What is an EPD?

An EPD, or expected progeny difference, is just what it sounds like. It is a prediction of the difference between the average performance of future progeny of an individual and the performance of a theoretical reference animal, an animal with an EPD of zero. To make this comparison fair, we assume similar environments and mates of the same genetic value. An EPD is then a prediction of progeny performance relative to some standard. It is expressed in trait units. For growth traits, the units are pounds.

For example, a particular sire might have an EPD of +1.5 for birth weight. This means that he is expected to produce calves 1.5 pounds heavier on aveage than the theoretical bull with a zero EPD for birth weight. More meaningfully, this sire is expected to produce calves 4.5 pounds lighter than a sire with an EPD of +6.0 (6.0‑1.5=4.5) or 4.0 pounds heaver than a sire with an EPD of –2.5 [1.5‑(‑2.5)=4.0]. As you can see, EPDs are designed to compare animals, nothing more. (Back to the top)

How are EPDs calculated?

EPDs are calculated using statistical procedures that fall into a general category called Best Linear Unbiased Prediction or BLUP for short. BLUP procedures properly weigh vast amounts of information on individuals and their relatives. (Back to the top)

What advantages do EPDs have over ratios?

Ratios are measures of individual performance and are calculated on a within herd basis. They contain no pedigree or progeny information and are really only appropriate for comparing animals within the same herd or contemporary group. EPDs, on the other hand, are calculated from all sources of information – pedigree, own performance and progeny. EPDs are comparable across herds. (Back to the top)

How can we compare EPDs on animals in different herds if their calves have been raised in very different environments?

BLUP procedures account for environmental differences through information on the genetic relationships between animals in different herds. This is similar to having “reference sires” in each herd. The more closely related the animals in two herds are, the better job BLUP procedures can do in adjusting for the differences in environment between those herds. (Back to the top)

How much do EPDs depend on pedigree vs. own performance vs. progeny information?

The relative importance of these sources of information depends on the amount of information available from each source. The EPDs of young animals will be based entirely on pedigree and individual performance. EPDs for animals with large numbers of progeny will be based almost totally on progeny information. (Back to the top)

If I mate a bull only to the best cows, won’t that make his EPDs look better than they should?

No. The procedures used to calculate EPDs account for the merit of the mates to which an animal is bred. (Back to the top)

What does an EPD of zero mean in terms of actual performance?

The actual performance of progeny of a sire with EPDs of zero will depend heavily on the environment in which those calves are raised and the genetic value of the dams. To take weaning weight as an example, we can expect the zero EPD sire to produce heavier calves in better environments. And if a cow herd is genetically fast growing, we can expect heavier weaning weights from the zero EPD bull than if the herd were less growthy.

It is not possible, therefore, to generalize about the absolute performance of progeny of zero EPD animals, or for that matter, the absolute performance of progeny of animals with an EPD. The only good way to get a feel for what particular EPD levels mean in your own herd is to try well-evaluated sires and establish your own baselines. Having done that, you should be able to predict how new sires will do by comparing their EPDs against those of the sires you have used and are familiar with. (Back to the top)

Where does the zero point come from?

For starters, zero is not breed average (though it can sometimes be close). In the Red Angus analysis, zero is a function of the performance of foundation animals, animals with no recorded parents. As such, the zero point or base, as it is often called, does not change over time as the breed changes genetically. This is handy because it means that a given EPD will always have the same biological interpretation: +10 for milk will always imply the same level of genetic ability for milk production. (Back to the top)

How do I determine breed average?

Probably the best way to get a feel for breed average is to study the statistics, which are published at the front of the sire summary. These vary from sire summary to sire summary, but at least they will provide the average EPDs and the range of EPDs for the listed sire in each trait. These should give you a good idea where the breed currently stands in terms of EPDs. (Back to the top)

What is a “significant” difference in EPDs?

A mistake that breeders often make is to interpret small differences in EPDs as being meaningful. A sire with a yearling weight EPD of +45 might be slightly genetically faster growing than a sire with a +40 yearling weight EPD (or he might not be, considering the error involved in prediction), but the truth is that a five‑pound difference will be undetectable in most herds. My advice is to divide EPDs into five categories: extremely high, high, moderate, low and extremely low. Once you have placed an animal in one of these categories for a given trait, forget about his or her precise EPD. (Back to the top)

What EPDs are best?

Beef cattle breeding would be a lot easier if we could assume that the highest EPDs are the best. Unfortunately, it’s not that simple. When we increase one trait, growth rate for example, we often pay a price somewhere else, perhaps in supplemental feed costs or reproductive performance. A delicate balance of levels of traits is required, and the balance differs with varying climatic, nutritional and economic environments. One of the most enduring challenges in beef cattle breeding is to determine the best balance of trait levels (and associated EPDs) for a given environment and clientele. (Back to the top)

What is meant by accuracy?

