Castration has been performed in the pork industry to reduce aggressive behaviour and to prevent boar taint in meat. This has usually been done by physical castration, and has the effect of reducing feed efficiency and increasing fat gain over lean gain. To increase efficiency and lean gain, entire males are being considered for use, but to avoid boar taint they would need to be slaughtered before sexually maturing – which reduces economic profitability. Studies of boars in group housing systems have found little difference in final weight or average daily gain when compared to barrows, but final lean gain has been higher and final fat content lower. This effect is more pronounced when boar are housed individually. The reduced performance in groups may be due to increased aggression and sexual behaviour, which reduces feed intake. Immunocastration – or immunizing against gonadotrophin releasing factor – reduces the presence of boar taint compounds, reduces aggressive behaviour, and improves growth. Immunocastration allows the boar to have a growth rate and feed intake similar to a barrow, but the feed efficiency and lean gain closer to an individually housed boar. The decrease in boar taint compounds and increased carcass weight appears after 2 weeks and lasts for 4-8 weeks after the second vaccine administration, although the skatole pattern is not yet confirmed. Due to the increased protein deposition in boars, their lysine requirement will be higher than for barrows. As well, it appears to be higher than for gilts, although the boar’s efficiency in lysine use seems to be better. Immunocastration is one solution to achieving a boar’s higher feed efficiency and lean gain, while maintaining a barrow’s temperament and reducing boar taint.