Differential rates of genetic change in litter productivity, growth and carcass traits were estimated for the pig populations of six British breeding companies from their relative performance between 1972 and 1981 in the Meat and Livestock Commission's Commercial Pig Evaluation. Pigs were grown on one of two feeding regimens and slaughtered at one of three live weights (61, 93 and 118 kg).
Important differentials were found for a number of traits. Annual rates of change in litter size at birth differed by 0·34 pigs between the two extreme populations. The range between populations for live-weight daily gain (ad libitum feeding) was 20 g, for lean weight daily gain (restricted feeding) it was 6 g, and for lean weight food conversion (feeding regimens pooled) it was 28 g food per g gain. The average annual rate of change of the six company populations was compared with the rate achieved by the purebred Large White breed over the period 1972 to 1977. For mean piglet birth weight the rate was 30 g higher for the company populations, whilst for carcass weight daily gain it was 24 g higher.
There was little evidence for population × feeding regimen interactions except that companies ranked differently in average daily food intake, live-weight daily gain and carcass weight daily gain on the two feeding regimens. On limited evidence, population × slaughter weight interactions were also unimportant.