Arey DS. and Brooke P. 2006. Animal welfare aspects of good agricultural practice: pig production. CIWF TRUST.UK.
Arey DS, Sancha ES.1996. Behaviour and productivity of sows and piglets in a family system and in farrowing crates. Applied animal science joumal 50.135-145.
European Commission.2001. Official Journal of the European Communities. L316/1-4. http://eur-lex.europa.eu/pri/en/oj/dat/2001/1_316/1_31620011201en00010004.pdf