Department of Animal Science

North Carolina State University

Expected Progeny Differences (EPDs) are estimates of genetic merit for animals based on their own performance and production record, or their progeny with performance records. EPDs can be used by seedstock producers to identify strengths and weakness in their own herds, evaluate new sources of genetics, plan matings, make selections and culls, set price on animals for sale, and manage risk in using new unproven animals. Commercial producers can use EPDs to evaluate seedstock suppliers, evaluate and select individual pigs, determine the value of an animal for their operation, and manage the risk of new unproven genetics. Listings of EPDs are available from breed associations, seedstock suppliers, and artificial insemination studs.

This presentation will:

EPDs are estimates of genetic merit based on performance and production records. Performance records are measurements of an animals phenotype and are made up of two components: genetic and environmental. Genetic improvement through selection operates only on the genetic component of the animals record. The environmental component is not passed from parent to progeny and, therefore, needs to be accounted for when determining the genetic value of an animal. Some environmental factors such as parity of sow or sex can be adjusted for mathematically. However, other factors such as health, management and nutrition are only accounted for when EPDs are calculated.

After performance records are collected on pigs in a contemporary group and adjusted for known sources of environmental variation that information can be combined with all other available records in the population to estimate across-contemporary group or across-herd EPDs. The total set of performance records and pedigree information is combined in a statistical procedure known as Best Linear Unbiased Prediction (BLUP) by an "animal model" that describes gene flow over time and the biology of the trait. The pedigree information allows the performance records of progeny, cousins, sibs, parents and grandparents to help predict the genetic merit of the individual. Comparisons can be made across herds where genetic ties are present and when sires have been used in multiple herds through artificial insemination.

It is important to realize that the estimates of genetic merit are "estimates", they are not exact and they can change as more information is known. Genetic merit is reported as either an EPD or an estimated breeding value (EBV). The EBV is an estimate of the total genetic merit of an individual for a specific trait. The EPD is one-half of the EBV and can be interpreted as the genetic merit that the individual will provide to its offspring. Four different EPDs are commonly reported, two for growth and two for maternal traits. The growth EPDs are backfat depth and days to 230 pounds. For both backfat and days a negative EPD is more desirable, indicating less backfat and fewer days to reach 230 pounds. The maternal EPDs are number born alive and 21 day litter weights. For maternal EPDs a positive number is more desirable, indicating more pigs born alive and heavier litters at 21 days. EPDs and EBVs are reported in the units of measure for each trait. For example, a backfat EPD will be reported in hundredths of an inch (millimeters in Canada), and a litter weight EPD will be reported in pounds of pigs.

Accuracy values are usually reported for each EPD. The accuracy value tells how many pieces of information were used to estimate the EPD. An accuracy (repeatability in Canada) will range from 0 to 1.0 and is an expression of the reliability of the EPD. The closer the value is to 1, the higher the accuracy. Low accuracy values are caused by very few pieces of information on the animal being evaluated or by low heritability for a given trait. As more information is used in the genetic prediction the accuracy value will increase. The number of herds where a sire has progeny, the number of pigs tested, and the number of daughter records reported may also be reported. The numbers of records give an indication of the usage of a particular individual. If an animal has EPDs that meet a producer's selection goals it should be used regardless of the accuracy value. Figure 1 shows that the genetic change in progeny is greatest when EPDs are used, as compared to selection on adjusted data.

EPDs are commonly combined into three types of indexes; 1) Terminal Sire Index (TSI), 2) Maternal Sire Index (MSI or MLI) and 3) Sow Productivity Index (SPI). The TSI ranks animals on a combination of growth rate and backfat and should be used when selecting boars to produce terminal market hogs. The MSI ranks animals on a combination of all four traits with greater emphasis placed on maternal traits. The MSI is a good index for commercial producers to select replacement gilts or boars to produce replacement gilts. The SPI ranks animals on a combination of number born alive and litter weight and should be used when emphasis is placed solely on maternal traits.

