More than 50m hectares of some of the best land in the world is infested with glyphosate-resistant weeds, potentially rendering this valuable herbicide obsolete.

And according to agronomists and researchers, without changes in farming practice, it is set to get worse, with one predicting that all US arable crops will be affected by 2050.

All five of the big global exporting countries – US, Brazil, Argentina, Canada and Australia – have herbicide resistance problems, says Steve Powles, director of the Australian Herbicide Resistance Initiative in Perth.

However, these exporting countries have a crucial role in feeding an increasing population, which is predicted to grow by more than 30% by 2015.

Of these, he highlights the USA, Brazil and Argentina, being big adopters of GM glyphosate-resistant technology.

The US and Brazil already have herbicide-resistant weed problems and he believes that Argentina is at real risk of following suit.

See also: Call to ban glyphosate on milling wheat

Affected area

The US has 100m hectares of field crops and 70% of this is cropped with glyphosate-resistant GM crops.

“The latest survey suggests 34m hectares have some weeds that are resistant to glyphosate,” says Prof Powles.

The cost to farmers is huge.

Glyphosate-resistant weeds have tripled herbicide spend for US corn and soya bean growers, with the average cost for soya beans increasing from $75/ha (£49) before the problem to $250/ha (£165).

In Brazil, there are 60m hectares of cropping and 30% of this area is affected by glyphosate resistant weeds while in Argentina, there is no survey data.

Prof Powles estimates that more than 50m hectares of the world’s best crop land is already infested with glyphosate-resistant weeds in the three countries and is, therefore, a threat to global food production.

“Glyphosate is a one in 100 year discovery and it is lamentable that it is being driven to redudancy by a system of farming.”

What weed species?

In Brazil, weed researcher Fernando Adegas says 34 weeds are listed as resistant and this year, there has been the first report of glyphosate-resistant palmer amaranth (palmer’s pigweed).

This weed can grow up to 5cm a day, reaching 4m in a season and a single plant can produce 1.8m seeds in one season.

Two other key weeds are sourgrass and conyza (horseweed) and having both on farm will result in a 400% increase in herbicide costs.

“We are now seeing the problem of Roundup (glyphosate) resistant corn volunteers in Roundup Ready soya and resistant soya volunteers in Roundup Ready soya,” says Dr Adegas.

In the US, palmer amaranth is now a US national issue and 29 states are now infested with this weed, says Jason Norsworthy, professor of weed science at the University of Arkansas. Resistant waterhemp is now found in 16 states.

Cause

Prof Norsworthy explains that herbicide resistance is not a new occurrence.

“Resistance is not unique to glyphosate, nor is it relatively new. For example 2,4-D resistance developed in wild carrots in Canada back in the 1950s and atrazine resistance in common groundsel in 1968 in the US.”

However, he says it’s the heavy use of glyphosate that is resulting in the problem. Roundup Ready is a simple and effective system, accounting for about 89% of the US corn area and 94% of soya.

“Soya is cropped mainly with corn, therefore, rotating corn with soya sees glyphosate on top of glyphosate on top of glyphosate,” says Prof Norsworthy.

GM cotton, which is grown on 35m hectares, has an extended growing season and requires repeated applications.

“It is often grown continuously and again, you see glyphosate on top of glyphosate.”

Prof Powles believes change is needed to halt the spread and that it is an entirely manageable problem.

It is not a transgenic issue, but the over-reliance on a system and overuse of one herbicide. “I have been on farms where they only use one herbicide across the whole farm.”

He believes diversity is the key, in terms of actives used and cropping along with non-chemical measures.

“Some weeds have multiple resistance, suggesting herbicides alone are not sustainable.”

On Prof Powles’ farm in Western Australia, glyphosate still works and he is confident it will keep working for the next generation, by applying a diversity of herbicide actives and combining this with non-herbicide approaches.

He admits it is not easy and his measures include harvesting weed seed with a chaff cart behind the combine.

Future

The rate of spread of glyphosate resistant weeds in the US shows no signs of slowing, says Prof Norsworthy.

“If farmers continue to rely on one technology and one herbicide, we will lose this herbicide,” he warns.

He predicts that by 2020, all US row crops will be infested by glyphosate resistance (see graph).

In Canada, winter wild oats (Avena fatua) are the most abundant grassweed and Linda Hall, professor at the University of Alberta fears glyphosate resistance could become a problem.

The latest survey in 2011 indicated that 44% of the population has some form of herbicide resistance and while none were found to have glyphosate resistance, she predicts it will be one of the next weeds to become resistant to it in Canada.

She points to the increased reliance on glyphosate preseeding in Canada and widespread adoption of glyphosate/glufosinate-resistant oilseed rape, which now accounts for 7.6m hectares.

A greater concern for Prof Norsworthy is multiple resistance, with stacking of resistance to different modes of action.

He says some populations of palmer amaranth are resistant to three modes of action while some waterhemp plants have been found to have resistance to five modes of action.

USA

Mississippi arable farmer Bowen Flowers recalls that it started with having to disc up 40ha of cotton, as he could not eradicate the weeds.

“It happened in just two years and at first we didn’t think it was bad and all of a sudden, we saw the whole field affected.”

The key weed is the difficult to control amaranthus palmeri (pigweed), which can grow 2in in a day.

“We had to go back to using older actives and residuals. It means we are having to spend more, with the herbicide cost increasing from $35/acre [£23/acre] to more than $100 [£65/acre],” he says.

It also means herbicide timing is more critical, with moisture required for residuals to work.

Argentina

Weed resistance in Argentina means it is no longer viable to grow soyabean in some areas of the country.

Back in 1996, Roundup Ready GM soya changed the way weeds were managed, says grower and agronomist Santiago Del Solar.

But this heavy reliance on glyphosate has resulted in resistant weeds, the main one being conyza, which has added another layer of costs on his 11,000ha farm north west of Buenos Aires.

It has forced him to go back to older herbicides to regain control.

The result is the soya area is shrinking, as it is not profitable with the extra cost of weed control.

Guidelines to keep UK resistance free

New guidelines for minimising the risk of glyphosate resistance development were published by the Weed Resistance Action Group (Wrag) earlier this year.

Roundup manufacturer Monsanto believes these should form the basis for all glyphosate use from now on, to safeguard the vital weapon for grassweed control.

Despite being commercially available for over 40 years and used widely in almost every sector, no resistance to glyphosate has so far become apparent in the UK, stressed Monsanto’s Barrie Hunt.

“Yet resistance is known to have developed in other parts of the world. This and the pivotal value of glyphosate in the face of growing resistance to almost every other piece of herbicide chemistry we have today makes it imperative to do everything we can to sustain this position.”

The Wrag guidelines focus on four key areas:

1. Prevent survivors Repeat applications to surviving plants pose the highest risk
2. Maximise efficacy Use the right dose for size of weed and on actively growing plants
3. Use alternatives Maximise cultivation or other non-chemical control and use other herbicides in sequence
4. Monitor success Work with agronomists and suppliers, remove survivors and test seed samples of survivors.

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This article is based on papers given by global weed experts at the Bayer CropScience Weed Resistance Global Symposium held earlier this month in Paris.