The accuracy value reflects the amount and relevance of the information used to calculate an individual EPD. Accuracy values range from zero (very poor) to one (extremely accurate). Accuracy measures the reliability of an EPD or the degree of risk associated with using a particular animal on the basis of its EPD. Highly accurate EPDs are very reliable; there is little risk that the progeny performance of an individual with high accuracy values will, on average, be much different than the EPDs indicated. On the other hand, the average progeny performance of an individual with low accuracy values may be quite different from what his EPDs suggest. A common misconception is that accuracy values tell us how variable an individual’s offspring will be. They don’t. Accuracies simply tell us whether the estimate is based on good hard data or whether it is little more than a guess. (Back to the top)

What is good accuracy?

The answer varies according to each person’s feelings about risk. Here is my breakdown:

Low: less than .40‑unreliable, but still a best guess

Low/medium: .40 to .60 – worth looking at but risky

High/medium: .60 to .80 – quite trustworthy, make comparisons with some confidence

High: greater than .80 – good accuracy, compare with confidence

(Back to the top)

What are direct and maternal EPDs?

Traits like weaning weight have what is called a maternal component. This means they are affected not only by a calf’s own genes, but by genes in the calf’s dam that influence the environment she provides for the calf. For weaning weight, we assume the maternal component is primarily a function of milk production. So we call the EPD for the maternal component of weaning weight the milk EPD.

The calf’s own genes for growth have a direct effect on its weaning weight. We call the EPD that predicts the effect of these growth genes the EPD for weaning weight direct or simply the weaning weight EPD.

There are, then, two basic types of EPDs for maternally influenced traits: 1) a direct EPD which estimates the value of genes passed on from an individual to its offspring which directly affects offspring performance, and 2) a maternal EPD which estimates the value of genes passed from an individual to its daughters which affect the environment those daughters provide for their offspring. (Back to the top)

What is a total maternal EPD?

Neither the direct nor maternal EPD predicts entirely the performance of offspring of an individual’s daughters since that performance will be a function of both direct and maternal effects. A third EPD, the total maternal EPD, combines direct and maternal predictions. For weaning weight, the total maternal EPD predicts the relative performance of progeny daughters of an individual, taking into account both the inherent growth of those calves and the milking ability of their dams. (Back to the top)

Who calculates EPDs?

BLUP procedures require the solution of many thousands of simultaneous equations, and for that reason BLUP analyses are performed at universities where the necessary computer facilities and the expertise are located. These analyses are normally done once or twice a year. (Back to the top)

Can I get EPDs for animals not listed in the sire summary?

Yes. EPDs for all animals, males and females, from newborns to those long since dead are updated each time a BLUP analysis is performed. Many of these are routinely reported on weaning and yearling reports and dam summaries. EPDs are also available from the RAAA on the internet at http://old.redangus.org/epdlook-new.html. (Back to the top)

What is an interim EPD?

New performance data arrive at the breed association daily, and so EPDs, especially for young animals, rapidly become out of date. To mitigate this problem, interim EPDs are calculated by the breed association as new performance information is processed. Interim EPDs combine the EPDs of a young animal’s parents with that animal’s new performance record and an estimate of the average genetic value of his or her contemporaries. In cases where no new observation has been recorded, interim EPDs may simply be pedigree estimates. Like any EPD, interim EPDs can be compared against EPDs of animals in other herds.

Typical examples of an interim EPD would be the EPDs for yearling weight that a breeder receives after reporting yearling data on his calves. These EPDs contain pedigree information from the most recent university analysis and combine that information with the new post weaning performance records. Interim EPDs do not have accuracy values associated with them. RAAA prints a “P” or “P+” in the accuracy column depending on whether the EPD is a pedigree estimate or contains individual performance information. Actual accuracies will be low. (Back to the top)

What is stayability?

The probability that a cow will remain in the herd for a given number of parities or to a given age, with the condition that the cow was given the opportunity to become part of the breeding herd (e.g. calved at least once). Stayability data consists only of calving records. Stayability is affected by breeders’ culling criteria. Given the primary emphasis in most herds on pregnancy, stayability is likely to be more a measure of sustained fertility than anything else. Stayability EPD for bulls are the prediction of the genetic differences between their daughters probability of staying in the herd to at least the age of six years. (Back to the top)

What are heifer pregnancy EPDs?

Heifer pregnancy EPDs estimate differences in daughters’ ability to conceive to calve as a two year old. Just like the stayability EPD, heifer pregnancy EPDs are expressed in terms of a percentage difference. For example, two heifer pregnancy EPDs , 5 and 10, differ by 5 percent. Daughters of the bull with the EPD of 10 are 5% more likely to conceive than daughters of the other bull assuming both sets of daughters are raised and managed in the same environment. In years past, scrotal circumference EPDs were used to infer a daughter’s age at puberty which is a factor in her ability to conceive to calve as a two year old. The heifer pregnancy EPD allows producers to select on the actual trait of interest instead of selecting only on related traits. Heifer exposure data is voluntarily submitted by RAAA members to estimate these EPDs. (Back to the top)