Response to selection for decreased backfat thickness at restrained intramuscular fat content in Duroc pigs

Intramuscular fat (IMF) content is a relevant trait for the pig industry and consumers. However, selection for IMF has the undesired correlated effect of decreasing lean growth. A selection experiment was performed to investigate the effects of selection against backfat thickness (BT) at restrained IMF. Barrows from a purebred Duroc line were allocated into a selected (n = 165) or a control (n = 185) group based on their litter predicted EBV. Litters in the selected group were selected against BT at 180 d at restrained IMF in gluteus medius (GM) whereas those in the control group were chosen randomly. Realized selection intensities and genetic responses for BT, IMF in GM, and BW were estimated using a 3-trait multivariate animal mixed model under a Bayesian setting. Correlated responses for other traits were estimated similarly but using a 4-trait model, where other traits were added to the previous 3-trait model 1 at a time. Selected pigs had less BT than control pigs [-1.22 mm, with highest posterior density interval at 95% (HPD95; -2.47, -0.75)] with restrained decrease in IMF, both in GM [-0.16%; HPD95 (-0.36, +0.05)] and in LM [-0.15%; HPD95 (-0.37, +0.09)]. However, the realized selection intensity for IMF in GM denotes that the restriction on IMF was incomplete [-0.18; HPD95 (-0.36, +0.02)]. Selection decreased BW [-1.64 kg; HPD95 (-2.47, -0.75)] but increased carcass lean weight [+0.66 kg; HPD95 (+0.14, +1.22)], indicating that the response in BT offsets the unfavorable correlated response in BW. Selected pigs were shorter [-0.50 cm; HPD95 (-0.81, -0.20)] but with similar ham weight and loin depth. These results provide evidence that lean weight can be improved restraining the genetic change in IMF. However, they also stress that a complete restriction on IMF is difficult to achieve unless selection is practiced on a big population where IMF is accurately predicted.