Fine Mapping QTL And Candidate Genes Discovery For Residual Feed Intake On Chromsomes 5, 15, 16, And 19 In Beef Cattle. M.K. Abo-Ismail * , E. J. Squires * , K. C. Swanson * , D. Lu * , Z. Wang † , J. Mah † , G. Plastow † , S. Moore † and S. P. Miller * . Introduction Feed efficiency remains a trait of great importance to beef producers and breeding values can be estimated for selection purposes when the trait is measured (Schenkel et al. (2004)). However in practice selection is limited due to the cost of measurement making this trait a candidate for selection with molecular genetic information (Dekkers and Hospital (2002)). High density SNP panels are becoming more powerful and providing encouraging results in beef and dairy cattle (Hayes et al. (2009)). When prediction across breeds is required as is often the case in beef cattle, Marker Assisted Selection (MAS) based on direct or functional mutations increases the genetic gain (Dekkers (2004)). Different whole genome association studies to detect putative quantitative trait loci (QTL) for feed efficiency have been conducted (e.g. Sherman et al. (2010)). A whole genome scan (WGS) with the Illumina BovineSNP50 Beadchip has identified a large number of regions associated with residual feed intake (RFI) (results not published). Information from the WGS can be coupled with an understanding of the biological function underlying genes to further enhance the list of candidate genes for fine mapping. The objectives of this study were: (1) to map QTL for RFI (2) estimate the proportion of variation in RFI explained by the detected QTL and (3) identify the candidate gene or the flanking genes to elucidate the biological meaning behind the underlying potential QTL. Material and methods Animals and phenotypic data. Phenotypes were obtained from detailed studies of growth and feed intake during the post-weaning period on 589 heifers (20), steers (338), and bulls (231). Animals were crossbred, with breed composition formed by Angus (39%), Charolais (11%), Piedmontese (9%), Simmental (25%), and Limousin (2%). Animals originated from one of three University of Guelph cooperating herds in Ontario, Canada. Calves were weaned at approximately 200 days of age, and then transported to the Elora Beef Cattle Research Centre for various post-weaning nutrition trials. Average daily gain (ADG), metabolic midpoint (MWT) and individual feed intake were determined. RFI was calculated * Centre for Genetic Improvement of Livestock, University of Guelph, Canada. † Faculty of Agricultural, Life and Environmental Sciences, University of Alberta, Canada.