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Critical Review on Animal Welfare in Commercial Farming


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.


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Arey, D. and Edwards, S., 1998. Factors influencing aggression between sows after mixing and the consequences for welfare and production. Livestock Production Science, 56(1), pp.61-70.

Conte, S., Lawlor, P., O'Connell, N. and Boyle, L., 2012. Effect of split marketing on the welfare, performance, and carcass traits of finishing pigs1. Journal of Animal Science, 90(1), pp.373-380.

Eur-lex, 2019. Council Directive 2008/120/EC of 18 December 2008 laying down minimum standards for the protection of pigs. Official Journal of the European Union [ONLINE] Available at: [Accessed 17 September 2020].

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McGlone, J., 1985. A Quantitative Ethogram of Aggressive and Submissive Behaviors in Recently Regrouped Pigs1. Journal of Animal Science, 61(3), pp.556-566.

Mellor, D., 2016. Updating Animal Welfare Thinking: Moving beyond the “Five Freedoms” towards “A Life Worth Living”. Animals, 6(3), p.21.

Peden, R., Turner, S., Boyle, L. and Camerlink, I., 2018. The translation of animal welfare research into practice: The case of mixing aggression between pigs. Applied Animal Behaviour Science, 204, pp.1-9.

Rydhmer, L., Hansson, M., Lundström, K., Brunius, C. and Andersson, K., 2013. Welfare of entire male pigs is improved by socialising piglets and keeping intact groups until slaughter. Animal, 7(9), pp.1532-1541.

Spoolder, H., Geudeke, M., Van der Peet-Schwering, C. and Soede, N., 2009. Group housing of sows in early pregnancy: A review of success and risk factors. Livestock Science, 125(1), pp.1-14.

Stukenborg, A., 2011. Investigations on agonistic behaviour in pigs kept under commercial farm conditions. Institut für Tierzucht und Tierhaltung der Christian-Albrechts-Univ. zu Kiel. pp.5-85.

Verdon, M., Morrison, R. and Hemsworth, P., 2018. Forming groups of aggressive sows based on a predictive test of aggression does not affect overall sow aggression or welfare. Behavioural Processes, 150, pp.17-24.

Weng, R., Edwards, S. and English, P., 1998. Behaviour, social interactions and lesion scores of group-housed sows in relation to floor space allowance. Applied Animal Behaviour Science, 59(4), pp.307-316.



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