Adopt an herbicide resistance management plan to fight profit-robbing horseweed

By Vicky Boyd


California recently gained the dubious distinction of being the 11th state in the nation to confirm glyphosate-resistant horseweed, also known as marestail.

Regardless of the herbicide program you’re using or the weed spectrum you’re trying to control, the recent discovery should drive home the need for a strong herbicide resistance-management program, says Rick Roush, director of the University of California’s Integrated Pest Management Program in Davis.

It should include scouting, rotating herbicides with different modes of action, appropriate application timing and cultural controls, such as plowing or mowing, to control escapes.

And using the same weed-control method, even if it’s mechanical, year after year is the worst thing you can do.

If you’re in a football game and you pass on every down, wouldn’t the defense catch on pretty soon?” Roush says. “But if you keep using different plays, you keep the defense off balance.”

Chuck Foresman, Syngenta Crop Protection technical brand manager for nonselective herbicides, agrees and recommends a pre-emptive approach.

This problem is real, and it’s growing. If it’s my vineyard, I don’t want to be surprised. I don’t want to go out and find a weed population that I can’t control. Then, all of a sudden, it becomes a big problem with the costs of additional herbicides that I hadn’t budgeted for.

I think that’s the impetus--you want to be preventive and manage the potential for glyphosate resistance.”

Horseweed explodes

Since 2003, UC weed researchers Kurt Hembree and Anil Shrestha have noted unusually large numbers of horseweed in Central Valley vineyards and most frequently in raisin vineyards. They attributed the proliferation to raisin growers trying to reduce costs and scrimping on weed control rather than widespread herbicide resistance.

The weed, which can grow as tall as 8 to 10 feet, is hard to miss since it typically towers over vineyards. It prefers undisturbed areas, such as along roadsides or canal banks or between vines where plowing misses.

In 2005, the manager of a Dinuba, Calif., irrigation district contacted Hembree and Shrestha about horseweed along a ditch bank that wasn’t being controlled by glyphosate. Laboratory tests confirmed the weeds could tolerate up to four times the label rate of glyphosate without being killed.

The irrigation district had been using glyphosate continuously for 15 years.

Strive for year-round control

Even if you don’t have herbicide-resistant biotypes in your vineyards, Shrestha, a UC integrated weed management specialist based at the Kearney Research and Agricultural Center, says you should be concerned about horseweed. Left uncontrolled, it can reduce young vine growth harbor pests, such as the glassy-winged sharpshooter.

One of the challenges in controlling horseweed, Shrestha says, is it germinates over several months and can be found growing in vineyards nearly year-round.

Probably the most critical time for horseweed control, Shrestha says, is spring to early summer, when it can compete with vines for nutrients and potentially reduce yields.

But even in late summer, you should pay attention to horseweed to minimize seed production and since large plants can impede harvest. Regardless of the season, you need to scout your vineyards.

You always need to monitor your vineyard for weed escapes, to evaluate your weed program’s success and also if you have a resistance situation,” Shrestha says.  

A resistance-management strategy

In Australia, where weed resistance problems are more severe, Roush says Chris Preston, a weed scientist with the University of Adelaide, recommends avoiding glyphosate use one year out of three.

In places where people had more mixed strategies, they still have yet to pick up any resistance,” says Roush, who spent eight years at the Cooperative Research Centre for Australian Weed Management in Adelaide before returning to the United States 2 1/2 years ago. “In Australia, once the things have revolved to be resistant to glyphosate, it’s a bit late to start throwing all of these strategies at it.”

Foresman says Syngenta has a similar strategy. In Roundup Ready corn or soybeans for example, the company recommends two applications of glyphosate in one field every two years.

Australian researchers also recommend against tankmixes as a resistance-management strategy, since you’re still exposing the weed to the chemical.

For a tankmix to work really well, if the weeds survived the glyphosate application, they will get killed by the other herbicide,” Roush says. “That will work if you get 100-percent control. When was the last time you got 100-percent control? Often 10 percent survive. Selection will continue every year.”

And even mechanical control methods have their limitations, Roush says. Australian researchers have noted in areas where growers rely exclusively on mechanical control methods, such as mowing, the weeds evolve to thwart them. The weeds eventually grow more prostate to avoid mowing, and seed heads stick flat in the ground.

Protect what we have

By managing for glyphosate resistance, Foresman says you’re also helping other herbicides, such as Gramoxone, used as rotational products. If glyphosate is no longer effective in a program, growers may increase the use of another herbicide, hastening weeds developing resistance to it.

These modes of action we have available right now are the ones we will have for quite some time, because nobody is finding any new ones,” Foresman says. “Even if we found one today, it would take us 10 years to get it to the market.”

The last herbicide with a new mode of action registered by the Environmental Protection Agency was in 1998. Although new herbicides have come on the market since then, they’re combinations or premixes of existing modes of action.

Contact Vicky Boyd at (209) 571-0414 or

What is herbicide resistance?

Herbicide resistance is simply a weed’s inherited ability to survive an herbicide treatment. By using the same chemical or chemicals with the same modes of action repeatedly, you kill the sensitive plants, allowing the tolerant ones to remain and reproduce. What starts out as only a few resistant plants quickly becomes the dominant population.

And weeds with high reproductive potentials, such as horseweed, lend themselves to herbicide resistance because of the large numbers of seeds and eventual progeny they produce, says Anil Shrestha, a University of California integrated weed management specialist based at the Kearney Research and Agricultural Center near Parlier. One horseweed plant can produce up to 200,000 seeds annually.

Other traits leading to a high potential for herbicide resistance are short seed dormancy, widespread seed dispersion and annual growth--all traits shared by horseweed.

Scientists refer to the resistant weeds as resistant biotypes since they are the same morphologically as the sensitive ones.

The first case of heribicide resistance in the United States was documented in Washington state in 1968. It involved common groundsel resistant to altrazine and simazine.

Since then, 303 biotypes within 182 weed species worldwide have become resistant to 18 herbicides.

The triazine herbicides involve the greatest number of resistant weeds.

Lately, resistance to glyphosate, which is marketed under at least 50 different brand names including Roundup and Touchdown, is gaining attention. Among the states reporting confirmed horseweed resistance to the broad-spectrum herbicide are Delaware, Kentucky, Tennessee, Indiana, Maryland, New Jersey, Ohio, Arkansas, Mississippi, North Carolina, and most recently, California.

Glyphosate-resistant horseweed is estimated to cover more than 2.5 million acres in the United States.

In Ohio, marestail from a handful of sites is resistant not only to glyphosate, but also to Classic and First-Rate--herbicides that are ALS inhibitors. This is what scientists refer to as multiple resistance, because the weeds are resistant to chemicals with different modes of action.In addition to horseweed, six other weeds worldwide, including common ragweed, have confirmed resistance to glyphosate.

And true resistance shouldn’t be confused with herbicide failures, Shrestha says.

In the case of resistance, you can do everything right in terms of timing, sprayer calibration, water pH and temperature, and yet the weed isn’t controlled.

With an herbicide failure, you weren’t able to control the weed because you may have sprayed when the weeds were too large, used water with the incorrect pH, treated the weeds when they were under stress or incorrectly calibrated the sprayer.