Inbreeding in pig herds

Inbreeding occurs in all closed breeding populations, and is increased by:

• small herd size
• long working life of boars
• performance testing and selection programs.

The effect of these factors can be calculated and remedial steps taken to slow down inbreeding, such as by introducing unrelated breeding stock.

Inbreeding is the mating of related animals. The progeny of these matings have an increased chance of inheriting copies of the same genes from their sire and dam. The level of inbreeding (LOI) is a measure of this chance and theoretically ranges from 0% to 100%. The average LOI rises over time in any interbreeding population (e.g. breed, strain) and the more closely related the parents, the faster this rise will be.

Effects

• Animals whose parents are closely related may suffer inbreeding depression.
• The most striking effect is on traits connected with reproduction, early growth and survival.
• The extent of inbreeding depression varies between different populations (e.g. breeds, strains).
• Inbreeding depression depends on the actual LOI and on the time it takes to reach this level. A rapid rise results in greater inbreeding depression than a gradual rise.
• Any estimate of the effect of inbreeding can only be a general guide.
• One study of several inbred lines of pigs found that an increase of 10% in LOI reduced the number of piglets born per litter by 2.5% and reduced the body weight at 160 days by 3kg. These values are considered high because the rise in inbreeding was rapid in this study.

Calculating

All members of the same breed are related if their pedigrees are traced back far enough. However, when calculating inbreeding, we usually pick a starting point not too far back and assume all parents at that point were unrelated (i.e. LOI is 0%). We then work out the increase in LOI from that point to the present. For example, analysing pedigrees of Australian large whites and landrace born in the 1970s showed that, relative to their first herd book records, the LOI was 7.5% for both breeds. In the 1980s, unrelated genes from overseas animals were infused into our local large whites and landrace and this reduced the level of inbreeding. This allowed the establishment of a herd of large whites or landrace with only 5% LOI.

Breed crossing

When 2 unrelated populations are crossed, the LOI in the crossbred (F1) offspring falls to 0%. When the F1s are crossed among themselves to produce an F2 generation, the LOI rises to 50% of the level in the parent populations.

For example, if both landrace and large white had an average LOI of 5%, an F1 gilt from a cross between these breeds would have an LOI of 0%. Crossing this F1 gilt to an F1 boar would produce F2 offspring with LOI at 2.5%.

LOI would rise further in generations following the F2 (i.e. F3, F4, etc.). This can be slowed in the short term by avoiding close matings.

For example, LOI is increased by 25% in the offspring of full siblings (same parents) and by 12.5% in the offspring of half siblings (one common parent). However, levels of inbreeding in the longer term are determined more by herd size and other factors.

Herd size

In a herd closed to the entry of breeding stock from outside, the rate of increase in inbreeding will depend on the herd's size; being higher for small herds than for large herds.

Selection

A herd selection program increases inbreeding, as animals that perform well are likely to be selected as breeder replacements and are also likely to be related.

Two ways of avoiding an unacceptable rise in LOI are to:

• increase the rate at which boars are replaced
• obtain some boars from outside the herd.

Boar replacement rate

Keeping boars for a shorter time reduces the chance of their mating with daughters and this leads to a reduction in LOI.

Introducing breeder replacements

A way of slowing the rise of inbreeding in a herd is to introduce some breeder replacements. If these are obtained from similar but unrelated herds, we can estimate their effect on a herd's LOI.

Acceptable limits of inbreeding

• A certain amount of inbreeding is unavoidable in a herd selecting its own breeder replacements.
• The benefits of a selection program based on performance testing far outweigh any adverse effect of inbreeding, provided the level is held down.
• An upper LOI limit of 10% over 15 years is suggested.
• When the herd reaches this upper limit, LOI is reduced by outcrossing to stock of other herds carrying out similar programs of selection. Performance results will show which herds these are.
• It is equally important to keep the rate of increase in LOI to a minimum.
• At least 10% of all matings in the herd should be performance-tested AI boars (when these are used to provide the outside matings) to maintain satisfactory genetic improvement.