Review Formalised review of environmental enrichment for pigs in relation to political decision making Marc B.M. Bracke a, * , Johan J. Zonderland b , Petra Lenskens b , Willem G.P. Schouten c , Herman Vermeer b , Hans A.M. Spoolder b , Hay J.M. Hendriks d , Hans Hopster a a Animal Sciences Group of Wageningen UR, Division Animal Resources Development, Research Group Animal Welfare, Wageningen University and Research Centre, P.O. Box 65, 8200 AB Lelystad, The Netherlands b Applied Research of the Animal Sciences Group of Wageningen UR, P.O. Box 2176, 8203 AD Lelystad, The Netherlands c Agrotechnology and Food Innovations, P.O. Box 17, 6700 AAWageningen, The Netherlands d The Ministry of Agriculture, Nature and Food Quality, National Reference Centre, P.O. Box 482, 6710 BL Ede, The Netherlands Accepted 29 August 2005 Available online 6 December 2005 Abstract The EC Directive 2001/93/EC states that: ‘‘Pigs must have permanent access to a sufficient quantity of material to enable proper investigation and manipulation activities, such as straw, hay, wood, sawdust, mushroom compost, peat or a mixture of such, which does not compromise the health of the animals’’. In order to help determine what is sufficient material for weaned and growing pigs, a literature review was conducted in a transparent and formalised way, systematically collecting relevant information in a database and translating this information into welfare-relevance. In total, 54 experiments reported in 47 references were selected for analysis. These references contained 200 statistically significant and welfare-relevant findings. A cross-table was constructed showing how classes of enrichment materials significantly affect classes of measured parameters. The classes of enrichment materials were metal objects, rubber, rope, wood, mineral blocks, roughage, substrates, straw and compound materials. The classes of welfare parameters were object-directed behaviour, pen-directed behaviour, tail and ear biting, aggression, (other) harmful social behaviour, activity www.elsevier.com/locate/applanim Applied Animal Behaviour Science 98 (2006) 165–182 * Corresponding author. Tel.: +31 320 238205; fax: +31 320 238094. E-mail addresses: marc.bracke@wur.nl, m.b.m.bracke@id.wag-ur.nl (Marc B.M. Bracke). 0168-1591/$ – see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.applanim.2005.08.021