Successful Palmer amaranth control in 2016 begins now. This fall, cotton growers can take several steps to reduce weed populations and save money next year.
When rotating cotton behind corn or grain sorghum, don’t let emerging pigweed grow and go to seed after grain harvest, says University of Tennessee weed scientist Larry Steckel. Eliminate the extra seed now.
Fall ground work and herbicide applications help knock the seed bank down enough that you don’t have to fight as many pigweeds and save money the following year, according to Steckel.
“If you farm no-till, you might want to apply a shot of something like Gramoxone, and depending on the time of the year, maybe add a residual,” he says. “In conventional or minimum-till systems, disking reduces pigweed seed production. Also treat and/or disk field borders, drainage ditches and fields that were flooded and took out some of the crop this season. We want to prevent pigweed seed from going into the soil seedbank in all areas.”
Cover crops have proven very helpful in suppressing pigweed and other weeds, especially winter annuals like marestail. Steckel says growers can get another 30 to 40 percent control of pigweed with cover crops, depending on how early they’re planted and field coverage. In Tennessee and the rest of the northern Delta, the best time to plant cover crops is as early as possible — such as right behind the picker.
“We’ve researched legumes and grasses and found the best pigweed suppression with a blend of wheat or cereal rye and a legume like vetch or crimson clover,” Steckel says. “The biggest problem with those is if you get thick ground coverage you could have difficulties at planting getting cotton seed/soil contact because the crop is planted shallow anyway.”
Steckel and his colleagues have studied banding, like a 10-inch band of an early burndown from an RTK unit in February, and planting back into those strips at planting time. That practice enhances good seed/soil contact. Then growers burn down the whole field close to planting like normal.
To get the best weed suppression, Steckel typically recommends terminating a cover crop three to four weeks ahead of planting. “With complete cover crop ground coverage, you shouldn’t have any pigweed coming up at burndown,” he says. “On bare spots where the cover crop seed was washed or blown away, and pigweed is emerging, then you need to apply something like Gramoxone.”
This fall, Mid-South growers can reduce the soil seedbank by roguing plants and removing them from the field. And in the spring, they can make an at-planting application of something like Cotoran, Caparol or Liberty. “Once the EPA grants registration, growers will have more tools, 2,4-D/dicamba cotton, for controlling glyphosate-resistant pigweed,” Steckel adds.
Steckel can be contacted at [email protected].
On corn fields that will be rotated to cotton in 2016, growers can make a post-harvest burndown application following corn harvest, says North Carolina State University weed scientist Alan York.
“We need to constantly chip away at the soil seedbank,” he says. “If you get corn out early enough and have Palmer amaranth that came up late, you still have a long way to frost and the weed can make seed. Applying straight paraquat (Gramoxone, other trade names) seems to be as effective as anything.
“Now is also a good time to take stock of your situation. Note the problem fields, what worked well this year, what didn’t work well and tweak things next year.”
Some Southeast growers also plant cover crops to suppress pigweed. “For a lot of suppression of weeds, you need to form a lot of mulch bed,” York says. “A few growers grow massive cover crops, such as 7-foot high rye, and roll them down, which definitely has a major impact on weed suppression. But even a little cover crop helps some.”
York can be contacted at [email protected].
Don't let your guard down
At this time of the year, don’t let your guard down; that is, don’t let any late-emerged pigweed produce seed, says Texas Tech University weed scientist Peter Dotray. “We have documented that pigweeds that emerge as late as Sept. 1 are capable of producing 500 seeds per plant,” he says. “So for those who have been battling pigweed all year and have done a good job, finish the job and make sure no new seed is allowed to be produced.”
Many Southwest growers use layby treatments that burn down weeds and provide residual activity, including Direx, Layby Pro and MSMA. Additionally, some harvest aid products, such as paraquat, do a good job of burning down pigweeds.
“Later in the spring, make sure you start clean,” Dotray adds. “In the Southwest, we normally don’t see pigweed until close to planting. Eliminate any emerging pigweed either by applying a burndown and/or incorporating yellow herbicides. Some growers incorporate the dinitroaniline herbicides, Treflan and Prowl, in February, March or April, depending on soil conditions. They use full rates of the yellow herbicides and often incorporate them, sometimes using two-pass incorporation. They incorporate in one direction with maybe a spring tooth harrow and come back in a different direction and incorporate again before they bed or hip up.”
Additionally, some growers use a tillage implement like a rod weeder at planting or just a few days before to help them plant cotton without any emerged pigweed plants.
Dotray can be contacted at [email protected].