Here we present the de novo assembly of the B. napus cultivar Tapidor and comparison with an improved assembly of the B. napus cultivar Darmor -bzh . Both cultivars were annotated using the same method to allow comparison of gene content. We identified genes unique to each cultivar and differentiate these from artefacts due to variation in the assembly and annotation. We demonstrate that using a common annotation pipeline can result in different gene predictions, even for closely related cultivars, and repeat regions which collapse during assembly impact whole genome comparison. After accounting for differences in assembly and annotation, we demonstrate that the genome of Darmor -bzh contains a greater number of genes than the genome of Tapidor. ; Other ; The authors would like to acknowledge funding support from the Australian Research Council (Projects LP110100200, LP130100925, LP140100537 and DP160104497), from the UK Biotechnology and Biological Sciences Research Council (BBSRC) to GK (BB/E017797/1) and IB (BB/E017363/1) and from the National Basic Research and Development Programme of China (2006CB101600). This work was supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia, the Australian Genome Research Facility (AGRF), the Queensland Cyber Infrastructure Foundation (QCIF) and the Australian Partnership for Advanced Computing (APAC).