By Steve Boyles
Expected Progeny Differences (EPD) are the most reliable tools to generate directional change in traits. However, like all tools, they must be used correctly and require some degree of background knowledge to ensure proper use.
Breed Averages
Every breed provides breed averages for every trait with a published EPD. Breed average, as the name implies, is the average EPD for a given trait within a specific population (e.g., breed). Breed averages are rarely zero, but instead reflect a point in time or a set of historic animals.
Percentile Ranks
Breed averages can serve as a barometer relative to how an animal compares to other animals in a breed. Percentile ranks serve as a more refined gauge of how an animal compares to other animals in the same breed. Like breed averages (50th percentile), percentile ranks are available for every trait with an EPD. Percentile ranks indicate what proportion of animals have an EPD that is better or more desirable than a given value. As an example, an animal with an EPD in the 10th percentile means that 90% of the population has an EPD for that trait that is considered less desirable than the EPD of this animal. Note that depending on specific goals of a breeding program, extreme values may not be desirable and animals that have higher percentile ranks (e.g., 50th-99th percentile) may be desirable.
Possible Change
Possible change values allow producers to construct confidence intervals or ranges around an animal’s EPD. Possible change is inversely related to accuracy; as accuracy goes up, possible change goes down. As compared to accuracy, possible change represents a more tangible tool to determine the risk associated with the possibility of an EPD deviating from the animal’s true genetic merit as a parent. Most breed associations publish a possible change table. Possible change values are unique to each breed and each trait.
Economically Relevant Traits and Indicator Traits
The key questions that every farmer/rancher needs to answer are:
- What are my breeding/marketing goals?
- What traits directly impact the profitability of my enterprise?
- Are there environmental constraints that dictate the minimum, maximum or optimal level of performance that is acceptable for a given trait in my enterprise?
Once these three questions are answered, sire selection becomes much simpler. The answers to these questions inherently lead a producer to the traits that are economically relevant to their enterprise. We call these traits economically relevant traits (ERT). Fundamentally these are traits that are directly associated with a revenue stream or a cost. All traits that are not ERTs are indicator traits, or a trait that is genetically correlated to an ERT but not an ERT itself.
Classic examples of indicator traits include ultrasonic carcass measurements and birth weight. Ultrasonic carcass measurements are a non-destructive measure of traits such as intramuscular fat percentage (IMF). Producers do not receive premiums for IMF levels, rather premiums (and discounts) are applied to quality grades.
Assuming that carcass maturity values are the same, actual carcass marbling is the driver of quality grade. Although IMF is genetically correlated to carcass marbling it is not the ERT. Birth weight is another great example of an indicator trait. Selection to decrease birth weight in an attempt to reduce the prevalence of dystocia is practiced by numerous commercial bull buyers. However, birth weight does not have a direct revenue source or cost associated with it. Calving ease is the trait that has a cost associated with it. Calving ease is related to the level of assistance needed during a calving event. Although the two are related, the genetic correlation between calving ease and birth weight is between -0.6 and -0.8, suggesting that birth weight only explains 36-64% of the genetic differences between animals for calving difficulty. Other traits that producers can evaluate Growth, Reproduction, Carcass and Management traits.
Source : osu.edu