Aggression in Commercial Farm Pigs as an Indicator of Poor Welfare
Animal welfare is an important consideration not only in terms of legislation and it is crucial to identify signs of poor welfare (Fraser, 2008). There are several indicators of poor welfare ranging from physiological to behavioural changes (Broom, 1986). One of them is an increased aggression that represents a major animal welfare issue in commercial pig farming, which currently receives an increased attention (Appel et al., 2013; Peden et al., 2018).
Although aggression in general could be considered an expression of natural behaviours, aggression caused by intensive farming practices is in dispute with the five freedoms of animal welfare (Mellor, 2016). From a natural perspective, aggression can arise when a new hierarchy is being formed and thus could be reduced by less frequent regrouping of pigs in farms and maintaining the already established hierarchies (Peden et al., 2018; Verdon et al., 2018). For instance, Rydhmer et al. (2013) found that pigs showed less aggression while being kept in intact groups compared to controls and that maintaining intact groups during growth and slaughter resulted in an improved welfare. However, besides regrouping being a usual farming strategy (Appel et al., 2013), it cannot be avoided completely due to the EU legislation (Eur-lex, 2019). On the contrary, Conte et al. (2012) found that split marketing strategy, which removes the heaviest pigs from the group first, resulted in a decrease in aggression during feeding and improved welfare of the animals compared all-out strategy. To decrease aggression levels in pigs, Verdon et al. (2018) considered the distribution of aggressive animals among groups. However, they found no significant effect of group composition on pig welfare. The latter indicates that aggression could be a more complex behaviour, which, besides establishing hierarchies, can be affected by other environmental factors regardless of regrouping (Appel et al., 2013). For instance, it could be argued that animals deprived of space would show more aggressive behaviours compared to those with increased space allowance (Arey and Edwards, 1998; Stukenborg, 2011; Weng et al., 1998). However, Appel et al. (2013) found aggression to be stronger by a newly mixed group on a larger space compared to those with only 66.7% of that allowance. Both spaces in the study exceeded the minimum requirement by law. This could be explained by absence of sufficient bedding, another factor influencing overall pigs’ welfare (Spoolder et al., 2009). Moreover, aggression in pigs represents an increased risk of injury and infection, yet another welfare issue (McGlone, 1985) that Rydhmer et al. (2013) demonstrated in their study, where frequently regrouped pigs suffered more of skin lesion caused by repeated fights compared to intact groups.
In conclusion, aggression in farm pigs was found to indicate poor welfare in relation to stress from an excessive regrouping and thus reduced chance of establishing a naturally functioning hierarchy among a group. Nevertheless, regardless of regrouping strategies, aggression also indicates poor welfare arising from other factors. Therefore, to examine the overall welfare, aggression should be assessed in combination with other welfare indicators.
References:
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