EPDs are very effective tools to evaluate your own herd or those of potential seedstock suppliers. Figure 2 (PIH-9) describes the importance of evaluating seedstock suppliers. The rate of genetic improvement in commercial herds will parallel the genetic progress made by the seedstock supplier. The first three lines (A, B, C) of the graph show the expected improvement in genetic merit when the seedstock suppliers are making genetic progress. The genetic merit of commercial herds (B and C) follows that of the seedstock supplier. By purchasing the highest-ranking boars available, the commercial herds are able to approach the genetic level of the seedstock supplier. Purchase of boars from seedstock suppliers where genetic improvement is not realized (D and E) results in inconsistent genetic progress. This is because the genetic merit of the boars purchased is not improving. Commercial herds that purchase average boars from genetically improved seedstock suppliers have an advantage over commercial herds that purchase the very top boars from unimproved seedstock suppliers. For this reason, identification of seedstock suppliers is of primary importance. Selection of individual animals from these herds is secondary.

When selecting seedstock suppliers, first pinpoint the genetic traits that your operation needs. Then review the genetic improvement programs followed by the potential seedstock suppliers. A sound genetic improvement program should include four points (PIH 9); 1) accurate complete performance records, 2) assessment of genetic merit, 3) indexes, and 4) selection of the highest ranking boars and gilts based on the selection indexes. It is also important that emphasis be placed on reproductive and structural soundness of individual animals and herd health when selecting seedstock.

The first step in evaluating herds with EPDs is to compare the herd of interest to other herds in the same breed. Study the rankings in traits of interest for animals used in the herd you are evaluating. For example, if your goal is the improvement of backfat the seedstock supplier you select should be using herd sires ranked among the top 10% for the breed.

The second step is to compare the seedstock suppliers average genetic merit to that of the breed. Select a supplier that has above average genetic merit for the traits of importance in your operation.

The genetic trend in figure 3 also illustrates the important concept of genetic base. A genetic base is defined as a group of animals whose EPDs average zero. This group can be arbitrarily defined for each population. In figure 3 the average genetic value of animals born each year for backfat is decreasing . Sire A is an above-average boar born in year 1 that is .05 inches leaner when compared to all animals born that year. Sire B is a below-average boar born in year 8 that is .05 inches fatter when compared to all animals born that year. However, because of genetic trend caused by selection, boar B is superior to A by .15 inches. If the base in the genetic evaluation is set so that all animals born in year 1 average zero, the EPD of boar A would be -.05 and the EPD of boar B would be -.20. With the base set at year 1 most animals in the breed would have negative EPDs for backfat. Now, assume year 8 is selected as the base year, the EPDs of A and B would be +.20 and +.05 , respectively, and most EPDs would be positive. A base chosen in the middle years would tend to give approximately equal proportions of positive and negative EPDs. The actual EPD values of boars A and B change dramatically with changes in base. But remember, the difference in genetic merit between the boars is the same regardless. The genes do not change! The comparison of two animals EPDs is unaffected by the base. Regardless of base boar B is superior to boar A by .15 inches for backfat. Because of genetic trend and the genetic base, comparisons with EPDs should only be made between two individuals or between an individual and the average genetic value for the current year. Do not assume that 0 is always average.

EPDs can also be used as a decision making tool when planning breeding programs in both seedstock and commercial herds. Decisions that can be aided by the use of EPDs are selections, cullings, and matings.

When selecting animals based on EPDs it is always desirable to select the highest ranking animals available for the trait or index of interest. However, this does not imply that only the number 1 sire in a breed should be used. Any boar in the top 50% is above average and will provide genetic improvement to most herds. Commercial producers should concentrate on identifying superior sires through across-herd sire summaries and then selecting top ranked sons from on-farm programs.

Culling decisions made with EPDs will be most common in seedstock herds. To maximize genetic progress animals should be culled on the basis of their EPD when an animal with a superior EPD is available for replacement.

Mating decisions can also be made based on EPDs. Planned matings can be either positive assortative (high to high and low to low), negative assortative (high to low and low to high) or random. Positive assortative mating would be desirable among the top 5% of seedstock animals in order to produce offspring of extremely high genetic merit. This would also be the situation on a commercial farm using a rotaterminal program to produce its own replacement gilt's, where the top 15% of sows are identified and mated to superior maternal line boars. Negative assortative matings would be used in an attempt to produce greater uniformity among the offspring and for matings that provide complementarity across traits. An example would be the use of terminal boars superior for growth and backfat on maternal sows to produce a large volume of lean, fast growing market hogs.

