Sixty pigs, equal numbers of boars and gilts of the two breeds Large White (LW) and Gloucester Old Spot (GOS), in litter groups of three, were divided equally between an initial slaughter group and two treatment groups; H, given a diet with 149 g/kg crude protein (CP) at a daily level of 0·1 W0·75; and L, given a diet with 193 g/kg CP at a daily level of 0·075 W0·75. The cumulative total amount of feed given to each pig was the same, 175 kg, but H received more energy per day. All pigs received the same CP per day but L received more CP in total. Pigs started on experiment at 12 weeks of age. The treatments differed mainly in their ability to promote fat deposition: H pigs grew faster than L, due mainly to a greater deposition of fat (as measured by the weight of subcutaneous fat produced) rather than lean (as measured by the weight of hind limb lean produced). H pigs had a poorer feed efficiency than L pigs.

The breeds were similar in most aspects of production and body composition over both treatments but GOS tended to be slower growing and fatter than LW, with shorter carcasses, thicker backfat, and smaller subcutaneous fat-free hind limbs. Boars were faster growing, leaner and more efficient than gilts, with a higher proportion of bone in the subcutaneous fat-free hind limb and a lower muscle: bone ratio (boars 4·74, gilts 5·28).

For several aspects of carcass composition the potential benefits of the lean types (LW and boars) over the fat types (GOS and gilts) were realized on treatment L but not on H. There was a significant breed × treatment interaction for lean content of hind limb: LW had higher values than GOS on L but both breeds were similar on H. Both LW and boars had significantly less subcutaneous fat than GOS and gilts respectively on treatment L but similar amounts on H.