Incidence of splayleg pigs in Nebraska litter size selection lines1

Genetic parameters for the splayleg (SL) condition were estimated from 37,673 records of pigs from six lines derived from a Large White–Land-race base population. Random selection for 22 generations was practiced in Lines C1 and C2. Line C2 was derived from C1 at Generation 8. Selection lines were as follows: 1) Line I, selected 11 generations for an index of ovulation rate and embryonic survival followed by 11 generations of selection for litter size; 2) Line IOL, derived from Line I at Generation 8 and which underwent eight generations of two-stage selection for ovulation rate and number of fully formed pigs per litter followed by four generations of litter size selection; 3) Line COL, derived from Line C1 at Generation 8 and selected eight generations in two stages for ovulation rate and number of fully formed pigs followed by four generations of litter size selection; and 4) Line T, selected 12 generations for increased testis size. From logistic models, it was found that boars were 224% more likely to have SL than gilts (P < 0.01). Decreases in birth weight, dam age at puberty, dam nipple number, and dam embryonic survival, and increases in dam litter size and inbreeding increased the odds of SL (P < 0.05). Direct and maternal heritabilities of SL were 0.07 and 0.16, respectively, and the correlation between direct and maternal effects was −0.24. Correlations between direct genetic effects for SL and number born alive, nipple number, birth weight, age at puberty, and embryonic survival were −0.19, −0.36, 0.23, −0.19, and −0.32, respectively. Except for the correlation of 0.32 between maternal effects for SL and direct effects for number of live pigs, correlations of SL maternal genetic effects with direct genetic effects of other traits were less than 0.11. Annual direct genetic trends (%) for SL in I, IOL, COL, T, C1, and C2 were −0.003 ± 0.003, 0.121 ± 0.012, −0.273 ±0.009, 0.243 ±0.014, −0.274 ±0.004, and 0.086 ±0.008, respectively; annual maternal genetic trends (%) were 0.106 ±0.004, 0.508 ±0.019, 0.383 ±0.015, 0.527 ±0.024, 0.188 ±0.005, and 0.113 ±0.012, respectively. Annual genetic maternal trend in Line I after Generation 12 was 0.339 ±0.014. Maternal breeding value for SL is expected to increase as a correlated response to selection for increased litter size and increased size of testes.