Selected portions of sire summaries are presented In Tables 1 and 2 showing how EPDs are reported. Each table is followed by various production scenarios where you are asked to identify the most ideal boar to use in each situation.

Table 1. Across-herd sire summary of selected Duroc sires.

Source: December 1994, Duroc Across -Herd Sire Summary

PURPOSE: Natural service, terminal sire to be used on a three-breed terminal cross breeding system. SITUATION: Commercial operation (250 head, Yorkshire-Landrace sow base) integrated with corn production. Sows are selected for high reproduction and are moderate in growth and average in backfat. Sows are reared in total confinement. All offspring are to be marketed.

PRIORITIES: Improve percentage lean. Maintain growth.

A. Which of the above boars best fits this situation?

PRIORITIES: Improve growth rate.

B. Which of the above boars best fits this situation?

PRIORITIES: Your packer has said you must improve percentage lean.

C. Which of the above boars best fits this situation?

PRIORITIES: A balance of growth and fat.

D. Which of the above boars best fit this situation ?

ANSWERS: A) The boar best suited to improve percentage lean while maintaining growth rate is B. His EPDs of -1.00 (Days) and -.10 (Backfat) fit this situation very well, in that the other low backfat boars will add days to market. B) The greatest improvement in growth would be from boar D. He is expected to reduce time to market by nearly 8 days and have a small increase in backfat. C) When the goal is an improvement in percentage lean boars B and E would be expected to produce the greatest improvement but A, C, and G would also improve leanness. D) For a balance of growth and fat any boar but H should be used. The TSI combines fat and growth with the economic values and any boar with a TSI greater than 100 should be considered.

Table 2. Active sire list of selected Yorkshire sires.

Source: 1994, American Yorkshire Club National Genetic Evaluation

PURPOSE: Herd boar in multiplier portion of a 1000 sow commercial operation that produces its own F1 York x Landrace gilts for replacements. SITUATION: Herd operates with a good supply of feed and management skills. Gilts are kept as replacement females, males are sold as barrows. PRIORITIES: Postweaning growth and backfat. Litter size and milking ability. Soundness.

A. Which of the above boars best fits this situation?

PURPOSE: Herd boar in a seedstock herd that supplies gilts and boars to seedstock herds and AI studs. SITUATION: Seedstock herd operates with a good supply of feed and management skills. Gilts are sold as replacement females or retained for within-herd selection. Boars are sold to produce replacement females. PRIORITIES: Litter size and milking ability.

B. Which of the above boars best fits this situation?

PURPOSE: AI sire to be used as a terminal sire on a three-breed rotational cross-breeding system. SITUATION: Commercial operation (500 head, Duroc-Landrace sow base). Sows are reared in large outside concrete lots. Most offspring will be marketed on a carcass merit program, some gilts will be retained. PRIORITIES: Improve backfat. Maintain maternal traits and growth rate.

C. Which of the above boars best fits this situation?

ANSWERS: A) In a commercial operation producing replacement females that balance all four traits boar A would be given preference as his MLI is at least 14 points greater than any other boar. However any of the boars listed could and should be used, as they are all among the top 20% in the Yorkshire breed for MLI. B) For a seedstock herd where the maternal traits alone are of the greatest importance boar A would again be selected. Any boar with the exception of F could be considered. C) In a rotational program with emphasis on backfat boar C would be selected.

EPDs should and are used to determine value and price. Commercial producers can use EPDs to decide how much they are willing to pay for boars and gilts. Seedstock suppliers can use EPDs and indexes for pricing. EPDs can be used to determine if you are getting value for your money. Table 3 gives some standard economic values for various traits. If you know the value of these traits for your individual situation that would be more accurate.

Table 3. Economic values.

Sources: 1994 NSIF Guidelines (recommended); 1994, Stewart, et al.

