This thesis aimed to define a suitable definition of ewe longevity for the Australian sheep industry and determine the merit of incorporating the trait into the Australian sheep breeding objectives. Therefore, the fundamental requirements for incorporating ewe longevity in the Australian sheep breeding objectives, the genetic parameters and economic values were estimated for ewe longevity and stayability traits followed by an estimation of response to selection.

The first part of this thesis deals with the data exploration of the MERINOSELECT and LAMBPLAN maternal databases within the Sheep Genetics and the estimation of genetic parameters for the ewe longevity and stay ability traits in Merino and maternal breeds. The majority of the flocks submitting data to the MERINOSELECT and LAMBPLAN maternal databases do not have sufficient recording patterns to derive longevity. However, the contemporary groups with regular recording patterns were selected. The contemporary groups were defined as the site × flock × year of birth. These contemporary groups of ewes with regular recording patterns within the MERINOSELECT database were; 1) born since the year 2000, 2) had spent a minimum of 3 years in the flock, 3) had their own annual weight (weaning, postweaning or yearling) or wool record and reproduction record (from 2 yrs) up to 6 years of age, 4) contained at least 30 ewes, and 5) at least 70% of the animals were assigned a sire (chapter 3).The ewe longevity or time in flock (TIF) was defined as the period between birth and the last available production record. The stay ability traits were defined as the presence of a ewe in flock up to certain periods of time. The heritability estimates of the ewe's longevity and stayability traits were moderate if not corrected for the ewes’ production and reproductive performance. However, after correcting for these traits, the ewe's longevity and stayability traits were lowly heritable. The correlation between the ewes’ longevity and stayability traits was strong.

The correlation between ewe longevity and production and reproduction traits was estimated via a series of bivariate analyses. The analysed production traits were weaning weight (wwt), post-weaning weight (pwt), post-weaning C-site fat (pcf), post-weaning eye muscle depth (pemd), post-weaning faecal egg count (pfec), yearling weight (ywt), yearling Csite fat (ycf), yearling eye muscle depth (yemd), yearling faecal egg count (yfec), yearling greasy fleece weight (ygfw), yearling fibre diameter (yfd), adult greasy fleece weight (agfw) and adult fibre diameter (afd). The reproductive traits analysed were fertility (fert), litter size (ls), number of lambs born (nlb), ewe rearing ability (era) and number of lambs weaned (nlw). The ewes’ TIF was lowly heritable and correlated to the production and reproduction traits. Therefore, a breeding objective was to be formulated that considers longevity as an objective trait, which requires calculating the economic value of the ewes’ TIF trait.

The second part of this thesis deals with the estimation of economic value and response to selection. The economic value of the ewes’ TIF was large across the fine wool Merino, dual purpose Merino and maternal production systems. Ewe longevity has a positive correlation with the current breeding objectives suggesting that selection on the current breeding objectives will improve ewe longevity across the three production systems within the Australian sheep industry. However, including longevity trait in the breeding objectives will further increase the overall genetic gain, and particularly improve genetic gain for longevity. In the maternal production system, the genetic gain of the growth and carcase traits slows down a little after including the longevity trait in the breeding scenarios. The results showed a 2 to 3% increase in the total dollar response across the three production systems after incorporating the ewes’ TIF trait into the breeding objectives. However, incorporating genomic information of TIF into breeding objectives increased the overall response by 13 to 16% across three production systems. The results suggest that selection based on the current breeding objectives will improve ewe longevity within the Merino and maternal production systems but noticeably higher genetic gain can be attained if the genomic information of ewes’ TIF is incorporated into the breeding objectives.

The final chapter discusses the research findings and concludes with recommendations for future research areas. These recommendations include important encouragements like improving the data quality, the importance of recording culling reasons, estimating accurate genetic parameters of ewe longevity and the potential of incorporating the ewe longevity as a trait in the Australian sheep breeding objectives to achieve higher genetic gain. This thesis contributes significantly to define ewe longevity and using the genetic parameters in the Australian sheep breeding objectives.