Although for most traits, there is a difference between phenotypic expression in ewes versus rams, they have been traditionally treated equally in breeding programs. This thesis has investigated whether there is any variation between them in how they inherit growth and whether there are any benefits to be gained from divergent selection objectives. A bioeconomic model was used to show that divergent selection for growth in ewes and rams could increase the feed efficiency and profitability of a self-replacing production system. The magnitude of the increase was found to be influenced by the age structure of the flock, fertility, base weight (average of both sexes) and the annual feed availability and price cycles. To test whether divergent selection for growth is possible, sex specific genetic parameters and genetic correlations between ewes and rams were estimated. Accordingly, a series of univariate, multivariate and random regression models using combinations of direct additive genetic, maternal additive genetic, maternal permanent environmental and direct permanent environmental effects were fitted. The results from these analyses revealed that ewes have larger heritability and smaller phenotypic variances than rams and that the intersex genetic correlations were less than unity and decreased with age (range 0.59 to 0.97). Furthermore, the phenotypic and BLUP selection responses predicted using these parameters revealed that rams had higher selection responses than ewes. The results for fat depth (corrected for live weight) were the sole exception to these results. Fat in ewes was deeper and had a lower heritability and higher phenotypic variances than rams. Little variation in sexual dimorphism was observed between the breeds used in this study despite variation in selection history/objectives and founder effects in each breed. In conclusion, these results reveal that it is useful to select for divergent weight, fat and eye muscle depth objectives in ewes and rams and that sexual dimorphism can have a positive effect on the feed efficiency and profitability of a self-replacing flock. Other potential uses of sexual dimorphism in livestock breeding including in species other than sheep, single sex production systems (using sexed semen), reducing product diversity (equal ewes and rams) and the influence of recording/selecting a single sex were also discussed.

This study investigated whether the prenatal maternal environment in dairy cattle influences the postnatal milking performance of the resulting daughters and grand-daughters. Linear mixed models were used to analyse whole season milk production from ~46000 Jersey and ~123000 Holstein Friesian cows in their 1st and 2nd lactations. Variation in the prenatal environment was associated with a small but significant (P<0.05) proportion of the total phenotypic variation (0.010 to 0.015) in all traits in Holstein Friesian cows and in the first lactation milk volume (0.011) and milk protein (0.011), and the second lactation milk fat (0.015) in the Jersey breed. This indicates that the prenatal environment does influence the adult performance of the subsequent daughter. Associations between daughter performance and dam and grand-dam traits indicative of their prenatal environment were also estimated. A one litre increase in the dam's herd test milk volume was associated with a 7.5 litre increase in the daughters' whole season milk yield and a 1% increase in either the dams' herd test milk fat or protein percentage was associated with a reduction in daughter whole season milk volume (-49.6 and -45.0 litres for dam fat and protein, respectively). Similar results between the grand-dam herd test traits ansd the daughters' whole season milk production were observed with a 1% increase in either grand-dam milk fat or protein percentage associated with a reduction in daughter whole season milk yield (-34.7 and -9.7 litres for fat and protein, respectively). This study revealed that the prenatal environment of the dam and the grand-dam can influence milk production in the subsequent daughters, though the effects are small. The similarity of the results between the dam daughter and the grand-dam daughter analyses suggests that the majority of the prenatal maternal effects are mediated by epigenetic mechanisms.