by Dr. Lauren Hyde, IGS Geneticist and Dr. Wade Shafer
Q 1. How do you balance EPD selection and actual phenotypes?
Expected progeny differences (EPD) have a clear advantage over less sophisticated predictors such as raw weights and contemporary group ratios. Performance data are subject to environmental influences, such as available feed and management, and cannot be compared from one herd, or even one pasture, to another. Contemporary group ratios are more informative than actual or adjusted measurements, but they don’t take into account differences in performance from selective mating, nor do they include comparative performance data on related individuals in other herds. EPD are calculated using complex statistical models and methodology. The statistical models use all known information on a particular animal to calculate its EPD. This information includes its own performance data, such as birth, weaning, and yearling weights; records on its progeny and other relatives; and genomic data on all of these animals. Performance records are adjusted for such factors as age and sex of the animal and age of the dam prior to being evaluated. These adjustments allow performance records to be fairly compared in the analysis. In addition, genetic merit of mates is accounted for so that animals’ progeny records are not influenced by superior or inferior mates. The statistical analyses used for EPD calculation also account for the effects of environment (e.g., nutrition, climate, geographical location) that exist between herds. These environmental effects can be estimated due to the widespread use of artificial insemination (AI). Through AI, the same bull can be used in several herds throughout the world. These common sires create genetic links among herds with differing environments and serve as the foundation for large-scale genetic evaluation. For these reasons, animals with published EPD within an evaluation may be directly compared regardless of their age and origin. Finally, the genetic relationships that exist between various traits are also considered in the EPD calculations. Growth traits provide a classic example of this. Some genes affect growth only during certain stages on an animal’s life, while others affect growth in general. These genes affect two or more traits, and as a result, the traits are genetically correlated.
Read more on the ASA link here