Breeding trials for swede (Brassica napus var. napobrassica) in 2000–2010 showed that 0·85 of the incidence of brown heart (BH) in the trials was associated with genotypes that are progeny of Ag31, Or13 and Me77c. In order to investigate this and the effect of treatment with boron (B), established varieties and improved parent lines carrying male sterility (ms), and their F1 hybrids (test hybrids), were grown in a field trial in the UK in 2011 and subjected to four B treatments (0·00, 1·35, 1·80 and 2·70 kg B/ha). The results confirmed that BH incidence and severity was affected by genotype but could be ameliorated by B application. Genotype Ag31 was very susceptible while Or13 and Me77c were of intermediate susceptibility and the hybrids between susceptible parents were also sensitive. Genotypes Gr19 and Ly01 were highly resistant even in the absence of B application. Hybrids between resistant and susceptible lines were highly resistant. The use of ms had no influence on BH. Resistance to BH was a dominant trait: homozygous dominant (BHBH) or heterozygous (BHbh) genotypes confer this trait, while susceptibility is recessive (bhbh). Some quantitative variation existed, suggesting that resistance was not a single gene effect. There was a significant negative correlation (r=−0·632) between root B content and the severity of BH in susceptible genotypes. Severe BH was associated with 12–21·5 μg B/g of root dry weight at zero B applied. Moderate discolouration was associated with 19·5–24·8 μg B/g recorded at moderate B applied and only Ag31 showed BH at 2·70 kg B/ha. Resistant varieties had root contents of 23 μg B/g or more while susceptible varieties required a minimum of 31 μg B/g to offset BH.