The major components of economic performance in pigs - growth rate, feed conversion ratio and carcass merit - have been the targets of selection for a number of years in the closed herds of breeding companies. Over the past 20 years substantial progress has been made for these traits. Declining fatness in closed herds may lead to a decline in the genetic variation for growth and carcass traits, and thus reduced opportunities for selection. This could arise either as genetic variation is exhausted in closed lines, or as physiological selection limits are approached. In this study genetic variation in closed lines with a history of selection for growth and carcass traits was investigated.

The data were collected from two closed lines between 1979 and 1985 by the Cotswold Pig Development Company Ltd. The two lines were housed on a single farm in the same environment and had a history of selection based upon an index of growth rate and fat depths. There were 167 sires with 4722 male and 5386 female progeny in line A, a synthetic breed based largely upon the Landrace breed, and 164 sires with 4160 male and 3887 female progeny in line B, derived from the Large White breed.