The first national sire evaluation in beef was published in the early 1970s comparing 13 sires for a limited number of traits. As time has gone on, both the number of animals and the number of traits with EPD have increased.
More recently, a more defined focus for EPD has been encouraged.
This new focus has been on Economically Relevant Traits, or ERT, as they are sometimes called. Economically Relevant Traits, as the name implies, are those traits that have a direct economic impact to the producer. Traits such as weaning weight and carcass weight are ERT because there is a direct monetary value associated with these traits.
Other traits, such as birth weight, do not have a direct economic value associated with them. For instance, an increase in 1 lb of weaning weight increases the producer’s income, but a decrease in 1 lb of birth weight does not directly affect the income or expense of a producer. Instead, birth weight is used to indicate the probability of dystocia, or calving difficulty, which does have an economic impact. For this reason, birth weight is not an ERT but is what is called an indicator trait. Newer EPD, such as direct and maternal calving ease, are the ERT for which birth weight is the indicator.
Source: NBCEC.org