Battling herbicide-resistant weeds

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Managing herbicide-resistant weeds is a growing problem in the Unites States that will eventually force growers to adopt strategies they either have never used before or used a long time ago. The concept is reality, according to Stephen Powles, Ph.D., director, Australian Herbicide Resistance Initiative.

"Herbicide resistance in Australia is more advanced than in the United States," Powles said. "But the U.S. is kind of at a crisis point. Australia had to learn the hard way how quickly resistance can develop. Diversified programs and being smart about whether to use chemical versus non-chemical methods have helped the Australian farmer, and the U.S. farmer could learn from that experience. It was a very painful lesson for Australia."

Today, Australian farmers use multiple strategies to combat weeds. Some of those strategies include judicious cultivation, harvesting weed seeds and hand weeding.

Powles described how many Australian farmers have accomplished reducing weed seed. "Farmers added a metal part to their harvesters that allows the machines to either spit weed and other harvest debris into chaff carts or into narrow windrows that later are burned to destroy weed seed," Powles said.

Australia has been using alternative weed management strategies for 10 years, according to Powles. He likens the changes farmers had to adopt as being similar to the changes the world had to face since the terrorist attack of 9/11. "After 9/11, the world faced huge changes in security," he said. "That day showed everyone that the world had changed. The same is true with herbicide-resistant weeds and farmers today. Herbicide-resistant weeds have forced us to take measures we didn't have to take previously. So, now we must adapt."

Powles was interviewed while in the United States on a trip sponsored by Syngenta, and many of his philosophies are incorporated in information at

Research in the U.S.
Powles has coordinated herbicide-resistant weed research and the best methods for preserving herbicide effectiveness with researchers all over the world. In the United States, he's worked with Jason Norsworthy, Ph.D., weed scientist, University of Arkansas, and Stanley Culpepper, Ph.D., Extension agronomist, University of Georgia.

"We are currently working with researchers from Australia to identify tactics that have been effective there and the potential use of those in U.S. cropping systems," said Norsworthy. "There is a wealth of non-chemical tactics that have been used in Australian cropping systems. First, one must realize that the wheat production system of Australia is quite a bit different from how we produce crops in many areas of the U.S.

With that being said, research at destroying weed seed present at harvest was initiated in Australia and is being implemented. We are currently initiating research based on the successes and failures that have occurred in Australia."

Norsworthy explained that he is using modeling techniques to understand the factors that contribute to the evolution of glyphosate resistance, particularly glyphosate-resistant Palmer amaranth, which is now widespread in the South, and glyphosate-resistant barnyardgrass, which has not yet evolved into a major problem yet. The modeling work has helped identify knowledge gaps in understanding the biology of these two weeds. Modeling has been instrumental in helping to develop concepts that can be tested in the field as well as understanding the value of non-chemical approaches to weed management.

Culpepper explained he has coordinated with North Carolina State University on research in the United States for alternatives as well as helped the USDA's Natural Resources Conservation Service launch a pilot project this year. The project is in its first year of examining the impact a heavy cover crop has on reducing the emergence of pigweed.

Top Strategies for Control
Norsworthy offered his top 10 tips for managing resistant weeds. 

  • Understand the biology of the weeds.
  • Use diversified approaches to weed management with a goal of reducing the soil seedbank.
  • Plant into fields that are free of weeds at planting.
  • Overlay residual herbicide prior to canopy formation (don't allow residual herbicides to become ineffective before deciding to apply another herbicide).
  • Use multiple efficacious herbicides against the most troublesome or resistant-prone weeds in the field.
  • Apply the labeled rate of the herbicide, which also means that timing is critical.
  • Integrate cultural and mechanical techniques into current weed control programs. There are many strategies that can be used. The effectiveness and applicability of these strategies will differ by crop, weed and region.
  • Prevent seed production! This will likely mean some hand removal.
  • Manage weeds along field margins to prevent an influx of weeds into a field.
  • Minimize the in-field movement of weeds.

Both Culpepper and Norsworthy agree that the strongest strategy is knowing the plant's biology and understanding its strengths and weaknesses.

"We have found the few weaknesses that glyphosate-resistant Palmer amaranth has," Culpepper explained. "By using its biological weaknesses, we can devise a complex management system that understands that the life of this seed is short lived. We know that it has a shallow emergence depth. If the seed is deeper than three inches, it won't germinate. And the seed requires lots of sunlight to germinate. So, if we can use residue to block the sunlight from reaching the soil, then Palmer amaranth emergence is immensely reduced."

Norsworthy explained the benefit of integrating multiple strategies. "In regards to barnyardgrass and Palmer amaranth, rotating from cotton to drill-seeded/double crop soybean is an effective management strategy. The presence of the wheat crop in double crop soybean suppresses early season emergence of summer annual weeds. Secondly, the delayed planting of soybean means that fewer weeds will emerge in the soybean crop and in turn be exposed to herbicides such as glyphosate in Roundup Ready soybean. Thirdly, drill seeding soybean will ensure that crop canopy formation occurs four to five weeks sooner than the conventional practice of seeding in rows, which will in turn suppress weeds that emerge within the crop and also result in little to no weed emergence once canopy formation occurs (i.e. few weeds in soybean that will need to be controlled or exposed to selection by a herbicide)."

In Norsworthy's example, at least four non-chemical tactics are employed (crop rotation, delayed planting, physical suppression from the wheat and increased competitiveness by drill-seeding soybean).

"The goal here is preventing escape weeds (those few individuals that may have evolved resistance) from producing seed," Norsworthy said. "Once a resistant individual produces seed, it becomes quite difficult to regain effective use of the herbicide to which resistance has evolved."

Rising Costs Will Force Change
Culpepper explained that alternative strategies will continue to grow in popularity due to the rising cost of controlling herbicide-resistant weeds.

"Glyphosate-resistant Palmer amaranth has already forced us to change in Georgia," Culpepper said. "In Georgia, it's gone from $20 per acre for herbicides to $63 per acre now plus additional costs in hand weeding. In fact, in 2010, 92 percent of our cotton growers hand weeded 54 percent of our cotton crop at a cost of nearly $16 million. This is following an expense of $15 million for hand weeding during 2009. Although these programs are very effective, that input level is not sustainable long term for our farmers. They will have to find more economically sustainable alternatives."

The search for these alternatives by growers are underway and being pursued aggressively.

Norsworthy agreed with Culpepper that hand weeding is increasing in the Mid-South. "Last year in Arkansas, 70 percent of cotton acres had at least one hand weeding," he said.

"We will definitely use more herbicides as a result of glyphosate resistance," Norsworthy said. "As we move back to older herbicides and techniques, some producers will be forced to have fewer acres to achieve the needed level of management. Also, some of the newer technologies such as Liberty Link will aid in the management of glyphosate-resistant weeds in crops like soybean and cotton."

Although herbicide-resistant weeds are more of a challenge in the Mid-South and Southeast than the Corn Belt, both Culpepper and Norsworthy warned that as resistance spreads, those states will likely need to adopt more of these strategies than they ever thought they would have to.

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