Expected Progeny Differences: Why, What, When, and How, Part 2

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Expected Progeny Differences: Why, What, When, and How, Part 2

by Randie Culbertson, PhD, IGS Lead Geneticist

This article is the second of a two-part series discussing what, when, how, and why of EPD. In this article, we will focus on the “how” and “when” of EPD. When making decisions for purchasing bulls to add to your bull battery, how do you know which bull would work best for your breeding objective? Sale catalogs are full of pertinent information. But how do you use this information and when should you use the information provided.

The general rule, when considering an animal on their own individual performance, look at the phenotype. When considering an animal as a potential parent and for improving the next generation, look at the EPD!


It is important to remember that EPD are a tool for comparison. An EPD is a reflection of how we expect an animal’s progeny to perform on average in comparison to the average performance of progeny from other animals. Let’s consider two bulls: Bull A has a weaning weight EPD of 95 pounds and bull B has a weaning weight EPD of 102. This means that if we look at 100 calves from each bull, on average, bull B’s calves will weigh roughly seven pounds heavier than bull A’s. This does not mean that all of bull B’s calves will be heavier than bull A’s calves. Some calves will perform better than others. This difference in performance among calves can be due to differences in environment, the dam’s genetics (she contributes 50% of the genes to her offspring), and the rule of independent assortment. But when we look at all the progeny from each bull across different contemporary groups, we will see that bull B’s calves on average are heavier.

In the last article we discussed how the addition of information can result in an EPD change. When considering younger animals, there is a risk of their EPD changing as more information enters the genetic evaluation. This is compared to older animals with progeny information already incorporated in the calculation of their EPD, resulting in less EPD movement. The level of information included in the estimation of EPD is indicated by the accuracy. Alongside each EPD that is published, an accuracy is also published. An accuracy is defined as the relationship between estimation of an animal’s EPD and the “true” EPD for that animal. More simply put, an accuracy is reflective of the amount of information provided on that animal, and is the level of risk associated with each EPD. The lower the accuracy, the less information provided for the EPD estimation, and the higher the accuracy, the more information used for EPD estimation. As an accuracy begins to approach 1, this would mean that these animals have a significant amount of information included in the evaluation, and that their EPD are close to the true genetics for that trait. In addition, as accuracy increases, the amount of potential change for an EPD decreases.

Possible change is an easier way to interpret the amount of change likely in an EPD. When considering a young bull with low accuracy, it is important to consider the amount of change that could potentially occur as data for this specific animal enters into the genetic evaluation. Younger bulls will have a larger possible change range than older bulls with higher accuracy. As an EPD approaches an animal’s true genetic value, ⅔ of the time this value would be within the possible change range, but ⅓ of the time the true EPD will fall outside of this window. This doesn’t mean that each animal’s EPD will change by this amount, but it is the potential change that could occur to an EPD. It gives an indication of the level of risk for each trait and accuracy level. 

Along with EPD, accuracies, and possible change, a percentile rank for each animal is also published. The percentile rank is the ranking of an animal based on their EPD in comparison to all animals within the breed population of ASA’s registry. Percentile ranks range from 1 to 99, and the lower the number, the higher the ranking of the animal. For example, a bull whose weaning weight EPD is in the 5% percentile rank means that this bull is in the top 5% for weaning weight based on his EPD. Percentile rank does not take into account accuracy; therefore, if this bull has a low accuracy, there is a potential for this bull’s percentile rank to change (either up or down) as more information enters the evaluation.


When is it appropriate to use an EPD? When genetic improvement is your goal! EPD are a tool for genetic improvement and should be used when an animal is being considered as a parent for the next generation. Looking at a bull or heifer’s EPD is an evaluation of that animal’s merit on a genetic level, and is the expectation in the performance of their calves on average. When selecting animals as replacements or looking at purchasing a bull for your herd, the value of those animals is their genetic potential for producing future generations of calves with high performance.

When is it appropriate to use phenotypes? Phenotypes are important when the performance of the individual animal itself is being considered. When terminal steers enter a feedlot, their phenotype for feedlot and carcass performance become crucial for profitability, but these steers themselves will not be producing the next generation of calves. The genetics of these steers will not be passed on to the next generation. However, the phenotypes on these particular calves are extremely valuable. Not only for operational profitability, but in the context of genetic improvement, these phenotypes are extremely valuable information to the genetic evaluation, as well as improving the genetic prediction of the bull and dam of the calves.

Read part I.


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