Heterosis in pigs

Members of the same population of pigs (e.g. breed or strain) are usually related and, therefore, somewhat inbred. When 2 populations are crossed, the level of inbreeding in the offspring falls to zero and those traits that suffered from inbreeding depression in the parent populations improve. This improvement is called heterosis.

Heterosis is the recovery of performance depressed by inbreeding in the parent populations. The degree of heterosis for the same trait varies among strains, breeds and environments.

As with inbreeding depression, heterosis is most often seen in traits of low heritability, particularly those connected with reproduction, early growth and survival. It occurs least often in traits of high heritability, such as carcass characteristics. Heterosis is usually greater if the genetic difference between the crossed populations is wide. Crossing breeds should give more hybrid vigour (i.e. improved biological traits in offspring) than crossing strains within the same breed.

Types

Maternal heterosis

Maternal heterosis benefits the individual pig through the hybrid state of its dam. It has the greatest effect when the individual pig is dependent on its dam i.e. from conception to weaning. Because of the economic importance of the number of pigs weaned per sow, maternal heterosis is the most important of the 3 types.

Offspring heterosis

Offspring heterosis benefits the individual pig itself due to its own hybrid state. It affects the pig's growth and survival throughout its life, but mostly after weaning when it is independent of its dam.

Paternal heterosis

Paternal heterosis results from the hybrid make-up of the sire. It shows itself by improving mating success, including through increased libido and conception rate. Evidence for this type of heterosis is limited, but it appears that boars can be somewhat inbred before their ability to produce offspring is harmed.

Calculating

The degree of heterosis is the difference between the performance level of the offspring and the average performance of its parents. It is usually expressed as a percentage of the parents’ performance (average effect).

For example, 1 study found litter sizes of 8.5 and 8.3 pigs for large white and landrace. Sows of the cross between these breeds produced 9.1 pigs per litter, which was equivalent to 0.7 pigs per litter (or 8%) higher than the purebred average of 8.4 pigs per litter. The degree of heterosis was therefore 8% (0.7 divided by 8.4 and multiplied by 100).

This shows that the performance of a crossbred is made up of the average effect of its parents (8.3 and 8.5) and the degree of heterosis appropriate to that cross (8%). It follows that a high value for heterosis may not overcome a poor to average effect of a parental breed.

Values

This table:

• summarises estimates of the degree of heterosis for important pig traits
• lists values from crosses of all breeds and from those involving only large white and landrace breeds
• separates maternal and offspring heterosis.

Key points:

• maternal heterosis benefits only the number of pigs born and weaned
• offspring heterosis benefits growth traits
• paternal heterosis values are not given due to insufficient information
• as the heritability of the traits rises, heterosis values fall and maternal heterosis has no effect on post-weaning growth, efficiency or fatness.

Predicting crossbred performance

This example shows how to predict the number of pigs weaned per litter for the three-way cross of a duroc sire over a large white and landrace-cross dam.

If average breed effects for large white and landrace are 8.5 and 8.3 respectively, and maternal heterosis is 5%, then the expected number weaned from a large white and landrace-cross sow mated to any breed of sire is 105(%) x 8.4 = 8.8. When the breed of the terminal sire (i.e. duroc) is different from that of the dam's parents (i.e. large white and landrace), all offspring are crossbred. This slightly raises their chance of surviving to weaning.

Key points for breeders

It’s difficult to accurately predict how much heterosis to expect from a given cross (such as backcross, criss-cross and three-breed combinations). This is because the wide variation in values from the different studies that made up the averages (even between crosses of the same breeds) is not shown. This results from variation in strains of a breed crossed and in the testing environments used.

Any values given for degrees of heterosis are a guide only, and the choice of breeding system depends on management, pig health, required level of recording and cost considerations (such as the maintenance of own purebreds or the purchase of first cross-breeders).

Most Australian producers use combinations of large white and landrace, and some use a terminal sire such as the duroc.

The average effects of the purebreds that made up the cross are easier to predict than the degree of heterosis.

You can request information on average breed effects for growth and carcass traits can be calculated by those who coordinate central genetic improvement programs. Australian programs provide a service that calculates estimated breeding values for individual pigs from within or across herds.