Effects of genotype, diet and sex on backfat depth in pigs measured physically at different carcass sites and ultrasonically at different liveweights

Pigs of both sexes, from a line selected for low backfat depth and an unselected line, were grown on 3 diets. Backfat depths were measured by ultrasonics at '3 liveweights during growth and by optical probe and scale at 5 sites on and off the midline of the carcass after slaughter. Ultrasonics and carcass measurements were equally effective at detecting the effects of line, diet and sex on fat depth. Average fat depth was higher in females than males, higher in the unselected line than in the selected line, and increased with dietary energy-lysine level. The difference between sexes, lines and diets increased with liveweight. Changing diets affected carcass fat depths at different sites of measurement unifonnly, but both line and sex varied in their effects on fat depth at different sites.