Johnsongrass, the latest entry to the lengthening list of glyphosate-resistant weeds — in both Arkansas and Mississippi — was announced in mid-March. It is the first glyphosate-resistant warm-season grass found in the United States.

“We're not trying to push resistance in these weeds,” says Bob Scott, Arkansas Extension weed specialist. “But there's close to 5 million acres of Roundup Ready crops that get two or three applications of Roundup every season. Plus, we're using Roundup as a burndown.

“It's inevitable that such weeds are produced. It's hardly a surprise.”

Johnsongrass found in a Crittenden County soybean/wheat field is the fifth glyphosate-resistant weed discovered in Arkansas. The others: horseweed (also known as marestail), common ragweed, giant ragweed and Palmer amaranth, a pigweed.

The resistant Mississippi johnsongrass was found in the northern Delta, outside Clarksdale. Even though the weeds were both found last year, there is no known connection between the sites.

“We haven't found any other johnsongrass that worries me,” says Trey Koger, Mississippi Extension soybean specialist. “Last year, we did look at some populations in Washington County. But when plants were brought to the greenhouse and screened, they proved susceptible. I think the initial control failures were a function of drought stress rather than herbicide resistance, thank goodness.”

However, the Clarksdale-area johnsongrass is resistant and has received “a lot” of glyphosate over the last few years.

“From a weed control standpoint, all it's gotten in-season is glyphosate. And from what I understand, the Mississippi population may be more glyphosate-tolerant than the one in Arkansas.”

Last year, Scott says, a few populations of Arkansas johnsongrass were suspected resistant to glyphosate. Only plants from the long-time Roundup Ready soybean field in Crittenden County actually were. No other populations are currently being tested.

As in previous cases of resistance, Extension agents alerted Scott to the weeds. “The farmer called the agents in when he had trouble controlling johnsongrass. We ended up screening four populations. Three were highly susceptible in the greenhouse — dead from 11 ounces of Roundup WeatherMax.”

However, control of the resistant population required between 44 ounces and 88 ounces of Roundup WeatherMax. The field rate is 22 ounces, so the preliminary belief is the grass has a 2X to 3X level of resistance.

Much of the Arkansas plant material is now with Jason Norsworthy, a University Of Arkansas weed scientist stationed in Fayetteville. “He'll re-run the studies and come up with very accurate dose/response curves,” says Scott. “That'll give us a real picture of how resistant this is.”

In greenhouses he works in Lonoke, Ark., Scott tested both rhizomes and seed from the infested field. Having found the resistance trait in both seeds and rhizomes “is a confirmation of the heritability of the resistance. However, we're currently growing some survivors out to seed. Those will be tested to double-check the heritability. We will also go to the field this summer to do field evaluations.”

Control options

The Mississippi johnsongrass was found in a field that's been in a soybean/wheat rotation. Soybeans are on the acreage annually with wheat two of the last four years.

“Almost all they've used as a burndown is Roundup,” says Rick Cole, Monsanto technology development manager. “Same is true for what was used on the beans. Other products have been used in the wheat, but Roundup use has been heavy for at least four years.”

The Mississippi site is a small field — no larger than 5 acres — that was “aggressively managed. They managed to spray graminicides on it and were treating the problem rapidly” and with success.

Cole is unsure of the level of resistance. “From a practical standpoint — whether 3X or 8X — it's still an issue for the grower. We had a difficult time (when bringing) in seeds and rhizomes to make sure they grew out in the greenhouse in an environment that simulates the field. We weren't so concerned with the level of resistance as much as confirming it actually was resistant and couldn't be controlled by the normal use rates.”

Testing confirmed the resistance and “it wasn't a situation where it was on the fence.”

The Arkansas johnsongrass was screened against Valent's Select herbicide — an ACCase inhibitor — and found to be susceptible to the chemistry.

“So the farmer will be able to (continue using) the Roundup Ready technology,” says Scott. “One option would be to tank-mix something like a reduced rate of Select Max. I believe the full rate is 16 ounces, and he may be able to use less than that. There's a rate for volunteer corn control, for example, that he may be able to follow.”

Glyphosate-resistant johnsongrass isn't unmanageable. However, for any unfortunate farmer with the pest, it will be more costly to control.

“If you're already tank-mixing and burning down and doing things for a broadleaf resistance problem, adding a grass resistance problem becomes significantly more expensive,” says Scott. “We're very concerned about that.

“It's a guess, but a full rate of Select costs about $20 per acre. In a tank-mix for johnsongrass control, you're probably looking at $8 to $12 more per acre.”

Scott expects more problematic johnsongrass to emerge shortly. “Our experience has been that once a single population of a resistant weed is found, others pop up right after. I don't think this grass will ever be as significant a problem as resistant horseweed has been — or as glyphosate-resistant pigweed will be. But for those with this grass, it won't be fun. They'll have to re-evaluate their control program.”

Control tips

Although ubiquitous in much of the country, johnsongrass is actually a non-native species. Its seeds are fairly large and aren't carried on the wind as easily as marestail seed. Johnsongrass rhizomes often propagate as much as the seed.

As far as control tips, Cole would “stick with the Roundup Ready system” and watch the crop closely to ensure adequate johnsongrass control is gained. If problems develop while other weeds are being killed, “I'd take a proactive approach and (apply) something like Select Max, Assure II — some sort of an ACCase inhibitor over-the-top in soybeans. That would be true for cotton also. I'd get on top of it right away. The last thing I'd want is for the johnsongrass to go to seed.

“One of the best cultural practices to use in an area with johnsongrass is to make sure that, when traveling from field to field — whether with cultivation or harvest equipment — that equipment is clean and you aren't dragging any rhizomes or rhizome pieces.”

Further, with the new problem weed a grass species, “there are a number of products for soybeans, cotton and corn that are fairly effective at controlling seedling johnsongrass post-emergence.”

Pre-emergence, effective products include Harness, metolochlor and others. The “yellow” compounds such as Prowl and Treflan can also be utilized.

Next up?

What weed is likeliest the next to be found glyphosate-resistant? “In research circles, many believe that, given its history in the state, barnyardgrass is likely the next to (gain) resistance,” says Scott. “We've already got barnyardgrass that's Facet- and Propanil-resistant. Already, there's tremendous pressure on both ALS and glyphosate technology. There's no barnyardgrass being tested currently, but with its genetic diversity it's a solid bet.”

One possible positive with johnsongrass is easy identification. “As we travel our operations, a lot of weeds don't stand out visually,” says Koger. “Pigweeds and other species are like that. But because of the johnsongrass architecture and lighter green shade, it's easier to pick out. Hopefully, we'll use that to our advantage and producers will keep a closer watch on it now.”