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The challenge

Groundsel (Senecio vulgaris) was the first triazine (e.g. simazine, atrazine) resistant weed to be identified worldwide in the late 1960’s in the USA. Triazine resistance became the most common type of resistance worldwide between 1970 and 1990, being identified in over 65 species. Triazine-resistant populations of eight weed species, including groundsel, have been recorded in the UK.

EU regulatory action meant that the triazine herbicides, atrazine, cyanazine and simazine could no longer be used in the UK after 2007. Consequently, triazine resistance might be viewed as being of only academic interest now. However, triazinone herbicides, such as metribuzin and metamitron, are still widely used for weed control in potatoes, sugar beet and other crops and these have the same mode of action as the triazines, being inhibitors of photosynthesis at PSII. Both triazines and triazinones are in the same (C1) HRAC mode of action class.

Following reports of poor control by metribuzin, samples of groundsel seeds collected in 2010 from four asparagus fields in Worcestershire and Warwickshire were screened for resistance to metribuzin, metamitron and simazine. Results confirmed triazine resistance and partial resistance to both triazinone herbicides in all four populations. These were the first documented case of resistance to triazinone herbicides in the UK. Target site resistance was the most likely mechanism, being a consequence of a point mutation of the psbA gene resulting in a Ser264→Gly substitution. This needs to be confirmed, but is the mechanism of triazine target site resistance in virtually all cases that have evolved in the field worldwide, and is well documented as conferring very high resistance to triazine herbicides and moderate cross-resistance to the triazinones.

Management Options

  • Wherever possible, try to avoid using the triazinone herbicides as the sole or main means of control of broad-leaved weeds. Preferably use in mixture, sequence or rotation with alternative modes of action. This may be difficult in minor crops where few alternatives are available and in perennial crops, such as asparagus, which are grown on the same land for many successive years with limited opportunity for rotating herbicides with different modes of action.
  • If alternative herbicides are unavailable, consider non-chemical methods, such as inter-row hoeing and hand rogueing, to reduce dependence on herbicides.
  • If resistance to triazinone herbicides, such as metribuzin and metamitron, is suspected, collect a seed sample and have it tested. Fat-hen (Chenopodium album) is the weed species most commonly found to have resistance to both these herbicides worldwide, including several other European countries. Resistant populations have been found in potatoes and/or sugar beet in Belgium, Bulgaria, Greece, Norway, Poland and Sweden. Resistance is suspected to occur in the UK, although this has not yet been confirmed.
  • A good indication of the other weed species most likely to evolve resistance to metribuzin and metamitron are those which were prone to becoming resistant to triazine (simazine and atrazine) herbicides historically. A good global source of information to consult is the HRAC International Survey of Herbicide Resistant Weeds (http://weedscience.org).
  • Detecting resistance early can help greatly as if resistance is confined to a relatively localised area, it may be possible to destroy this area with glyphosate or cultivations and hence minimise further spread.