Increasing numbers of pigs are being housed in groups as producers transition away from use of gestation stalls. Remixing of pigs is a common management technique that can cause aggression between pigs as social groups are disrupted. This aggression is often intense and can lead to stress and injury, both of which are concerns for animal welfare and production efficiency. To reduce the amount and severity of aggression, changes to the environment as well as the genetics of the pig need to occur. This study used behavioral and genotypic tools to better understand the underlying genetic component to aggression in order to identify individuals best suited for group living environments. Previous research validated the use of skin lesions as an indicator trait of aggression. We estimated genetic parameters of skin lesions immediately following remixing with unfamiliar individuals, as well as 3 weeks following remixing, at 3 age groups in which mixing in a commercial setting may occur (at weaning, at move into grow-finish pens, or at sexual maturity). We estimated lesion score heritabilities at multiple ages and examined how these differed across multiple regions of the body. Genetic and phenotypic correlations between the various body regions within and across different ages were computed to compare underlying relationships. Genetic correlations between lesions and growth traits were also obtained. This study found moderate heritability estimates for lesion scores, suggesting that there is a significant portion of lesion score variation that can be attributed to genetic components. Genetic and phenotypic correlations were highest between periods closest together in time, and between body regions next to one another. Additionally, there were no undesirable genetic correlations between growth traits and lesions suggesting that skin lesions can be genetically selected upon to reduce aggression without having adverse effects on growth rates or body composition traits. An additional step in furthering our understanding of the genetic mechanisms of aggression is identifying regions of the genome that are associated with aggressive traits. This study performed genome-wide association analyses on skin lesions and identified regions on chromosome 11, which were associated with accumulation of anterior and central skin lesions immediately following mixing into grow-finish pens. Furthermore, this study examined video observations to determine total time pigs were engaged in delivering and receiving damaging and non-damaging aggression. Heritabilities of many agonistic behaviors were estimated, and values were small to moderate and overall of lower magnitude than previously estimated for skin lesions. Genetic and phenotypic correlations between groups of behaviors and skin lesions were obtained at both 24 hours post-mixing and 3 weeks post-mixing. At mixing, reciprocal aggressive interactions were genetically and phenotypically positively correlated with lesions to the anterior and central regions of the body. Delivery of one-sided aggression was also positively correlated with anterior lesions. Genetic and phenotypic correlations 3 weeks post-mixing were difficult to interpret, potentially due to small population size and low numbers of lesions observed at this time period. In conclusion, this study provides further information about the underlying genetic components of aggression. This knowledge will help guide genetic selection to reduce levels of aggression by helping determine the most optimal traits to select for the greatest potential impact on genetic change while reducing any potentially negatively associated correlated traits.
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