Now is a good time to map out a weed control program for the whole 2019 season, especially for controlling resistant weeds. And the plan should be flexible to respond to weather and other conditions, say university weed scientists who offer several best management practices for controlling these weeds.
Larry Steckel, University of Tennessee Extension weed specialist, says glyphosate-resistant pigweed is offset by good options such as cotton varieties with technology traits that provide resistance to dicamba, 2,4-D, and Liberty over-the-top applications.
“We have some good postemergence options for pigweed,” he says. “However, we’re now having problems with glyphosate-resistant grass species, including junglerice, johnsongrass, and goosegrass, which are the most prevalent around the Memphis area.
“Several growers have been adding Select to Roundup, which has worked fairly well, but we have some fields where that tank mix is not working well now. So, we now recommend never letting these glyphosate-resistant grasses emerge — we want to start clean. Cotoran and Caparol are some of the main herbicides we use, and some growers use Brake on the grasses. And early postemergence, when growers are going out with Orthene for thrips, is a good time to put out Dual or Warrant. Then two weeks later, spray again, never letting these grasses emerge.
“In some fields we’re wondering if clethodim (Select) is working. We’re now looking at some of the aryloxyphenoxypropionate (fops) like Assure or Fusilade.”
PIGWEED AND RAGWEED
Charlie Cahoon, North Carolina State University Extension weed specialist, says herbicide-resistant Palmer amaranth and ragweed are public enemy No. 1 in cotton. He is especially concerned about PPO-resistance.
To minimize resistance, he recommends using multiple modes of action upfront. “We’ve been recommending pre-emergence tank mixes for a long time,” he says. “Some growers use just one herbicide, such as Reflex or Direx, but we recommend mixtures such as Reflex + Direx or Reflex + Warrant or Reflex + Brake. We definitely have fewer weeds to manage later if we apply a good pre-emergence herbicide mixture up front.
“At postemergence, Liberty works very well on both pigweed and ragweed — if we can spray timely. We also get good control with XtendiMax and Enlist. And we remind cotton growers of the importance of using and overlapping postemergence residuals such as Warrant, Dual, and Outlook — especially when managing Palmer amaranth.
“However, these three residual herbicides don’t work on ragweed. Ragweed is an earlier-emerging species, so we can really improve control by adding a residual, normally Valor, with our burndown in March. Since we’re concerned about PPO-resistant ragweed, we normally suggest something like Cotoran or Cotoran + Reflex at planting.”
MANAGE WITH RESIDUALS
Wayne Keeling, Texas A&M AgriLife Research weed scientist, reminds growers that both the Enlist and Xtend systems again require users to undergo training this year. “Regardless of the cotton technology planted, starting clean is critical,” he says, “especially in no-till fields that have winter weeds that need to be cleaned up prior to planting. Plant into a clean seedbed, either through tillage and/or herbicide applications.
“And do everything possible with residuals — PPI, pre-emerge, or postemerge — to control Palmer amaranth and take the pressure off the number of escapes that we have to deal with from a postemergence standpoint. To avoid or minimize resistance problems, we need to manage weeds with residuals and not overly rely on the newer technologies. Overlay the residuals, rotate chemistries, and make postemerge treatments that clean up after you have done a good job with residual herbicides.
“Finally, remember to time applications by weed size. I know we get busy and there are limited spray days, but we need to treat weeds when they’re small and actively growing to maximize herbicide efficacy.”
Palmer amaranth is Arizona cotton’s worst weed problem across the state, according to Bill McCloskey, University of Arizona Extension weed specialist. He recommends starting with a clean field, primarily through tillage.
“However, if you’re one of the few state growers who are double-cropping and no-tilling, then you need to use minimum-till and an effective burndown treatment. Just make sure no resistant weeds are out there when you plant.
“Use pre-emergence herbicides; pendimethalin and Prowl work great for us. You can also use Caparol and Diuron. The point is to use a pre-emergence herbicide to give you the first mechanism of action.”
In a postemergence weed control program, McCloskey recommends using multiple mechanisms of action. “For example, if you don’t have resistant Palmer yet, either the Enlist Duo premix of 2,4-D and Roundup or Enlist One tank mixed with Roundup will work,” he says.
“Dicamba — either Engenia or XtendiMax — tank-mixed with Roundup also works well. If you already have resistant Palmer amaranth, which is resistant to glyphosate and ALS inhibitors (Staple and Evoke), I recommend going with Enlist One tank-mixed with Liberty, because you can’t tank-mix Liberty with Engenia or XtendiMax.
“Remember to spray when weeds are small for best control. Additionally, use one or two cultivations. Even just one early pass across the field can make a big difference with a big resistant weed population. And if possible, make a layby application.
“Finally, have zero tolerance for weed seed production, particularly if you have Palmer amaranth. Sanitation includes cleaning equipment.”