Divergent selection lines for lean growth on a restricted feeding regime, in Large White and Landrace pigs, were established to complement the lean growth selection lines on ad-libitum feeding. This study estimated the direct and correlated responses after four generations of selection and the corresponding genetic and phenotypic parameters.

The selection objective for lean growth on restricted or scale (LGS) feeding was to obtain equal correlated responses in growth rate and carcass lean content, measured in phenotypic s.d. The selection criterion included measurements of growth rate and ultrasonic backfat depth.

Large White (LW) and Landrace (LR) boars and gilts were purchased from eight British nucleus herds and boars from national artificial insemination centres in 1982. Homozygous or heterozygous halothane positive pigs were not included in the experiment. The base populations consisted of 31 LW and 19 LR sires and 57 LW and 67 LR dams. Within each population, there were high and low selection lines with a control line, each consisting of 10 boars and 20 gilts, with a generation interval of 13.5 and 12 months for LW and LR pigs. Animals were performance tested in individual pens from 30±3 kg for a period of 84 days and fed a high energy (13.8 MJ DE/kg DM) and high protein (210 g/kg DM crude protein) pelleted ration. Daily food intake was equal to 0.75 g/g of the daily food intake for ad-libitum fed pigs and the total food intake was 134 kg for LW pigs and 150 kg for LR pigs. On average, 3 boars and 3 gilts were tested per litter. The total number of pigs tested per line and average inbreeding coefficients at generation four, by population are given below.