Soybean and grain crops are susceptible to invasions from winter annual weeds like Italian ryegrass early in the growing season, and perennial summer weeds such as johnsongrass all season long. Both are competitive with the crop for vital resources and, in the case of johnsongrass, can leave fields overgrown and difficult to harvest. Three LSU AgCenter researchers are working on solutions that include tips for producers at being both proactive and patient with herbicide applications.

Stephen Ippolito, Prashant Jha and Donnie Miller have been studying invasive grassy weeds for decades combined. Along with their graduate students, they have focused on a variety of tactics to improve yields for farmers in Louisiana and elsewhere. Lately, their research has focused on solving the problem of herbicide-resistant populations of these two weeds.

“We have started seeing johnsongrass resistance to glyphosate and ACCase chemistry, so there’s nothing much we can do postemergence other than Liberty and Capreno as a tank-mix,” Jha said. “And again, Liberty is a contact herbicide. It doesn’t translocate to the rhizomes, so that’s not the solution.”

Liberty is a nonselective, postemergent herbicide widely used to control tough broadleaf and grassy weeds but works only on the areas it touches and does not travel to roots. Whereas, glyphosate, the industry standard for years, is absorbed by the plant and moves through the plants into the roots.

Jha went on to say that, while a field may look good after a May application, when observed again in June, the plants could have died because the rhizomes are still alive, having not been affected by the Liberty application. Soil residuals like S-metolachlor or pyroxasulfone can be a good option to control seedling johnsongrass but will not do much to rhizome-derived johnsongrass plants.

Miller added that while historically Italian ryegrass control programs prior to crop planting have been built around glyphosate, the annual grass has become resistant to it over the years. Unfortunately, he emphasized the weed has also started to become resistant to effective alternative herbicide chemistries as well.

“Resistance has been documented in the past to both the ACCase and sulfonylurea herbicides and most recently to paraquat,” Miller said. “So, we’ve just about run out of effective postemergence application options.”

Miller described the problem as being like a super virus that has been treated with the same medications for long periods of time.

“What the old medicine used to kill in susceptible populations isn’t even fazed anymore in resistant populations,” he said.

According to the researchers, one issue is that the introduction and widespread adoption of the Roundup Ready technology in the 1990s significantly slowed development of new herbicide chemistries over the past two decades in the agrichemical industry. Miller said that it can take 10 to 12 years at a minimum to develop a new compound and get it registered through the Environmental Protection Agency.

Alternative strategies, such as cover crops like cereal rye, can be effective at reducing the in-season selection for postemergence herbicides, according to Jha.

“Planting cereal rye, with the fall residual herbicide applied after emergence, provided us with the 90% to 95% control,” he said.

Miller said the combination of herbicides such as S-metolachlor and fall-planted cover crops as weed combatants is slowly catching on with producers because it’s a bit of a foreign concept. So, it is incumbent on the experts to keep promoting the benefits of the combination as a control method.

For his part, Ippolito has been working with trifluralin, a widely used, selective preemergence dinitroaniline herbicide, specifically in the realm of johnsongrass control.

“I intend to expand that a little bit out as well. I’ve been doing some work looking at kind of a heavy preprogram just to see if anything else has some activity,” he said.

Ippolito went on to say that a preemergence application followed by a postemergence application, regardless of the crop, is vitally important for resistance management. He said preemergence applications reduce selection pressure on postemergence applications.

Jha echoed his sentiments saying that, while it may not be cost-effective today, in the long-term growers need to be proactive and patient in managing herbicide resistance.

“Those who have been proactive, they’re going to make more money at the end of the five years as compared to those who aren’t,” Jha said. “We may have limited tools, but we have to figure out the best way to use them.”