Glyphosate-resistant johnsongrass has been identified in northern Argentina. While not an immediate threat to U.S. crop fields, the development once again points to the necessity to prevent or slow a weed's ability to evolve herbicide resistance.
“I deal with weed resistance stewardship from a global perspective,” said Michelle Starkey, Monsanto Roundup Stewardship lead in an Aug. 29 interview. “So I've been talking with our Monsanto reps down in Argentina quite a bit about this. I've been working with them as they begin their research.”
Most of Argentina's corn and soybeans are grown in the central part of the country. Even so, farther north where the resistant johnsongrass was found, there are plenty of soybeans grown.
“It's an area that's been in production agriculture. They began (growing) Roundup Ready soybeans there probably about seven years ago. Prior to that (cropland) was mostly in things like dry, edible beans.”
Can you explain how it progressed?
“We began the investigation after a farmer complaint in about 2004. We were initially a bit confused because there were some inconsistent results. We went back and did additional field tests and tested the weed seed here in the United States. Once we were fairly sure there was something to report, we began additional communications both in Argentina and (here).”
How might this translate to the United States? “What we can say is there's no way to predict what the next resistant weed will be — whether glyphosate-resistant, an ALS-resistant weed, any resistant weed. There are places where glyphosate-resistant weeds have shown up in other countries and we have the same weeds in the U.S. but (they haven't become resistant).”
While the johnsongrass find is “significant,” Bob Scott pointed out there are at least two other grasses confirmed resistant to glyphosate: goosegrass (in Asia) and ryegrass (in Australia).
“Johnsongrass is a species that causes problems for us in Arkansas. (But) broadleaf signalgrass, barnyardgrass and others would probably be worse weeds to find glyphosate resistance in.
“It's interesting a weed like johnsongrass would develop resistance. It can reproduce both vegetatively and by seed,” said the Arkansas Extension weed specialist. “There's already johnsongrass resistant to the ACCase herbicides (graminicides) such as RiceStar, Clincher, Select and Poast.”
Andy Kendig, Missouri Extension weed specialist, said johnsongrass “hits me a lot like pigweed — just before Roundup Ready crops became the norm, resistance to a lot of chemistry was popping up in johnsongrass.
“It's always tough to talk about going back to traditional chemistry. But it's even tougher when you have to go back to something that was showing preliminary signs of resistance.”
While there are products yet to be developed to address horseweed and Palmer pigweed (the dicamba gene, Reflex labeling in cotton), “johnsongrass has a much smaller arsenal. At last count, the grass-killer resistance did not include Select — except you could see the resistant johnsongrass would tolerate a fractional rate of Select better than the ‘wild’ type. Basically it's good news that we were able to kill it. But the grass killers were more along ‘single enzyme site,’ the type it's easier to (develop) resistance (to).”
Also, Kendig said, ALS resistance had been found in a number of locations. “Gosh, here I am again talking about Treflan. I remember Treflan being considered a johnsongrass herbicide (with all the other weeds just being a fringe benefit). And with $2.50 diesel and normal farm labor issues, no one is interested in incorporating something like Treflan. MSMA would be a fair option for a cotton grower, but that's about it on the chemistry list.”
Monsanto is suggesting several agronomic practices for Argentine farmers worried about resistant johnsongrass.
“We're making a point to say, ‘Start with a clean field,’” said Sharkey. “Either do a burndown application or tillage. We want to make sure (the Argentine) growers are scouting their fields for any problems. That's very similar to what we (recommend) here.”
The farmers also need to use the correct herbicide or tank-mix at the proper time for the weeds. Following an application, they must make sure herbicides have been effective.
“Down there, the point is also being made that the weeds need to be controlled (prior to seed being set). There's a lot of custom harvesting there, so farmers who might have (glyphosate-resistant johnsongrass) in their fields need to watch where the custom harvesters are going. Any area of the field with this johnsongrass should be (harvested last).”
Another recommendation is to use certified seed. “That way they'll know where their seed is coming from and the (weed) seed isn't in what they plant.”
Monsanto wants their customers “to be very successful with the Roundup Ready system. So we continually put out messages and recommendations to help them. A couple of things towards that in the 2007 Technology Use Guide — a publication that goes out to every farmer licensed to grow Roundup Ready crops — contain weed resistance recommendations.
“(Last fall), Monsanto also launched its own weed resistance Web site (www.weedresistancemanagement.com). Farmers can go there for additional recommendations.” The company has also been working with commodity groups on weed resistance educational modules.
“We take this very seriously. The newsletter (announcing the resistant johnsongrass) makes the point that Monsanto took the initiative to begin research in Argentina. Monsanto also took the initiative to update the International Survey of Herbicide Resistant Weeds (www.weedresistance.org).”