Roundup-resistant Palmer amaranth has been bad in the Texas High Plains and Rolling Plains this year. And it could be worse in 2015.

Weather will play a role, say weed scientists Peter Dotray and Wayne Keeling, both with Texas AgriLife Extension in Lubbock. Dotray also has teaching and research responsibilities with Texas Tech.

“Rain made it seem like the light switch just turned on,” in early summer, Dotray said. “If it’s dry next year, resistant-weed infestations may not be as bad, but we know the source of plant resistance is here.”

Keeling said the job farmers have done this year also will play a role in what they face in 2015. If they had escapes in the field in October, they can expect to see problems next spring unless they take precautions—before the resistant seeds germinate. “They will need to apply residual herbicides at as high a rate as the label allows.” The key, he adds, is to have the herbicide in place before the resistant-weed seeds germinate.

Acceleration of glyphosate-resistant weed pressure has been rapid, Keeling says. “In 2011, we saw resistance start in a little pocket in the High Plains. It seemed to be a niche problem. Over the last three years it became widespread. We thought escapes would be a problem, but this year, it just blew up.”

They agree that early rainfall over the Memorial Day weekend provided near perfect conditions for resistant Palmer amaranth weeds to germinate and get a foothold in cotton and other cropland. “Conditions were ideal to germinate pigweed seed,” Keeling says. That early and heavy rain also may have limited efficacy of pre-emerge herbicides, and possibly prevented timely follow-up as fields dried out. “The two-week extended rainfall period increased the number of weed escapes. That’s not all that unusual here. We often see erratic rainfall patterns and long dry periods.”

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Management is crucial

Management decisions also likely played a role, Dotray adds. “I’ve seen fields side-by-side that were managed differently—residual herbicide use in one field but not in the other. Some fields are extremely clean and others are extremely weedy and most were somewhere in between.”

He says some farmers may have “let a few weeds go and those fields will likely be worse next year.”

Keeling and Dotray say producers who continued to rely on Roundup-only this year may have faced an uphill battle all season as they tried to manage resistant pigweed. “It was an eye-opening experience. A lot of farmers spent a lot of money and some I believe simply walked away,” Dotray says. Cultivation and hoe labor added to production costs.

They say farmers will need to rethink how they manage weeds and may need to go back to older chemistries and technology to clean fields up after the weed explosion this year.

“Roundup still does a good job on many weeds, including some Palmer amaranth,” Dotray says. “But we need to take the pressure off Roundup with residual herbicides that have a different mode of action.”

Roundup-only will not work with resistant pigweed as widespread as it has become in just the last three or four years. A systems approach that includes preplant and pre-emerge herbicides, residual herbicides applied after planting or with Roundup applications and some old technology, including cultivation, hooded sprayers and rope-wick applicators, may be necessary.

Also, within a year or two farmers may have new chemistries and new systems to incorporate into those programs to add other layers to weed management practices.

Farmers may have to spend a little more money to add residual herbicides, Dotray says. “But those herbicides will be critical to help manage resistance.”

Keeling says some farmers, pressured by declining cotton prices, may have eased off on pre-emergence and residual herbicides. “Cutting back on herbicide applications early is a false economy.”

Zero tolerance

Dotray says “zero tolerance” will need to be part of the weed control goal. “That was a concept I heard about 20 years ago, and now the Mid-South and Southeast areas are practicing this idea. Ninety-eight to 99 percent control of Palmer amaranth isn’t good enough. We need to strive for complete control when dealing with pigweed resistance and make sure resistant plants don’t produce seed.”

If farmers can take out 80 percent to 85 percent of their weeds with yellow herbicides and get another 10 percent to 15 percent with residual at-plant materials that leaves only about 5 percent, and takes a lot of pressure off Roundup and other postemergence herbicides.

“Some may think we have weeds resistant to those yellow herbicides but these herbicides are still an effective foundation for good control,” Dotray says.

Incorporation will be a key to pre-emergence herbicide success, however. “They must be properly incorporated to get the best activity,” Keeling says. Mechanical incorporation is better than water, he says, but in no-till situations, irrigation can be effective.

Starting clean next spring will be critical for effective resistant-pigweed management, especially following a year with heavy pressure. “Producers have one chance to start clean,” he says. “Get the weeds controlled ahead of the crop.”

Some farmers may have to re-evaluate tillage practices as they clean up resistant weeds. Transgenic varieties made weed control in reduced-till and no-till production much more manageable, but with reliance on Roundup and a surge of resistant pigweed, many may have to go back to the plow for a year or two to break the resistance cycle. “Producers may not have to deep plow or cultivate every year,” Dotray says, “but tillage may be a way to break up resistance even in reduced- and no-tillage systems.”

But he cautions producers to go back into no-till production carefully. “Resistant weeds show up even on the best -managed farms. Be vigilant and adopt a zero-tolerance level. Be more aware that resistant weeds are here.”

Keeling and Dotray also note that new technology is on the horizon to help manage resistant weeds.

“I think we have reason for some optimism,” Dotray says. “New technology will help, but we cannot simply replace Roundup-only with Clarity-only or 2, 4-D-only systems. I don’t think we will.”

Systems approach

He and Keeling recommend a systems approach that incorporates the new technology, scheduled to be available in 2016, with residual herbicides, possibly some cultivation and wise use of Roundup.

One new product, Zidua from BASF, is already available and several companies are mixing it with other products for broader spectrum weed control. “It’s targeted for pigweed,” Keeling says, “and is extremely effective.”

He says a limited amount of Monsanto’s XtendFlex cotton will be available in 2015 with wider availability expected in 2016. Dow’s Enlist system for cotton is also expected in 2016.

“We have to get to 2016,” Keeling says.  “That’s why residual herbicides are so important.”

Dotray says producers should assume that resistant Palmer amaranth is in every county and possibly in every field, and individual farmers need to develop Best Management Practices on every farm. “Liberty needs help. Dicamba will need help; 2, 4-D will need help. We need to consider all products and all technologies. We can no longer consider glyphosate as our silver bullet, and no new silver bullet is coming.”

As those new products make their way into the mainstream, he’s also learning more about weed biology and ecology. “How long does the seed persist in the soil? How quickly can plants produce seed at the end of the growing season? How many seed will a weed produce? We need to learn more about our weeds.”

He says producers have already learned a lot about product stewardship and those lessons have to carry over to new products and new technology.

Dotray and Keeling agree that farmers, especially cotton farmers, need to start thinking now about how to approach weed control next year and consider spending a few extra dollars on yellow herbicides to start clean and take some of the pressure off Roundup. Both say resistant as well as non-resistant weed populations have been difficult to manage this year.

“It’s been interesting,” Dotray says.