Herbicide resistance in weeds requires producers to react and adapt
By Stuart Estes
With the advent of herbicide-resistant, particularly glyphosate-resistant, varieties of crops nearly 20 years in the past, it seems as though Mother Nature has finally caught up; the weeds that used to threaten the conventional crops of the past are now beginning to threaten present-day crops by exhibiting herbicide resistance.
This is especially true across the mid-South region of the country where herbicide-resistant varieties of corn, soybean and cotton have been heavily utilized. One state where herbicide-resistant weeds pose a major threat is Arkansas.
Herbicide-resistant weeds such as horseweed (marestail), ragweed, johnsongrass and ryegrass have all been found in the state, but the bane of herbicide-resistant agriculture in Arkansas is glyphosate-resistant Palmer amaranth, according to Jason Norsworthy, Ph.D., a weed scientist and professor in the Crop, Soil and Environmental Sciences Department at the University of Arkansas.
“It’s had a major impact on production agriculture in the state,” Norsworthy said.
Also known as pigweed, the Roundup-resistant Palmer amaranth has only recently become such a problem for agriculture.
“Five years ago you would have only found pigweed in 10 or 15 states,” Norsworthy said. Now, glyphosate-resistant pigweed is found in 24 states.
A 2011 survey of the soybean and cotton acres in Arkansas found that 61 percent of soybean acres and 85 percent of cotton acres were affected by glyphosate-resistant pigweed.
Pigweed’s hardy nature and proclivity for procreation make it an especially troublesome weed considering its resistance to glyphosate.
“Under ideal growing conditions, it will grow 3 inches per day,” said Norsworthy, also noting that a fully mature plant can stand 6 to 7 feet tall and produce more than 1 million seeds.
Once the glyphosate-resistant pigweed gained a foothold in the fields, it didn’t take long for the weed to completely take over.
“You used to see fields that have one weed,” said Norsworthy, meaning that was the one resistant weed that spawned the eventual takeover. “It overwhelms the crop.”
With such a productive and harmful pest causing problems, producers must deal with an ever-shortening list of ways to combat the threat.
“The options for farmers to control this weed become less and less,” Norsworthy said. “We have areas in the mid-South that have basically lost the use of glyphosate.”
Producers have some options to stop the spread of not only glyphosate-resistant Palmer amaranth, but of herbicide-resistant weeds in general.
“It’s a zero-tolerance program,” Norsworthy said. “Growers have to go out and make sure they prevent seed production at all costs.”
For farmers to be truly successful at controlling herbicide-resistant weeds, a prevention mindset should be adopted instead of a management mindset.
“At the end of the day we have to draw the seedbank down,” Norsworthy said. “We have to do a better job of being proactive.”
In terms of timing, it is important for producers to not wait too late in the season to begin combatting herbicide-resistant weeds.
“We must start clean at planting,” said Norsworthy, also mentioning that it is important to start with a residual herbicide and make follow-up applications throughout the growing season.
The decision of what applications of herbicides that farmers make to control herbicide-resistant weeds needs to be carefully calculated.
“The key is to have two effective modes of action to prevent the resistant weeds,” Norsworthy said.
The task of finding effective modes of action becomes harder as weeds begin to exhibit new resistance to different herbicides.
“Each year we are losing herbicides because of resistance,” said Norsworthy, noting the last new mode of action was discovered in 1982.
Producers also need to be mindful of the basics of crop production as they strive to combat herbicide-resistant weeds.
“I encourage people to not forget about sound agronomic practices,” said Norsworthy, which include fostering proper canopy growth and implementing tillage methods when needed.
Crop producers must take into account the areas surrounding their crops as a place for herbicide-resistant weeds to escape management.
“We talk about managing fencerows and ditches,” Norsworthy said. “We have to be mindful of our field boundaries.”
Unfortunately, the ultimate answer to the problem of herbicide-resistant weeds is not a simple one.
“The more diversity we bring to a program, the more success at preventing resistant weeds,” Norsworthy said. “No one likes complexity, but at the end of the day it is going to take some complexity.”
“There is no silver bullet,” Norsworthy said. “We will never see another herbicide like glyphosate.”
With the heyday of glyphosate-resistant crops possibly in the past, producers must proactively seek out prevention and management practices that will allow for future success regardless of the effects of herbicide-resistant weeds.