Growers have been battling weeds since the dawn of farming, according to David Yarborough, Ph.D., wild blueberry specialist and professor of horticulture at the University of Maine in Orono, Maine.

“Back in China when they first were developing agriculture, they used to hand-weed the rice seedlings,” Yarborough said. “The weed seedlings mimicked the rice seedlings so they couldn’t tell the difference between the weed and the rice when they were trying to weed it. So it enabled the weed able to escape.”

Whether it was the weed population increasing, becoming indistinguishable from the rice seedlings, or whether an herbicide was killing a weed through a chemical formulation, Yarborough said that weeds can develop resistance regardless of the process used to kill it.

“There really is only one cause [which] is selection pressure. That doesn’t really have to do anything with an herbicide. Herbicides just happen to be what’s putting pressure on the plants,” Yarborough said.

However, there’s one additional consideration where herbicide resistance can develop. “They can also come via transfer of herbicide-resistant material,” Jatinder S. Aulakh, assistant agricultural scientist II with the Windsor, Connecticut-based The Connecticut Agricultural Experiment Station, said. “This can consist of seeds, pollen or other material from plants used in reproduction.”

How herbicide resistance develops

Herbicides attack and kill plants through a mode of action. As Dwight Lingenfelter, Extension associate of Weed Science at Penn State University, gives an example of a photosynthesis inhibitor, which halts photosynthesis in a plant. For this mode of action, the herbicide acts on a “site of action,” which is a particular enzyme.

Lingenfelter uses atrazine as an example to illustrate how the site of action functions within the herbicide’s mode of action. “It’s considered a photosynthesis inhibitor. It actually does its inhibition on the quinone site, a specific site within the photosynthetic process.”

Herbicide resistance often happens through enzyme mutation, what Lingenfelter refers to as the “lock and key method.”

“The herbicide fits nicely into the certain enzyme and actually would stop production of a key amino acid and that would eventually stop protein production,” Lingenfelter said. “That enzyme would be altered so that herbicide could not be the key to fit in that lock. So the herbicide has no activity to stop the function of the plant for the enzyme to keep producing, even after the herbicide is even applied.”

Other ways herbicide adaption can take place, as Lingenfelter said, is by herbicide sequestration within the plant or by increasing enzymes an herbicide is intended to block.

Weed resistance in New England, New York and Pennsylvania

Aulakh speaks to the current state of herbicide resistant weeds in New England.

“We have two weed species, common lambsquarters and redroot pigweed, which are known to be resistant to photosystem II inhibitor herbicides,” Aulakh said. Herbicide resistance can occur “if a grower is continuously relying on a single mode of action or one herbicide year after year, and he’s not rotating his herbicide use with other available chemistries, and he’s also not employing other weed management matters, cultural and mechanical matters.”

Using the State of Connecticut as an example, according to The International Survey of Herbicide Resistant Weeds, redroot pigweed, a dicot weed, is known to have resistance to atrazine and cyanazine, both photosystem II inhibitors. However, other types of herbicides including alachlor, metolachlor and pendimethalin are all noted methods of action to work against this weed.

Herbicide spectrum and resistance considerations

When applying herbicides, application considerations depend on what it kills and how often it’s used. For example, Yarborough explains that glyphosate is a broad spectrum herbicide, one that kills both broadleaves and grasses. Yarborough said that there is evidence of Roundup resistance seen because it’s being used in many applications in a single season and on multiple crops over many acres on genetically engineered crops.

“Glyphosate inhibits amino acid synthesis or protein synthesis. There’s a specific enzyme, EPSP, and glyphosate affects that enzyme and stops production of it,” Lingenfelter said. “In turn, that enzyme is necessary to build amino acids and those amino acids build proteins in that plant. Once you stop that EPSP enzyme, it can’t build three key amino acids and protein production is stopped within that plant.”

When there is incidence of glyphosate resistance, Yarborough recommends going back to 2,4-D. Although it’s an older material, he said because it has a different mode of action than glyphosate, it can combat glyphosate resistance because it takes off excessive selection pressure.

Herbicide selectivity

Determining which herbicides work also depends on the type of crop that’s being grown.

“It has to do with how they work. [With] the photosystem inhibitors, which all plants have photosystems, that will be a fairly broad spectrum. [However] there are particular materials that only work on dicots and monocots. Regulator type [herbicides] like 2,4-D, work on dicot or broadleaf weeds,” Yarborough said, adding that’s why it’s safe to use on corn.

