The heat period at which gilts are bred affects first reproductive performance and overall sow productivity. However, most studies which have examined this subject have confounded chronological age (i.e. days) with physiological age (i.e. number of heat periods experienced) and the importance of each factor remains unclear. The Dynamic Service System (DSS) is a recently developed variant of group mating and the optimum heat period at which gilts should be bred has never been established. This study tested the hypothesis that the physiological age at which gilts are first mated in a DSS will affect their first reproductive behaviour (i.e. mating frequency and quality) and performance.