Genetic Progress Through Selection

Selection is a directional process whereby only a portion of the population is allowed to reproduce. If the selected individuals are genetically superior to the population average for a trait, then their offspring will be expected to perform above average for that trait. The basis for selection is the genetic similarity between parent and offspring. Each parent transmits a random one-half of its genes to its offspring. The genetic merit of the offspring is dependent on the genetic makeup or breeding value of its parents.

The goal is genetic improvement, but the genetic composition of an animal cannot be directly measured. What is measured, the phenotype, results from the interaction of the animal's genetic make-up (genotype) with its environment. In performance testing, the animals eligible for selection are measured phenotypically for a particular trait. Because animals have different parents, variation in genotypes exists among animals. Environments also vary among animals. Therefore, variation in phenotypes for measured traits will be found. To improve selection accuracy, environmental variation needs to be minimized so that differences among animals are genetic in nature to the greatest extent possible. The proportion of the phenotypic variation due to genetic variation is called heritability. Some traits, such as carcass merit, are highly heritable (50%), but others such as reproduction efficiency are not as heritable (15%). The other (50% , 85%) differences is due to environmental effects.

Breeding Value and Expected Progeny Differences

The actual genetic merit of an animal is its breeding value, which is the sum effect of all its genes. How the breeding value is expressed by each pig's individual phenotype is dependent on the environmental conditions under which it is raised. The concept of breeding value relates to selection through the fact that genes occur in pairs. Selected animals transmit a sample one-half of their genes (one of each pair), or one-half of their breeding value, to each offspring. For this reason, the expected difference between the progeny of an individual and the original population is one-half the breeding value of that individual. Many genetic programs express the genetic merit estimate as expected progeny differences (EPD), which is one-half the animal's estimated breeding value (EBV). Therefore, EPD = 1/2 EBV.

These EPDs can be based on direct measures of animal performance, and/or on measures of performance of relatives to the animal in question (ancestors, sibs, progeny). To calculate EPDs, all available information is combined in a statistical procedure known as best linear unbiased prediction (BLUP). The EPD is a prediction of how progeny of an individual are expected to perform relative to the group or population average (disregarding the other parent). The EPD for progeny resulting from the mating of a specific male to a specific female is the sum of the EPDs of the two parents.

Expected Progeny Differences and Estimated Breeding Values are just that, ESTIMATES. The more sources of information that can be used in the estimates, the more accurate these estimates will be. The reliability to be placed on the EPD is associated with the accuracy of each EPD. Traditionally accuracy is defined as the correlation between the EBV of an individual and its "true" breeding value. An accuracy value close to 1.0 indicates higher reliability for the EPD. The accuracy value reported in most genetic evaluation programs is a function of the possible change or variation for that particular trait. The accuracy values published with EPDs reflect the amount of information available in the genetic evaluation. Breeders should refer to the EPDs to decide whether an animal should be selected for their breeding program and then consider the accuracy value to determine how extensively to use the animal. If an animal has EPDs that meet a producer's selection goals, it should be used regardless of the accuracy value. A producer may wish to limit the use of an animal with low accuracy, whereas a boar with many progeny and hence a higher accuracy may be used more extensively. Accuracy values are most effective as a tool for risk management because regardless of accuracies, EPDs are the best estimates of genetic value available.

A positive EPD is desirable when selecting for traits such as number of pigs born alive or 21 day litter weights because a large positive number translates into more pigs per litter or more pounds per litter. When selecting against backfat or days to market weight, however, a negative EPD is desirable because we wish to reduce the time required to reach market weight and the amount of backfat.

EXAMPLE: Comparison of boar A and boar B for days to 250 lbs. If boars A and B are mated to large numbers of randomly selected sows, what would be the expected difference in their progeny for growth rate? Given that the EPDs for days to 250 lbs are -5.8 for boar A and +.3 for boar B, the genetic potential of these two boars as sires can be compared. Simply subtract one EPD from the other to get the difference between the two, [-5.8 - .3 = -6.1]. The -6.1 says that the progeny of A are expected to reach 250 lbs., on the average, 6.1 days faster than the progeny of B.

Predicting Genetic Progress

The widespread use of performance testing within herds is the first essential step to swine improvement. Records of performance provide a basis for comparing pigs managed alike within a herd. Large environmental differences due to location, management, health, and nutrition are more likely to exist among herds or among different management groups within herds. Genetic differences among herds do exist, but only through carefully controlled evaluations can these differences be assessed. To identify genetically superior individuals within a breed or population, it is necessary to first identify high-ranking individuals within herds. The principal features of effective performance programs are:

    1. Animals of a given sex and age are provided equal opportunity to perform.
    2. Animals are measured for economically important traits.
    3. Records are adjusted for known sources of variation.
    4. Records are used in selecting replacements (boars and gilts).

The annual rate of genetic improvement is dependent on: 1) the proportion of observed differences among animals for a certain trait that is due to genetic causes (heritability); 2) the difference between selected individuals and the average of the herd or group from which they came (selection differential); 3) the genetic association of the trait with other traits upon which selection is based (genetic correlations); and 4) the average age of parents when the offspring selected for the next generation are born (generation interval). Genetic progress per year is illustrated by the following formula:

 Expected genetic progress per year = heritability x selection differential
                                              generation interval

Thus, the greatest rate of genetic change is observed for traits with the highest heritabilities that are the most favorably correlated to other traits under consideration, expressed in animals exhibiting the best phenotype and bred at the earliest possible age to obtain replacements from their offspring.

Selection Indexes

Profitability in a swine enterprise is influenced by many factors and several traits need to receive emphasis in a well-designed breeding program. The problem arises in determining the appropriate emphasis to place on each trait to allow for identification of the genetically superior animals.

Traits are measured in different units (number of pigs, pounds per day, inches, etc.), are not of equal economic importance, and are not genetically influenced to the same degree (different heritabilities). Each of these factors compounds the problem of determining the appropriate emphasis to apply to each trait in a breeding or selection program. The purpose of a selection index is to assign appropriate emphasis to each of the various traits to provide a single value for use in comparing different animals. NSIF recommends the use of indexes where appropriate. Indexes may be based on differences in phenotype, or may consist of EPDs weighted by their economic values.

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