2 February 1996

Atrazine resistance brings big headaches

Herbicide resistance to atrazine – the standard chemical for weed control in forage maize – and upper limits on dose rates could force growers to use other herbicides to control grass and broad-leaved weeds. Jonathan Riley reports

WHERE maize has been grown for a number of years, resistance to atrazine in the weed population builds up, says maize Growers Association agronomist Simon Draper. As a result growers may see an upsurge of weeds, particularly black nightshade and orache.

"Also because of the levels of atrazine found in watercourses, the maximum recommended rate in one season is 3 litre/ha," he says.

"At these levels there is a requirement for a follow up post-emergence spray and weed control programmes should be balanced from pre-cultivation to post-emergence."

He advises that before cultivating couch grass and other grass weeds should be controlled with a mix of glyphosate and a cationic wetter.

"Given that the total amount of atrazine that can be applied is 3 litre/ha, incorporate atrazine into the seedbed at 2 litre/ha at cultivation and apply the remaining litre post-emergence.

"If conditions are certain to be dry, atrazine will give better control when incorporated after drilling. Cyanazine, a chemical similar to atrazine, is more effective with most weed species, except fat hen, under very dry conditions but has to be applied at 5.2 litre/ha, so increasing costs," says Mr Draper.

"Pendimethalin, which can be applied either pre- or post-emergence, is particularly effective at controlling black nightshade after drilling. But only when there is plenty of soil moisture and ideally wet weather is needed for the three weeks after spraying."

Post-emergence, pendimethalin can be mixed with atrazine to improve the efficacy of atrazine. The mix can be applied up to fourth leaf stage but, as with its use pre-emergence, the chemical needs high levels of soil moisture to be most effective.

Oil, pyridate and bromoxynil, can also be mixed with atrazine for post-emergence weed control.

Oil improves action for contact use of atrazine but fails to improve the action where there are resistant weeds. Also the control of weeds such as black nightshade and orache is still limited.

"If atrazine has not been applied pre-emergence, its use at 3 litre/ha plus 5 litre/ha of oil will control large weeds particularly under dry conditions.

"To improve black nightshade, orache and fat hen control, pyridate can be used. It is most effective when most weeds have germinated and safe up to the first leaf stage of maize. Applied at 2 litre/ha it can be mixed with atrazine at 2 litre/ha," says Mr Draper.

"This cannot be done where 2 litre/ha of atrazine have already been applied pre-emergence and although control of these weeds is improved with atrazine at 1 litre/ha other mixes should be considered," he says.

The contact herbicide bromoxynil mixed with atrazine has proved successful in controlling black nightshade, fat hen, chickweed, polygonums, mayweeds, and limited control of orache in trials in the UK and France.

Mr Draper advises that bromoxynil should only be applied when weeds are visible, ideally at the cotyledon stage, and applications split to coincide with new flushes of weeds.

He recommends a rate of 1.4 litre/ha bromoxynil plus 2 litre/ha of atrazine (unless 2 litre/ha were applied pre-emergence) followed with a further 1 litre/ha of bromoxynil.

However, to avoid scorch, bromoxynil should be applied when maize is not suffering stress. It must not be applied until the crop has reached two leaf stage or if the plant has been growing rapidly.

Spraying in the evenings will minimise the risk of scorch. Particular care must be taken if control is planned as a single application at 2.4 litre/ha.

"Approved last year for use in maize was fluroxypyr which controls black nightshade well and has some effect on volunteer potatoes, docks, black bindweed and nettles," says Mr Draper.

Initial recommendations are for rates of 1 litre/ha between third and sixth leaf stage of the crop. For black nightshade acceptable control was achieved at 0.5 litre/ha.

"But fluroxypyr should not be applied after the buttress roots become visible. Trials last year suggested fluroxypyr caused scorch, particularly if the crop was stressed and some weakening of the stems was reported," says Mr Draper.

"Where there are heavy infestations of thistles clopyralid could be applied at 0.5 litre/ha and repeated with a second application to achieve thistle control."

But the product is expensive, he warns. &#42