Pesticide sales globally topped $40.5b (£25.18b) in 2008, and of those sales, 47% were generated from herbicides. Here in Great Britain the 2010 DEFRA pesticide usage survey revealed that although fungicides dominate the crop protection market, herbicides assume a 32% market share.
This highlights the importance both globally and domestically of herbicide products for maintaining crop yields, so it is quite alarming that herbicide resistance to certain herbicidal modes of action is now becoming so widespread.
So what is herbicide resistance? It can be defined as: "the inherited ability of a weed to survive a rate of herbicide that would normally kill it".
It occurs in two ways. Enhanced metabolism resistance (EMR), the most common form in the UK, is when the weed develops the ability to detoxify itself of the herbicide.
Target site resistance (TSR) is when a mutation blocks the site specifically targeted by the herbicide's mode of action and results in complete resistance at that particular site, but the weed remains susceptible to other modes of action.
Herbicide resistance now occurs so widely in the UK it is impractical to record individual cases.
Both grassweeds, and to a lesser extent broad-leaved weeds, are becoming increasingly difficult to control with the available chemistry where resistance is found.
The highest risk modes of action include ALS (acetolactase-synthase) inhibitors and ACCase (acetyl Co-A carboxylase) inhibitors, with weed populations having developed both target site and enhanced metabolism resistance to both sets of chemistry.
Worryingly, ALS inhibitors (such as Atlantis) account for 32% of grassweed herbicide use in the UK and ACCase inhibitors such as the "fops", "dims" and "dens" 18%, which has led to the vast rise in the biggest weed threat facing growers - resistant blackgrass.
"Of the 20,000 farms known to use herbicides to control blackgrass, we have estimated that at least 80% of those will have some level of resistance to at least one herbicide," says Richard Hull of Rothamsted Research.
"Perhaps the second most common herbicide-resistant weed in the UK is Italian ryegrass, but there has never been any evidence of ALS target site resistance (TSR) in that.
Most populations will have some enhanced metabolism resistance (EMR) though, and ryegrass populations do show high levels of TSR to ACCase chemistry," he explains.
Complaints about problem wild oat populations have largely died out, continues Mr Hull, as the Atlantis and Broadway products have been so effective.
"They are also a self-pollinating species, which doesn't allow the spread of the resistance genetics.
"The out-crossing capabilities of blackgrass is one of the drivers behind the rapid spread and escalation of resistance across the UK," he adds.
Broad-leaved weeds are less of an issue, but there is ALS TSR in chickweed, predominantly in Scotland where there is a trend towards spring cropping and subsequent reliance on sulfonylurea herbicides. "It seems to be TSR or nothing in broad-leaved weeds from what we can see at the moment," says Mr Hull.
"There are no shades of grey like we see in grassweed populations.
"ALS resistance in poppy populations tends to occur down the east coast of the UK, (see map, above) which at present we don't have an explanation for and from sampling we have only found four or five populations of ALS-resistant mayweed," he notes.
When Bayer CropScience introduced ALS inhibitor Atlantis to the UK market in 2003, the product was so effective at controlling blackgrass in winter wheat that its use was soon almost universal across the wheat producing area.
It is not surprising then that a high-resistance risk product such as Atlantis has taken less than a decade to lose efficacy in some blackgrass populations.
"There is no doubt that there has been some irresponsible use of the product, which has made the issue of resistance inevitable," says Chris Cooksley, combinable herbicides campaign manager at Bayer CropScience.
"However, Atlantis resistance is not endemic and it is still effective in many areas.
"We would hope that stewardship advice to slow the build up of resistance will have been taken on board," adds Mr Cooksley.
The difficulty that growers face where resistance to Atlantis has been found is the lack of alternatives.
"Cross-resistance has developed, meaning that the resistant populations are likely to be resistant to chemistry from the same group," continues Mr Cooksley.
"Broadway products or Unite will very often be as ineffective as Atlantis where ALS TSR is present, as they are also ALS inhibitors, so they are certainly not a solution. We need to look for different modes of action if we are to sustain post-emergence blackgrass control," he says.
This is an opinion echoed by Dow AgroSciences' principal biologist for cereal herbicides and insecticides, Dilwyn Harris, who points out a new herbicidal mode of action with activity on blackgrass is the Holy Grail. "Where Atlantis has broken down to TSR, pyroxulam products will not be effective either, unfortunately, so there is no alternative.
"We are actively commited to research and development to find a new mode of action, but until then, where resistance is present in blackgrass, we will have to use all the tools available," he adds.
Aramo - tepraloxydim
Atlantis - iodosulfuron + mesosulfuro
Avadex - tri-allate
Broadway Star - florasulam + pyroxsulam
Broadway Sunrise - pendimethalin + pyroxsulam
Crawler - carbetamide
Hurricane - diflufenican
Unite - flupyrsulfuron + pyroxsulam
"Using sequences of residuals is the most important chemical aspect, as this ensures that there is as little blackgrass as possible for the post-emergence herbicides to take out.
"Where the post-emergence ALS inhibitors are used, apply them early on small plants to give them the best possible chance of working," says Mr Harris.
Risk with residuals?
With limited and increasingly less effective post-emergence products, there is an increasing reliance on the "stacking" of pre- or peri-emergence residuals for weed control, in particular blackgrass. It asks: are they also at risk of resistance build-up?
"Resistance is increasing in post-emergence herbicides, so we are stacking more and more residual herbicides as our main method of blackgrass control. For example, in 2010 flufenacet applications in wheat had over taken Atlantis, demonstrating our increased reliance on this chemistry," says Mr Hull.
There are levels of EMR resistance to all pre-emergence herbicides, including tri-allate and prosulfocarb, but of concern to growers and advisors will be the news that Mr Hull has been able to select for resistance to flufenacet and pendimethalin under controlled conditions at Rothamsted Research, showing a decline in efficacy of around 5% per year due to the build up of EMR.
"That is the worst news, but comparisons with what is actually happening out in the field are not as pessimistic," explains Mr Hull.
He points out that although it is very hard to evaluate the performance of pre-emergence herbicides in the field due to variations in conditions and masking from other actives used in control programmes, data released from chemical manufacturers back up his own research.
"It is telling us that these actives are only losing efficacy at a rate of less than 2% per year. However, this average decline may not solely be attributed to an increase in resistance, as there were fluctuations in control linked to autumn rainfall. The increasing reliance on pre-emergence sprays is sustainable, it seems, in the short to medium-term, but probably not in the long term," says Mr Hull.
The less specific mode of action of flufenacet makes it lower risk for resistance and consequently should be the core of most pre-emergence treatments, but Mr Cooksley reminds us that we will be in much trouble without it. "Where would we go?" he asks.
"There is certainly a decline in efficacy, but we would hope that it bottoms out eventually and will always give a reasonable level of control. Stacking will help maintain high levels of control for longer and growers should make application an art form," says Mr Cooksley.
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Broad-leaved weeds are much less of an issue when considering herbicide resistant populations, but in Scotland and Northern Ireland where there is a higher proportion of spring crops - in particular spring barley - ALS target site resistance has developed in populations of chickweed.
Article sourced from Farmers Weekly