Farm Progress

Researchers have confirmed six weed species in California that are resistant to the popular non-selective herbicide glyphosate.

Greg Northcutt, Contributing Writer

March 8, 2017

6 Min Read

Never an easy job, the task of controlling weeds in California vineyards, orchards and fields continues to challenge California’s grape growers and other crop producers.

One reason is the ability of weeds to develop resistance to a particular herbicide through gene mutation.

“Herbicide resistance is an increasing problems and is getting worse,” says Paul Verdegaal, University of California Cooperative Extension viticulture farm advisor for San Joaquin County

To date, he reports, researchers have confirmed six weed species in California that are resistant to the popular non-selective herbicide glyphosate. Four have been known for some time: horseweed (marestail), hairy fleabane, rigid ryegrass and annual ryegrass.

Recently, however, junglerice and Palmer amaranth in the Central Valley have been added to the list. What’s more, several others, which are becoming more difficult to control, are suspected of developing glyphosate resistance. They include goosegrass and, in the central San Joaquin Valley, the summer grasses, sprangletop and witchgrass.

But herbicide resistance isn’t the only reason weed control is becoming more challenging. In some cases, regulations designed to protect water quality limit the types of herbicides which can be used to control a particular weed threat, he notes. Changing cultural practices are also hampering weed control success.

Many growers, for example, have shifted from furrow or flood irrigating their fields several times a season to applying less water much more frequently through drip systems.

“This increased frequency and more uniform application of water can increase the breakdown of the active ingredient in a herbicide, allowing more “escapes” to grow and encouraging growth of the more difficult-to-control weeds, especially perennials like field bindweed and nutsedge, in the vine rows,” Verdegaal says.

To help get the upper hand on weeds, he recommends selecting a variety of tools and practices in developing and implementing your weed control plan. He likens it to another aspect of the more familiar Integrated Pest Management (IPM), designed to limit crop damage from insects and disease. Similarly, the intent of integrated weed management is to combat weed threats by minimizing the ability of weeds to evade or resist any single control measure, but, with one key difference.

Typically, IPM measures are often implemented when the area infested by a given disease or the population of an insect pest reaches a certain economic threshold. By contrast, an integrated approach to controlling weeds focuses on the long-term, with more emphasis on zero tolerance. That is because weeds can build up long-lived seed banks, which can provide a source of weeds and potential resistance for decades.

“It requires a slightly different mind-set than the familiar IPM,” Verdegaal says. “Integrated Weed Management isn’t something you do just once or twice a season. Instead, it’s a year-round strategy for controlling problem weeds.

He offers six tips for developing and following an integrated weed management program:

Mix it up

Rotating use of herbicides among products with different active ingredients and modes of action is essential for reducing development of herbicide-resistant weeds and making the most-effective use of other available products.

If you apply glyphosate to treat an emerging weed threat, next time you could spray a different non-selective material, like glufosinate. Each controls weeds by targeting a different plant enzyme, making it more difficult for weeds to adapt to and overcome the effects of both active ingredients.

Grower combination mixes or combination products, which contain two different herbicides, can be another option for reducing chemical resistant in weeds. “This way you get two modes of action with one spray,” Verdegaal says.

Include mechanical and cultural weed strategies

Despite the importance of rotating use of herbicides among different classes of chemicals, it’s only the first step in an integrated weed management program, Verdegaal notes

Any practice that reduces the success rate or efficiency of a herbicide application reduces the cost-effectiveness of what can be an already-expensive product. Plus, this loss of effectiveness can help survivors develop resistance to the chemical.

That’s why he recommends adjusting your cultural practices, as needed, to minimize any breakdown of a herbicide’s active ingredient. For example, to prevent a herbicide from breaking down due to over-irrigation, consider soil difference across the entire field or block. Then, change your irrigation intervals and application rates in these areas to prevent wet spots from developing.

Verdegaal suggests using caution when applying nitrogen, not just to minimize any harm to water quality, but also to improve weed control.

“When, where, how much and in what form you apply nitrogen can affect weed growth indirectly,” Verdegaal says. For example, nitrogen applied before vines are ready to take it up, can be used by weeds to benefit their growth.

Mowing, disking, cultivation, in-row plowing and other such practices to keep berms clean can help reduce herbicide resistance by eliminating trash, which can block effectiveness of some herbicides in certain soils. He also notes that the cost of a walk-through by a worker or small crew using shovels or a backpack sprayer can pay off by catching persistent or potentially resistant weeds.

Improve your timing

“Sometimes, loss of weed control isn’t so much herbicide resistance as it is improper timing of your spray,” Verdegaal says. “Even when herbicide resistance isn’t a problem, traditional weeds, like willowherb, prickly lettuce and sow thistlecheeseweed (Mallow) can get out of hand, if the timing of your application is off.”

Proper timing also applies to mowing, he adds. For example, mowing a cover crop too soon can reduce its competitive effect in controlling weeds, particularly annuals. Mowing too late can allow some weeds to complete their life cycle and produce seed. Prolific seeders can also encourage development of herbicide resistance.

Write it down

Verdegaal advises growers to keep good records of their herbicide applications. “That means making notes of your success and failures as well problem areas in your vineyard,” he says. “This information can help you make more effective use of your herbicides while preventing over-use of any particular product, reducing the risk of a weed becoming resistant to that material.”

Think and act long-term

Making weed control a year-round priority can help you stay ahead of noxious and perennial weeds, Verdegaal says.

That may mean concentrating on controlling noxious weeds, but ignoring less threatening species.

“In the not too distant past, it was common to control all the weeds in a field, leaving a ‘moonscape’ with only the crop growing after treatment,” he says. “But, not all weeds compete with a crop. Those can be left to grow, reducing your herbicide costs and development of herbicide resistance.”

Heed the seed threat

A critical component of long-term weed control is preventing weeds from going to seed. “Otherwise, you may win the battle by killing the weeds you already have but lose the war by enabling the weed problem to continue for years to come,” Verdegaal says.

Even though herbicide products can add different modes of action to help counter development of weeds resistant to them, such materials can only go so far in keeping weeds controlled.

“While new herbicides can improve success in managing weeds, they can give you a sense of regaining control,” he says. “But seeds can pose a problem for a long time.”

For example, one year of seeds can produce seven years of weeds and some weed seeds can remain viable in the soil for 50 to 100 years.

“A single nutsedge plant can produce around 400 to 500 tubers or nutlets and up to about 2,000 seeds in a year,” he says. “At that rate, it doesn’t take long for the weed to overcome any kind of control strategy.

“You can’t afford to lay off your weed control efforts for even just a year or two, because the proliferation of new weeds from seeds can put you right back where you started. Lasting success requires an ongoing year-round control program and really staying vigilant and responding to any threat.”

About the Author(s)

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like