Castration performed without anaesthesia in piglets is a painful operation and it represents a potential animal welfare issue (Prunier et al., 2006). In an attempt to improve animal welfare, the Norwegian Parliament has passed a law banning the castration of piglets from 2009. Cessation of castration will relieve the animals from pain during and after surgery, but the rearing of entire males poses other welfare issues as entire male pigs are more aggressive and sexually active than castrates (Ellis et al., 1983; Giersing, 1998; Cronin et al., 2003). This could result in another animal welfare problem where high rank male pigs badger their pen mates of lower rank with biting, chasing and head knocks. Sexual behaviour (mounting of both gilts and boars) has also been reported as problematic, and can occasionally result in a high frequency of serious leg injuries to pen-mates (Rydhmer et al., 2004). It is important to recognise these problems and to find alternative rearing methods, which might ensure better animal welfare. Mixing of unacquainted pigs is usually followed by fighting (especially during the initial hours) until a new rank order is established in the group (Petherick and Blackshaw, 1987; Moore et al., 1994). The fighting behaviour tends to be more prolonged and serious when the weight differences between pigs are small (Rushen, 1987; Moore et al., 1994; Andersen et al., 2000; Schmolke et al., 2003). Mixing pigs is a very common practice in Norwegian pig production (and in many other countries also) both after weaning and when transferred to the fattening unit. The intention of the mixing is to reduce and balance the weight differences between pigs within the pens and to make the most out of available space. According to previous reports (Fredriksen et al., 2004), this practice can exacerbate sexual maturation of entire male pigs compared to when rearing them in sibling groups, in farrow-to-finish pens. Moreover, aggressive and sexual behaviour increases during puberty while mixing and moving animals. Furthermore reestablishment of the hierarchy among the pigs in the pen provokes aggressive and sexual behaviour. The hypothesis for the present study was that by keeping littermates together in stable groups, this process might be avoided and the onset of puberty delayed, thus resulting in a lower frequency of unwanted behaviour, lower skin lesion scores and consequently improved animal welfare. The frequencies of aggressive and sexual behaviour among finishing pigs were recorded in three different groups in one herd; farrow to finish-group (entires + gilts, siblings), mix-group (entires + gilts, mixed) and castrate-group (castrates + gilts, siblings). Frequencies of skin wounds were recorded in this herd and in an additional herd (only farrow to finish-group and mix-group) shortly before the animals were sent to slaughter. Higher skin lesion scores in the entire male pig groups were in accordance with the behaviour data. Rearing entire male pigs in sibling groups reduces aggressive behaviour, though the frequency of the aggressive behaviour bouts was still higher than it was in the castrate-group. The frequency of skin wounds in the farrow to finish-group was reduced to similar levels as in the castrate-group. The findings suggest that rearing entire males in sibling groups may be an appropriate management strategy for improving animal welfare in entire male pig production.

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