The period between weaning and breeding
is a very critical time in the life of a beef female. At weaning she is
between 7 and 10 months old and weighs, in general, 350 to 650 pounds.
Some six months later, she is exposed to the bull or to artificial insemination.
Hopefully, most of these heifers are bred in the first 21 days and 80 percent
or more are pregnant after a 45 day breeding season. Growing programs for
weaned replacement heifers must be adequate to allow enough gain from weaning
to 13 months of age to allow a high percentage of heifers to being cycling.
Since most beef breed replacements will need to gain 240 pounds between
weaning and breeding, the heifers must gain at least 1.33 pounds per day.
It should be emphasized that replacement heifers need to be fed separately
from the rest of the herd. Because of their size and age, as well as higher
nutritional demands, they simply cannot compete with the rest of the cow
herd, nor can they be expected to efficiently utilize poorer quality forages
and still breed as yearlings.
If there are wide ranges between the smaller and larger heifers, they
should be divided into two feeding groups to reach their desired weight
by breeding time. The days between initial weighing and beginning of the
breeding season are then calculated. The average daily gain necessary to
reach the desired breeding weight is determined, and the heifers are fed
to attain that average daily gain. The addition of approved levels of ionophores
such as monensin or lasalocid to the ration will improve the average
daily gain and enhance onset of puberty.
Onset of puberty is affected by age, weight, breed, and adverse environmental
stresses such as temperature and parasitism. Of these factors, weight is
the one that most producers can readily influence. Researchers and ranchers
have observed that high percentages of heifers will not reach puberty until
they have reached a minimum weight. These weights usually represent about
65% of the potential mature size. Therefore, the first target weight to
consider is that at the beginning of the breeding season. If the heifers
weigh about 55% of the mature size producers can expect only 50% of them
to be cycling at the beginning of the breeding season. However, about 90%
of most beef breed heifers will be cycling when they weight 65% of their
mature weight. Many ranchers have not recently weighed the adult cows in
their herd to know what average mature weight to expect. Therefore most
commercial ranchers would underestimate the mature size and underestimate
the target weights for the heifers. Recent data from the American Angus
Association (with records of over 20,000 cows) indicates that average mature
size in the seedstock portion of their bred is about 1,200 pounds. Heifers
from 1,200 pound mothers will need to weigh about 780 pounds by the start
of the first breeding season. If the mature size of the herd is 1,100 pounds
then the heifers can be about 715 pounds when breeding begins. And only
heifers with potential mature size of 1,000 pounds can be expected to cycle
at 650 pounds. These weights will not be exact since there is considerable
variation within breeds, but the data show that large cattle must be fed
for greater growth rates than smaller cattle.
Table 1. Puberty Weight (lb) of Heifers by Breed (assumes small to moderate
frame).
Weight
Weight
Estimated
at 50%
at 90%
mature
cycling
cycling
weight
