Studies on a Two Trait Selection Experiment in Pigs

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Abstract
Correlation responses in litter size, average piglet weights and sex ratio in 1st 8 generations were studied. Three groups of traits were analyzed for total born, liveborn at 21 days and at 42 days of age. The experiment included 2 selection lines, high and low, on an index combining backfat thickness and daily gain. The high index line (HP line) was selected on high growth rate and low backfat thickness. The low index line (LP line) was selected on low growth rate and high backfat. A control line (KG line) was also maintained. Litters (2131) were analyzed. The record for each litter was expressed as deviation from annual averages in the control line, adjusted for significant parity and seasonal effects. In HP line the correlated genetic response was positive for litter size at birth and all average piglet weights. In LP line there was a significant decline in litter size and average piglet weight at all stages, except for non-significant decline in piglet weight at 42 days (weaning). Sex ratio showed no response to selection. Heritabilities for litter size and average piglet weight traits were based on paternal half-sib analyses. Two sets of reproductive performance records were included, based on 1st litter only and average performance of 1st and 2nd litters. Low estimates were found for heritability of litter size at birth based on 1st litters. Genetic correlations between reproductive traits and selection index were calculated from analyses on covariance among half-sib groups and responses obtained in the experiment. All correlations to the index were positive except for piglet weight at weaning in covariance analyses. Correlations decreased from birth to weaning in both litter size and average piglet weights. all estimates were based on pooled data for all selection lines. The only signs of different genetic relationship between reproductive traits and index were for litter size at 21 and 42 days, when there was a decline in these 2 traits in both selection lines.