When selecting for trait improvement, it is important to take the time to think about all the traits that are important to your operation and find a balanced approach for selection in all of those traits. While this may seem like a daunting task, there are some simple steps you can follow.
Formulate a breeding objective
Breeding objectives are a simple way to outline what traits in your operation lead to the most profit. For example, in a breeding objective, you might identify that your objective is to purchase crossbred replacement females and breed them to a terminal bull and sell all the progeny at weaning.
Using this objective, you have now identified that you need to focus your heifer selection on maternal traits and your bull selection on weaning weight, while placing some emphasis on direct calving ease. These traits will all lead to more live calves on the ground and more calf to sell.
You might also want to take note of details like whether you’re breeding your bull to heifers or cows. Heifers need emphasis on calving ease direct, but if you’re breeding to cows and keeping replacement females, you should probably be more interested in selecting on calving ease maternal.
Why the fuss over selecting on only one trait? Selection doesn’t happen in a vacuum, so the decisions we make in one trait often impact other traits. This phenomenon happens because traits often share genetic correlations.
Think of it as having a giant pile of assorted color paperclips and your goal is to select only the blue paperclips out of the pile. It is an impossible task without moving a few other paperclips, because they share a physical relationship with one another. Selection for traits works the same way. When we choose to select for one trait, we will often make changes in other traits that we may not have intended.
Understand genetic correlations
Genetic correlations give us information about how traits are related, either by genes that impact more than one trait (like DGAT1 which impacts milk volume and milkfat), or because there may be genes in close proximity in the genome that tend to be inherited together (Table 1).
Sometimes these relationships are favorable, and by selecting on one trait, we get a favorable side effect.
These correlations are helpful to breeders and are the basis of how we can use indicator traits in genetic evaluation to select for economically important traits. As an example of how indicator traits work, consider the relationship between birth weight and calving ease. We can select for lower birth weights to increase calving ease because they share a genetic correlation.
However, calving ease is the economically important trait, not birth weight. Birth weight is just an indicator for what we really care about: getting live calves on the ground.
Sometimes genetic correlations have less favorable impacts called genetic antagonisms. Consider the relationship between birth weight and weaning weight. We typically want to increase performance at weaning, but if we only select for increased weaning weight, we get an increase in birth weights due to this antagonism, which can cause an issue with calving difficulty.
Use independent culling
How do we get around these relationships? Sometimes the answer is that we can’t. If one gene is impacting multiple traits, that’s a relationship we can’t change. However, when these relationships are caused simply because genes are impacting two different traits and simply close together in the genome and often inherited together, we can “break” those relationships and overcome genetic antagonisms.
In order to “break” these effects, the answer is to simultaneously select for all the traits that are economically important. Various options can be employed, including independent culling and utilizing selection indices.
Independent culling essentially sets minimum thresholds for a trait and any animal selected for mating would have to exceed those criteria (Figure 2).
In this example, there was only one bull in the group of 10 candidate sires that met the criteria set by the independent culling levels (top 10 percent of the breed for calving ease direct and weaning weight). However, there were bulls that just missed the cutoff for either weaning weight or calving ease that might actually be a better fit for that program and be more profitable.
Try a selection index
As you probably already surmised, there are issues inherent with using independent culling levels. A better alternative is to use a selection index. A selection index is essentially a combination of expected progeny differences (EPDs) weighted by their economic importance to a defined breeding objective. Because of this, you have a single number on which to place selection.
The key to using indexes well is to make sure the goals of the index align with your operation’s goals and then simply select for the largest number (more money saved or earned). Indices provide a very simple and easy way to practice multiple-trait selection and balance the economic importance of a variety of traits relevant to a herd.
Instead of practicing single-trait selection, practice balanced, multiple-trait selection either on a set of EPDs or by using a selection index. While challenging, new selection tools are making the process more straightforward than ever before, allowing you to focus on the other challenges of beef production. 
PHOTO: It is important to take the time to think about all the traits that are important for your operation when selecting an animal for breeding. Staff photo.
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Megan Rolf
- Assistant Professor
- Animal Breeding and Genetics - Kansas State University
- Email Megan Rolf
