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As male pigs reach puberty, they start producing andosterone, a male sex hormone, and skatole, a digestive by-product formed in the intestines. The production of andosterone and skatole is responsible for boar taint, an unpleasant odour and taste found in meat from some (not all) entire male pigs. The risk of boar taint cannot be completely eliminated by slaughtering entire male pigs before they reach puberty. Overseas, boar taint prevention has consisted of physically castrating all male pigs whereas in Australia early slaughter and, more recently, immunological castration are more common, with physical castrates only making up a very small percentage of castrated pigs. Physical castration, with or without anaesthetic or pain relief, is not preferred on animal welfare grounds.
Where it is considered necessary to castrate male pigs in order to prevent boar taint, the RSPCA supports immunological castration. ‘Immunocastration’, as it is referred to, requires two doses of vaccine at least 4 weeks apart with the second vaccination administered 4 to 5 weeks before slaughter. The active ingredient in the vaccine is a protein that delays the onset of puberty by stimulating the pig’s natural immune system to produce antibodies that inhibit testes function. By delaying the onset of puberty, male pigs cannot produce andosterone and skatole, which in turn significantly reduces the risk of boar taint being present in the pork product. Immunocastration significantly decreases the sexual and aggressive behaviour normally associated with testosterone production such as fighting, pushing, head butting and mounting which may result in stress, injuries and skin lesions. Therefore, the animal welfare benefits of immunocastration not only include the elimination of painful physical castration methods but also significantly less aggression and other sexual behaviours relating to the onset of puberty. In some pigs, the effect of the vaccine will wear off within 8 weeks of the last vaccination and sexual behaviours will gradually return along with boar taint in the pork product.
Not all entire male pigs exhibit sexual and aggressive behaviour and not all pork product from entire male pigs exhibits boar taint. Research looking into differences between pig farms has found that management factors play a role in the level of mounting, aggressive behaviour and skin lesions among entire male pigs. Good stockpersonship, rest and routine, appropriate housing, feeding practice, sufficient feed and water are all associated with reduced sexual and aggressive behaviour. Boar taint is also lower in clean pens with good quality flooring and with smaller group sizes. The role of management factors should be recognised and pig farmers should aim to provide an optimal environment for entire male pigs prior to considering castration as a method to control undesirable behaviours and prevent boar taint.
Further work to eliminate boar taint and reduce sexual and aggressive behaviours in entire male pigs is ongoing and includes dietary supplementation, feeding system, genetics (e.g. low boar taint sires, genetic markers for boar taint), lighting program, and pen design. Work to detect boar taint on the slaughter line is also ongoing.