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dfp-ronsmith-steckel.jpg Ron Smith
Larry Steckel, University of Tennessee Institute of Agriculture weed scientist, prepares for a presentation at the recent Conservation Systems Conference in Memphis, Tenn.

Preventing emergence is weed control goal

Prevention best option for weed control.

Don't let it come up.

Simply put, that's still the best strategy for managing hard-to-control weeds and grasses in row crops.

It's not necessarily easy, according to weed scientists speaking at the recent National Conservation Systems Cotton and Rice Conference in Memphis.

University of Tennessee Institute of Agriculture weed scientist Larry Steckel and PhD candidate Clay Perkins discussed emerging weed control issues during a conference breakout session.

Steckel said glyphosate-resistant Palmer amaranth (pigweed) remains a concern, but he's fielded more calls recently on failures in grass control.

Widespread adoption of Xtend soybean and cotton varieties, Steckel said, may account for fewer calls on dicamba drift. "But we are getting a lot of calls on resistant grasses."

Perkins said main culprits are jungle rice and barnyard grass. Goosegrass and prickly sida pose control challenges as well.

Perkins said he's seen a lot of jungle rice and barnyard grass escapes in Roundup and dicamba treatments in soybean and cotton fields.

The two grasses are similar, he said. Barnyard grass is taller and is awned. Jungle rice is awnless. Jungle rice sometimes shows purple striping and has a purple base and a flat stem.

Resistant grasses

Perkins screened both species in 2018 for glyphosate resistance. "We identified a significant amount of resistance to glyphosate."

"Jungle rice is widespread and moving," Steckel added. "It moves by combine and water; waterfowl also transfer it. We've recently identified jungle rice in middle Tennessee, and we have never seen it there before."

He said they've seen escapes with Roundup applied at 66 ounces per acre.

Antagonism, Perkins said, might be a factor. A mix of Roundup and dicamba results in reduced control. "We see some antagonism with Enlist but not as bad as with dicamba," he said.

Sequential application offers a better solution, but the sequence makes a difference.

"With glyphosate alone, we get escapes," Perkins said. "Glyphosate and dicamba is worse."

Reversing the order, however, improves control. "By applying dicamba first and waiting 72 hours to apply glyphosate, we get near 100 percent control," he said.

He added that weeds at one location escaped clethodim at 44 ounces per acre.

Goosegrass, Seckel said, also escapes Engenia and Roundup. "Adding clethodim is no help. The only way to control it is with separate applications.

High populations

"We have seen a lot of intense populations of goosegrass and barnyard grass. We emphasize the importance of residual herbicides. We have to keep these grasses from coming up."

Fields with multiple resistant grass and weed species offer more challenge. "We can control them," Steckel said, "but it isn't easy."

He said Halex GT has not been a good option for glyphosate-resistant johnsongrass in corn. Going back old school with Accent or Steadfast has produced better control, Steckel said.

Grass control BMP

Perkins offered a Best Management Practices list for grass control.

• Try to keep grass from emerging.

• With cotton, use Prowl, pre-emerge. Dual Magnum, applied post-emergence, is also an option.

• Soybean options: Authority Elite, Authority Supreme, Boundary, Broadaxe, Prefix and Zidua Pro.

• Also for soybean: Anthem Maxx, Dual Magnum and Zidua.

• Fusilade and Assure are better for johnsongrass than clethodim.

• Clethodim is still effective on most annual grasses.

Perkins said Brake is another option. "We see no reason to get away from Dual and we will lean more on Select postemergence."

Pigweed problems

Palmer amaranth pigweed continues to cause problems, Steckel said, "but was not as troublesome in 2019 as grasses. But some pigweed plants have not been as susceptible to dicamba. We saw some escapes when treating 3-inch pigweed. It may be ramping up tolerance. We saw some pigweed in Xtend soybeans and better control across the road with Liberty Link.

"Palmer amaranth control in 2019 often required two applications (of dicamba) and even then control was sometimes sketchy.

"With spray period limitations, it will be difficult to get two applications in," Steckel said. "We can do it in cotton but not in soybeans."

The standard 0.5 pound dicamba rate has not provided as effective Palmer control in 2019 as it did in early years.

"Some applicators have reported pushing the dicamba rate as a response to the 0.5-pound rate showing less Palmer efficacy," he added.

He reiterated the caution not to tankmix glyphosate and/or clethodim with dicamba.

Grass control failures

Most failures, however, have been with grass control. Steckel and Perkins said 67% of weed control calls have included jungle rice and 50% have included barnyard grass.

"Lack of grass control results from a combination of glyphosate resistance and dicamba antagonism with glyphosate," Steckel said.

Some fields, he and Perkins said, included multiple grass species as well as Palmer amaranth.

Prickly sida escapes are showing up in plots treated with XtendiMax and Roundup. "We've gotten a lot of calls on prickly sida escapes," Steckel said. "Python and Classic are good options in soybeans and Staple can help in cotton."

As planting season nears, farmers should begin planning weed control strategies, Steckel and Perkins said. Pre-emergence control should be the overriding goal. If they don't come up, they don't cause problems.

TAGS: Herbicide
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