An expected progeny difference (EPD) is the relative difference in performance expected in future progeny of an individual, compared with the future progeny of another individual. An EPD describes the average merit of a gamete produced by an individual (sperm or egg) for a particular trait, e.g. birth weight, weaning weight, marbling score. EPDs are used to describe the portion of genetic merit that is transmitted from parent to offspring. Each EPD is reported in the units of measure for that specific trait. EPDs are derived from multiple sources of information including pedigree relationships, an animal's own performance record, and progeny performance records. An animal's progeny performance is the most valuable source of information for computing EPDs. They are normally developed by the breed association based on performance data submitted by the breeders. EPDs are breed-specific; EPDs can only be used to compare animals of the same breed.
An example of how EPDs can be used is to compare bull A with a weaning weight EPD of 30 lb to bull B with a weaning weight EPD of 48 lb (remember, both bulls must be of the same breed). If the cows these bulls were mated to were of equal average genetic merit and were managed alike until weaning, you'd expect bull B's calves to average 18 lb (48 - 30 = 18 lb) heavier at weaning time. EPDs are the most accurate means of comparing the genetic differences between animals. For more help, refer to specific breed association Web sites to locate their sire summary or evaluation. Complete descriptions of the each breed's genetic evaluation is included in the preface materials of their sire summary along with examples of how to use EPDs, trait definitions, summary statistics, percentile tables, and genetic trends.
