Using Expected Progeny Differences in a Breeding Program

Source: Manitoba Agriculture, Food and Rural Initiatives

The role of purebred breeders is to improve the genetic quality of the seedstock cattle that they sell to commercial beef producers. Expected progeny differences (EPDs) have become a popular genetic improvement tool in recent years. This article is intended to explain some important factors that must be considered when EPDs are being used in a sire selection program. Excellent background information regarding EPDs is available from the breed associations listed at the end of this document

The EPDs for a yearling bull calf appeared recently in a purebred magazine:

birth weight = 3.1
weaning weight = 9.1
yearling weight = 12.4
milk = 10.4

However, these EPDs are not particularly useful.

You must be able to answer three important questions before a set of EPDs can be used. For the sake of this article, we’ll call this bull “Enigma.”

Question 1. Who calculated the EPD?

Some breed associations publish a single North American EPD for the entire breed. However, sometimes the sire lines used in Canada are not used extensively in the U.S. Consequently, there may not be enough genetic “linkage” between Canadian and American herds to create a common EPD for the whole breed. As a result, some breeds publish separate Canadian and American EPDs. Unless they are published in a joint “North American EPD”, a Canadian EPD and an American EPD cannot be compared directly, and EPDs from different breeds cannot be compared to each other.

Question 2. What is the breed average for each EPD?

Whether an EPD value is above average depends on what the breed average EPD value is. The breed average EPD value is not always zero. Most breeds use a fixed base. This means that the breed average EPD changes from year to year. If the breed average EPD for weaning weight is +20, a bull with a +15 EPD is expected to sire calves that weigh 5 lbs less than calves sired by a breed average bull. Put simply, a positive EPD doesn’t always mean above average. Be aware of what the current breed average EPD is for each trait. Current EPD values for a number of breeds are shown in the table. Most breeds update their EPDs twice per year.

Pretend that the breed averages for our bull Enigma were 3.3 for birth weight, 9.4 for weaning weight, 47.0 for yearling weight, and 9.1 for milk. Comparing Enigma’s EPDs to the breed average is very informative. He is near the breed average for birth weight, milk and weaning weight, but is far below average for yearling weight. Average or below average EPDs are not necessarily bad. This is a moderate bull, and will likely produce moderate sized replacement cows that do not have excessive feed requirements. But he is probably not an ideal terminal sire due to his below average yearling weight.

Question 3. What is the accuracy of the EPD value?

The EPD value only tells half the story. The accuracy value is equally important. The accuracy of an EPD tells you how much data was used in the calculation. Accuracy values range from 0 to 1. The higher the value the more reliable it is, or the less likely the value of the EPD will change significantly in the future.

Initially, data from parents, ancestors and half sibs is used. So it was theoretically possible to calculate a weaning weight EPD for Enigma as soon as he was born, based on the EPDs of his relatives.

EPD= +15lbs, ACC = 0.10

But this prediction is not very accurate, since it is based solely on the performance of its relatives, and does not include Enigma’s performance. Most breed associations will not release EPDs that are based strictly on pedigree data. Once Enigma has been weaned, the weaning weight EPD will be adjusted and the accuracy will increase because of the added information.

EPD = +11 lbs, ACC = 0.25

Once Enigma has sired calves of its own, the EPD will continue to change and the accuracy will continue to rise. Once 40 progeny records have been added, the EPD value will stabilize and the accuracy value will begin to plateau.

EPD = +12 lbs, ACC = 0.70

Accuracy values should be provided alongside the EPD value. If accuracy values are not provided, ask the seller for them. A low-accuracy EPD is simply a preliminary estimate on a young sire, and it may change when data from his calves becomes available. For example, a weaning weight EPD with an accuracy of 0.30 may vary by 10.7 lbs from the true value. A high accuracy EPD (ACC = 0.70) belongs to a proven sire. A weaning weight EPD with an accuracy of 0.65 may vary by 7.2 lbs from the true value. This is a substantial improvement from the unproven sire mentioned above, and is much more valuable than the individual bull’s actual weaning weight. Even a low accuracy EPD provides more genetic information than using only the actual measurement.