However, ACCase inhibitor type herbicides impact monocots, which include corn, and not dicots due to “different chemical pathways,” as Yarborough explains. “You couldn’t use these ACCase inhibitors in corn because it would kill the corn.”

Read more: What’s Right for Your Farm: Herbicides or Fungicides?

Example of herbicide use leading to resistance

Aulakh said that pre-emergence weed control for apple growers can be accomplished with applications of pendimethalin, such as dinitroaniline. However, if the same herbicide is used year after year, weeds can develop resistance. Similarly with post-emergence weed control, using glyphosate between rows continuously can develop resistance. This is evidenced by California growers experiencing glyphosate-resistant rigid rye grass and horseweed weeds. Regardless of the type of herbicide or how it’s applied, he brings up a bigger-picture recommendation to reduce herbicide resistance.

Glyphosate provides a nonselective, broad spectrum of weed control against grass and broadleaf weeds. Although Aulakh said there are not any mass-grown crops in Connecticut that are resistant to it, glyphosate should still be used only in certain cases.

Glyphosate can be used as part of ground management for orchards, as the University of Massachusetts pointed out in its 2012 New England Tree Fruit Management Guide. Glyphosate is a recommended herbicide that can remove perennial grasses and weeds during the creation of an orchard floor. Application caution is needed as the report advised that glyphosate should not touch bark, leaves and rootsuckers, as it can negatively impact recently these parts of a tree and recently pruned sites.

“Our use of glyphosate is mainly as a directed or filtered application sometimes in orchards and even Christmas tree plantations, but not over our crop lands, because they are sensitive to it,” Aulakh said. “If we want to safeguard this herbicide [and] keep it as effective tool for a long time in our weed-management kit, we need to integrate other weed-control methods.”

When glyphosate is not the right choice, because it’s no-selective or as Aulakh said, “it can kill any plant material it comes in contact with that is green, actively growing at the time of its application,” there are other herbicides that work more narrowly.

Clethodim or sethoxydim are recommended as grass herbicides as they don’t generally impact broadleaf plants. Conversely, 2,4-D and Triclopyr are recommend as herbicides for broadleaf applications as they generally don’t impact malevolent grasses.

An example of an herbicide that’s effective on broadleaf weeds for watermelons, tomatoes and cucumber crops is halosulfuron.

“Even if you can rotate your herbicides or then mix two different modes of action, then you can drastically reduce the chances of selection of herbicide-resistant weed,” Aulakh said. “The best method is to have knowledge of all the available herbicides and other known chemical or mechanical or cultural techniques, which you can employ for combating weed in your situation.”

Aulakh explained how crop rotation can help increase the use of different herbicides. By using different herbicides, by growing different crops, it decreases the risk of increased resistance, along with different crops providing nonchemical herbicide control.

Nonchemical strategies to combine with chemical herbicides

  • Crop rotation. As Aulakh explained, although there are few pre-emergence herbicides for carrots, there are postemergence herbicides that can selectively control grasses. Using crop rotation by growing corn the following year Aulakh said that broadleaf weed herbicides can be used pre- and post-emergence, where they couldn’t be used in a carrot crop. Growing crops can also provide opportunities to rotate herbicides and provide nonchemical suppression through their growth schedules and their growth cover.
  • Lingenfelter recommends using two steps of tillage as another nonchemical approach to combat herbicide resistance. Performing primary tillage can bury surface weed seeds between 6 and 10 inches underground, making them less likely to grow immediately. With a corn crop, “then you have secondary tillage or cultivation, which would occur in the crop itself, [when] the corn is 8 or 10 inches tall,” Lingenfelter said. Cultivators would “cut off or tear out to control the weeds as would be controlled as they’re small — you’re dislodging them or uprooting them.”
  • Weed resistance development. “Along with using surface mulching to smother the weeds and prevent seed germination that sanitation can reduce malevolent weeds,” Yarborough said. He pointed out that during harvest weed seeds can attach to equipment. If machinery is sanitized through steam cleaning, weed seeds can transfer from field to field.”
  • Cover crops. Another nonchemical weed management approach is to use cover crops. Yarborough pointed out that a cover crop, such as winter rye, can start to be grown in the fall after the crop is harvested. Acting as a “living mulch,” Yarborough explained that cover crops prevent weeds by outcompeting weeds for soil resources, along with blocking sunlight that would germinate them otherwise. Along with helping out with erosion resistance in the winter months, the cover crop could be plowed in, and either re-plowed again as weed seeds are disturbed or use a nonselective herbicide such as glyphosate to kill any residual weeds.

Read more: Conquering Realities of Herbicide Resistance