Before considering those specific factors, there are some management strategies and mathematical techniques that are important tools for improving rate of genetic change in meat goats. They are discussed briefly in Appendix G.

Selection and genetic change for breeders

The effectiveness of selection is typically measured by the rate of genetic change that results. The task of selection is not a simple one. You would like to maximize the rate of genetic change. The challenge is to choose those breeding goats with the best breeding values as parents. Unfortunately we can not know breeding value in advance and so we must work with predictions. Fortunately we do know the factors affecting the rate of genetic change from selection in meat goats. Our focus will be change in the mean or average breeding value of a population caused through selection. There are other factors that cause genetic change in a population, including mating systems such as inbreeding and crossbreeding. The four factors are involved in genetic change through selection include:

• Accuracy of selection
• Selection intensity
• Genetic variation
• Generation interval

Genetic change is directly proportional to the first three, and inversely proportional to generation interval. Knowing about these four factors will help you develop selection strategies and design an appropriate breeding program.

Accuracy of selection

You should be particularly interested in the accuracy of breeding value prediction. The reason for this is because the more accurately you can predict breeding values, the more likely that the animals you choose to be parents will actually be the best parents. Accuracy ranges from zero, when there is no performance information used, to almost one, when there is an abundance of information. Accuracy is never negative.

Accuracy of selection depends on a number of factors:

• Heritability – the higher the heritability of a trait, the better each piece of performance information is as a predictor of underlying breeding value. Any steps a breeder can take to increase heritability will increase accuracy of selection:
  • Managing animals uniformly
  • Taking careful measurements and checking measuring instruments
  • Adjusting for known environmental effects
  • Using contemporary groups
• Genetic prediction technology – accuracy can also be increased by using more information and more sophisticated genetic prediction technology. Performance information on individuals and potentially large numbers of relatives can be combined to provide more accuracy in predicting breeding value. For this to happen, breeders must be dedicated in recording information in the first place.

Differences in accuracy of selection can be large. Selection based only on individual phenotypic record, particularly if the trait under selection is lowly heritable, is not very accurate. Alternatively, selection of potential parents (usually bucks) on the basis of expected progeny difference (EPDs) derived from large volumes of progeny data is very accurate. The key elements here are collecting the data, and accumulating large amounts of it. These are key elements for breed directors who want to involve the newer technologies to genetic prediction — need data and large amounts of it.

Selection intensity

Selection intensity measures how particular meat goat breeders are in deciding which individuals are selected. If selection criteria are reasonably accurate (reasonable predictors of underlying breeding values, then intensely selected parents should be far better than average, genetically. The next generation should be equally superior and the rate of genetic change should be fast. Mathematically, selection intensity is determined by the difference between the mean performance level of those goats selected to be parents and the average performance level of all potential parents within the herd. This difference is sometimes called the selection differential, wherein the potential impact of the selected parents is expressed in the units of the trait or characteristic. Selection intensity will regularly be different on the sire side as compared to the doe side. Because parents contribute equally to the genotype of the offspring, both the sire and the dam will contribute to the selection intensity involved. Keep in mind however, that because a single sire contributes genes to the whole herd, his impact is proportionally larger than individual does. This means that selection of herd sires is very important to genetic change.

Accuracy and intensity are independent concepts. You can select intensely regardless of the accuracy of the performance data. Even it you have good indicators of what is the best animal, if the accuracy is not high there will be a slower rate of genetic change. It should be evident then that when the product of these two factors in rate of genetic change is a small number, the rate of change will be slow. This relationship is direct.

Genetic variation

This is the raw material with which meat goat breeders work. It refers to the variability in breeding values within a population for a trait under selection. With lots of variation, the range between the best animals and the worst is large. The best animals are far superior genetically to the worst. If there is little genetic variation, then even the best individuals will be only a little better than average, so will their offspring, and the rate of genetic change will be slow. Unlike accuracy and selection intensity, genetic variation is not a factor that is easy to manipulate. It tends to be relatively fixed within a population. A lot of genetic variation exists in most populations of meat goats, so the loss of genetic variation is unlikely to be a constraint to genetic change.

Generation interval

The shorter the generation interval, the faster will be the rate of genetic change. Generation interval refers to the amount of time required to replace one generation with the next. Although it will be influenced by environmental factors, including management, it is somewhat species specific and tied to the biology of the reproductive rate. The generation interval in meat goats will be shorter than in cattle. It will not be as short as that of laboratory mice. Both the gestation length and the nursing period are shorter, and the age at puberty is lower. Mice are capable of producing 150 generations in the time it takes humans to produce one. This is why mice are such desirable animals in genetics laboratories where theories of genetic change are tested.

In populations that are closed to genetic material from the outside — the generation interval can be defined as the average age of parents when their selected offspring are born. This “average age” definition works well for herds that are truly closed (providing all their own replacements — male and female). It works well for entire species or breeds. It becomes inappropriate when animals are imported from outside a population. For example, if semen from an outside buck were introduced into a closed herd, the age of that buck would have little bearing on the amount of time required to replace one generation with another. Why? Well, because that buck might be chronologically old, but if he is genetically superior, then in respect to the genetic level of the herd, he is relatively young. The range for generation interval for meat goats is 3 to 5 years.

The mathematical equation which incorporates all these elements into one function is as follows:

This equation is conceptually simple. However it tends to become more complex when consideration is give to the fact that accuracy, selection intensity, and generation interval are often different for bucks versus does. In dairy goats where major production traits are associated with milk secretion, it will take additional time to obtain data from daughters to achieve the desired level of accuracy before final selection.

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