NEWPORT — University of Arkansas System Division of Agriculture researchers have selected dicamba-resistant pigweed to document how genetic resistance develops and how the industry must work to protect the few remaining weed-fighting options.
Bob Scott, an extension weed scientist, emphasized that the finding was the result of controlled greenhouse studies and not a confirmation of anything found in any field.
“Through experimentation in the greenhouse, we selected a population of pigweed that is tolerant to the herbicide dicamba at a field rate,” Scott said. “This pigweed population was not found to be resistant to dicamba in nature or in any field.”
Scott’s colleagues, Jason Norsworthy, division weed scientist; Parsa Tehranchian, Norsworthy’s post-doctoral associate; and Stephen Powles, professor of plant biology at the University of Western Australia, designed the greenhouse experiment to examine the potential for the future of resistance.
The researchers began with dicamba-susceptible pigweed collected from the field. The researchers exposed three generations of pigweed to sublethal doses of dicamba, “which, of course, is a recipe for resistance development,” Scott said.
The first two generations were still susceptible to dicamba. The third generation was resistant. Scott emphasized that this resistant population is highly controlled and will never be released to the field.
“The research that was conducted in the greenhouse that resulted in a dicamba-resistant pigweed illustrates how multiple resistances have developed,” Scott said. “One pesticide quits working, so we replace it with another, and so on and so on, until you are left with a weed population or insects, for that matter, that can tolerate multiple modes of action.
“This is the inevitable result of using a single
effective mode of action to control a given pest. As new technologies emerge, we must be diligent in our efforts to stop this cycle.”
It was pigweed’s emerging resistance to glyphosate that drew international attention to the issue of herbicide resistance. With glyphosate out of the picture as an effective control on a growing number of farms, soybean and cotton farmers looked to dicamba to manage the notoriously difficult weed with the release of Xtend soybean and cotton. The Xtend crops can tolerate dicamba deployed to control weeds.
Scott said the greenhouse findings, plus the lack of emerging control options, paint a frightening picture for growers.
For farmers, the number of tools they have for weed control is dwindling quickly, and it’s not just that the pigweed is resistant to one class of controls, but “some of these populations have now been found to be resistant to as many as four formerly effective modes of action,” Scott said.
The researchers, in their conclusion, said their finding “strongly suggests that there will be sizable evolutionary consequences if dicamba is not properly stewarded in dicamba-resistant crops, such as applying it repeatedly in a manner that provides less-than-complete control.”
“This is of great concern to us,” Scott said.
For more information about crop production, contact a county extension office or visit www.uaex.edu or arkansascrops.com.