HERBICIDE RESISTANCE in broad-leaved weeds is not as big an issue as it is in grassweeds, but growers should be aware of the threat, warns Stephen Moss of Rothamsted Research.

Resistance to sulphonylurea (SU) herbicides may only occur in chickweed and poppies in the UK, but across Europe it has been found in corn marigold, too, notes Dr Moss.

 “There”s no reason why it shouldn”t occur in other broad-leaved weeds before long,” he says.

An isolated case of mayweed resistance found in Lincs last year adds weight to his concern.

Chickweed resistance has been confirmed on 15 farms in Scotland and England, he adds.

 “But we urgently need to update the figures, as it may well be more widespread than this.”

 Although resistance is to SUs, there are signs that cross-resistance to florasulam (as in Boxer) also exists.

 “Boxer is an ALS inhibitor, but not an SU, so this is of interest. We”re not sure how often you get this cross-resistance, or how many of the populations resistant to SUs also have it.”