Fitting Estimated Progency Differences (EPDs) Into Your Selection System
Estimated Progeny Differences (EPDs) are powerful genetic evaluation calculations. But how do you actually use these values or fit them into your selection program on the farm?
One thing to remember is that the EPDs are only an additional tool for your toolbox. In reality, EPDs take some of the information that you normally like to keep in your head and translate that into a number for each animal. For example, your favorite ewe is the favorite for a number of reasons. Perhaps, she always has twins that are a good size and her mother also had a lot of twins and the daughters you have kept from her also have good sized twins. It is easy to recall this information for your favorite ewe(s) and maybe even for your worst ewes. But, if you have 100 ewes it may be difficult to be sure how that information relates for each and every one of those 100 ewes. EPDs take all of the information on how the ewe performed and how the ewe's relatives have performed and give the ewe a ranking compared to other ewes of the same breed.
The amount of information used in the EPD is limited compared to what many producers keep in their memory for favorite animals. Generally, EPDs are fairly specific for example, number of lambs born per lambing. This doesn't take into account the fact that the ewe was in the wrong group and didn't get flushed like the rest of the group for one lambing. It also doesn't consider that the ewe is on an accelerated lambing program and she's been averaging 3 lambs born per year but only 2 lambs born per lambing.
EPDs are based on statistics. The statistics tend to use a lot of averaging of numbers. As a result, if you use the EPD values, on average you will make more improvement in the trait than if you do not. However, the average is made up of individual animals. Individuals do not always fit averages very well. So, don't get discouraged if the values on one animal don't fit with the performance that you see on the farm. If the accuracy on the animal is low, the EPD value can change dramatically as more information is added.
Using the EPDs
The EPDs that are available may not take into account some of the traits that are important to the breeding program on your farm. You may still need to consider some average raw statistics in addition to the available EPDs in order to meet the selection criteria that are important to your operation.
Included in the traits not considered by the current EPDs are conformation and physical soundness traits. One strategy for selection is to pick on paper twice as many animals as you want to keep, using the EPDs and any other raw statistics necessary and then go out to the barn, sort those animals out, and then choose animals based on physical conformation and soundness.
Some traits may have an optimum in your operation. One obvious example is number born. In some operations it may be ideal if all ewes had triplets but it probably wouldn't be better if all of the ewes had quadruplets or quints. In this situation, one of the easiest ways to determine what kind of EPD value is best for your operation is to look at the EPD values on animals in your flock that are your ideal for the trait. Then aim for the kind of values that these animals exhibit.
EPD indexes are the best way to select for several traits at once. These indexes take into account how the traits interact with each other and allow for the best progress on all traits at the same time.
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