The most damaging weed in the UK is about to become resistant to the main defence farmers have against it – the weed-killer glyphosate. And the situation is similar in many other countries around the world, with having evolved resistance to at least one herbicide.
Many weeds have evolved resistance to several different kinds of herbicides, and some are set to become resistant to all the herbicides used on particular crops. That is bad news for farmers, consumers and wildlife. These superweeds will cause massive crop losses and push up food prices. They will also speed up climate change and harm wildlife as even more land is converted to farmland to make up for the lost crops.
“It is not a matter of if but when we are going to be losing chemical control of these weeds,” says Adam Davis of the University of Illinois at Urbana-Champaign.
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Massive losses
In Europe alone, will cause yields of wheat, barley and oilseed rape to fall around 10 per cent, causing losses of around €2 billion for farmers and pushing up food prices.
An additional 2 million hectares of farmland would be needed to compensate, resulting in higher greenhouse gas emissions and a loss of wildlife habitat. The loss of habitat is the single greatest threat to wildlife and biodiversity.
The latest threat is a weed called blackgrass. “It can totally infest a field to the extent you can barely see the crop,” says David Comont of Rothamsted Research in the UK. By outcompeting crop plants, it causes yields to plummet.
Strains of blackgrass (Alopecurus myosuroides) have already evolved resistance to many herbicides, and some are resistant to several at once. The herbicide glyphosate is often the last line of defence. And now Comont’s tests on blackgrass collected from more than 100 fields in the UK show that it is evolving resistance to glyphosate too.
In the US, several weeds are on the brink of becoming resistant to all herbicides that can be used on particular crops. There are eight classes of herbicides that are used by soybean growers, says Davis.
Some strains of a weed called tall waterhemp (Amaranthus tuberculatus) are now resistant to five of the eight and could interbreed with nearby strains resistant to the other three. In Kansas, a strain of another weed called Palmer’s amaranth (Amaranthus palmeri) is resistant to six general types of herbicide.
“This is a very large problem,” says Davis. “Many different weed species in many different parts of the world and many different cropping systems are developing resistance.”
Both Davis and Comont compare the situation to the rise of antibiotic-resistant superbugs. The cause is the same – the overuse and misuse of antibiotics and herbicides leads to rapid evolution.
But evolution can be prevented. The key is to combine lots of different methods – integrated weed management – so weeds never have a chance to evolve resistance to any one method. That might mean rotating crops, growing crops at a different time of year, combining herbicides with a different mode of action and so on.
Davis compares it to baseball – Ěýif a pitcher throws the ball exactly the same way each time, it’s easy for the batter to hit the ball. If the pitcher varies the throw, it’s much harder.
The trouble is that farmers who do the right thing . “There are no economic signals that are providing a disincentive for this overuse,” says Davis.
It is possible to control weeds without herbicides, as organic farmers do but it is more difficult and more expensive, and yields are usually lower. For instance, hand weeding is effective, but very costly because it is so labour intensive. Various mechanised weeders are available, but none are perfect.
One device that is catching on is the , developed in Australia where resistance has long been a major problem. This is towed behind combine harvesters to grind up weed seeds before they fall to the soil. But weeds can even evolve resistance to this, by dropping their seeds earlier.
Last month at the launch of a major biodiversity report saying that a million species could go extinct, chair Bob Watson said part of the answer is “” of farming. The idea is to maximise yields from farms in the least damaging ways so that as much land as possible can be left for wildlife. This will be harder if herbicides cannot be used occasionally.
New Phytologist
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