Step 1: Know which weeds you have. Step 2: Kill them by any means necessary.
Doing the same thing over and over again and expecting a different result isn’t just one definition of insanity; it’s also a recipe for herbicide-resistant weeds.
Over the past two decades, the convenience and effectiveness of post-emergence glyphosate applications led many farmers to abandon past approaches to weed control, which included using a variety of herbicide and tillage options. Increasing pressure from glyphosate-resistant weeds like Palmer amaranth, waterhemp, giant ragweed and kochia have weed experts promoting a return to more diverse practices, similar to those used by past generations of farmers.
“Most of our older farmers who have a base knowledge of all these other herbicides have retired,” says North Dakota State University (NDSU) Weed Control Specialist Richard Zollinger. “Many farmers today only know glyphosate, so we’ve got some training to do.”
Diversifying your operation to combat herbicide resistant weeds can seem overwhelming. With the help of NDSU weed scientists, here is a top-12 list of tips for managing weeds to reduce the likelihood of herbicide resistance on your farm.
- Scout fields before and soon after herbicide applications – Correctly identify weeds and use whatever means are necessary to kill weeds that escape or germinate after chemical application.
- Diversify crop sequences – Crops with different lifecycles, such as winter annuals, perennial crops and summer annual crops, offer different planting and harvest times, more herbicide options and decreased risk of herbicide-resistant weeds.
- Consider weed biology and ecology – Consider tillage, crop sequence, soil fertility, planting date, crop competition, weed-seed longevity and herbicide response as you build your weed-management plan.
- Use effective pre-emergence herbicides – Apply effective pre-emergence herbicides at full rates and include multiple modes of action. Pre herbicides reduce weed emergence and allow flexibility in timing of post herbicide applications.
- Use effective post-emergence herbicides – Apply herbicides that include multiple modes of action in tank-mixes or in sequential applications.
- Use full herbicide rates – Full rates kill weeds and dead plants cannot produce resistant progeny. Reducedrates allow plants with low-level resistance to survive and produce offspring with higher levels of resistance.
- Spray weeds when they’re small – Small weeds, those less than 3 inches tall, are generally more susceptible to herbicides than large weeds.
- Practice zero tolerance – Scout fields after row closure and kill uncontrolled weeds, including by pulling them manually, if necessary. Seed from escaped weeds will contribute to the weed seedbank.
- Control weeds in field perimeters and non-crop areas – Weeds surviving a partial herbicide dose on field borders can be a repository for the introduction of resistant weeds into a field. Control weeds in all areas of the field where crops are not growing, including field edges, fence lines and waterways.
- Rotate herbicides with different modes of action – Diverse crop rotations can introduce herbicides with different modes of action to delay herbicide resistance.
- Use good sanitation – Clean tillage and harvest equipment to ensure weed seed will not be transported between fields.
- Evaluate – Review your weed-management results at the end of each season and revise to improve weed control next year.
These practices that can help, but fighting herbicide resistance starts with farmers knowing what they are up against.
“If you don’t know the enemy, how can you prepare to fight them,” asks Van Larson, a crop consultant based in Rochester, Minnesota.
Weed species respond differently to herbicides and tillage practices, so scouting and understanding which weeds are present sets the stage for farmers to develop a plan for eradicating the problem.
“Our growers deal with two to four different grassy weeds and two to six broadleaf species,” says Louisiana State University AgCenter Weed Scientist Daniel Stephenson. “One might be resistant, but you still have five others to deal with. Not all species are controlled by one product, so you have to know what weeds you have.”
Take Action is an industrywide effort involving agricultural organizations, agri-businesses and researchers to fight herbicide resistance. The Take Action website features resources to help farmers identify weeds while highlighting options for treating them.
When it comes to diverse practices farmers should employ, weed researchers are nearly unanimous in their call for the use of herbicides with different sites of action and different chemistries. Many recommend starting off with a pre-emergence residual herbicide, followed later by a post-emergent herbicide with a different chemistry. Increased tillage is also an option for control of some weed species.
“Farmers need to use every option available,” Larson adds. “It needs to be effective, but also reasonable to implement.”
Integrated weed management uses multiple strategies for controlling weed populations, but not all are realistic for all farmers. However, adopting even a few diverse practices can keep weeds off balance and give growers the upper hand in battling herbicide-resistant weeds.
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