Don't overlook the risk of ALS-resistance in broad-leaved weeds

With target site resistance to ALS inhibitors (sulphonylureas or SU’s) confirmed in chickweed, poppy and mayweed in certain locations across the UK, a resistance management strategy needs to be put in place to ensure these weeds remain manageable.

Sarah Mountford-Smith of BASF points out that over-reliance on spring sulphonylureas, which has been routine for many years in the UK, must be avoided.  “Herbicide programmes for broad-leaved weeds must include other modes of action to prevent and control ALS-resistant populations becoming more widespread. Farmers are used to doing this when managing resistant black-grass and wild-oats. They now need to do the same for broad-leaved weeds before the problem gets out of hand.”

AICC’s trials Manager Mike Thompson has already had test results confirming SU resistance to chickweed, poppy and mayweed.  “The problem of broad-leaved weed resistance is increasing year on year. I have seen resistant poppy problem arise on farms growing spring barley where weed control has relied upon SU chemistry almost exclusively. I am also seeing resistant chickweed on black land, particularly where Atlantis (another SU) has been used for black-grass control and SU for broad-leaved weed control.”

Lynn Tatnell of ADAS points out that ALS inhibitors can now be used in every part of your rotation.  ”The fixation of treating black-grass with a SU grass-weed herbicide may be inadvertently leading to a build-up of resistant poppies. And in spring cropping, ALS-inhibitors are used very widely. “

Mike Thompson is recommending an immediate change in approach, starting in spring barley but across the rotation.  “Many growers have repeatedly used SU’s as the only herbicide in spring barley. Here we need to introduce Crystal pre-emergence as an EAMU or use straight Stomp Aqua. In winter cereals we need to push for more pendimethalin, as a straight or in coform as in Picona or Crystal.”

Containing picolinofen and pendimethalin, Picona is a useful herbicide for preventing ALS-resistant poppy and chickweed.  “Picona will also give long lasting control of fumitory, crane’s-bill and Shepherd’s purse. It is recommended in winter wheat and barley, with an EAMU in spring barley. Picolinofen is in the HRCA group F1 and pendimethalin in HRAC group K1, so two actives with different modes of action. It is a non-ALS herbicide which controls both sensitive and resistant chickweed and poppy, making it vital for future resistance management strategies for broad-leaved weeds.” says Sarah Mountford-Smith.

Crystal, containing pendimethalin and flufenacet, is an ideal choice, controlling both sensitive and resistant poppy and chickweed as well as providing excellent control of a wide range of grass and broad-leaved weeds. The complimentary residual and contact activity of pendimethalin and flufenacet provides long-lasting control of annual meadow-grass and poppy yet to emerge, gives valuable contribution to the control of black-grass and some problematic weeds such as fumitory, chickweed, pansy, speedwells and crane’s-bill.

Sarah concludes that the demise of many active ingredients in the autumn, such as IPU and next year chlorotoluron, has left fewer options for grass and broad-leaved weed control.  “Sole reliance on ALS chemistry must be avoided to prevent a widespread increase in ALS-resistant broad-leaved weeds, as we are experiencing in black-grass. Growers need to put a resistance strategy in place. This will include keeping weeds at low population levels, using herbicides with different modes of action, avoiding reliance on ALS-inhibitors alone and starting the broad-leaved weed control programme in the autumn.

In the UK grass-weed resistance is the big issue, but resistance to ALS herbicides in broad-leaved weeds is the biggest single type of resistance in the rest of the world. Resistant poppies were first seen in the UK in 2000 and by 2010 resistance was confirmed on 25 farms in 9 counties. Resistant chickweed has been confirmed on 40 farms in 13 counties, 7 of which are in Scotland. Cultural control is difficult as the resistant weeds emerge in both the autumn and spring and generally produce an enormous seed return.