26 January 1996

Conflict over fop ability to tackle resistant blackgrass

By Andrew Blake

CONFLICTING views on the use of so-called "fop" herbicides to control "difficult" or "resistant" blackgrass emerged at last weeks annual conference of the Association of Independent Crop Consultants.

Mike Read, for AgrEvo, manufacturer of fenoxaprop (as in Cheetah), openly criticised rival firm Ciba, maker of clodinafop (as in Topik and Hawk). "We are not happy with Cibas recommendation of fops on confirmed resistant sites," he said.

Earlier Dr Stephen Moss of IACR-Rothamsted noted that the comparatively rare "target-site" type of resistance specific to "fops" and "dims" (as opposed to the "enhanced metabolism" type) had recently been found in Suffolk and Northants. "It is now on nine farms in five counties," he said.

With its typical symptoms of the odd plant surviving alongside others killed and patchy distribution, it needed detecting early, he advised. "You could very soon get into a situation where all fops and dims fail. It is a small problem now but is potentially very serious."

Cibas Jim Butchart conceded that clodinafop would not control blackgrass with target-site resistance, but to date relatively few farms had that problem. "If it became more widespread maybe we should look again."

He outlined trials showing good results from using the herbicide in combination with others such as tri-allate and trifluralin but stressed that "timing was of the essence". The cut-off time for applications was effectively the beginning of weed tillering (GS12-21). "After the end of February, you are wasting your time on resistant blackgrass," he said. With clodinafop having a very sharp dose response curve, he advised delegates not to "mess about" with rates and to stick to mineral oil additives.

This seasons experience with Hawk had shown "a very high level of control on resistant sites. But do not use another fop in spring on the same target weed in the same year," he warned.

With 300 confirmed cases of blackgrass resistance and "probably" two or three times that in practice in the UK, AgrEvo took the threat "very seriously", said Mr Read. But it was important to talk about resistance to specific products and to keep problems in perspective, he stressed. "We need to raise the profile but not to cause panic."

The key to control was that each site would require a specific strategy, based on assessing individual risks according to the "three Cs" of cropping, cultivations and chemical rotation.

Isoproturon still had an important role to play, especially as it seemed to work very well on sites with confirmed resistance to fenoxaprop. Of 100 samples from known and new resistant sites, only 1% displayed "cross resistance" to the two chemicals, he noted. "We were amazed."

Mixing half rates of fops and dims was no solution, he added. At best the approach gave only 50% control on known resistant sites.

Herbicide-resistant blackgrass is widespread – Dr Stephen Moss.

Few farms had the target-site type of resistance, said Ciba Agricultures Jim Butchart.