Palmer amaranth, which has been dubbed a “super weed” by some, is definitely living up to its reputation. The weed already has documented resistance to six different modes of action: glyphosate, ALS-inhibitors, HPPD-inhibitors, PPOs, microtubule inhibitors like Treflan, and photosystem II inhibitors like atrazine and metribuzin. At least a few biotypes have resistance to multiple modes of action at once.
Scientists are still unsure of the mechanism behind the resistance in some cases. Work at the University of Illinois last year found Palmer amaranth resistant to PPOs also had a genetic mutuation that allowed it to withstand glyphosate. With each successive generation, the mutation frequency was higher, meaning it took higher rates of PPO herbicides to control it.
In other cases, they haven’t pinpointed the exact change within the weed that allows it to withstand herbicide applications. It also appears that some Palmer amaranth populations resistant to glyphosate have more tolerance to other herbicides.
Whether this is true resistance or the herbicide was never really effective on Palmer remains unanswered.
But you don’t have to be a scientist to realize that Palmer pigweed resistance to herbicides is a growing problem and one that shouldn’t be taken lightly.
Extension weed specialists such as Bob Scott with the University of Arkansas continue to preach overlapping residuals and rotating effective modes of action. That’s the operative word: effective modes. You’d be just wasting money if you rotated ALS-inhibitors and glyphosate in a field with multiple resistance to those herbicides.
Even with the most diligent integrated weed program that includes mechanical control and crop rotation, the job won’t be easy. A single female Palmer pigweed plant under optimum conditions can produce up to 460,000 seeds annually, according to University of Georgia research. Preventing the plants from going to seed in the first place should be job one.
The glyphosate-resistance trait also can be carried on pollen at least 1,000 feet to fertilize susceptible female plants. As a result, a portion of the offspring will be glyphosate resistant.
In addition, University of Missouri research has found that geese and ducks can spread resistance by feeding on glyposate-resistant amaranth seed, traveling to another location and passing it out the other end.
Growing up, we liked to joke that should a nuclear explosion go off, two things will survive: cockroaches and my mom’s fruitcake. Based on the research of weed scientists, I think Palmer pigweed should be added to that list.