Farmers, field agronomists and weed scientists across the United States increasingly are concluding that populations of important weeds have adapted to glyphosate herbicide to the point that its days as a standalone broad-spectrum weed killer are numbered.
As a result, controlling weeds in agricultural crops is becoming more complicated. But growing awareness of the challenge and adoption of more sophisticated control strategies are helping growers tame the new tougher-to-control weed variants.
"Glyphosate will continue to be an important component of weed control programs," says Bill Johnson, a weed scientist at Purdue University. "As far as being the sole herbicide applied on a field, those days are numbered."
The severity of the challenge and the weed species that are no longer adequately controlled by glyphosate vary by geography. But the result is the same.
Take Arkansas, for example. A massive — and largely uncontrolled — outbreak of Palmer pigweed in the state in 2010 has convinced Extension Weed Specialist Bob Scott that virtually all Palmer pigweed (also known as Palmer amaranth) now should be considered glyphosate-resistant.
"The days of using glyphosate alone are over in Arkansas," Scott says. "Glyphosate is still an effective herbicide on many weeds, but not Palmer pigweed. In the future, it will be part of a programmed approach, much like before the days of glyphosate-tolerant crops."
Southwestern Minnesota agronomist Dorian Gatchell has reached the same conclusion. As control of lambsquarters and other broadleaf weeds with glyphosate-only programs worsened in recent years, his customers gradually began including herbicides with alternative modes of action into weed management programs.
"We went from a one-pass program to a two-pass and then to a three pass at high rates," says Gatchell, agronomy manager for Equity Elevator and Trading Co., Wood Lake, Minn. "It still didn’t control the lambsquarters. That should tell you something. Glyphosate alone is gone."
Other weeds to watch in various geographies include giant ragweed and waterhemp, Johnson says. "Issues with giant ragweed and waterhemp are growing very rapidly," he adds.
From difficult to control to resistance
The evolution of weed populations from easy to control to harder — or impossible — to control isn’t a mystery. Weed scientists have documented the process with a range of herbicides. For some weeds, such as ragweed, the difficult-to-control or resistance process begins with a naturally occurring variant in a weed population that is resistant to the herbicide. This one-in-a-million variant (or a billion, or a trillion or less, depending on the weed species and herbicide) then pollinates — potentially passing herbicide resistance to susceptible types. Then it and its cross-fertilized neighbors produce seed. Assuming the same herbicide is applied repeatedly, the population of uncontrolled variants gradually builds until it makes up a large percentage of that weed species in a field.
"Gradually, the weeds get harder to control," Johnson says. "At first you might notice patches where you are not effectively controlling the weeds. And the patches grow. And then (through continued selection for more resistant types), they become resistant."