Trey Koger wasn't surprised by the report of possible glyphosate-resistant horseweed in DeSoto County, Miss., during August 2002. As a weed biologist at the USDA-ARS Southern Weed Science Research Unit, Stoneville, Miss., he had been studying ways Roundup Ready cropping systems affect weed populations and trigger weed shifts.
Within a few days of receiving the report, Koger and Dan Poston, weed scientist with Mississippi State University Cooperative Extension Service, visited the suspect north Delta fields. They collected seed which were planted in a greenhouse for confirmation tests.
Over the winter, Koger concluded that populations of the weed — commonly known as horseweed or marestail — were 10 to 15 times more tolerant to glyphosate than susceptible plants of the same species. Resistance was clearly the problem.
Koger and several cooperators collected seed from about 100 populations of resistant and susceptible horseweed in Mississippi, Arkansas, Tennessee, and Delaware during 2003.
After inducing seed from about 75 populations to germinate, he initiated a series of greenhouse and lab experiments. At least five significant findings have already evolved from the work.
“First,” Koger said, “we've proved that resistance is caused by poor glyphosate translocation. Something in the plant prevents glyphosate from moving as freely as it does in susceptible horseweed.”
Resistant horseweed can be killed with glyphosate at rates eight to 12 times heavier than farmers use — Koger has done it. Although that's not an economical option and is off-label, he said, it means resistance is not caused by genetic mutation, and that's good news. In different weed species, genetic resistance to other herbicides has enabled plants to survive rates hundreds of times greater than those used commercially.
Koger's second finding concerns shikimate, a naturally occurring compound plants use to develop essential amino acids. Glyphosate blocks the shikimate pathway, causing such heavy accumulations of the substance that they are toxic to the plant.
“In our experiments, the shikimate level rose quickly following glyphosate application to susceptible plants at the 1X rate,” Koger said. “In resistant plants it increased somewhat, but not nearly as quickly.”
He believes this further supports the conclusion that glyphosate does not translocate in resistant plants to the extent it does in susceptible plants.
Koger is enthused about a third area of research involving development of two assays that promise to speed up the process of confirming resistant horseweed. The tools would enable growers to learn fairly quickly if resistance is the problem when they have horseweed control failures with glyphosate herbicides.
One test, developed in cooperation with USDA-ARS weed scientist Dale Shaner in Fort Collins, Colo., requires lab equipment. Growers would collect plant samples, place them in a solution to keep the leaf tissue metabolically active, and air express them to a testing lab.
“Even if no susceptible plants were available for comparison, we could make a fairly reliable judgment as to whether the plants were resistant,” Koger said. This test can be done overnight.
The second test is based on a simple procedure farmers or consultants could use. Leaves are dipped in a solution for two to three days. Resistant plants develop some leaf margin injury, but remain fairly healthy and green; susceptible plants show extensive injury.
The validity of both assays has been proven in greenhouse and field tests, Koger said. Both are currently moving through the patent process.
A fourth effort undertaken with Poston and a Mississippi State University graduate student was aimed at learning more about horseweed germination.
“Since this species is a winter annual, you would assume that it always germinates in the fall and overwinters before making a burst of spring growth,” Koger said. “That's consistent with what people have observed in the field.”
But Koger and his colleagues found that horseweed germinates year-round as long as moisture is ample. In these studies, horseweed populations were counted with meticulous accuracy, then killed with Liberty herbicide. Counts were taken every two weeks, and populations were killed again.
“We learned that horseweed develops a huge flush in the fall, fails to germinate when it's really cold, but develops multiple flushes as temperatures rise during March and April,” Koger said. “Additional flushes come during the growing season as adequate moisture is available.”
Koger believes the tendency for new flushes to develop after burndown and before planting suggests the need for growers to revise their strategy. “We know that preplant burndown applications of glyphosate should be enhanced with a growth hormone herbicide, such as 2,4-D or Clarity, when horseweed is present,” he said.
“But that combination is effective only against weeds that have emerged at time of application. I believe a third component — a residual herbicide — should be used in the burndown application to prevent additional flushes.”
He said that an early burndown, such as in mid-February, is a great tool for controlling winter weeds when they are small. “It provides a clean seedbed earlier, so growers can plant into warmer soil,” he said. “In addition, it reduces chances for herbicide drift to small corn, often planted in late February or early March.”
But, with glyphosate-resistant horseweed, an early burndown not containing some type of residual herbicide may be followed by a new flush that emerges between burndown and planting. Those weeds must be dealt with at planting or in-season.
A fifth area of information concerns the timing of tillage to control horseweed. In the research, done jointly with Poston, horseweed-infested plots were tilled in September, November, January, and March.
“September tillage reduced populations, but tremendous amounts of horseweed emerged later. It was obvious that fall tillage doesn't eliminate the problem,” Koger said.
November-tilled plots had a spring flush, but at lower numbers than plots tilled in September. The same was true for plots tilled in January. Plots tilled in March had very low populations, but more plants emerged in April and May.
In one treatment, plots were tilled in September and sprayed with Ignite herbicide in March. “Fall tillage followed by spring burndown is typical of what a grower might do, but horseweed still emerged in April and May,” Koger said.
Other studies being conduced by Koger seem to offer the potential for more knowledge. For example, he's trying to learn if glyphosate is metabolized (broken down) by resistant horseweed.
“Typically, glyphosate is very stable in a plant. If resistant horseweed breaks down glyphosate, that would make the more plant more resilient to the herbicide,” he said. Conclusions from this work are expected soon.
Koger has used a high-powered electron microscope to examine leaf surfaces of susceptible and resistant horseweed. He looked closely at the leaf hairs, stomata and leaf crevices to see if they might be related to resistance.
So far, this work in cooperation with Krishna Reddy, also a USDA-ARS weed scientist at Stoneville, has lead to optimism followed by disappointment.
“Resistant populations from Arkansas and Mississippi had more leaf hairs per surface area than the susceptibles,” Koger grins. “We thought we were really on to something. But we found no difference in number of leaf hairs on resistant and susceptible plants from Tennessee and Delaware.” This work is expected to be concluded fairly soon, as well.
In a third experiment, Koger has used an additive to enhance glyphosate activity. The compound, called amads, is sulfuric acid-based, and is an ingredient of some cotton harvest aid products. The addition of this product to glyphosate killed resistant horseweed in the greenhouse, and the technique is being investigated in the field to see if it works there.
Koger plans to use radioactive glyphosate spiked with amads this winter to see if translocation in resistant plants is enhanced, or if plants are simply burned to the point that they die.
“Amads is not commercially available to growers,” said Koger. “Furthermore, it's corrosive to equipment, due to its acidity. But we want to know if it aids translocation in resistant plants.”
The scientist considers Roundup Ready technology a “phenomenal tool.” To preserve its usefulness, he said, farmers should rotate crops and use herbicides with various modes of action. He also considers it important to control weeds when they're young, small, and most susceptible to herbicides.
“If you see resistance developing, take prompt action to nip it in the bud,” he said. “The problem will only get worse, if ignored. Prevention is critically important, especially with horseweed, because the seed are so widely dispersed by wind currents.”
Koger is convinced that glyphosate resistance is manageable. “But it's going to take more awareness and a new level of management,” he said. “Farmers must adjust their weed control programs — they can't rely on glyphosate alone once they have resistance.”
A native of Oklahoma, Koger earned his bachelor's and master's degrees at Oklahoma State University before getting his Ph.D. under weed scientist David Shaw at Mississippi State University.
Bill Barksdale is a freelance writer who lives in Cotter, Ark. e-mail: firstname.lastname@example.org