6 September 1996

IPU reduction prompts weed resistance worries

Following last weeks views on isoproturon (IPU) herbicide by members of the IPU UK Task Force (Arable, p55), we relay those of Cyanamid

A MARKED reduction in the top application rate for isoproturon (ipu), which could be imposed in autumn 1997, might well accelerate the build-up of herbicide-resistant blackgrass, says the firm.

Company staff reason that the lower rate will encourage more autumn use of fop and dim herbicides to which the weed can develop resistance with worrying ease. But they stress the proposed maximum ipu dose of 1500g/ha of active ingredient (equivalent to 3litres/ha of product) need not lead to more widespread resistance or a worsening blackgrass problem.

Indeed, technical services manager Peter Tayler, claims blackgrass control can be improved and the likelihood of resistance avoided or significantly cut by tank mixing 1500g/ha of IPU with suitable herbicides from a different chemical group. This has been clearly demonstrated in three years trials at the firms development sites near Basingstoke, Cambridge and Thirsk, he claims.

"A significant reduction in the top IPU rate could be a bad blow because this herbicide forms the basis for grass weed control on some 2.5m ha of cereals each year," says Mr Tayler. It also has the advantages over fops and dims of residual activity and of controlling annual meadow grass and key broad-leaved weeds.

"Our trials have shown that the level of blackgrass control is markedly reduced when 1500g/ha of IPU is used." But adding 2.5litres/ha of Stomp (pendimethalin) boosts control to beyond that from the current full rate (2500g/ha) of IPU and offers the bonuses of longer activity and a much wider weed control spectrum, he says.

The trials also show that the same IPU/pendimethalin tank-mix gives better blackgrass control than 3litres/ha of IPU plus 2litres/ha of trifluralin (as in Treflan) and much the same result as the more expensive sequence of 22.5kg/ha of triallate (Avadex) granules followed by IPU/trifluralin.

Herbicide product manager Dick Holden points out that the resistance is increasing. Worldwide the number of resistant species has tripled since 1982 to 113, and the area affected has risen 10-fold. Eighty species are resistant to more than one herbicide, he adds.

In the UK the main threat is resistant grass weeds, says Mr Holden. "But it has been estimated that the incidence of affected fields could be up to eight times the reported number.

"Many farmers assume that agrochemical manufacturers will develop an endless stream of herbicides to overcome the problem, but they are wrong," he says. "Over 70% of the herbicides used for grass weed control have only two modes of action and that tends to encourage the resistance problem."

There may not be any new herbicides to save the day, so it is essential to exploit existing ones intelligently, says Mr Holden. To assist in this Cyanamid has developed a weed management strategy and set up an allied Weed Management Club to explain resistance and help farmers to prevent or successfully live with it. The rapid increase in membership to about 1500 is seen as a clear indication of growers concern.

But like preventable human health problems, resistance is something many growers tend to disregard until it has happened, according to the firms weed management specialist Dr Antony Goulds.

"The trouble is that once youve got herbicide-resistance you have to sharpen up your management and spend more on control to allow you to live with it."

In parts of the US and Canada it is seriously cutting farm incomes, he maintains. Despite that a recent Cyanamid survey suggests UK growers still do not take the problem seriously enough. Although 33% of nearly 1000 respondents had "experienced weed resistance" only 35% of that proportion had had a test done to confirm or disprove it.n


&#8226 Reduced IPU dose need not lead to resistance

&#8226 Suitable tank mixes enhance IPU performance

&#8226 Limited chemical alternatives

&#8226 Resistance threat must be taken seriously.

Below: Blackgrass resistant to fop, dim, and isoproturon herbicides in a Cyanamid mixed chemistry trial at Abingdon, Oxon. Tests for resistance should be more widely used, staff suggest.

Growers must tighten up blackgrass management to keep crops this clean, warns Dr Antony Goulds.