Response of imidazolinone-tolerant volunteer oilseed rape to herbicides and herbicide mixtures used for broadleaved weed control in sugar beet

Due to a higher proportion of oilseed rape (Brassica napus) in sugar beet rotations, volunteer oilseed rape can occur in increased numbers in subsequent crops. The sulfonylurea triflusulfuron is widely used for broad-leaved weed control in sugar beet, but may no longer be effective to control imidazolinone-tolerant volunteers when imidazolinone-tolerant oilseed rape is grown, which is due to a cross-tolerance to ALS-inhibiting herbicides. Aim of the study was to evaluate six different herbicide strategies for the control of tolerant volunteer oilseed rape. As a result, imidazolinone-tolerant oilseed rape showed a distinctive cross-resistance to triflusulfuron in the field. Mean herbicide efficacy was 14 % and was only slightly increased by combination with metamitron. Imidazolinone-susceptible and -tolerant oilseed rape varieties showed different response to the herbicide treatments. A significantly higher reduction of plant fresh mass (shoot) > 90 % was caused by herbicide treatments that included the active ingredients desmedipham, phenmedipham, ethofumesate, chloridazon and quinmerac. The results showed that triflusulfuron is no longer suitable to control volunteers if they are derived from imidazolinone-tolerance expressing varieties. However, alternative herbicides are available. Generally, increased attention has to be paid to volunteer management within the whole crop rotation if imidazolinonetolerant oilseed rape is grown. Appropriate tillage strategies after oilseed rape harvest have to be followed by effective herbicide treatments in the subsequent sugar beet, for example a mixture of metamitron, phenmedipham, desmedipham, ethofumesat and lenaciel.