Genetic improvement for dairy production in sheep and goats

Functional traits have become important for efficient breeding schemes in the dairy goat and sheep industries, mainly in Mediterranean countries, due to increased costs of production relative to milk prices and consumers demand for safe, quality food and attention to animal welfare. The challenge facing the European dairy sheep and goat sector is to cost-effectively produce typical cheeses attractive to the consumer, i.e. of high quality and perceived to be safe, while maintaining production in less favoured rural regions. The emphasis for functional traits related to udder morphology and health, at the moment on a quantitative genetics basis, has resulted from the knowledge established during the last decade that selection on milk traits only, as practiced for several decades for breeds benefiting from efficient breeding schemes, would lead in the long term to “baggy” udders that are more difficult to milk by machine and more susceptible to mastitis. At the same time another window has been opened based on new molecular tools allowing the detection and mapping of genes of economic importance in farm animals. To date, marker- or gene-assisted selection (MAS/GAS) has been applied in dairy small ruminants either for introgression of a major gene such as the Booroola mutation or for selection of major genes such as the polled mutation and αs1-casein gene in goats, or the PrP gene for scrapie resistance in sheep. These applications clearly showed the need for balance over time between selection for polygenes and the major gene for a given trait, or between increasing frequency of favorable alleles of a major gene while maintaining selection for other traits and the genetic variability within the breed. It showed that the selection for major genes will be more profitable at the breed level if an efficient breeding scheme is already running to be able to account for these optimizations over time. Moreover attention is also turning to the mapping of quantitative trait loci (QTL) for production and functional traits. Results are promising since numerous QTL have now been detected, mainly in dairy sheep, showing that cattle results can be partly transferred to dairy small ruminants. But QTL fine mapping is a crucial next step before any application of MAS/GAS because of the need to dramatically reduce genotyping costs for these species. Finally, given the large differences among existing breeding programmes for dairy sheep and goats, the ability to use these new technologies and molecular knowledge in the breeding schemes will probably be breed dependent at least in a near future.