Since January 2003, the provision of appropriate environmental enrichment to pigs of all ages has been a legal requirement in the EU. The Directive states that: ‘To enable proper investigation and manipulation activities, all pigs must have permanent access to a sufficient quantity of material such as straw, hay, wood, sawdust, mushroom compost, peat or a mixture of such which does not adversely affect the health of the animals’. European animal welfare legislation is built on the principle that all animals have intrinsic value. Animals should therefore be able to express species-specific behaviour. The pig has a propensity to express nosing, rooting and chewing behaviours as a result of both exploratory and feeding motivation (Fraser et al., 1991; Day et al., 1995, 1996b. When individuals are not able to express these behaviours in barren housing environments, the activity may be channeled towards inappropriate stimuli such as pen-mates (Lawrence et al., 1993). Straw has been reported to improve the welfare of pigs housed in barren environments, because it can be used as a recreational substrate, as a nutritional substrate and as a bedding material (Fraser et al., 1991; Arey and Bruce, 1993; Lyons et al., 1995; Van de Weerd et al., 2005b; Tuyttens, 2005). However, it is still not really clear how straw functions to modify behaviour and what it exactly is about straw that makes it so effective in occupying a pig (Van de Weerd, 2005a). Therefore it is important to gain knowledge about the properties of straw that are behaviourally rewarding for pigs (Day et al., 2001b). The acquisition of such knowledge can then be applied to facilitate the construction of environmental enrichment devices which meet the behavioural needs of pigs, whilst also being compatible with partly and fully slatted housing systems (Van de Weerd et al., 2003). The present study explored the efficacy of providing pigs with small quantities of chopped straw, as opposed to full-length straw, since the former material could be used in partly and fully slatted systems with less risk of blockage of the liquid slurry handling facilities. To investigate this, groups of growing pigs were exposed to treatments where the provision and length of straw varied. Twenty-four groups of growing pigs were exposed to one of four treatments (no straw bedding, full-length straw, half chopped straw, and fully chopped straw). Both pen-mate- and straw directed behaviours were recorded using ad libitum sampling. It was found that the length of straw affected both the quantity and quality of straw-directed behaviours. The provision of straw of any length reduced the occurrence of behaviours such as nosing other pigs, aggression and tail-biting compared with when straw was absent. Chopped straw increased the prevalence of behaviours such as licking and decreased the prevalence of behaviours such as picking, suggesting that pigs were not able to manipulate the chopped straw in the same way as full-length or half chopped straw. In addition, levels of tail-biting were higher in groups that were provided with chopped straw than in groups with full-length or half chopped straw. It is concluded that the use of chopped straw in growing/finishing housing systems, whilst better than no enrichment, is inadvisable because of the possibility that levels of adverse pen mate directed behaviours will increase.

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