Commercial recommendations for dietary lysine specifications in diets for pigs supplemented with ractopamine (RAC) is 0.56 g available lysine/MJ digestible energy (DE; King et al., 2000). A recent study confirmed that when RAC supplemented diets are formulated to 0.56 g available lysine/MJ DE, growth rate and feed efficiency are improved (Dunshea et al., 2005), together with an increase in lean tissue deposition in both sexes, although fat deposition was reduced in boars and remained constant in gilts. The aim of this experiment was to determine whether the current dietary lysine recommendations are sufficient to optimize the response in feed efficiency (FCR), growth rate (ADG) and tissue deposition in boars and gilts offered high and low doses of RAC in diets
