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The ABCs of EPDs

It's been 16 years since the first expected progeny differences, or EPDs, were published for beef cattle. Initial producer skepticism has changed to widespread acceptance. By 1993, EPDs were the genetic selection tool of choice for most cattle producers in the U.S.To use EPDs effectively in selection programs, a clear understanding of what EPDs can and can't do is essential to avoid costly mistakes.What

It's been 16 years since the first expected progeny differences, or EPDs, were published for beef cattle. Initial producer skepticism has changed to widespread acceptance. By 1993, EPDs were the genetic selection tool of choice for most cattle producers in the U.S.

To use EPDs effectively in selection programs, a clear understanding of what EPDs can and can't do is essential to avoid costly mistakes.

What Does An EPD Tell Us? An EPD value predicts the genetic transmitting ability of an animal as a parent. The genetic makeup of an animal will never change, but the amount of information we know about that animal can. Additional information can change our predictions about that animal as a parent.

EPDs are obtained from genetic evaluation systems based on Best Linear Unbiased Prediction (BLUP) theory. Genetic evaluation systems based on BLUP theory use performance records, such as birth, 205-day and 365-day weights, along with pedigrees, to estimate EPD values.

These systems simultaneously estimate EPD values for direct and maternal traits, incorporate all relationships among animals being evaluated, and use information from correlated traits in multiple trait evaluations. These genetic evaluation systems estimate EPD values on sires, dams and young (non-parent) animals that are comparable across herds within a breed.

An EPD estimate on an animal incorporates the animal's records and its parent records, progeny records and sibling records, the animal's genetic ability and the environment in which the animal was raised. To help separate genetic ability from the influence of environment, producers supply information about the environment in which an animal was raised through the formation of contemporary groups.

A contemporary group is a group of animals born and raised together under the same management. Producers separate cattle into contemporary groups by determining which calves were raised in similar conditions and grouping them together.

Contemporary groups are also determined by sex, so heifer calves are compared with heifer calves, bull calves with bull calves and steers with steers. Calves remaining in the same contemporary group after birth must also be measured for production record keeping on the same day.

All contemporary group definitions after birth are usually sub-groups of the birth weight contemporary group. Different sub-groups account for death, selling of animals or the use of different management techniques. Animals can't be added to an initial birth contemporary group after it is formed because animals added later were raised under different environmental conditions.

When we are careful about contemporary grouping, environment becomes a common denominator and performance can be compared equally among calves in the group. If contemporary groups are incorrectly defined, the environment portion of the record is not accurately partitioned, causing incorrect estimation of EPD values, which can cause incorrect rankings of individuals and incorrect selection decisions.

Used To Compare Differences EPDs are easy to use and understand if we don't forget the word "difference." Comparing EPD values allows us to obtain a difference. The difference is used to predict differences in performance of an animal. For example, comparisons could be made between two or more bulls, between a bull and the breed EPD average or between a bull and the EPD average of his birth year.

Consider an example using weaning weight. A commercial producer is interested in producing pounds of weaned calf and has the choice of purchasing bull A (with a weaning weight EPD of +40 lbs.) or bull B (with a weaning weight EPD of +10 lbs.). The difference between bull A's EPD and bull B's EPD is 30 lbs. Thus, the producer could expect bull A's progeny to weigh on average 30 lbs. more than bull B's progeny at weaning (Figure 1).

This variance is due to the genetic differences between bulls for weaning weight, which are passed on to progeny. The commercial producer interested in increasing pounds of weaned calf should strongly consider purchasing bull A.

All breed associations with genetic evaluation programs have EPD values for birth, weaning and yearling weight and maternal milk. Some breed associations have many more EPDs, including calving ease, gestation length, scrotal circumference, stayability, yearling hip height, mature height, mature size and carcass traits.

To effectively use EPD values in a selection program, we must know what the current breed averages are for each trait. This will allow us to know where the breed is and whether individuals selected as replacement females or potential sires will produce the results we want. A breed's sire summary is the best place to find current breed averages for each trait.

Although we may be tempted to compare EPD values across breeds or compare individual animals across breeds, EPD values for specific breeds can't be used for across-breed comparison because the genetic base varies from breed to breed.

Genetic base is a group of animals whose EPDs average zero. A base can be set by forcing EPD values of animals of a particular birth year and breed to average zero. Or it can be set by allowing the EPD values of the base animals (those unknown parents at the top of the pedigree in a breed) to average zero. Although choice of a base is arbitrary, once the base is set, all EPD values are relative to that base. No two breeds have the same genetic base.

