SAC Research

To assess whether aggressiveness is partly modulated by the genotype of the animal, the first objective was to devise and validate a rapid test of aggressiveness for use on a large number of animals under commercial conditions. The second objective was to use this test to estimate the heritability of the aggressiveness trait in grower stage pigs and to examine the phenotypic relationship between aggressiveness and relevant production traits.

Can aggression be measured rapidly?

For selection against aggression to be feasible, we need to be able to measure the trait rapidly and accurately on a commercial scale. A method of measuring aggression at mixing was developed and shown to truly reflect aggressiveness by examining videos of the pigs’ behaviour at two different units.

Does aggressiveness have a genetic basis?

Using the aggression test, the post-mixing aggressiveness of 1100 pigs was measured at 28 kg on a commercial sire-line nucleus farm. Using the pedigree information of these animals, a heritability value for the aggressiveness trait was estimated.

It was established that aggressiveness is a moderately heritable trait (heritability = 0.22). This heritability is slightly less than that for daily weight gain, but greater than that for survival and fertility traits. Aggressive characteristics in pigs can therefore be passed from parents to offspring.

If selection pressure is placed on aggressiveness, the average aggressiveness of the herd should fall by up to 5% per year (see Figure 2) during the early years of selection. Over a longer time period, the response to selection may be less than 5% / annum as the variation in aggressiveness between animals in the breeding population is reduced.

Selecting against aggressiveness is likely to be commercially feasible only if it does not reduce genetic change in other traits too much. There was no obvious relationship between the productivity and aggressiveness of a pig so selecting against aggressiveness should not remove the best performing pigs from the population.

The breeding companies involved in this study are examining how selection against aggression might be achieved efficiently and how to weight the trait with respect to other important traits in their selection programmes. The wider welfare implications of selection against aggression will be considered in a new project at SAC.