Re-emergence of Pre-emergents

Posted by .

The 2007 corn and soybean harvest is winding down, which can mean only one thing: the pesticide TV commercial season is upon us. Winter time is when farmers make decisions about what seeds and chemicals they will use come spring, and input suppliers know it. This year, keep an eye out for ads that promise better “weed resistance control” or something of that nature. Weed resistance has become a major issue in the “crop protection” game, and that means we are going to see more toxic chemicals in our fields as farmers struggle to deal with water hemp that shakes off a spray of herbicide like it’s a gentle spring shower. It wasn’t supposed to be this way.

When Monsanto first introduced crops genetically engineered to resist being killed by the herbicide Roundup in the 1990s, it made the argument that this was a boon for the environment. For one thing, Monsanto claimed, this technology would help protect the environment by reducing reliance on herbicides which are much more toxic and stable than glyphosate, the main ingredient in Roundup. Some older style, “pre-emergent” herbicides are applied to the soil before the crop plants emerge. As a result, they have a “residual” quality about them, meaning they can stick around long after they are first applied.

That’s good news for weed control, but can be bad news for the environment: the longer a chemical sticks around, the more chance it has of causing ecological mischief. For example, atrazine, a highly residual herbicide, can be found in water long after it’s applied. Researchers such as Tyrone Hayes have found that low levels of atrazine can cause serious health problems in amphibians. And as Hayes highlighted in LSP’s Ear to the Ground podcast 42, there’s even research tying atrazine exposure to human health problems.

Indeed, glyphosate is a relatively benign herbicide and is volatile, quickly breaking up in the environment after it’s applied. When farmers plant herbicide-resistant crops, they can spray glyphosate later in the season, when the corn or beans are well established and weeds are thriving. Since you are applying the chemical directly to growing weeds, instead of to soil that will eventually produce weeds, in theory you can get away with using much less herbicide per acre.

But glyphosate has proven to be its own worst enemy. First soybean farmers and then corn farmers adopted the “Roundup Ready Package” whole hog, and the USDA estimates that today at least 90 percent of this country’s soybean acres are being raised using a herbicide-resistant system (around 36 percent of U.S. corn is herbicide resistant).

The problem is all that glyphosate is now producing herbicide-resistant weeds, despite assurances by the pesticide industry during the early 1990s that the complex formulation of the chemical would outsmart weeds indefinitely. But the laws of natural selection could have predicted such an occurrence: the more you use a product that kills, whether it be a herbicide, bug killer or antibiotic, the more likely a few weeds, aphids or bacterium will outsmart it and survive. And when they reproduce, there is a good chance at least a few of their offspring will have inherited that ability to fend off the killing effects of a certain chemical.

In the case of crop production, glyphosate-resistant weeds have been popping up in spots around the world, including in the Midwest. In 2007, fields were found in southern Minnesota where glyphosate wasn’t able to control giant ragweed and water hemp at four to eight times the rate recommended on the label, according to the Oct. 12 edition of Farm and Ranch Guide magazine.

There are a couple of strategies for fighting superweeds. These days, the one that’s being promoted heavily by pesticide company experts and university extension educators involves diversifying the herbicidal arsenal and throwing a mix of pesticides at weeds.

It’s a sound strategy: the less uniform the chemical use, the less likely a superweed will adapt and reproduce. The problem is, what’s being recommended is a heavier reliance on pre-emergent herbicides that stick around for a long time. It would be one thing if farmers were being told to replace glyphosate with these more toxic, residual herbicides. Instead, the recommendation is that they use glyphosate and pre-emergent herbicides together as a “chemical package.” So it’s no accident that at a recent field day on glyphosate-resistant weeds at the University of Minnesota Southern Research and Outreach Center, a “biological research and development scientist” for pesticide giant Syngenta was on hand to discuss the issue.

His advice? Buy more pre-emergent herbicides from Syngenta.

Researchers are now studying the “farm-level economic impacts of combining pre-emergent products and glyphosate.” When phrases like that begin to be thrown around, that’s extension economist talk for: “The price of chemical weed control is about to go up.” And that puts a damper on another argument Monsanto made for adopting Roundup Ready technology: it was supposed to save farmers money.

Leave a Reply

  • (will not be published)