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Abstract The aim of the present study was to investigate differences in reproductive and body traits during successive parities between two genetic lines. The LP line was constituted by means of selection of animals having an extremely high number of parities (at least 25) and an average reproductive performance compared to the V line selected for litter size at weaning during 31 generations. The two lines were found to have an equal reproductive performance in the first three parities, but the LP line had higher reproductive performance from the fourth parturition onwards. The low reproductive performance after the third parity in the V line was suggested to be caused by constrained environmental conditions in the test station. A line by parity interaction was also observed for body weight, since body weight declined going from the third to the fourth parity in the LP line but not the V line. Thus, it was concluded that hyper selection for reproductive longevity and average prolificacy successfully delayed reproductive senescence, and that this newly founded line showed less environmental sensitivity, which might have been mediated by a higher body reserve.

Abstract Rates of inbreeding (ΔF) in selected populations were predicted using the framework of long-term genetic contributions and validated against stochastic simulations. Deterministic predictions decomposed ΔF into four components due to: finite population size, directional selection, covariance of genetic contribution of mates, and deviation of variance of family size from that expected from a Poisson distribution. Factorial (FM) and hierarchical (HM) mating systems were compared under mass and sib-index selection. Prediction errors were in most cases for ΔF less than 10% and for rate of gain less than 5%. ΔF was higher with index than mass selection. ΔF was lower with FM than HM in all cases except random selection. FM reduced the variance of the average breeding value of the mates of an individual. This reduced the impact of the covariance of contributions of mates on ΔF. Thus, contributions of mates were less correlated with FM than HM, causing smaller deviations of converged contributions from the optimum contributions. With index selection, FM also caused a smaller variance of number of offspring selected from each parent. This reduced variance of family size reduced ΔF further. FM increases the flexibility in breeding schemes for achieving the optimum genetic contributions.