
found by Lewis, of the Oklahoma experiment station, to be accom- 
panied with a rapid decrease in both the number and vitality of the 
sperm cells. It is a common belief that fertilization by the weakened 
sperm cells, formed after excessive service, will result in unthrifty 
young, but the experimental evidence does not support this view. 
O. Lloyd- Jones and F. A. Hays, of the Iowa experiment station, made 
extensive experiments on rabbits to test this question. After too 
frequent service they found a marked decline in the percentage of 
pregnancies induced and ultimately a decline in the size of litters, 
