Pig traits and genetics

The theory of quantitative genetics enables you to improve traits under complex inheritance, such as growth rate.

This theory rests on 4 main concepts:

• heritability
• selection differential
• generation length
• inbreeding.

Heritability

Any trait expressed by an individual pig is the sum of the effects of its environment and the effects of the genes it inherited. The percentage of the total variation (i.e. genetic + environment) that is genetic is a measure of the heritability.

• Heritability % = 100 (genetic variation) ÷ (genetic + environmental variation)

Heritability is also the percentage of the parents' superiority that is passed to their offspring (see Figure 1. It is more effective to improve traits with a high heritability than those with a low heritability.

Heritabiity values

Many favourable traits in pigs, particularly growth rate and carcass leanness, have medium to high heritability. The performance of an animal is also a very good guide to the genes it carries for these characteristics.

Computer-based selection

For traits that have low heritability (mainly those relating to reproduction), an animal's performance tells us very little about the genes it carries and passes to its offspring.

As an individual has some genes in common with its close relatives, we could examine the performance of these relatives to learn more about traits that have low heritability. This can be done more easily with a genetic evaluation system like the PIGBLUP.

Reproductive characteristics are very difficult to improve using selective breeding because of low heritability. However, specialist breeders have achieved genetic gains for litter size using best linear unbiased prediction (BLUP) technology, such as PIGBLUP.

Manual selection

Focusing selection on a greater number of objectives, such as litter size, will reduce the gains made in other traits.

A suitable strategy to maintain a reasonable level of performance for most commercial herds is through:

• good management
• a low rate of inbreeding
• the culling of breeding animals with poor reproductive histories
• the occasional introduction of a boar (via artificial insemination) from a herd with a high estimated breeding value for litter size.

In contrast, the higher heritability traits are relatively easy to improve by genetic means. There are now many examples of substantial genetic improvement in these types of traits, particularly growth and leanness. This improvement has occurred mainly through the selection of herd replacements based on growth and fat measurements made on the animals before breeding age.

Selection differential

The selection differential is a measure of the performance superiority of selected stock over the average of the group from which they were selected.

Generation length

The generation length is the time it takes for 1 generation to be replaced by the next.

Inbreeding

Animals whose parents are closely related may suffer inbreeding depression. This has the most striking effect on traits connected with reproduction, early growth and survival. A certain level of inbreeding is unavoidable in a herd selecting its own breeder replacements. However, the benefits of a selection program based on performance testing far outweigh any adverse effects of inbreeding if the level is kept low.

Predicting genetic improvement

The annual improvement in any trait under selection in a breeding program can be estimated using the following formula:

• Annual improvement = [(selection differential × heritability) ÷ generation length] − inbreeding depression.

The conditions for fastest improvement are obvious.

High rate of improvement:

• selection differential
• heritability.

Low rate of improvement:

• generation length
• inbreeding.