Aiming At Pigweed

Different Modes Of Action Will Keep Cotton Fields Clean

RECTOR, ARK.
   Multiple methods of cotton weed control, especially to control pigweed, were presented recently by Dr. Tom Barber, University of Arkansas Division of Agriculture extension weed scientist.
   “Before we get into pigweed control, I want to share a little about strategies we use for burndown, getting ready for in-season weed control,” he began. “Burndown is a crucial time, this year we have a winter like we haven’t had in many years. Some say it’s one of the coldest winters they’ve experienced so some of our winter annuals that we’re usually dealing with prior to cotton planting aren’t even in the field now; so we don’t have a lot of vegetation out there currently, especially up in northeast Arkansas.”


Dr. Tom Barber, University of Arkansas Division of Agriculture extension weed scientist,
presents multiple methods of cotton weed control.
Photo by John LaRose, Jr.

   Since these winter annuals may come up a little later farmers need to start early and have a plan. That plan starts at burndown. The time of burndown, usually within 30 days before planting is anticipated, will probably include a residual just to keep the field clean until planting.
   “The trick in weed control in cotton especially in Arkansas, is starting with a clean slate so we want to start especially without any pigweeds in the field,” Barber said. “This past year, as far as percentage acres planted, we had the highest percentage acres planted to a LibertyLink tolerant variety. So almost 50 percent of our acres were planted to a variety that we can spray Liberty over the top; we’ve seen that shift from several years coming out of a straight Roundup Ready Flex market moving into more LibertyLink, again due to the pigweed issues.”
   Last year there was a big shift into a variety of cotton that can survive Liberty and glyphosate over the top. That shift over time came about because varieties in that technology are increasing as far as yield potential.
   “Liberty over the top of the cotton to control pigweed is really the only herbicide option we have for post emergence pigweed control at this time. It’s a tremendous tool for us and pigweed management in cotton,” he noted.
   Every cotton producing county in Arkansas has glyphosate resistant pigweed present so farmers have reason to be concerned. Those who plant a variety that can only accept Roundup are really limiting themselves into a residual control aspect only.
   “We have to overlay or overlap residuals from burndown through planting and then through the rest of the season,” Barber advised. “So we will make several applications of a residual herbicide starting with Reflex preplant. After a half an inch of rain, we can plant behind the Reflex application. If we put Reflex out pre, then we will see some injury on our soils, some significant injury. We’ve had to replant several fields because of that, so Reflex preplant, wait for a half inch of rain, then we can plant when the field is ready.”
   At planting, farmers need to spray something like Cotoran, Direx or Caparol preemerge just ahead of, or just after the planter. So two residuals are needed up front in a Roundup only system for cotton.
   “Then we’ll follow that with Warrant and Dual with our Roundup applications in season,” he continued. “Then follow that with Post Direct as we need it with more residuals and then a layby. This year I’m looking forward to a new herbicide label that we have in cotton, Fierce. Fierce is a combination of Valor and Zidua (pyroxasulfone) and I think it’s going to give us another good option for pigweed control an layby, or once the cotton reaches 16 inches in height.”
   In a LibertyLink system, Liberty will work over the top to control pigweed, but timing is crucial. If pigweed isn’t controlled at less than five inches tall then the percent of control goes way down.
   “We have to calibrate our sprayer for maximum coverage and then we have to time that Liberty application to the pigweed size,” Barber advised. “Really our programs don’t change a lot. If I’m looking at a LibertyLink system or a Glytol LibertyLink system, which we can spray both Liberty and Roundup over the top, what I’m looking at is still using a preemerge or still using that preplant of Reflex. Doing both will help tremendously; if we continue the preemerge program that we have in the Roundup system, that will really buy us time moving into our over-the-top Liberty application.”
   Reflex preplant is a crucial application in cotton production in Arkansas. Using Cotoran, Direx, Caparol pre is also needed.
   “Then come back with Liberty over the top when those preemerges break, and again we have to catch that pigweed small and the problem is it could be four inches today and in two more days it’ll be six inches,” Barber said. “So we really have a small window of application that we can work with and that’s what makes it a little more difficult.”
   One helpful effort in the LibertyLink system is to spray 29 ounces today then come back in seven to ten days with the second application. That greatly improves pigweed control.
   “Our standing recommendation, especially if pigweeds are over four inches tall, is to come back with a second application of Liberty to clean up the field,” he said. “Then we can come back with our layby application and we should be pretty good through the rest of the year.
   “The problem with any of these systems that you’ve heard me discuss several times in this interview is residual herbicides,” Barber added. “There are several things that residual herbicides need for them to be efficient and provide the best control. One is activation. Once we spray a residual herbicide, it’s not doing any good until it is activated. We need rainfall or overhead irrigation to activate these to get the best control or any control out of some of them; and then if it rains too much we can decrease control due to soil movement or leaching.”
   Environment plays a big role in how effective preemerges are; however, they are still crucial in the fight against pigweed in Arkansas. Residuals are keys to rotating herbicide mode of action and fighting to prevent future weed resistance.
   “If we only spray Liberty and nothing else over the top, whether we’re talking about cotton or soybeans, we’re going to quickly lose Liberty herbicide and build resistance to Liberty in pigweed; so my take-home message today in cotton is we’re residual herbicide-based at this point, and I don’t see this changing any time even when we move into the new technology,” Barber said. “Whether we’re talking about the Roundup Xtend system or the Enlist system, I don’t think we’re going to move away from residual herbicides. Nothing that we have in the pipeline that I can see in the next several years is going to be a silver bullet for pigweed control, so we have to always come up with a game plan before we plant.
   “We have to be prepared. We need that preemerge, we need that residual and we must continue to use them in season to buy time,” he said.∆
   BETTY VALLE GEGG-NAEGER: Senior Staff Writer, MidAmerica Farmer Grower