Selection for lean growth rate and correlated responses in litter traits in a synthetic line of Yorkshire-Meishan pigs

Selection for lean growth rate (LGR) was conducted for four generations in a synthetic line of Yorkshire-Meishan pigs to study the effectiveness of se lection for LGR and correlated responses in litter traits. Lean growth rate was estimated from ultrasound measurements of 10th-rib backfat thickness a nd longissimus muscle area. In the selection line, 7 boars and 20 gilts wit h the highest LGR were selected to produce the next generation. The generat ion interval was 13 mo and the average selection differential per generatio n was 1.1 phenotypic standard deviation units. A contemporaneous control li ne was maintained by randomly selecting 5 boars and 15 gilts. Data from a t otal of 1057 pigs sired by 58 boars and out of 133 sows were available from the two lines. Selection responses were estimated from deviations of the s election line from the control line using least squares (LS) and by multipl e trait derivative-free restricted maximum likelihood analysis using an ani mal model (AM). The estimate of response to selection per generation using LS was 9.4 +/- 0.95 g d(-1) for LGR. The corresponding estimate from the AM was 9.8 +/- 0.51 g d(-1). Correlated responses in litter traits were regre ssed on generation. For the LS method, regression coefficients were negativ e but not significant (P > 0.05) for total number born. number born alive, and number at 21 d and at 42 d. Significant, positive correlated responses occurred in 42-d litter weight and 21-d piglet weight (P < 0.05). For the A M method, the regression coefficients were also negative, but were not sign ificant (P > 0.05) for number alive at birth, at 21 d, and at 42 d. A signi ficant positive correlated response occurred only for 42-d litter weight (P < 0.05), Although results are based on a population of limited size, it ca n be concluded that selection for LGR in a synthetic line is effective and should have little effect on litter traits.