In the following examples, economic values are used to determine which of the two boars (Table 4) would be a better buy. If a terminal boar is needed Boar B would have an advantage of $0.88 per pig produced (129.9 - 121.1 = 8.8 x $.10 = $.88). If you assume that the boars would be used to produce 700 market hogs the total economic advantage for boar B over boar A is $616.00. On the other hand, if a maternal boar was needed to produce replacement gilts Boar A would have an advantage of $35.30 per litter (150.6 - 115.3 = 35.3 x $1.00 = $35.30) produced by daughters used in the herd. If 100 daughter were kept in the herd for an average of 2.2 parities the total maternal advantage of boar A over boar B would be $7,766.00.

Table 4. Which boars is a better buy?

Economic values can also be used to compare differences between individual traits. For example if growth rate was most important in the terminal sire, boar A has the advantage. The economic advantage of boar A for days could be calculated as (-4.4 - -1.2 = -3.2 ) the 3.2 day advantage is multiplied by $0.17 per day and is equal to $0.54 per pig produced.

EPDs are "estimates" of genetic merit based on varying amounts of information and may change over time as new information is added. The amount of new information will change the accuracy and how well progeny perform will change the EPD. The accuracy value reflects the amount of information that was available to calculate each EPD and should be used as an aid in deciding between two animals with similar EPDs. The closer the accuracy value is to 1.0, the more likely the performance of offspring will be close to the EPD and the less likely the EPD will change drastically with the addition of new information. Table 5 contains ranges of possible changes in EPDs depending on accuracy, for four traits evaluated by the STAGES program for Yorkshires.

Table 5. Possible changes (+/-) associated with

accuracies for growth and maternal traits.

Source: NSIF - FS13

In order to manage the risk of using unproven sires with low accuracy, limit the use of low accuracy boars and use proven, high accuracy boars extensively. Another option is to select and use larger groups of low accuracy boars, increasing the accuracy of the group average. Tables 6, 7, and 8 (Sullivan, 1992) illustrate these two methods of using accuracy (repeatability) to manage risk. The goal in both methods is to maintain a herd average for EBV index of at least 110.

Table 6 illustrates how risk decreases with higher repeatability. By using high repeatability boars there is less risk that the EBV is very different from the boar's true genetic value. For example the boar with a 40% repeatability and a 125 index, there is a 26% risk that he is truly under 110. In contrast, a well proven boar (90% repeatability) with the same index has only a 7% risk that his true genetic value is less than 110.

Table 6. Effect of repeatability on risk.

Source: Sullivan, 1992

Risk with low repeatability animals can virtually be eliminated by using several young boars, rather than relying on one or two exceptional herd sires. In Table 7 we see that a single boar with a repeatability of 40% and an EBV index of 125 does have a 26% risk of having a true genetic merit below 110. However by using five young boars with an average index of 125 and a repeatability of 40% the risk is cut to just 7% that the average genetic merit of all five boars is truly below 110.

Table 7. Effect of using multiple sires on risk with low repeatability (40%).

Source: Sullivan, 1992

It is also easier to find higher indexing young boars because of genetic progress. Table 8 shows that if the average of the purchased young boars was an EBV index of 130 with a repeatability of 40% this to can have an important affect on risk. The risk with five boars is only 3% and the risk with 10 boars is less than 1%.

Table 8. Effect of using multiple sires with higher EBVs on risk with low repeatability (40%).

Source: Sullivan, 1992

Both of these strategies reduce risk. However, the use of several young sires allows for a greater reduction in potential risk and may offer more potential for even faster genetic improvement. The approach with risk management is not to use any one boar too extensively. Some young boars selected may turn out to be very poor, but remember, there is an equal chance that some will turn out to be much better than expected. Regardless of accuracy, the EPD is the best estimate of genetic merit currently available.

The EPD is the most accurate estimate of genetic value that is currently available and should be used when possible, to evaluate seedstock suppliers, make selection decisions and to evaluate the economic value of superior animals. To make genetic improvement in a herd, use performance evaluated seedstock selected from at least the upper 50% of boars and 75% of gilts in a herd that is realizing genetic progress through the use of a performance testing and genetic evaluation program. Commercial producers should require seedstock suppliers to follow a sound genetic improvment program and then use genetic information when purchasing breeding animals along with acceptable health, reproductive soundness, and skeletal structure.