Evidence backs pendimethalin use

Has residual herbicide pendimethalin lost some activity against blackgrass?

Some agronomists believe so, but manufacturer BASF is suggesting the evidence doesn’t support that view.

The loss of activity story is one ProCam’s Nick Myers has heard from some agronomists within his firm.

“There is a feeling it is not as consistent or reliable as it was a couple of years ago.”

Frontier technical manager Bob Mills has had similar comments.

“Generally it appears to be fine, but there is a perception there is a slight change in sensitivity.”

That perception started a couple of seasons ago when growers were using Lexus + Stomp, but has spread also to the pre-emergence herbicide Ice, he says.

“Agronomists in Lincolnshire are suggesting it hasn’t been giving them what they expected.

“It is all anecdotal rather than scientific.

The feeling is maybe more enhanced metabolism resistance is creeping into pendimethalin, but it is perception rather than absolute fact.

“However, the agronomists would say it is fact that it [pendimethalin] is not working so well.”

BASF does not deny pendimethalin is affected by enhanced metabolism resistance.

“But it affects virtually all herbicides, not just pendimethalin,” the firm’s Will Reyer says.

The first cases in pendimethalin were found in the 1980s, he says.

“But there seems little movement in resistance since then.”

Mr Reyer cites the conclusions of a survey by WRAG’s Stephen Moss of the results of resistance testing carried out between 2000 and 2005.

“He stated there had been no major shift in pendimethalin resistance.”

Its multi-site activity should help prevent resistance building, he says, while using it on small plants that shouldn’t have the stature or reserves to detoxify herbicides should mean very little selection pressure is exerted.

The perception pendimethalin use has increased dramatically over the past few seasons is also a myth, Mr Reyer suggests.

“While the area of blackgrass treated per se is increasing – it reached more than 900,000ha last season – the proportion receiving a pendimethalin treatment is actually falling.

Two years ago 34% was treated at least once, last year it fell to 26%.”

And very little was treated twice in the same season, he adds.

“It is not being over-used.”

Even if its use was increasing ,preliminary analysis of data emerging from a current WRAG project examining resistance mechanisms suggests continual use of pendimethalin doesn’t drive the build-up of resistance, according to ADAS weed expert James Clarke.

“There is no evidence that pendimethalin activity is declining with repeated use, although there is evidence its activity is reduced by the development of enhanced metabolism resistance.”

Fortunately, severe cases of enhanced metabolic resistance seem to be few and far between, he says.

And even where it is present pendimethalin is still effective, BASF regional agronomist Clare Tucker says.

“At the Peldon site, which is triple ‘R’, Crystal last year gave 84% control.”

And at another triple “R” site for enhanced metabolism resistance in Lincolnshire full rate pre-emergence Crystal gave 77% control while 4 litres/ha of Stomp gave 42%, she points out.

To get the best out of pendimethalin use it pre-emergence, she advises.

It is a message Agrovista’s Mark Hemmant endorses.

“The place to use is as early as possible – for us that’s pre-emergence on blackgrass.”


BOXTEXT: If the perceived poorer control from pendimethalin is not down to increasing enhanced metabolism resistance a second possibility could be increased microbial breakdown of the herbicide in the soil, ProCam’s Nick Myers suggests.
“There is a school of thought it could be enhanced biological breakdown.”
A similar theory has been applied to IPU, which after several years’ break from farms, appears to be working better than expected in some situations, he says. “It could be that is what is happening to pendimethalin, although I don’t have any scientific evidence.”