With the success of glyphosate-resistant crops, including corn, soybeans, cotton and alfalfa, came a disincentive to develop herbicides with new modes of action, according to Washington State University Research Weed Scientist Rick Boydston.
“It is just hard to compete and come out with new products that can match that cost-effective technology,” Boydston said.
As a result of that and other factors, Boydston said, it’s been a quarter of a century since a new mode of action has been introduced
in herbicides. Now, however, with resistance developing to glyphosate has come new incentives to, if not develop new modes
of action, to at least look at old
To that end, Boydston and others have been looking at the older chemistry linuron, or Lorox, over the past year in a multi-state trial involving researchers in Oregon, Washington, Indiana and Wisconsin.
“We worked to get good data from all the different growing regions in order to make a decision to go ahead with this project in mint,” Boydston said in a presentation at the Mint Industry Research Council’s Annual Meeting, January 26 in San Diego.
The researchers looked at different rates and at pre-emergence and post-emergence applications on peppermint and scotch spearmint and analyzed responses in different soil types, including muck soils, sandy loam and silty loam soils.
“We also did a trial with Lorox on native spearmint and it performed similarly,” Boydston said.
When measured against Sinbar, the researchers found no significant difference as far as mint injury between the two compounds in California, Oregon, Washington and Wisconsin when applied pre-emergence. In Indiana researchers had mint injury, but Boydston said, that could have occurred because of application timing.
“In Indiana the researchers thought they got it on a little too late and maybe the combination of the soil type and the later application caused the plant injury,” he said.
When applied post-emergence, Lorox caused more plant injury than Sinbar in all areas tested. “We got up around 20 percent injury with the one-half and one-pound rate when applied post-emergence,” Boydston said.
As for yield response, however, that did not seem to be effected. “Eventually the mint grows out of most of that injury and you don’t see it in yield response,” he said, noting that yields were not significantly different between the trials.
Also, when applied pre-emergence, the researchers found no significant yield differences between the Sinbar, the control or the Lorox treatments.
Use of a surfactant tended to increase injury at all sites tested, he said.
Boydston also looked at Lorox applied between cuttings of double-cut mint in Washington. “We did see a fair amount of injury when putting it on between cuttings,” he said, noting injury was between six and eleven percent when the Lorox was applied at a rate of one pound between cuttings.
But the researchers found no significant yield reduction at that low rate, especially when putting Lorox on prior to any regrowth to mint or prior to any additional irrigation after the first cutting.
“And at the two-pound rate,” he said, “it tends to thin out the mint stand and it doesn’t come back in, doesn’t regrow after that first cutting as it does without the herbicide. So, we do have some injury potential there and potential to weaken our stands and thin the stands out with that between-cutting application.” At the lower rate, the trend for injury was there, he said, but yield reductions were not apparent.
Researchers also looked at how Lorox performed in tankmixes involving currently labeled mint herbicides, including Prowl, Spartan and Chateau.
Boydston reported that in tests in California the most injurious tankmix was Lorox with Prowl. In Washington, Indiana and Oregon the tankmixes with Spartan and Chateau “caused a little more injury,” he said. “It is mainly from Spartan and Chateau and not the Lorox,” he said. In all cases, despite the early-season injury that sometimes occurred, yields were seldom significantly different, he said.
Boydston noted that the researchers hope to continue with the research into Lorox on mint with hopes of getting a pre-emergence label for the chemistry in a couple of years through the IR-4 process.
“Post-emergence applications are probably out of the question,” he said.
Pre-emergence Best Timing in Bushy Cinquefoil Control
Controlling bushy cinquefoil pre-emergence doesn’t seem to pose much of a problem. Once the biennial or long-lived annual weed is up, however, control becomes difficult.
The weed has become a problem in some mint operations in recent years, prompting Washington State University Research Weed Scientist Rick Boydston to look into control options.
In a presentation January 26 in San Diego at the Mint Industry Research Council’s Annual Meeting, Boydston said the weed shows up mostly in moist, wet areas of fields and in poorly drained areas.
“So, water management of your fields may be important to how you manage this,” Boydston said.
Boydston said he conducted trials in greenhouses a few years ago and found several herbicides provide pre-emergence control of bushy cinquefoil. “The problem is it comes up in late summer and fall and overwinters and it is very difficult to control post-emergence,” he said.
Boydston looked at several different post-emergence treatment compounds in his trials. Tough and Thistrol, or MCPA, performed best, he said, “But at 65 and 69 percent, it still was not very good.”
Tough alone provided “very little control,” he said. “And Thistrol alone was not a whole lot better. But when we combined those two, we got a little better control.”
Boydston said he is continuing to look at control options in greenhouse trials.
One herbicide he currently is testing is Sharpen. “It does pretty good in greenhouses against the weed,” he said. “Still, it didn’t totally kill it and we got some regrowth.” In subsequent greenhouse tests with Sharpen tankmixed with Thistrol, control was improved, Boydston said.
Additional trials conducted in 2017 will focus on Aim, Sharpen and Tough treatments, either alone or in combinations, he said.
The bottom line for bushy cinquefoil? “Post-emergent, it is pretty difficult to control with the chemistries we have,” Boydston said.