Making Mid-Season Weed Decisions

By mid-season, most growers know the level of glyphosate resistance in their fields. 

 

“You’re going to have to start making some decisions about using a total post-emergence system, or going ahead and getting the hoods hooked up to control the weeds in your field,” says Dr. Chris Main, University of Tennessee Associate Professor and Extension Cotton Specialist. “When using a hooded sprayer, add a residual herbicide with glyphosate, glufosinate, or paraquat to keep your fields weed-free the remainder of the season.”

 

Says Dr. Larry Steckel, Tennessee Extension Weed Specialist: “A lot of folks have gotten rid of their hoods and are very reluctant to get them back out again. I can understand why – they’re a pain to run, they’re slow, they breakdown a lot. 

 

“But in order to manage these glyphosate-resistant weeds, we’ve got to go back to the hoods,” he continues. “The old ones were good for what they did when we were basically running glyphosate and Direx through them. You didn’t need a lot of real good coverage with them. But with these Palmer pigweeds that you’re trying to take out with contact herbicides – Gramoxone or Valor, or Direx and MSMA – we need good coverage.”

 

Steckel adds that while the older models of hooded sprayers are “better than nothing,” the new-style Model 915 hooded sprayer from Willmar is more suited for today’s conditions. Using grower input, Willmar worked with Monsanto to develop the new sprayer design.

 

“We have a number of growers who have gone to the new hood styles that are really designed more for contact herbicides,” explains Steckel. “The new-style hoods are more upright and they have a skirt on the front so you don’t knock the pigweeds over, or break them off. The pigweeds go in sitting more upright and you can get good coverage. 

 

“Plus you’re not breaking off any of the stems and losing performance. They just do a better job. There are a lot of the new hoods in place in the Mid-South. They’re really built for Gramoxone under the hood, and that works really well on these big pigweeds. There aren’t many things that work well on big pigweeds, but that does.” 

 

Steckel says many of Tennessee’s growers did all the right things early, which could pay dividends as the season progresses. 

 

“We had a lot of herbicides put out pre-plant; a lot of Reflex put out pre-plant this year. A lot of it got rain, so it should be pretty timely, and it should give four or five weeks of residual (activity),” explains Steckel. “If they could get another (residual) overlay – like a Dual or Warrant – it should carry them to (applications with) the hoods. In a lot of cases, they won’t have to go over the top with something like Ignite, like some of them are thinking about doing with WideStrike cotton.”

 

Most hooded sprayers are also designed for post-directed applications, giving growers a one-pass opportunity. And Steckel says the size of the cotton and the weed species are the deciding factors on what herbicides to use post directed. 

 

“You can’t use a lot of the contact herbicides unless the cotton is barked up. If you can get it barked up – four to six inches tall – you can put Valor out and be just fine,” he explains. “But if you only have a couple of inches of bark, you don’t want to put anything out that will scorch the green material on the stems. If it’s not barked up, you need to move to something like Caparol and MSMA, or Direx and MSMA. Those aren’t going to hurt the cotton and will do a pretty good job on pigweeds if they’re not too big – five to seven inches tall.

 

“The other thing we need to look at now is that we’ve just identified glyphosate-resistant goosegrass in several counties in Tennessee,” Steckel continues. “So, if your glyphosate applications are not picking up goosegrass – and it should be obvious if it’s still growing – you have to think about switching gears and get a  graminicide out there, like Select or Fusilade or Poast. 

 

Over the Top

For growers who don’t have access to hooded-sprayer equipment, about the only option is to go to over-the-top using either a Roundup- or Ignite-based system.

 

Like both Main and Steckel, Monsanto recommends adding residuals when making applications of Roundup, and offers growers incentives that could add up to $20 per acre in herbicide cost savings. Through its Roundup Ready Plus program, rebates are offered on Valor or Reflex ($4 per acre); Cotoran, Cotton Pro and Caparol ($3 per acre for two applications), and Warrant or Dual (from $3 to $4.50 for two applications). A one-dollar per acre rebate is also offered on the contact herbicide Gramoxone Inteon. 

 

In an Ignite-based system, Main says Ignite should be applied at the first sign of pigweed emergence.

 

“All the research shows that the earlier you can get control, the less chance of yield loss you’re going to have,” he explains. “When the crop is about to bloom, or just after bloom, is when we start to see yield penalties when using Ignite on WideStrike cotton.”

 

Ignite applied to WideStrike cotton, Main says, provides a safety net when glyphosate is no longer effective. 

 

“If this is the first time a grower has experience with glyphosate resistant pigweed and he has planted a WideStrike cotton variety, he can spray Ignite over it and get enough control to manage a bad situation,” he continues.

 

“There’s another option for the grower who knows he has glyphosate-resistant weeds, or suspects that he’s going to have them this year. He’s made all the correct decisions – he’s put out pre-plant burndown with a residual, and it’s done a good job. But he’s had several inches of rain and planting has been delayed. All of his residuals have dissipated. Then his cotton is up to a stand and here come pigweeds. He knows they’re resistant, and his only option now is to come in with Ignite on WideStrike cotton, or on some of the increasing acres of LibertyLink cotton.”

 

Main adds that more LibertyLink varieties have been planted in Tennessee this year than in all other years combined. 

 

“As more varieties like Stoneville 4145LLB2 come to market, we will continue to see more acres of LibertyLink cotton,” he says. “Tennessee growers have recognized the value of an Ignite-based system, but they’re hesitant to go to full-blown LibertyLink right now. They’re waiting on GlyTol/LibertyLink so they can have protection from Roundup drift, or the accidental ‘oops, we sprayed our Liberty Link cotton with Roundup’ situation.”

 

GlyTol/LibertyLink varieties will be tolerant to both glyphosate or Ignite, and  experimental varieties are in field trials this year.