What About EPD Accuracy Values? Each EPD value has an accuracy value that represents the amount of information known about an individual when the EPD value was estimated. Accuracy values are given in decimals and range from zero to one.

Larger values (those closer to one) indicate a greater degree of accuracy. These values help determine the level of risk a producer may take in using a particular individual in a herd.

If we like an animal's EPD values, we need to then look at the accuracy values. If the animal has low accuracies, and we are not risk takers, we may decide to use that animal in a limited way, and find an animal with a higher accuracy to use more extensively. Possible change tables, also found in sire summaries, provide the amount an EPD value can change.

But high-accuracy levels don't guarantee all of a sire's calves will be identical. The following graphs using American Angus Association data illustrate this point.

Graph 1 depicts the birth weight EPDs of two bulls as well as the number of progeny (at each given birth weight) each bull produced. The numbers on the graph represent the number of progeny at that particular birth weight. Bull A has a birth weight EPD of !0.1 lbs. Bull B has a birth weight EPD of 6.1 lbs.

The expected difference on average of progeny between these two bulls is 6.2 lbs. However, as you examine the graph, bull A, with a birth weight EPD of !0.1 lbs., produced calves that ranged in weight between 50 lbs. and 100 lbs. Most of his 78 calves were between 57 and 73 lbs.

Looking at bull B, most of his 86 calves weighed between 80 lbs. and 95 lbs. at birth. However, he sired calves that weighed 55 lbs. and 110 lbs.

The difference in performance between bulls for birth weight is apparent. However, each bull has the ability to produce calves on the "outer fringes" of what is expected.

Graph 2 illustrates milk or maternal EPDs. Bull A, with a !16-lb. milk EPD, has 73 daughters in production who have produced 131 calves. Bull B, with a 24-lb. milk EPD, has 51 daughters in production that have produced 149 calves.

The American Angus Association looked at the average weaning weight ratio (at 205 days) of these daughters' calves. As seen in the graph, the average weaning weight ratio of calves from daughters of the !16-lb. milk EPD bull averaged a ratio of 95. However, some calves had extremely good weaning weight ratios.

The same can be said about the calves out of daughters sired by the 24-lb. milk EPD bull. On average, the calves ratioed 105. However, daughters out of this bull produced some calves that were far below that average.

On average, the differences between bulls can be seen. However, individuals can and will fluctuate in their actual performance depending on genes received from their sire.

When To Use EPDs The majority of U.S. beef producers are commercial producers. Commercial producers sometimes think they need EPDs on their crossbred cows in order to use EPDs effectively. This is not true. Choice of sires within your breed(s) of interest has a tremendous impact on the genetic improvement of the herd. You do not need to have registered cows, or straightbred cows, to use EPDs as a sire selection tool.

A good understanding of the herd performance level for reproduction, growth, maternal and even carcass merit, is first priority. Calving and weaning percentages, pregnancy percentage, pounds weaned per exposed females, calf death loss and average mature cow size are all examples of decision-making tools.

Using EPDs, we select sires to move the herd in the desired direction genetically. Some recommendations for using EPDs in commercial scenarios are shown in Table 1.

Each of these recommendations should be followed while considering the prevailing production conditions, including environment and marketing strategy, among other things. For example, rougher environmental conditions probably dictate avoidance of very high EPDs for growth or milk and, perhaps more importantly, to avoid high birth weights.

Growth EPDs should be geared to the needs of the market. Also, traits for which there are no EPDs can still be important. Traits associated with reproduction certainly fall into this category. For example, commercial producers should demand that the seller's bulls should have passed a breeding soundness examination.

EPDs are another selection tool to be used as part of a complete selection program. Bull buyer basics include not only performance information but other criteria like visual appraisal for structural correctness and a satisfactory breeding soundness exam, including semen evaluation. Scrotal circumference measurements and frame score are also important considerations.

Both commercial and purebred cow/calf producers have EPDs available to them as a powerful selection tool. The purebred breeder may obtain EPDs on each member of their herd by participating in cattle evaluation services available through their respective breed associations.

Cow/calf producers can use EPDs to enhance sire selection and add predictability to the genetics of their cattle using sire summaries, bull sale catalogs and other sources. EPDs allow fair comparisons of future progeny performance for bulls of the same breed.