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

¹ INRA UR337 Station de Génétique quantitative et appliqué F-78350 Jouy-en-Josas
² INRA UE332 Domaine de Galle, F-18520 Avord

^* To whom correspondence should be addressed. E-mail: pierre.bidanel{at}jouy.inra.fr.

	Abstract

Effects of selection for reproductive traits were estimated using data from 3 pig lines derived from the same base Large White population. Two lines were selected for 6 generations on either 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 (C) line. Genetic parameters for age and weight 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 weight at birth (LWB and AWB), at 21 d (LW21 and AW21), and at weaning (LWW and AWW), were estimated using restricted maximum likelihood 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 different 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 weights (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. Both OR and PS were negatively related to PSW and average piglet weights. 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 C lines at each generation using both 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 both 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 pre-weaning survival.

Key Words: genetic parameters, litter size, ovulation rate, piglet survival, pigs, prenatal survival

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