A combination of weather, changing tillage practices and herbicide resistance is creating trouble for farmers across the Southwest this summer. From South Texas, into the Texas High Plains and all the way into Kansas, weeds are proving difficult to manage.

“Weeds in cotton are all over the board,” says Kerry Siders, Texas AgriLife Extension integrated pest management specialist for Hockley and Cochran Counties, up in the Panhandle.

“We definitely have resistant pigweed,” Siders adds. “In fact, we have resistant pigweed in dryland acres where the Roundup Flex system was never used.”

He says some producers learned early on that Roundup would no longer be sufficient to control pigweed (Palmer amaranth). “These producers began using other herbicides such as the pre-plant incorporated yellow herbicides (trifluralin and pendimethalin) or the at-planting white herbicides (direx, Caparol, etc.), as well as other residual herbicides as Staple, Dual, etc.” 

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Siders says weed control is always a fluid situation and always some growers who will hesitate to change their programs. “With any pest issue you have a certain number who may not accept the situation until it is clearly a disaster and is jeopardizing the crop.”

Weather has been a critical factor this summer, especially in the drought conditions that have persisted in the High Plains for the last four years.

“The severe drought has caused weed issues as well,” he says.  And it’s not just the effect dry soil has on herbicides, though that plays a role. “The drought has caused producers to reduce tillage to maintain crop residues for erosion control,” Siders explains. “This has increased the numbers of some weeds such as marestail, which is best control by tillage.”

And dry conditions limit herbicide efficacy. “The dry soil makes it difficult to achieve good incorporation of the yellow herbicides.”

Siders says a lot of factors “have conspired to get us into this current ‘weedy’ situation.”

Oklahoma

Oklahoma farmers are dealing with a rash of weed problems as well, says Oklahoma State University agronomist Todd Baughman. An abundance of moisture has spurred weed growth and limited herbicide efficacy, he says.

“With the blessing of rain, weeds are a much bigger issue in Oklahoma this year. We are continuing to see glyphosate resistance increase as a problem throughout the state.  Where producers are aware of the issue they are using a pre-emergence program.  However, with the abundant rainfall in many places those products have broken down in many cases earlier than normal.”

Baughman says using different products remains the best option. “Where possible growers are using different post emergence products and adding additional residuals to the mix.  In dealing with Palmer amaranth (pigweed) you can't put enough residual out.”

He says controlling pigweed has to be job one in weed management strategies.  “Palmer is a key pest and if you don't control (pigweed), it doesn't matter what other weeds you have; it will out compete them all.”

South Texas farmers have been dealing with herbicide resistance for several years and are looking at various strategies to help manage those hard-to-control weeds.

Jason Ott, county Extension agent in Nueces County, says glyphosate resistant Palmer amaranth is a particularly worrisome weed.

“While growers can do several things to help mitigate development of herbicide resistant weeds, the most effective step is to incorporate a pre-emergent residual herbicide into their weed control arsenal,” Ott says.  A Cotton Herbicide Demonstration at the Corpus Christi Research and Extension Center this spring, in an area with heavy pigweed pressure, compared effectiveness of three pre-emergent herbicides (Caparol, Staple LX and Dual Magnum) to five post emergent herbicide programs. 

Effective treatments

Ott says the three “were extremely effective in reducing, and in some cases practically eliminating, weed competition for the first 35 days leading up to early post treatments.  Plots were re-evaluated 7 days after this application and Dual Magnum and Staple LX both continued to perform as well as or better than any of the early post treatments.”

He says Caparol had begun to lose effectiveness and post treatments were providing better control at 42 days after planting.    

Re-evaluation at 50 days after planting and prior to a mid-post application indicated that Dual Magnum was still just as effective in controlling weeds as an early post treatment of Roundup PowerMax, Roundup PowerMax with Prowl H₂O, Liberty, and Liberty with Prowl H₂O. The pre-emergent Staple LX application was as effective as the post Liberty treatments but not the Roundup PowerMax treatments 50 days after planting.  

“The early post application of Staple LX was not as effective in controlling weeds as any of the above post treatments or the Staple LX pre-emergent application. Little difference between the Caparol application and the untreated check could be observed at this point,” Ott says.  

Another evaluation 27 days after the mid-post application or 77 days post planting showed all the pre-emergent applications had fallen below the control level of any of the post application treatments. All post treatments were providing an equally high (better than 90 percent) level of control 77 days after planting.

Ott says Dual Magnum produced the best control (better than 60 percent) of any pre-emergent material. Staple LX (around 40 percent) was next. “No difference between the untreated check and Caparol could be detected 77 days post planting. For demonstration purposes, postemergence herbicide applications were not made to plots treated with pre-emergent herbicides, although logically growers would have made a post application 50 to 70 days after planting.” 

Ott says the demonstration shows that pre-emergent herbicides provide an effective tool to delay need for a postemergence herbicide application. “It also shows that the postemergence application herbicides in the study all had the same level of control 77 days after application.”

He says rotating chemistries between Roundup, Liberty and Staple LX offer growers a means of reducing weed resistance. And adding a tank mix partner like Prowl H₂O also may decrease potential for herbicide resistance development.        

Kansas

Rainfall also has created weed control issues in Kansas. Late spring and early summer rains have been a boon to crop conditions, says Kansas State University Extension agronomist Dallas Peterson. But weeds are also thriving. That includes Palmer amaranth, pigweed, which is becoming increasingly resistant to glyphosate herbicide.

“We have had numerous calls about poor control of Palmer amaranth with glyphosate this year,” says Peterson, a weed specialist with K-State Research and Extension. “Glyphosate-resistant Palmer amaranth was first confirmed in Kansas three years ago but seems to be exploding across central Kansas this year. Poor control doesn't mean you have resistance, but if the herbicide treatment provides good control of some plants and not others, you may have resistance.” 

It’s not a new problem for Kansas farmers and has been a challenge for many years, including resistance to atrazine and ALS inhibiting herbicides. Roundup was seen as a solution when Roundup Ready crops came on line in the late 1990s. But, as has been the case in other states in recent years, heavy reliance on Roundup has created a situation for glyphosate resistance to become a serious problem.

Producers need to alter weed management strategies, Peterson says.

“Farmers need to use an integrated approach to weed control that utilizes a variety of cultural practices and herbicide modes of action to help control weeds and minimize herbicide resistance. The use of effective pre-emergence residual herbicides is probably going to be very important to help manage Palmer amaranth in the future.”

And they need to be aware of the population explosion that’s possible with just a few escapes each season

“If a producer notices just a few scattered Palmer amaranth that have escaped a glyphosate treatment, it may even be worth removing those by hand to prevent seed production,” Peterson says. Otherwise, resistant biotypes will increase, spread across the field and to other fields through harvest and other means of moving seed.

“If poor control was achieved with glyphosate, it is probably best to assume that it is resistant and plan accordingly, both this year and in the future,” he says. 

Specialists across the Southwest agree that weed resistance has become a more serious problem in recent years, primarily as a result of relying on the same product year after year to control weeds.

A systems approach to weed control, including rotating products, and mode of action, going back to pre-emergence herbicides, and, in some cases, cultivation will be important factors to consider in dealing with resistance issues.

The demonstration plots in Nueces County, Texas, show that a multi-pronged approach is effective.