Direct responses to six generations of selection for ovulation rate or prenatal survival in Large White pigs.

Effects of selection for ovulation rate or prenatal survival were examined using data from 3 pigs lines derived from the same base Large White population. Two lines were selected for 6 generations on high ovulation rate at puberty (OR line) or high prenatal survival corrected for ovulation rate in the first 2 parities (PS line). The third line was an unselected control line. Genetic parameters for ovulation rate on the left, right, and both ovaries at puberty (ORPL, ORPR, and ORP, respectively) and at fertilization (ORFL, ORFR, and ORF, respectively), total number of piglets born (TNB) per litter, prenatal survival (PS = TNB/ORF), and PS corrected for ovulation rate (CPS = PS + 0.0180RF) were estimated using REML methodology. Responses to selection were estimated by computing differences between OR or PS and control lines at each generation using least squares and mixed models methodology. Average genetic trends were computed by regressing line differences on generation number. Realized heritabilities were estimated using standard procedures. Heritability estimates were 0.17,0.11,0.34,0.13, 0.09, 0.33, 0.14, 0.11, and 0.17 (SE = 0.01 to 0.03) for ORPL, ORPR, ORP, ORFL, ORFR, ORF, PS, CPS, and TNB, respectively. Realized heritabilities were 0.37 ± 0.08 and 0.10 ± 0.09 for ORP and CPS, respectively. The different measures of ovulation rate had strong genetic correlations (rg > 0.7). The ORF had midrange negative genetic correlations with PS and CPS (-0.45 ± 0.07 and -0.42 ± 0.08, respectively). The ORP also had an antagonistic genetic relationship with PS (-0.26 ± 0.07) but was almost independent from CPS (-0.02 ± 0.11). The TNB was moderately correlated with ORP and ORF (rg = 0.41 ± 0.09 for both traits). Average genetic trends in OR and PS lines were, respectively, 0.49 ± 0.10 and 0.11 ± 0.10 for ORP, and 0.43 ± 0.11 and 0.11 ± 0.11 for ORF. Responses to selection were slightly superior in the left than in the right ovary. No significant difference was found for PS or CPS in any of the lines. The TNB did not change in the OR line but significantly improved in the PS line (0.24 ±0.11 piglets/generation).