Recently, I got a producer request for assistance selecting a bull for his herd. He sent me the breeder’s EPD’s related to the bulls they had for sale. After studying the values, I asked the rancher the first basic question during bull selection: What characteristics are you trying to change in your herd? After a short conversation, I determined his needs and was in a better situation to offer sound advice. During our conversation, he mentioned that it was difficult for him to understand numerical EPD’s. Based in our conversation, I determined that it would be worth to share with you what EPD are and what those values meant.
When evaluating prospective breeding animals, it is helpful to have an estimate of their genetic transmitting potential. For most production traits, this estimate is best calculated using records of performance. Thru out the years the most improvement in genetic evaluation came with Expected Progeny Difference (EPD). The term expected can be misleading as it implies a high degree of certainty, which may or may not be true. Predicted or estimated would probably be better terms than expected. The basis of EPD is ratios within a contemporary group, but EPD has more scope and precision. With EPD, more valid comparisons can be made of animals across contemporary groups, not just within a single group. The first practical implementation of EPD came through National Sire Evaluation (NSE), conducted by some breed registry associations. The widespread use of popular bulls through artificial insemination, particularly in breeds first available in the United States in the late 1960s, allowed them to serve as so-called Reference Sires, the benchmark in NSE. The first National Sire Summary, comparing EPDs of 13 bulls, was published by one of those breeds in 1971. The only bulls that could be included in NSE were those with adequate numbers of progeny managed in contemporary groups where at least one Reference Sire was represented. Some often incorrect assumptions reduced the validity of the estimates. One of these assumptions was that bulls are not genetically related. Another was that bulls are mated to females of equal genetic merit. It was assumed that no progeny are culled before all records are collected and that breed averages for traits do not change over time. All of the breed associations that have EPD report four traits:
• Birth Weight — in pounds at birth, excluding maternal influence. Birth weight is the most important factor in Direct Calving Ease (see below).
• Weaning Weight — in pounds at 205 days of age, excluding maternal influence (evaluated as Milk below)
• Yearling Weight — in pounds at 365 days of age, excluding maternal influence
• Milk — expressed as pounds of weaning weight (not pounds of milk) due to maternal influence of an individual’s daughters, excluding genetics for growth to weaning (evaluated as Weaning Weight above). The use of “milk” is inexact because this is an estimate of all maternal influences on weaning weight, milk production being the major element. Total Maternal EPD, combining Milk and Weaning Weight, also is reported by some breeds. Total Maternal should be ignored and the two components considered separately unless a producer merely wishes to increase weaning weight without regard for what causes the increase. EPD values are calculated as average relative deviations, not actual levels, of the unit of measurement of the trait. Assume that one bull has a Birth Weight EPD of +4.2 and another bull of the same breed has -1.4. This means that, if used on genetically equal females managed under equal conditions, the first bull is predicted to sire calves averaging 5.6 pounds heavier at birth (the difference between +4.2 and -1.4). As another example, if one bull has a Weaning Weight EPD of +42 and another has +27, the predicted average difference between the two bulls is 15 pounds in weight of their calves at weaning. There are more EPD calculated and provided by different breeds. What is important to remember is that EPD does not predict performance level. If a bull has +4 Birth Weight, this does not predict that he would increase birth weights by 4 pounds, nor would a bull with -1 Birth Weight decrease birth weights by 1 pound. The two bulls are predicted to sire calves averaging 5 pounds difference. The actual average birth weights, depending on other factors, might be 75 pounds and 70 pounds or 95 pounds and 90 pounds or any other average difference of 5 pounds. EPD predicts comparative differences, not level of performance. If the EPDs of both parents are known, they can be combined to predict the relative performance of the progeny. For example, compare a sire of Weaning Weight +55 mated to a dam of +35 with a sire of +40 mated to a dam of +30. Their progeny would be predicted to differ in weaning weight by 20 pounds (55 + 35 minus 40 + 30). Breed associations calculate their own EPDs that are comparable only within the breed. However, EPDs of individuals of the same breed can be legitimately compared even if they are to be mated to another breed, or cross of breeds, as long as the proposed mates are the same. For example, the EPDs of two Charolais bulls can be compared for use in a herd of Brahman-cross females. There are some adjustment factors for comparing EPDs from different breeds, but they may be less reliable than within-breed EPDs. In most cases, producers should first determine which breed(s) to use and then decide which individuals to select from within the breed(s). All breed associations establish a base period when the breed-average EPD value for a trait is zero, and those bases differ for each breed. Selection changes genetic level over time. As time passes since the base was established, the breed average could differ increasingly from zero. Breed averages can vary considerably. For example, recent average Yearling Weight EPD in one breed is +11 and is +76 in another breed. These breed averages cannot be compared, so the values do not mean that the second breed averages 65 pounds heavier. Current breed averages can be used to see where an individual ranks within a breed. Maintaining a fixed base provides a benchmark that can be used to help determine the level of EPD in a breed that might be appropriate for particular production conditions. This benchmark would not be available if the breed average was reset to zero every time EPDs are recalculated. Once or twice a year, associations update individual animal EPDs, breed averages, distribution of EPDs within the breed, and genetic trends. The most recent reports should be used and EPDs from different reports cannot be compared. Suppose two individuals have Weaning Weight EPDs of +32 (0.62) and +46 (0.41). The values in parentheses are for Accuracy, which ranges between 0 and 1. (Accuracy usually is not calculated for Pedigree EPD, based only on parental EPDs, or for Interim EPD, based on pedigree EPDs plus the individual’s record.) Accuracy is influenced by the number of records, genetic relationship among individuals providing the records, heritability of the trait, and number of contemporary comparison groups. Accuracy is not related to variation in progeny. Progeny of low-Accuracy parents will vary no more, on average, than progeny of high-Accuracy parents. Also, difference in parental EPD is not related to progeny EPD variation. For example, consider a sire and dam both with Yearling Weight EPD of +60 compared to a sire with +80 and a dam with +40. On average, there is no difference in progeny variation from these two matings and both sets of progeny are predicted to average +60 EPD. So what is more important, the magnitude of EPD or Accuracy? EPD is an estimate of true breeding value in relation to other individuals in a breed. Accuracy is a measure of confidence that the EPD is the true breeding value. If a producer wants large and rapid change in a trait then EPD should be stressed, even if Accuracy is low. But if predictability is more important, higher Accuracy individuals should be selected. Regardless of Accuracy, EPD is the best estimate available of true breeding value.
Mario Villarino DVM, Ph.D.
Hopkins County Extension Agent for Ag and NR
1200B Houston Street
Sulphur Springs, Texas 75482