GENETICS OF THE INTERVAL FROM WEANING TO ESTRUS IN FIRST-LITTER SOWS - DISTRIBUTION OF DATA, DIRECT RESPONSE OF SELECTION, HERITABILITY

A selection experiment with a selection and a control line maintained for eight generations was set up to study efficacy of selection for a short interval from weaning to estrus after weaning the first litter a nd to estimate genetic variation in the Dutch Landrace population. Int ervals were recorded without truncation (i.e., intervals up to 234 d w ere observed). A mating scheme with 10 mating groups was used to avoid inbreeding. Distribution of intervals from weaning to estrus was desc ribed by a mixture of a normal and an exponential distribution. Betwee n lines, both underlying distributions were similar, but the contribut ion to the total distribution was different. From generation 4 onward, intervals were significantly shorter in the selection line than in th e control line. Variances within lines; generations, and mating groups varied to a large extent. Response per unit selection differential wa s calculated for original data and three sets of transformed data; bot h with and without correction for unequal subclass numbers. Standardiz ed responses were more precise and linear, when data were log-transfor med, and when corrected for unequal subclass numbers. The assumed best estimate of the realized heritability was .17. The heritability in th e foundation population was estimated at .36 +/- .05, using an animal model including all genetic relationships in the data. It was conclude d that genetic selection decreases the average interval from weaning t o estrus by reducing the number of sows with a record in the exponenti al part of the distribution.