Understanding Expected Progeny Differences (EPD)
By Megan Van Emon, Montana State Extension Beef Cattle Specialist
Expected progeny differences (EPDs) estimate the genetic value of one animal over another of the same breed. EPDs are calculated using statistical equations and models and use known information on that animal that is submitted by the breeder, which can include data from the animal itself, ancestors, relatives (i.e. brothers and sisters), and progeny. Adjustment factors are used to adjust for data such as age and sex. Another factor that is considered when calculating EPDs is the environment from which the animal comes. Most commonly, EPDs are used to compare bulls within the same breed.
Bull 1 has a weaning weight EPD of 30. Bull 2 has a weaning weight EPD of 17. If both bulls were bred to a similar group of cows, we would expect calves from Bull 1 to average 13 pounds (30 – 17 = 13) heavier than Bull 2 at weaning.
Numerous other EPDs are used to compare bulls, such as scrotal size, birth weight, ribeye area, feedlot merit, calving ease, mature daughter’s weight, milk, etc. Numerous other EPDs are available and can vary by breed. EPDs are calculated and reported in the same unit of measurement as the trait (i.e. birth weight in pounds; fat depth in inches).
EPDs do NOT predict the actual birth weight, weaning weight, index value, etc of an animal. They only predict the expected difference between animals. If you want to know the average performance for a specific trait within a breed, many breed associations have sire summaries.
An accuracy value is also included with each EPD trait. The accuracy is used to determine the reliability of the EPD and will be reported as a number between 0 and 1. Accuracy increases towards 1 as more data is reported for a specific animal. It is also important to note that as the accuracy moves closer to 1, the EPD of the trait for that bull will not change significantly in the future. Young bulls that do not have any progeny data rely on data from ancestors for accuracy, which is usually from 0.1 to 0.4.
EPDs can be compared across breeds, if the proper adjustment factors are used. Breed adjustment factors are developed at the Roman L. Hruska U.S. Meat Animal Research Center in Nebraska and are updated each year. The base of the comparison is to Angus EPDs. Comparing EPDs across breeds is less accurate than comparing within the same breed.
It is important to note that EPDs do NOT guarantee a specific difference on each animal. EPDs are constantly changing as more data is added for each bull. As more data is added to the system for a specific bull, extreme values are averaged out. Extreme values can occur, even in a high accuracy (high reliability) bull.
EPDs should be used as a management tool for your cow herd, but should NOT be the only tool. Physical appearance, or phenotype, should also be a tool used when selecting bulls.