Selection for components of litter size in swine: Response to five generations of selection and expected response for alternate selection methods

Response to selection was evaluated in a line of swine (LSI) selected for five generations on an index of ovulation rate (OR) and embryo(fetal) survival (ES) (I = 10.6 OR + 72.6 ES). A randomly mated control line (C) derived from the same base population was maintained. Ovulation rate and ES were measured by laparotomy on all LSI and one-half of C females. Embryo(fetal) survival (ES) was calculated as the percentage of ova which were present as fetuses (FETS) at d 50 of gestation (ES = FETS/OR). Realized heritability of the index was.35 $\pm$.20. Regressions of difference between LSI and C lines on generation number were.57 $\pm$.11 for OR, $-$.013 $\pm$.0096 for ES,.21 $\pm$.20 for FETS, 5.11 $\pm$.1.73 for index and.15 $\pm$.18 for the number of fully formed piglets born. Correlated response in age at puberty was an increase of 1.6 $\pm$.57 d per generation in LSI relative to C. Important correlated changes in growth and body composition were not observed. Heritability estimates were.17 $\pm$.07 for OR,.20 $\pm$.09 for ES and.08 $\pm$.10 for FETS. The estimated genetic correlation between OR and ES was $-$.51 $\pm$.23. Expected response for alternate selection schemes was evaluated. Schemes included selection for FETS and ES, two-stage selection for OR or ES and litter size, direct selection on deviation of observed FETS from FETS predicted from OR by polynomial regression or an exponential curve, index of OR and deviation of observed from predicted (regression) FETS, and index of OR and ES. Expected response was highest for the index of OR and deviation of observed from predicted fetuses (predicted by regression) for both the case of equal selection intensity (.26 piglets/gen.) and the case of equal farrowing spaces (.31 piglets/gen.)