When resistant-weeds emerge in the weeks prior to planting, an early spring burndown may be the best option for Midwest and Midsouth farmers. That’s particularly true when these weeds are impossible to control in-season.
To manage herbicide-resistant horseweed/marestail, Indiana corn and soybean producers have turned to two-pass burndown programs – either a fall burndown followed by one at planting or early spring burndown followed by one at planting.
“For us, marestail is the driver of our winter annual weed control programs,” says Bill Johnson, professor of weed science at Purdue University. “To control it, producers are becoming more aggressive with their programs. We’re starting to increase rates or add other herbicides to the tank or both. We’re also seeing more acres getting a second burndown treatment.”
The fall/at-plant burndown is the more common two-pass option, and in a lot of cases, the more effective in his state, according to Johnson.
“We get more consistent activity on marestail in the fall rather than in the early spring timing,” he specifies. “We can also use higher rates of herbicides. But when we have mild winters, which we have about 70% of the time, I’d like to get more producers thinking about this two-pass spring program.”
Swat Horseweed/Marestail Early
For early spring burndown in no-till soybeans, Johnson suggests 2,4-D- or dicamba-based programs with Sharpen (saflufenacil), Valor (flumioxazin) Tricor (metribuzin) or an Authority- (sulfentrazone) based product.
“Most of that is going to go out with glyphosate, although we are seeing some paraquat-based programs down in the southern part of the state,” he adds. “Try to hit marestail from the rosette stage to 4-inches tall. Control is very variable on anything larger than that.”
For the 20% of corn that is no-tilled, Johnson suggests an early spring burndown with an atrazine premix with glyphosate and 2,4-D or dicamba.
Annual (Italian) Ryegrass, The Cover Crop
An early spring burndown is also critical for no-tillers taking out cover crops in advance of planting, adds Johnson. “If the rest of this winter is mild, we’re going to deal with marestail. If the rest of the winter is mild, and it’s cool and wet in the spring for an extended period, we’re going to struggle with burning down our annual rye cover crops.”
While Johnson discourages using annual ryegrass as a cover crop, the practice is widespread nonetheless. “Herbicide-resistant annual ryegrass (also known as Italian ryegrass) is a problem, particularly in the Midsouth,” Johnson says. “In the long term, I’m not convinced that we can continue to plant seed lots that aren’t contaminated with herbicide-resistant biotypes. I’m really concerned about getting a contaminated seed lot and then seeing the problem really take off on us.
“We encourage producers to try and kill the ryegrass cover when it’s small,” Johnson says. “Hopefully, you have some warm daytime air temperatures (for herbicides to work). Use the maximum rate of glyphosate and add either Basis or something with rimsulfuron or Sharpen to the tank.”
The Logistics Of Burndown
LSU AgCenter weed scientist Daniel Stephenson says the best timing for an early spring burndown is 4 to 6 weeks prior to planting. That gives the herbicides plenty of time to work and to ensure that producers are planting into bare ground.
Many producers follow this rule of thumb for corn. But for logistical purposes they often burn down cotton and soybean ground while they’re still in burndown mode for corn.
“So for cotton and soybeans, you’re now 8 to 10 weeks out and that does not work,” Stephenson observes. “When this scenario happens, new weed emergence is very likely. In that situation, tank-mix paraquat with your residual preemergence herbicide right before you plant the crop or soon after to eliminate weed vegetation that is present,” Stephenson says.
For management of all other winter annual weed species in the spring, Stephenson recommends a full label rate of glyphosate and no less than a pound acid-equivalent of 2,4-D. Producers may be tempted to trim the 2,4-D rate, but Stephenson says that will likely lead to incomplete “control of the annual winter weeds. When you come back to clean it up when the weeds are older, herbicides are not as effective.”
Stephenson also cautions producers about putting out a residual herbicide such as Valor, Dual Magnum (S-metolachlor) or Goal (oxyflurfen) when there is substantial green vegetation on the ground in the spring. “If you do, you’re only getting postemergence activity because the herbicide is contacting vegetation and not the soil. If you don’t see bare soil, you’re not helping yourself from a residual standpoint.”
Italian Ryegrass – As A Midsouth Weed
While Indiana producers plant annual/Italian ryegrass as cover crop to suppress weed growth, many Midsouth producers are desperately trying to kill it before it grows to an unmanageable size by spring. Glyphosate-resistant Italian ryegrass has spread rapidly through parts of the Midsouth over the last few years, particularly in Mississippi. It’s also appeared along rivers and waterways in Louisiana, according to Stephenson.
“Unfortunately, it’s in every one of our Mississippi River parishes, plus it’s spreading,” Stephenson says. “It’s also in the northwest part of the state following the Red River south. It’s not as bad as it is in Mississippi, but if we ignore it, that’s inevitable.”
For control of glyphosate-resistant Italian ryegrass, Stephenson recommends a residual applied in the fall as the best option. He says the next best option is applying a graminicide such as clethodim in January when Italian ryegrass is less than 4 inches tall.
Preliminary research suggests that clethodim-resistant Italian ryegrass is also spreading from Mississippi to Louisiana, according to Stephenson.
“If you have clethodim-resistant Italian ryegrass, a producer’s only choice in the spring is tillage or one to two applications of paraquat,” Stephenson says. “The LSU AgCenter has adopted Mississippi State University’s recommendations for glyphosate-resistant Italian ryegrass management. Their recommendations work when followed.”
Taking Out Cover Crops
Stephenson notes that Louisiana producers are reluctant to do fall burndown programs to control Italian ryegrass or other winter annuals because so much vegetation is removed by the herbicides, which exposes beds to weathering. But if weeds get too big, they’ll be more difficult to control in the spring. To address this problem, Stephenson and other scientists are studying the effects of planting a cover crop and spraying a residual herbicide after it emerges (Dual or Zidua) (S-metolachlor or pyroxasulfone) to provide winter weed control in the cover crop.
To kill the cover crops in the early spring, “it’s very important to use full rates of glyphosate and 2,4-D,” Stephenson says. “If you have legumes mixed in the cover crop, I’m suggesting adding 6 to 8 ounces of dicamba.”