ALS (sulfonylurea) resistant chickweed occurs in seven European countries and was first detected in the UK in 2000. By 2016, there were >50 confirmed cases of ALS (sulfonylurea) resistance in widely scattered counties comprising seven counties in Scotland, five in England and one in Northern Ireland. Studies showed that resistance was conferred by two ALS gene point mutations (Pro-197-Gln or Trp-574-Leu). The first of these mutations appears much more common and is associated with resistance to the sulfonylurea, metsulfuron-methyl, but not to the triazolopyrimidine, florasulam. In contrast, the latter mutation is associated with resistance to both herbicides groups.
- 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 chickweed means that the true number of cases is probably under-recorded. Seeds are best collected by pulling up mature plants and placing them upside down in paper bags in a dry place and allowing seeds to shed naturally. Don’t use polythene bags.
- Resistant chickweed appears to be mainly found in areas where black-grass is not such of a problem and where more spring crops are grown. In such situations there is likely to be less use of pre-emergence herbicides (mainly non-ALS) for grass and broad-leaved weed control and hence a greater dependence on post-emergence sulfonylurea and related herbicides, and hence an increased risk of resistance. A history of use of sulfonylurea herbicides and declining performance against chickweed, 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 fluroxypyr in cereals which remains highly effective.
- Florasulam is an ALS inhibiting herbicide but from a different sub-group (triazolopyrimidine) to the sulfonylureas and remains effective on the majority of populations of chickweed (which have the 197 ALS target site mutation). However, florasulam will not control chickweed populations with the less common 574 ALS target site mutation, so should be used with caution, unless the specific mutation is known, which will rarely be the case.
- Resistance to mecoprop has been found in chickweed in the UK in the past (1980’s), although all samples of ALS-resistant chickweed tested so far have been well controlled by mecoprop in screening assays However, many farmers and agronomists in both Scotland and Ireland are convinced that the efficacy of mecoprop on chickweed has declined in the field, although in some cases this may be due to use of lower than recommended rates. As resistance to mecoprop has been found in the UK in the past further investigations are warranted.
- 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 chickweed tends to be absolute, so assume that no control of this species will be achieved.