An Integrated Strategy Helps Counter Weed Resistance
“Selection pressure is a numbers game, and it’s influenced by the seed bank,” says Stanley Culpepper, Ph.D., an Extension agronomist of weed science at the University of Georgia. “You can’t let weeds go to seed, especially those that cost you money.”
No one knows this better than farmers in the South who have been battling glyphosate-resistant Palmer amaranth for 15 years. “This weed has cost Georgia’s cotton industry well over a billion dollars,” Culpepper says. “That’s why you hear stories like the father and son who were on their way to church, saw a pigweed in their field and stopped to pull it, even though they were wearing their good clothes.”
Sobering statistics also help tell the story. According to the International Survey of Herbicide Resistant Weeds, weeds have developed resistance to 23 of the 26 known herbicide modes of action (MOAs).* In the South, Palmer amaranth tops the list of most problematic resistant weeds. In the Corn Belt, the biggest threats include marestail, waterhemp, Palmer amaranth and giant ragweed.
“We don’t have a plethora of new tools coming onto the market to control these weeds,” says Bill Johnson, Ph.D., a professor of weed science at Purdue University. Johnson and many of his colleagues agree that the days of easy weed control are over.
“In some ways, it’s back to the future with weed control,” says Dane Bowers, technical product lead for herbicides at Syngenta. “There are no silver bullets on the horizon, so for a better future, we need to tap into practices that have historically worked well and integrate them into a program that includes today’s best herbicide technologies.”
Field-Tested Tips for Success
Aaron Hager, Ph.D., an associate professor of weed science at the University of Illinois, agrees that it’s more important than ever for growers to follow integrated weed-management (IWM) practices and prevent weeds from adding to the seed bank. “No seed, no weed,” he says.
Proven IWM practices include:
- Crop scouting. Basic weed identification is step one to knowing what weeds are invading your fields, Hager says.
- Starting clean. An effective pre-emergence herbicide helps control weeds before they take root. Boundary® 6.5 EC herbicide from Syngenta contains two modes of action and controls weeds, such as waterhemp and Palmer pigweed that are resistant to glyphosate, PPOs and ALS inhibitors. “This is one of the best products available to deliver proven early-season grass and broadleaf control in soybeans in part because it contains two non-glyphosate, non-PPO and non-ALS modes of action,” says John Appel, product marketing lead for herbicides at Syngenta.
- Incorporating multiple effective modes of action. Diversifying herbicide MOAs provides effective weed control and helps preserve the effectiveness of herbicides for future growing seasons. Field research shows that a tank mix can be more effective than rotating herbicides from season to season or within the same season. “When using an average of 2.5 MOAs per application, you are 83 times less likely to have resistance, compared to using only 1.5 MOAs per application,” Hager says. Tests show that Prefix® residual herbicide from Syngenta, which provides pre-emergence weed control in soybeans, works well with a tank mix for post-emergence spraying. “Prefix is all about flexibility,” Appel says. “With two modes of action, Prefix controls glyphosate- and ALS-resistant weeds. This helps maintain clean fields and increase yield potential throughout the season.”
- Knowing the group numbers. The Weed Science Society of America has adopted a herbicide-group numbering system to help growers select the right herbicide program for their farms. The system allocates a unique group number to each herbicide MOA. For example, Acuron® corn herbicide from Syngenta contains four active ingredients and three effective MOAs (Group 5, 15 and 27) for multitargeted control of the most difficult weeds in corn. “You don’t need to remember which herbicide belongs to which mode of action,” Johnson says. “Use the classification numbering system that’s listed on the labels of herbicide crop protection products.” This numbering system also shows the benefits of incorporating BroadAxe® XC herbicide from Syngenta into a soybean weed-resistance-management program. Classified as both a Group 14 and 15 herbicide, BroadAxe XC helps maximize soybean yield potential through early-season weed management and long-lasting residual control. “BroadAxe XC combines multiple modes of action and is especially strong on Palmer amaranth, waterhemp, morningglory and kochia,” Appel says.
- Using good stewardship. Following proper application timing, using full rates and acknowledging the importance of residual herbicides, regardless of the trait platform, are the keys to effective weed management in corn, soybeans and cereals. “Don’t pick weed-control strategies just because they’re cheap,” Johnson says. Managing resistant weeds is a “pay me now or pay me later” situation, Bowers adds. “It may cost a little more now to take action, but you can save yourself a lot of money, time and frustration later.”
- Adding diversified weed-management strategies. Time-honored practices like mechanical weed control and crop rotation can improve weed management. Additionally, agronomic practices—from narrow rows to increased plant populations—that help crops out-compete weeds can play a role. Culpepper also notes that cover crops can be part of an effective, diversified weed-control strategy.
“We need to tap into practices that have historically worked well and integrate them into a program that includes today’s best herbicide technologies.”
Responsible weed resistance-management practices also extend to a proprietary resistance model from Syngenta that’s designed to predict the sustainability of a specific weed-control program in the face of Amaranthus species with resistance to multiple herbicide MOAs.
“No one has ever taken this approach before,” says Joe Wuerffel, Ph.D., a research and development scientist in the weed-control group at the Syngenta Vero Beach Research Center in Florida. He is collaborating with full-time herbicide resistance modeler Chun Liu, who works at the Syngenta Jealott’s Hill research facility in the United Kingdom, on this computer model to assess how long it will take for resistance to develop.
“By incorporating field and greenhouse data, the system can mimic the full life cycle of every single weed seed in the model,” Wuerffel says.
The model simulates growers’ fields and focuses on soybeans and Amaranthus resistance. “It evaluates whether there’s low, medium or high risk of herbicide failure,” Wuerffel says. “It’s helping us predict the longevity of active ingredients in Syngenta crop protection products.”
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All these resources play a role in managing weed resistance. “The ability to manage resistance is in your control,” Bowers says. “The sooner you start, the better off you’ll be.”
*Note: Site of action is the more proper term, but mode of action may be more familiar terminology. Mode of action is the series of events that lead to plant death, while site of action describes the actual target site in the plant where the herbicide acts.
Resistance Fighter Promotes Sustained Herbicide Performance
Since the 1990s, the Resistance Fighter program has been raising awareness of the weed resistance issue and leading the way in finding measures to combat it.