An increasing number of Australian sheep breeders are joining ewes at 6-8 months of age, which is 6-12 months earlier than ewes are traditionally first joined. When joining at a young age, additional factors such as the attainment of sexual maturity must be considered. The age of sexual maturity is a challenging trait to measure with limited data currently available in sheep. This study explored several methods of analyzing age of first oestrus (AFO) data, an indicator trait for sexual maturity, and explored the relationship between AFO and early reproductive performance. Lambing records from 2218 Maternal-cross ewes joined naturally at 7-10 months were used, a subset of 906 ewes had AFO information collected through the use of teaser wethers. Heritability estimates for AFO were low (0.03 - 0.09) whilst estimates for number of lambs born and weaned at yearling age were 0.20 and 0.16 respectively. Genetic correlation between AFO and number of lambs born and weaned at yearling age were 0.45 and 0.51, respectively, but had high standard errors. Improving reproductive performance through the use of teasers to record AFO is not recommended, thus a need exists to find reliable measures for early reproductive traits including sexual maturity.

The successful breeding of ewes before the age of 12 months presents an opportunity to improve flock reproduction efficiency and increase returns for sheep producers. However, uptake of this practice is currently low in Australia and New Zealand. A contributing factor to this is that reproductive performance at 1 year of age is lower and more variable than in older ewes; between flocks and also from year to year. Whilst previous research has explored what factors contribute to successful reproduction at 1 year of age, there is no universally accepted measure of puberty and sexual maturity in sheep nor is there an accurate phenotypic predictor of yearling reproductive success. Although established genetic correlations between reproduction and production traits exist, reproduction has previously been analysed as a repeated records trait across parities, rather than treating first parity as a separate trait. Model studies show that genomic information offers the opportunity to select animals more accurately at younger ages. However the implications of a lower and more variable fertility rate of ewes mated prior to 1 year of age has been largely ignored. The aim of this thesis was to evaluate the impact of mating ewes prior to 1 year of age on flock genetic progress and to quantify the relationship between potential pubertal traits, yearling reproduction traits, reproduction traits at later parities and other key production traits.