Genetics And Reproduction
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The beef industry has made tremendous use of genetic selection to improve economically
important traits. Most recently, much of that progress has been through the implementation of expected progeny
differences (EPDs). Expected progeny differences make it possible to compare the genetic merit of two or more
animals for any number of traits. Most EPDs involve growth or carcass traits, because most producers are paid by
the pound, or in some instances by merit of individual carcasses. For the most part, selection of female
reproductive traits has been ignored even though reproduction plays a vital role in the economics of any beef
operation. Recent studies suggest that to maximize profit in a typical cow/calf operation, reproduction should
receive the greatest emphasis. Even in a fully integrated beef operation, reproduction warrants equal emphasis with
production and consumption traits.
To illustrate the importance of reproduction, consider the herd characteristic of
calves weaned per cow exposed for service. Contrast two herds, each with 50 cows. Assume an average weaning weight
of 475 pounds at a value of $0.65 per pound in each of the two herds. However, one herd has 85 percent calves
weaned per cow exposed while the other has 75 percent. That is equivalent to more than a $1,500 difference in value
in the current calf crop. This doesn't consider any other effects on profit, such as feed costs that might be
associated with cows that did not wean a calf. In this example, it is easy to see how reproduction is vital in
It has been difficult to apply genetic selection for female reproductive traits. There
seems to be limited consensus on how female reproduction should be expressed. Suggested traits include lifetime
pregnancy rate, postpartum interval and calving date. However, it is much easier to gather accurate data on birth
and weaning weights than on any of these reproductive traits. This presents a problem if EPDs were calculated for
female reproductive traits. Also, female reproductive traits appear to be only slightly heritable. Heritabilities
range from 0 to 1; values closer to 1 indicate that a trait responds better to selection.
The low heritabilities for cow reproductive traits suggest that selection response
based on those traits would be slow. However, there may be other traits that respond to selection and indirectly
affect cow reproduction.
Birth weight Obviously, heavier birth weights will lead to increased difficulty in
calving, or dystocia, resulting in additional losses of calves and possibly cows. Even though calving difficulty is
scored on a scale of 1 to 5, we might think of dystocia on more of a continuous scale. In other words, just because
we didn't have to assist a cow at calving doesn't mean she didn't experience dystocia as opposed to a "normal"
parturition, or eutocia.