Finisher pigs, i.e. pigs from about 50 kg to slaughter weight, are housed in single-sex groups because the dietary requirements for optimal growth differ for the sexes. In addition, in many countries male pigs not required for breeding are surgically castrated.

Research in the 1970s showed that surgical castration reduced growth efficiency. Consequently, the Australian pig industry ceased castration of pigs about 25 years ago. However pig nutritionists have recently found the growth efficiency of group-housed entire male pigs, to decline during the last weeks of growth for older/heavier pigs. It was suspected that increased sexual behavior and aggression reduced feed intake and thus impairing growth in entire males. Recently, a new technology, immuno-castration, was developed which involves using a vaccine to chemically "castrate" male pigs in the latter stage of the finisher phase of production, thereby eliminating the need for surgical removal of the testes. Two alternative methods of castration were compared with the help of The Observer: traditional surgical castration and the new technology of immuno-castration.  

[full paper]