The War Of The Weeds

When I was in high school, one of the scariest books I read was The Day Of The Triffids. A science fiction novel, the book described the fall of civilization as semi-mobile, carnivorous plants called Triffids relentlessly started to hunt and consume people, slowly taking over the planet in the process. What bothered me most about the book was its cover image — a lone human hand sticking out from underneath a pile of feasting Triffids.

Of course, now that I’m older, I realize the idea of plants somehow overrunning the world in apocalyptic fashion is purely fiction. Or is it?

Since the 2000s began, researchers and retailers have been noting the alarming increase in herbicide-resistant biotypes. Although weeds showing triazine and acetolactate synthase (ALS) resistance have been known for some time, biotypes with immunity to protoporphyrinogen oxidase-inhibiting (PPO) herbicides are new. Even more troubling, the world’s most popular herbicide — glyphosate — seems to have spawned a quickly growing hoard of weeds that aren’t controlled by it. These include marestail (horseweed), lambsquarters, and common and giant ragweed.

“No herbicide stays effective forever, and the honeymoon is definitely over for glyphosate and PPO products,” said Jeff Stachler, an Extension associate for The Ohio State University, at a recent industry field day. “Truthfully, 1999 and 2000 were probably the best times for weed control that we will ever see in our lifetimes.”

To back up this claim, Stachler pointed to some of his observations in his native Ohio crop fields. On his travels, Stachler has noted that 30% to 40% of the soybean fields in southern Ohio have giant ragweed present following a herbicide application. More than half of these same fields have marestail present post-application. “In most cases, the giant ragweed I’ve studied has remained viable after having 22 ounces of WeatherMax sprayed on it,” he said. “I’ve also seen this same survival rate with lambsquarters in my lab work.” Stachler added two other biotypes that bear watching are sweet clover (showing resistance to glyphosate) and kochia/Mexican fireweed (showing reported dicamba resistance).

Custom applicators attending Stach­ler’s talk agreed with his observations. Some reported using less than 1 gallon of glyphosate per acre back in the early 2000s. Today, control in those same fields requires 1.1 to 1.5 gallons to work. Perhaps more telling, 75% of CropLife 100 retailers report their growers have adopted some form of weed management in the past year.

To do our part, CropLife earlier this year run two inserts looking at Total Weed Management. Part III will appear in our November 2006 issue.

When it comes to curbing resistance, Stachler said that custom applicators and growers need to fundamentally change the way they control weeds. “If you use herbicides only, then you must achieve 100% weed control or future problems will occur,” he said. “If the weeds survive the herbicide, then you must remove them with your hand.”

This is great advice. While I don’t see the day when giant ragweed starts hunting people on the horizon, the vision of crop fields completely covered with resistant weeds is clearer unless things change. Based upon the evidence, that’s science fact, not fiction.

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