A comparison of alternative genetic models for litter size in pigs

Genetic and non-genetic parameters related to embryo production in superovulated Large White (LW) gilts

The aim of this study was to identify genetic and non genetic factors which might affect results of embryo production of Large White (LW) cyclic gilts from data collected in one herd during 6 years. Donors (n=1060) were synchronized with a progestogen treatment and luteolysis was induced 13-15 days later by 2 injections of cloprostenol. To stimulate follicular development 800IU eCG was then injected 24h later, followed by 500IU hCG 48h later. Donors were inseminated twice; depending on the onset of oestrus, the interval between hCG treatment and first insemination (hCGAI1) was either 24 or 41 h. Embryos were collected at 5-6 days after the 1st AI by flushing uterine horns. Traits of interest were the number of corpora lutea (CL), the number of flushed embryos (FE), the number of transferable embryos (TE) and the number of unfertilized embryos (UE). The average number of TE was 18.8 ± 9.0. The main sources of variation for CL, FE and TE were the season (P≤0.002) and hCGAI1 (P≤0.001) effects. For the interval of 24h of hCGIA1 the number of TE was increased by 4 compared with the TE obtained for the 41 h interval of hCGIA1. Maternal and paternal genetic effects were estimated using restricted maximum likelihood methodology applied to the univariate animal model, whereas genetic covariance components were estimated in bivariate models. Estimates of maternal heritability were 0.45 for CL, 0.32 for FE, 0.29 for TE and 0.05 for UE whereas for the paternal effect, heritabilities were very low (<0.06). Genetic correlation between CL, FE and TE variables were very high (>0.89) for the maternal effect. A breeding scheme based on CL selection in response to superovulation could thus improve the number of transferable embryos.

Correlated responses for litter traits to six generations of selection for ovulation rate or prenatal survival in French Large White pigs

Effects of selection for reproductive traits were estimated using data from 3 pig lines derived from the same Large White population base. 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 age and BW at puberty (AP and WP); number of piglets born alive, weaned, and nurtured (NBA, NW, and NN, respectively); proportions of stillbirth (PSB) and survival from birth to weaning (PSW); litter and average piglet BW at birth (LWB and AWB), at 21 d (LW21 and AW21), and at weaning (LWW and AWW) were estimated using REML methodology. Heritability estimates were 0.38 +/- 0.03, 0.46 +/- 0.03, 0.16 +/- 0.01, 0.08 +/- 0.01, 0.09 +/- 0.01, 0.04 +/- 0.01, 0.04 +/- 0.02, 0.19 +/- 0.02, 0.10 +/- 0.02, 0.10 +/- 0.02, 0.36 +/- 0.02, 0.27 +/- 0.01, and 0.24 +/- 0.01 for AP, WP, NBA, PSB, NW, NN, PSW, LWB, LW21, LWW, AWB, AW21, and AWW, respectively. The measures of litter size showed strong genetic correlations (r(a) >/= 0.95) and had antagonistic relations with PSB (r(a) = -0.59 to -0.75) and average piglet BW (r(a) = -0.19 to -0.46). They also had strong positive genetic correlations with prenatal survival (r(a) = 0.67 to 0.78) and moderate ones with ovulation rate (r(a) = 0.36 to 0.42). Correlations of litter size with PSW were negative at birth but positive at weaning. The OR and PS lines were negatively related to PSW and average piglet BW. Puberty traits had positive genetic correlations with OR and negative ones with PS. Genetic trends were estimated by computing differences between OR or PS and control lines at each generation using least squares and mixed model methodologies. Average genetic trends were computed by regressing line differences on generation number. Significant (P < 0.05) average genetic trends were obtained in OR and PS lines for AP (respectively, 2.1 +/- 0.9 and 3.2 +/- 1.0 d/generation) and WP (respectively, 2.0 +/- 0.5 and 1.8 +/- 0.5 d/generation) and in the PS line for NBA (0.22 +/- 0.10 piglet/generation). Tendencies (P < 0.10) were also observed for LWB (0.21 +/- 0.12 kg/generation) and AWW (-0.25 +/- 0.14 kg/generation) in the PS line. Selection on components of litter size can be used to improve litter size at birth, but result in undesirable trends for preweaning survival.

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 + 0.018ORF) 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 (r(g) > 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 (r(g) = 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).