by: Dr. Ronnie Silcox
Extension Animal Scientist University of Georgia

It was about 40 years ago that the beef industry was introduced to the Expected Progeny Difference (EPD). In the early days, data were limited and based on comparisons with a few reference sires used in designed programs. There has been much progress in the methods used to calculate EPDs, and today most breed associations provide EPDs on all animals in the breed. After 40 years, there is still confusion over how to use these tools.

It is important to start out with the realization that EPDs are a prediction of the genetic ability that an animal can transmit to its offspring. The animal's genetics do not change. Since the EPD is a prediction of the animal's genetics, an animal's EPDs can change as new information is included in the calculations.

EPDs are calculated by most major breed associations. These calculations involve complex statistical equations; but to get a little bit of a feel for how it is done, let's calculate the simplest EPD that can be calculated. Let's calculate a withinherd EPD for a young bull, where we do not know anything but individual performance. The first thing we do is compare the bull with his contemporaries. A contemporary group is a group of animals of the same sex in the same season that were raised under the same management conditions. The bull we are interested in had a yearling weight of 1,100 pounds in a contemporary group that averaged 1,000 pounds. First, we find the difference between the selected bull and the average (1,100 lbs - 1,000 lbs = 100 lbs). Our selected bull is 100 pounds heavier than average. This is called the selection differential. Some of that 100-pound difference is due to genetics, and some of that difference is due to environmental effects. From research done over many years, we know that about 40 percent of the observed differences in yearling weight are due to genetics; or heritability = .40. If we multiply the selection differential by the heritability (100 lbs X .40 = 40 lbs), we get an estimate of the bull's genetic difference from his contemporaries, or his Breeding Value. We figure that this bull has about 40 pounds more genetic potential than his contemporaries. A bull contributes only half of the genetics in a calf; the other half comes from the dam. So we need to take the bull's Breeding Value (40 lbs) and divide by 2 (40 lbs/2 = 20 lbs) to estimate how much of this genetic superiority he would transmit to his calves. We would expect this bull to produce calves that are about 20 'pounds heavier than the average bull in this contemporary group. In other words, we Expected his Progeny to have about a +20 lbs Difference when compared with the average bull in the group.

Comparing my calculations above with a breed association's calculation of EPDs is about like comparing a rowboat with the space shuttle. The EPD I calculated is not very accurate, and it is valid only in that one single-sire group; but the basic idea is the same. To make my EPD better, we need to develop an equation for every animal in the breed, living and dead. In each of those equations, we do compare the animal with its contemporary group; but we also include information on the ancestors (sire and dam), relatives (brothers and sisters), and progeny. In many cases, we also include information on the genetic relationship between traits. Before we do any of that, we also need to adjust records for things such as age of dam and sex. In some breeds, we wind up with a million-plus equations that we solve to get EPDs for every animal. Of course, when you actually start looking at the performance of an animal's offspring, instead of just looking at individual performance like I did above, you get a much more accurate predication of EPDs.

Because we use artificial insemination a great deal in purebred herds, some of the same bulls (or sons of those bulls) get used all over the county. These common ancestors and relatives give us a way to link herds together and allow us to calculate EPDs that can be used across herds.

If you send in a DNA sample on an animal, we can now add the genetic testing information to the equations, in addition to performance data. When we do this, we get what is called a Genomic-enhanced EPD. For a young animal with limited performance data, this inclusion of DNA results helps a great deal in improving accuracy of EPDs.

When I calculated my super-simpleminded EPD above, I compared the bull with the average of his group; so my average EPD for that group would be zero. Breed associations have been at this for close to 40 years; so breed average EPD, as reported by breed associations, is going to be based on some group of animals from the past. Today, it is important to realize that zero is usually not current breed average for a trait. In the Angus association, for example, zero is the average weaning weight EPD for animals born in 1979. The current breed average weaning weight EPD for young bulls in the Angus breed is +49. Most breed associations will have a list of breed average EPDs and percentile breakdowns on their website.

It is also important to remember that each breed association's EPD is based on calculations made with data from that breed. EPDs reported from one association are not directly comparable with those from another association. For example: In the Angus breed, breed average EPD for weaning weight EPD for active sires is +48; breed average EPD for weaning for active sires in the Limousin breed is +62. This does not mean that Limousin are better or worse than Angus. This is just a result of the fact that each association does its own calculations from a different starting point. Those differences do not mean anything. They are just an artifact of the way the different associations do their calculations. When I go to a sale that includes several breeds, I have to go back to the breed averages and recalibrate my mind as I move from one breed to another.

We have come a long way in calculating EPds. In the early days, EPDs were based on using a few reference sires in a few herds to evaluate a limited number of bulls. Over the years, the methods and equations have gotten much more advanced. We have recently added DNA testing results. Today, EPDs are more accurate predictions of progeny performance than ever, and we are evaluating a lot more traits; but the basic idea of comparing one animal with another animal in the same breed is the same.

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