GENOME WIDE ASSOCIATION STUDIES FOR MILK PRODUCTION TRAITS IN DAIRY CATTLE: A REVIEW

The selection of animals using molecular information is more reliable with increased accuracy of selection and higher genetic gain. Hence, there is need to use selection methods that are based on genomic studies. Genomic selection (GS) is a variant of marker-assisted selection method used for predicting genomic breeding values (GEBVs) of animals using high density genetic markers, such as single nucleotide polymorphisms (SNPs). The utility of genomic information in dairy cattle breeding schemes has now reached the level of accuracy that enables dramatic changes and improvements to breeding schemes. GS can increase the accuracy of selection, shorten the generation interval by selecting individuals at the early stage of life, and accelerate genetic progress. The application of GS in dairy cattle has been reported in many countries, including USA, Canada, Australia, Norway, New Zealand, Netherland, Denmark, Germany and Ireland with very promising results. Published results indicates that for dairy cattle approximately 1000 bulls are required in the reference population to obtain GEBVs with accuracies that compete with the accuracies of EBVs based on progeny testing for all traits. Use of genomically evaluated young bulls can accelerate the breeding cycle and increase genetic gain per unit time beyond what is possible with phenotypic selection. With denser marker panels, more sophisticated statistical tools and in the longer term, sequencing, it is expected that the accuracy of GEBVs will continue to improve and breeding schemes will utilize genomic information further at the expense of progeny testing. Current application of genomic selection is only the start of the genomic era in livestock production. To fully capitalize on the benefits provided by GS, breeding programmes may need to be redesigned substantially