There are currently 396 unique cases (species x site of action) of herbicide resistant weeds globally, with 210 species (123 dicots and 87 monocots). Weeds have evolved resistance to 21 of the 25 known herbicide sites of action and to 148 different herbicides. Herbicide resistant weeds have been reported in 63 crops in 61 countries.This is, of course, a direct result of genetically modified herbicide-resistant corn, soy, canola and other crops that were built to be resistant to weed killer so that farmers could spray their fields until the cows came home in order to keep down weeds. The best known such product is Monsanto's herbicide RoundUp, and the RoundUp Ready crops it also engineered.
Herbicide resistant crops are perhaps an appealing idea -- in year One. Anyone who knows anything about evolution knew that weeds would likely develop resistance, even as Monsanto assured farmers this wouldn't happen.
Farm Industry News writes:
The area of U.S. cropland infested with glyphosate-resistant weeds has expanded to 61.2 million acres in 2012, according to a survey conducted by Stratus Agri-Marketing. Nearly half of all U.S. farmers interviewed reported that glyphosate-resistant weeds were present on their farm in 2012, up from 34% of farmers in 2011. The survey also indicates that the rate at which glyphosate-resistant weeds are spreading is gaining momentum; increasing 25% in 2011 and 51% in 2012.The increase is fastest in the Midwest, though every state is seeing it. And, the number of species that are resistant is also increasing on each farm. In southern states, the proportion of resistant weeds is highest -- 92% of farmers reported resistant weeds in Georgia, for example.
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| Source: Stratus agri-marketing inc. |
In Mississippi, the first weed to become resistant was horseweed, or marestail, in 2004. Italian ryegrass, waterhemp, Palmer amaranth, Johnsongrass, giant ragweed, goosegrass and spiny amaranth have followed, according to Corn and Soybean Digest.
It is, of course, costly to spray for weeds; sometimes more than $100 per acre, and according to the Digest, farmers may have to apply herbicides 6 or more times a year. Thousand-acre farms are not uncommon in the Midwest, and resistant weeds have already put a lot of farmers out of business. And it's not just the cost -- sometimes the problem of resistance is so rampant on a farm that there's nothing that can be done to control the weeds.
Monsanto's RoundUp, of which glyphosate is the active ingredient, was seen by many as almost a miracle when it was first introduced. It was cheap and killed a wide spectrum of weeds, with no residual effect the following year. As a result, many farmers used it liberally, and happily. One such farmer was a cropping farmer in New Zealand where weeds have only now started to be resistant, although resistance is widespread in neighboring Australia. So much for miracles!
New Zealand is now in a position to prevent the spread of resistance -- one way is to use other herbicides on resistant plants. The problem is that these are not as environmentally friendly (and how friendly glyphosate is open to debate; we've blogged about this before, and the issues haven't changed), or as inexpensive to use.
But, as a group of researchers write in Weed Science:
It is clear to most weed scientists who are involved in herbicide research, and even those who are not, that the best way to reduce selection pressure for herbicide resistance is to minimize herbicide use. However, the “solutions” that have emerged in most recent meetings on herbicide resistance have usually involved more herbicide use—herbicide rotation, tank-mixtures, PRE- followed by POST-herbicides, “right-rates,” etc. To an unbiased observer, it would appear that many weed emperors are wearing no clothes.As is being found, to the surprise of no one who understands evolution, the use of different, and more toxic herbicides is creating races of weeds resistant to those chemicals, and it will always be thus. This is quite comparable to the problem of over-use of antibiotics, and even the use of single rather than combined chemotherapetutic agents to treat cancer patients (that leads to the evolution of resistant cells within the tumor).
And, the weed researchers continue:
Why are so many weed scientists and extension personnel recommending more herbicides to mitigate herbicide resistance problems? One speaker at a 2011 WSSA weed resistance meeting noted that because of his funding sources, it was difficult to talk about real solutions. At the same meeting, another expert suggested that the solution to herbicide resistance “all sits on herbicide diversity.” At a 2010 meeting of Pan-American and European weed scientists, the near-consensus solution for glyphosate resistance presented by speakers was to use glufosinate in the place of glyphosate. Industry strategy to manage glyphosate-resistant weeds is to develop crops with “stacked” herbicide-resistance traits (Green and Castle 2010; Wright et al. 2010).It's the likes of chemical purveyors such as Monsanto that got farmers into this mess. They aren't going to solve it in any rational way. "Are we as a discipline," these weed scientists ask, "so committed to maintaining profits for the agrochemical industry that we cannot offer up realistic long-term solutions to this pressing problem?"
The real solution, they say, if there is to be one, requires crop rotation with a greater diversity of crops, as that tends to increase the diversity of weeds in a field as well. Yes, farmers will use herbicides on these weeds, but it will be a range of different ones, and resistance will be slower to develop. And, farmers should use herbicides far less frequently than they now do, on schedules recommended by, who else?, the herbicide manufacturers. That would help in the short term.
But, in the longer term, the resistance roller coaster, and dependence on the chemical companies that manufacture both resistant plants and herbicides, needs to slow down.
More research on herbicide alternatives is required. Research on allelochemicals and biofumigants, diverse crop rotations, higher crop seeding rates, intercropping, competitive cultivars and planting patterns, physical weed control, weed seed destruction, and reducing weed seed and vegetative propagule dormancy is crucial for a sustainable future. Combinations of a diversity of tactics in integrated crop management systems augment herbicide-based weed control (Harker et al. 2009) and lengthen the useful life of valuable herbicide tools.Chemical companies that naturally have, or even that created, vested interests shouldn't be allowed the overpowering influence that enables them to continue to call the shots, because of course it's not in their interest to solve the problem except by more chemicals that depend on evolution not happening.
The problem is not with science per se, nor even with industrial-scale agriculture. The problem is with overly simple solutions and, equally perhaps, an over-confidence that industrialized science can itself always evolve a step or two faster than the organisms it's targeting.
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