In estimating the genetic merit of an animal, breeders are trying to determine the animal’s value as a parent, its breeding value. The phenotype of an animal (the traits we see and measure) is a combination of genetic and environmental effects. Genetic effects are the result of the genes inherited from parents. Environmental effects are the result of conditions the animal experienced, such as level of nutrition, stocking density, temperature, or health status.
The challenge to the breeder is to determine how much of an animal’s superiority (or inferiority) for a trait is due to additive genetic effects, since this is what will be passed on to its progeny through its own genes. Defining what constitutes genetic merit is an important first step in this process. This will be discussed further in NSIF-FS9, “Multiple Trait Selection for Pork Improvement,” but genetic merit can be defined as how an animal ranks, relative to other selection candidates, for its ability to produce superior offspring.
Favorable performance for a characteristic is an obvious way to rank animals; however, it should be done relative to other animals that are of similar age and housed and raised under similar conditions. It is best to compare an animal’s performance record to the average of the group of animals that they were raised with. This can be done by calculating performance deviations from the group average or the ratio of animal’s performance with the group average. For example, a gilt has an average daily gain of 1.9 lb/day for the grow-finish period while the other gilts of similar age and raised in the same building averaged 1.8 lb/day for average daily gain. The gilt in question would have a performance deviation of 0.1 lb/day, which is favorable and a ratio of 105.6 (1.9/1.8). This is the first step in evaluating an animal’s genetic merit for performance characteristics. For further examples see Table 1.
Two terms are often used in discussing genetic merit. The first term, Estimated Breeding Value (EBV) is the estimated genetic merit of animal, expressed as a deviation. The second term, Expected Progeny Difference (EPD) is one-half an animal’s Estimated Breeding Value and can be used to determine the expected performance change of progeny if the animal in question is used as a parent. Methods to estimate genetic merit in swine have evolved considerably this century. As breeders have been able to incorporate increasing amounts of information to assess an animal’s breeding value, methods have grown from simple visual appraisal to complex statistical techniques. The purpose of this fact sheet is to discuss these alternative methods.