ALS (sulfonylurea) resistant scentless mayweed occurs in three European countries and was first detected in the UK in 2002. By 2016, ALS-resistant mayweed had been identified on a total of 12 farms in five counties, Hereford & Worcester, Norfolk, West Midlands and Yorkshire in England and Angus in Scotland. Studies with the Scottish population showed that resistance was conferred by a Pro-197-Gln ALS point mutation, proving that ALS target site resistance was responsible.

Management Options (both species)

  • If resistance to sulfonylurea (or related ALS herbicides) is suspected, collect a seed sample and have it tested. The difficulty with collecting seed samples of mayweed means that the true number of cases is probably under-recorded.
  • As with other resistant broad-leaved weeds, a long history of use of sulfonylurea herbicides and declining performance against mayweed, when other susceptible weeds are still well controlled, indicates resistance is a likely cause.
  • If resistance is confirmed, use alternative (non-ALS) herbicides such as clopyralid which remains highly effective.
  • Florasulam is an ALS inhibiting herbicide but from a different sub-group (triazolopyrimidine) to the sulfonylureas and is likely to give effective control of populations of mayweed, provided they have the 197 ALS target site mutation. Very few UK populations have been characterised at the molecular level, so the frequency of the 574 ALS target site mutation, which is likely to confer resistance, is unknown. Hence, as with chickweed, florasulam should be used with caution against mayweed, unless the specific mutation is known, which will rarely be the case.
  • If sulfonylurea (or related ALS herbicides) continue to be used for control of other weeds, look out for signs of resistance in these other species. Resistance in mayweed tends to be absolute, so assume that no control of this species will be achieved.