21 February 1997

Knowing the enemy can keep blackgrass licked

Herbicide resistant blackgrass is becoming more of a problem. What can growers do to slow down its spread? Andrew Blake picks up some expert tips

WATCH out for herbicide resistant blackgrass. That is the warning from a leading researcher as concern over the spread of the weed grows.

Latest available figures (Arable, Jan 17) show that by 1995 it was present on more than 700 farms in 30 counties. "I realise that is only about 3.7% of the estimated 20,000 cereal farms with a blackgrass problem," says Dr Stephen Moss of IACR-Rothamsted. "But less than 10% of farms have had resistance tests done, so the true figure could be much higher."

Ignoring the possibility of resistance is shortsighted, he suggests. "You need to know your enemy in order to adopt appropriate control tactics. Trying to get rid of resistant blackgrass with inappropriate products can soon make matters much worse. All you do is select out resistant plants which seed and run riot in following crops."

The only sure way to identify the two types of resistance encountered so far in the UK is through a laboratory test. Some free testing has been provided through chemical firms, concedes Dr Moss. "But there is still a lot to be gained by having independent tests done, especially when it comes to interpreting the results.

"A resistance test is useful – even if it is done on only one field. It could set the alarm bells ringing to encourage a change of strategy. Without a test, growers are groping in the dark. People are in a much better position if they have had one done."

Cheaper test on way

However the apparently high cost (£60-100/field sample) seems a deterrent, he concedes. A new quicker and hopefully cheaper version is being developed at Rothamsted but is unlikely to be on stream this year.

Given that barrier, how else can growers protect themselves?

"It is rarely possible to confirm resistance solely on the basis of field observation," says Dr Moss. "But there are several good pointers to its presence.

"These include the level of weed control of other susceptible species. If these have been controlled, then resistance is a distinct possibility."

Live plants alongside dead ones are another sign of trouble. "But differences in weed growth stage, incorrect spray application and crop shielding can also be responsible," he notes.

Gradual decline in control over several years from the same formerly successful treatment also suggests resistance.

Herbicide records can be a giveaway, he adds. "Many farm records are poor or non-existent. But dont be too ready to discard old records. They can be useful in establishing the risk of resistance. "Repeated use of the same herbicide, or those with the same mode of action, favours selection for resistance."

Known problems nearby should serve as a warning. "If resistance involving the same herbicide has been positively identified in adjacent fields or farms, then it is highly probable that resistance is implicated where you have failed to get control."

Cultural history

Cropping and cultural history may also be valuable. Continuous or near-continuous cereals and minimum cultivation often go hand in hand with cases of resistance, he says.

Spotting enhanced metabolism resistance, with its characteristic partial control is well nigh impossible without a lab test. But for target site resistance there are some valuable in-field clues.

Picking up this type of resistance early is particularly important, stresses Dr Moss. "That is because it develops so much faster." Failure to change tactics once it is discovered may lead to complete loss of control in as little as two to three years, he explains.

"In non-cereal break crops, look out for blackgrass surviving fop or dim graminicides completely unscathed when other grass weed plants and volunteers are effectively killed.

"Watch out for small, dense patches of blackgrass where they havent been seen before." Often no bigger than 0.01ha, they arise from seed shed from a just few resistant individuals, he explains. Ignored they can soon spread resistance throughout the farm. "Such satellite patches are something growers need to be very aware of."n

Resistance tell-tale? A lone vigorous blackgrass survivor in cycloxydim-treated peas may be a sign of trouble.

"Trying to get rid of resistant blackgrass with inappropriate products can soon make matters much worse," says Stephen Moss

of IACR-Rothamstead.


IDENTIFYING RESISTANCE


&#8226 Lab tests for confirmation.

&#8226 Tests seen as expensive.

&#8226 Field pointers

– gradual loss of control

– individual unaffected plants

– satellite patches.

&#8226 Herbicide history.

&#8226 Cropping & cultivation background.

&#8226 Locally confirmed cases.