No accuracy numbers were shown for our bull, Enigma. But since he is only a yearling, we can be sure that no progeny data were available when the EPDs were published. Consequently, the accuracy of these EPDs is probably below 0.30 for all traits. Since this EPD may change as more data becomes available, you may not wish to breed this bull to a large number of cows.

Using EPD Values in a Breeding Program

EPDs are a valuable tool to aid purebred and commercial producers in selecting potential breeding stock. Cattlemen must clearly define their breeding objectives and determine what sort of bull is needed to meet these goals. Then carefully consider the EPDs of the bull as well as his relatives, if possible. The basic steps to choosing a herd sire are as follows:

1. Traits That Determine Your Bottom Line

Strict Cow/Calf Producer. If calves are sold at weaning time, birth weight and weaning weight are the primary considerations. Maternal traits (milk, stayability) and mature size must also be considered if replacement heifers are being selected.

Selling Calves as Yearlings. Birth weight and yearling weight are of primary importance. Maternal traits, such as milk and stayability and mature size are important if replacement heifers are retained. Stayability is the probability of daughters remaining in the herd for at least six years and have the opportunity to become part of the breeding herd.

Retaining Ownership Through to Slaughter. Birth weight and carcass traits (weight, yield grade, quality grade) are of primary importance, although maternal traits and mature weight must be considered if daughters are being kept back from this sire.

2. Don’t Forget the Other Traits

Keep in mind that EPDs are only available for a limited number of traits. Single trait selection can produce rapid results, but is not recommended because it can cause a decline in other traits. For example, simply selecting for extremely high yearling weights will probably cause birth weights to increase, and may cause calving ease to decline. Other factors such as structural soundness, conformation and temperament are equally important but are not included in traditional genetic evaluations because they are difficult to measure objectively.

3. Are You Satisfied With Your Herd’s Current Performance?

Which traits need improvement, and in what direction do they need to go? You must have production records in order to answer these questions (ex, smaller mature cow size, lighter birth weights,improved stayability).

4. Which Breed Will Match Your Needs?

Select a breed that will enhance the traits that you identified in step 2. Crossing two unrelated breeds is the simplest way to improve low heritability traits (such as reproductive traits) that are slower to respond to selection. This phenomenon is known as hybrid vigor. Moderate sized breeds tend to be superior for maternal traits, and can be used to moderate mature cow size; large framed breeds tend to be superior for growth traits. However, these generalizations do not always apply; there is as much variation within a breed as there is between breeds. This is where EPDs become useful.

5. Within That Breed, Which Bull Has EPDs That Meet the Goals Listed in Step 1?

It is clear from step 1 that the bull selection process becomes more complex as the number of breeding objectives increases. Rapid growth and superior maternal traits are not usually found in the same genetic package. Producers selecting their own replacements heifers while retaining ownership beyond weaning must make a decision. They can either find a sire that does all jobs acceptably well (a balance of maternal and growth traits), or split the cow herd, breeding some cows to a sire with strong maternal traits (calving ease, milk, stayability) to produce replacement heifers, and breeding the rest to a terminal sire (growth, carcass traits) to produce feeder or slaughter cattle.

Table 1. Expected progeny difference values for commonly used North American beef breeds

Note: EPDs from different breeds cannot be compared to each other. This is not an exhaustive list. EPDs are available for other breeds and there are additional EPDs for traits that are not listed here.

¹Canadian Angus Association: 142-6715 8th St NE, Calgary, AB T2E 7H7; Phone: 1-888-571-3580
²Canadian Hereford Association: 5160 Skyline Way NE, Calgary, AB T2E 6V1; Phone: 403-275-2662
³Canadian Charolais Association: 2320-41st Ave NE Calgary, AB T2E 6W8; Phone: 403-250-9242
⁴Canadian Simmental Association: 13, 4101-19th St NE, Calgary, AB T2E 7C4; Phone: 403-250-7979
⁵Canadian Limousin Association: 2320-41st Ave NE, Calgary, AB T2E 6W8; Phone: 403-253-7309