Plant-back restrictions, production systems, weed species and size are a few drivers of burndown timing and herbicide choice. Several university/Extension weed specialists have suggestions to help cotton growers maximize their burndown application.
Tom Barber, University of Arkansas weed scientist, says timing of the burndown partly depends on the type of cover crop a grower may have. “My No.1 concern is having enough cover crop biomass to protect against glyphosate-resistant Palmer amaranth. Cereal rye is one of my favorite cover crops that prevents light from getting to the soil to initiate pigweed germination. However, we can’t burn it down in March and expect to get any weed suppression benefit.
“Depending on weather and geographic location, we normally burn down cover crops early to mid-April. Generally, we burn down cereal rye two weeks ahead of planting. If you burn it down four weeks prior to planting, it will fall on top of itself and form a thick mat on the soil surface. The resulting biomass mat makes planting difficult.”
In Arkansas, glyphosate-resistant horseweed (marestail) is No. 1, and henbit is No. 2 as far as the severity of overwintering annual weeds that growers design their burndown programs around. “Again, depending on the area, horseweed is easier to control if we can burn down in March before it begins to bolt,” Barber says.
“Additionally, if we let glyphosate-resistant ryegrass go into spring, it will require two Gramoxone applications to take it out. So, we need to suppress it earlier before the second application. It all goes back to the weeds that are present. Generally, for fall annual weeds, I definitely like a burndown four weeks prior to planting minimum.”
For the most part, glyphosate (Roundup) plus either 2,4-D or dicamba (sometimes both) is used as a standard cheap starting point for winter annual herbicide options. Firstshot, and Sharpen are added to a program as a tank mix, depending on weed species present.
Sometimes, adding a residual herbicide really helps with horseweed control. “Valor has a very good fit in the burndown window from a residual standpoint,” Barber says. “In addition to giving some postemergence contact activity, it residually controls horseweed, henbit, and junk fall annual broadleaves. And if we don’t have PPO resistance, it will keep some of the early-germinating pigweed populations at bay.”
ARID CLIMATE BURNDOWN
Arizona typically does little burndown prior to planting because the lack of rainfall results in few weed issues prior to planting. However, there are some instances when a burndown is used.
“For example, we pre-irrigate a lot of our cotton ground three to four weeks prior to planting,” says Arizona Cotton Specialist Randy Norton. “If planting gets delayed, we can have a flush of weeds, such as morningglory and non-resistant pigweed, that we will burn down. We primarily use glyphosate at full labeled rates, which takes care of anything that might emerge prior to planting.
“We do have some pockets in isolated areas that have glyphosate-resistant pigweed. Fortunately, it’s not a widespread issue in our state, and we work to keep it that way by using residual herbicides, either at planting or immediately after planting.”
Additionally, some Arizona growers plant cover crops that they burn down, again primarily using glyphosate. In the southeast portion of the state, most cotton growers use oats as a cover crop, while other areas use other crops, including barley and triticale.
PLANT BACK RESTRICTIONS
Clemson University Weed Specialist Mike Marshall says a glyphosate/2,4-D burndown controls South Carolina’s most troublesome weeds, including primrose, radish, and marestail, while picking up other weeds, such as ryegrass.
“Adding a residual, such as Valor or diuron, also helps control early season pigweed emergence,” he says. “And in situations where a grower makes a last-minute decision to switch to cotton instead of another crop, he can use a Gramoxone/diuron program, especially if he is facing a 15-day or less plant-back. Diuron works well on pigweed and other small-seeded weeds, while providing good control of primrose and other winter weeds.”
Marshall says plant-back restrictions is the main driver of burndown timing. “Not only do you have to factor in the herbicide you use, but also its rate.”
TILLAGE STILL RULES
The majority of west Texas cotton is still conventionally tilled, so there is less burndown than in other regions of the cotton belt. But where growers plant into a cover crop or some kind of no-till residue, the key factor when using a burndown is weed size.
“The problems I see in most years, with something like horseweed, is when growers let them grow too big and they become difficult to control,” says Texas A&M AgriLife Research Weed Scientist Wayne Keeling.
“I recommend applying a burndown sometime in late February or early March. If it’s a no-till type field, just assume you have weeds present, even though they might not be too visible, and make an application. Horseweed and early-germinating kochia are easy to control when they’re really small. If you delay a month and you can see the weeds as you drive by, they’re much more difficult to control.
“New burndown options, such as 2,4-D cotton and dicamba cotton, will really help us. But whichever burndown program you use, spray early when weeds are small. You definitely don’t want to wait until two weeks before planting to burn them down.
“Being in a dry climate, we want to conserve soil moisture as much as possible. That’s another reason why it’s so important to control winter weeds as early as possible before they use much moisture. Cover crops like wheat or rye need to be terminated as soon as possible before planting in order to build back soil moisture.”