Genetic relationships among production traits and rebreeding performance

We studied effects of selection for increased daily gain, reduced backfat, increased number of piglets born alive, and increased 21-d litter weight on interval from weaning to farrowing (IWF) of two commercial populations of purebred Large White (LW) and Landrace (LR) lines with each represented in two farms. The analysis took into account that normal and prolonged intervals could be distinguished. Distributions of IWF were described by a mixture of a normal and an exponential distribution. Observed intervals were transformed to the probability of being a prolonged interval, using the mixed density function. Incidence of normal intervals was less in LR than in LW sows and was least in first-litter sows. Heritability estimates of IWF ranged from zero to .24 across parities and farms. Genetic trend within breed and farm was different from zero (P < .05) for each trait under selection, except backfat on Farm 2. Genetic correlations between the traits under selection and IWF were inconsistent across farms. Differences in estimated breeding value for the traits under selection between sows with a normal and a prolonged interval were significant only on Farm 1 in the Large White breed, when only observed intervals were analyzed. Including culled sows in the analysis as sows with a prolonged interval yielded consistent differences for average daily gain and backfat, such that genetic selection for improved production would increase the liability for a prolonged interval. Culling sows for delayed estrus apparently overcame this problem on Farm 2 but not on Farm 1, on which a trend of increasing incidence of prolonged intervals was observed.