EPDS AND INDICES TO MAKE DECISIONS FOR BULL PURCHASES by Chad Russell, doctorate student and Matt Spangler, Ph.D., University of Nebraska–Lincoln
Until the development of expected progeny differences (EPD), most purchasing decisions of bulls were based on phenotypic measurements and features as an estimate to how that bull’s calves would perform. Phenotypes (P) are the combination of genetics (G) and environment (E); (P = G + E). There are components of genetics that aren’t heritable, such as the unique combination of genes that results in hybrid vigor, or inbreeding suppression. These gene combinations will
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influence an animal’s own phenotype, but won’t be passed down to their offspring. EPD estimates only the heritable genetic component of that animal. So why would you want to make a purchase decision just on the genetic merit? For an example, let’s look at the relationship between an animal’s own birth weight and the portion of that phenotype related to their genetic merit for birth weight. If you were to take two bulls whose EPD for birth weight were -1.0 and -4.0 lbs. and you breed your herd to both of these bulls, in a hypothetical situation where the calves have the same environment, the equation P = G. As a result, the second bull’s calves would on average be 3 lbs. lighter at birth, all other factors equal, which may reduce the number of problematic births caused by dystocia. But that single birth weight you see in the catalog for that bull is influenced by his environment and his unique gene combinations, which aren’t heritable. Was he treated differently than other bulls, or weighed later or as soon as he hit the ground? EPD predictions use his own birth weight record as well as records from his half-sibs, parents, and any relevant
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