Now that we have a glossary to work with, we’re ready to find out how to use EPDs.
In this post, we’ll try to answer two questions:
- What does the “+10” in “WW +10” mean?
- How was the number “+10” calculated in the first place?
EPDs are most useful when comparing two animals. Let us suppose for this example that we are going to look for a Hereford bull.
If you recall, the first of the four basic EPDs is BW, or birth weight. We happen to know of two Hereford bulls with BW EPDs of -5.2 and +0.4. What does this mean? Well, there are two ways to think of this.
First, recall that the unit of measure for BW is pounds. But we cannot just assume that Bull A (BW -5.2) is going to knock about five pounds off the birth weight of his progeny; nor can we assume that Bull B (BW +0.4) will increase the birth weight of his progeny by nearly half a pound. Instead, we are comparing Bull A and Bull B to each other. The EPDs tell us that Bull A’s calves are lighter at birth than Bull B’s calves by 5.6 pounds on average.
We can also compare the EPDs of individual bulls to the breed average. Using the example above, suppose that we want to know how Bull A stacks up compared to other Hereford sires across the breed. We happen to know that the average EPD for birth weight in a given year is +3.3. This puts the average birth weight of Bull A’s calves 8.5 pounds lower than the breed average.
You may have seen an additional number next to the EPD, perhaps labeled Accuracy or ACC. This is usually a number between 0 and 1, and it reflects the amount of data that was available when the EPD was calculated. The closer the accuracy number is to 1, the more accurate the EPD is considered to be and the less likely it is to change over time as more data is collected.
How EPDs are Calculated
Calculating EPDs is an incredibly complex process and uses an enormous amount of data. Furthermore, it is a task that must be repeated frequently by each individual breed association so that the EPDs and averages are up to date. The process of collecting data and outputting EPDs is called a national cattle evaluation (NCE).
The data being collected falls into two broad categories:
- Progeny data.
Like begets like. For this reason, the performance of a sire’s ancestors, relatives, and progeny is a useful indicator of that sire’s potential. The NCE collects data on each sire’s parents, grandparents, great-grandparents, siblings, and offspring. Of course, a young bull may not have any offspring, or at least any offspring old enough to have associated performance data. In that case, interim EPDs are calculated primarily on pedigree data and the accuracy is set at a lower number.
All of this data goes into computer models for processing. Many factors besides the raw data are considered in the models. For example, the data is adjusted to account for the differing management practices between producers, changing genetic trends across the breed, and genetic linkage between the traits themselves, among other things.
While you might think that an EPD of 0 for any given trait would mean average, this is not the case. EPDs are assigned relative to a base population. An arbitrary year is selected, and the base population consists of all of the animals born in that year. The base population is assigned EPDs totaling 0, and current EPDs are reported relative to the base population’s EPDs. This is why the average birth weight EPD for a breed can be +3.3, as in the example above.
When the EPDs are finally calculated, they are reported along with the breed averages and percentile rankings. The percentile rankings simply tell us how a given sire stacks up in comparison to the rest of the breed. For instance, if our bull has a CED (calving ease direct) of +7.3, he might rank in the top 1% of the breed for that trait, while if his CED is +2.6, he might only be in the top 25%.
So now you have the basics of how to use EPDs. But like any other tool, this one has both advantages and disadvantages.
Next week: Pros and Cons