Members of the same population (e.g. breed or strain) are usually related and, therefore, somewhat inbred. When two 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.
As with inbreeding depression, heterosis is most often seen in low heritability traits, particularly those connected with reproduction, early growth and survival. It occurs least often in high heritability traits, such as carcase characters. Heterosis is usually greater if the genetic difference between the crossed populations is wide. Therefore, crossing breeds should give more hybrid vigour than crossing strains within the same breed.
On this page:
- Types of heterosis
- Calculating heterosis
- Values of heterosis
- Predicting crossbreed performance
Pigs have three types of heterosis: maternal, offspring and paternal.
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, by far, the most important of the three.
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 results from the hybrid make-up of the sire. It shows itself by improving mating success, such as through 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.
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 its parents´ performance (average effect). For example, one 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 0.7 or 8 per cent higher than the purebred average of 8.4. The degree of heterosis was therefore 8 per cent (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 per cent). It follows that a high value for heterosis may not overcome the poor-average effect of a parental breed.
Values of heterosis
Few Australian studies exist that compare purebreds with their crosses. Table 1 summarises estimates of the degree of heterosis in important pig traits. The data is from a large number of overseas studies using many breeds. The table lists values from crosses of all breeds and from those involving only large white and landrace breeds. It separates maternal and offspring heterosis. Values for paternal heterosis are not given due to insufficient information.
The values in the table show that maternal heterosis benefits only the number of pigs born and weaned, while offspring heterosis also benefits growth traits. As the heritability of the traits rises, heterosis values fall and maternal heterosis has no effect on post-weaning growth, efficiency or fatness.
|Trait||Heritability (%)||All breed crosses||Large white and landrace-cross|
|Post-weaning daily gain||35||6||0||5||0|
|Food conversion ratio||45||3||0||2||0|
The table does not show the very wide variation in values from the different studies that made up the averages, even between crosses of the same breeds. This results from variation in strains of a breed crossed and in the testing environments used. This means that it is very difficult to accurately predict how much heterosis to expect from a given cross (such as backcross, criss-cross and three-breed combinations). Therefore, any values given for degrees of heterosis are a guide only, and the choice of breeding system depends on management, pig health and cost considerations, such as the maintenance of own purebreds or the purchase of first cross-breeders, and required level of recording. 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, as information on average breed effects for growth and carcase 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.)
Predicting crossbred performance
The following 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 per cent, 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.
- Heterosis occurs when unrelated strains or breeds are crossed.
- 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.
- A hybrid dam improves her offsprings´ chance of conception and survival through maternal heterosis. This individual pig's own hybrid status improves its growth through offspring heterosis.
- Heterosis is highest for low heritability traits of reproduction and survival but lowest for high heritability traits of post-weaning growth and carcass fatness.
- Information about average performance and the degree of heterosis of the breeds crossed can be used to predict the performance of a crossbred.
- Using genetics to improve your herd
- Replacing breeding stock
- Performance testing
- Australian pig genetic improvement programs (Animal Genetics and Breeding Unit website)