University of Arizona a dot Cooperative Extension

Herbicide Resistance

by Ron Vargas,
Farm Advisor

This article was provided courtesy of the University of California Cooperative Extension.

Roundup Resistant ryegrass

Although there are few resistant weed populations in California, within the past couple of years, there have been two locations reported with rigid ryegrass (Lolium rigidum) biotypes that have exhibited resistance to glyphosate in Northern California almond orchard production systems and roadside sites.

An additional site has now been discovered in the Central San Joaquin Valley. An almond orchard that has been treated up to four times a year with glyphosate, for the past four years, is now exhibiting glyphosate resistant ryegrass. The 1 lb. ai or 1 quart per acre use rate has no visible effect. Some effects are noted with 3 lbs. AI of glyphosate, with considerable chlorosis and desiccation occurring at 5 lbs. AI per acre. It appears between 5 and 10 lbs. AI per acre is needed to completely control this population.

The use of herbicides in production systems have allowed growers to effectively and economically control weeds. Hard to control weeds, such as field bindweed and annual morningglory, can now be effectively managed in cropping systems. Selective herbicides have allowed the reduction, and in some cases, the elimination of hand weeding all together. Cultivation has been reduced in field and vegetable crops with some movement toward reduced or minimum tillage systems. The new generation of herbicides are much more environmentally friendly, controlling weeds with only ounces of active ingredients per acre as opposed to pounds per acre required by some of the older herbicides. The introduction of herbicide tolerant crops has provided growers with an additional option for effective control. But, for herbicides to remain effective and to sustain their use, attention and consideration must be given to Herbicide Resistance.

Weed resistance to herbicides is not a new phenomenon (Table 1), but is somewhat less known and experienced than insecticide or fungicide resistance. The first report of herbicide resistance occurred in 1960 with the discovery of Triazine resistant common groundsel. Since that time 216 weed biotypes around the world have evolved resistance to herbicides.

In California the greatest herbicide resistance problems have occurred in aquatic weeds in rice production in the Sacramento Valley (Table 2). Many of these weeds species have been selected for resistance to the sulfonylurea herbicide bensulfuron (Londax). Greenhouse work done by Steve Wright, UCCE Farm Advisor in Tulare County, showed that in some cases there was a trend for slightly higher barnyardgrass germination, and total seedling biomass weights in plots treated with both pendimethaline (Prowl) and trifluraline (Treflan). Rigid ryegrass (Lolium rigidum) has exhibited resistance to glyphosate (Roundup) in a northern California almond orchard. And, just recently, there appears to be another almond orchard in the Central San Joaquin Valley exhibiting ryegrass resistant to glyphosate. Although there are few cases of resistance in California there are many herbicides in use that have selected resistance in many weed species throughout the U.S. With the use of Staple in cotton, Shadeout in tomatoes, Upbeat in sugarbeets, Londax in rice, Pursuit in alfalfa and Assert in wheat, all herbicides that lead to rapid selection for resistant weeds, it is probable that the number of cases in California will increase. In addition the availability of Roundup Ready and BXN (bromoxynil tolerant) cotton may promote the sole reliance on one particular herbicide that will increase the selection pressure on weeds for resistance.

Definition of Resistance

Joe DiTomaso, UCCE Weed Specialist says: "Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. In contrast, tolerance can be defined as the inherent ability of a plant to survive a herbicide treatment at a normal use rate. In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering. Resistance may occur in plants by random and infrequent mutations; no evidence has been presented to demonstrate herbicide-induced mutation. Through selection, where the herbicide is the selection pressure, susceptible plants are killed while herbicide resistant plants survive to reproduce without competition from susceptible plants. Thus, the appearance of herbicide resistance in a field is an example of rapid weed evolution."

Factors Leading to the Development of Herbicide Resistance

Because weeds contain a tremendous amount of genetic variation that allows them to survive under a variety of environmental conditions the development of a resistant species is brought about through selection pressure imposed by the continuous use of an herbicide. Long residual preemergence herbicides or repeated application of postemergence herbicides will further increase selection pressure.

Factors that can lead to or accelerate the development of herbicide resistance include weed characteristics, chemical properties and cultural practices.

Weed characteristics conducive to rapid development of resistance to a particular herbicide include:

  1. Annual growth habit.
  2. High seed production.
  3. Relatively rapid turnover of the seed bank due to high percentage of seed germination each year (i.e., little seed dormancy).
  4. Several reproductive generations per growing season.
  5. Extreme susceptibility to a particular herbicide.
  6. High frequency of resistant gene(s), (e.g. Lolium rigidum).

In contrast, weed species less likely to develop resistance generally have 1) a slower generation time, 2) incomplete selection pressure for most herbicides, 3) ability to adapt to changing environment, 4) lower fitness for resistant biotypes, and 5) extended seed dormancy in the soil. These factors increase the number of susceptible biotypes in the population.

Herbicide characteristics which lead to rapid development of herbicide resistance in weed biotypes include:

  1. A single site of action
  2. Broad spectrum control.
  3. Long residual activity in the soil.

Cultural practices can also increase the selective pressure for the development of herbicide resistant biotypes. In general, complete reliance on herbicides for weed control can greatly enhance the occurrence of herbicide resistant weeds. Other factors include:

  1. Shift away from multi crop rotations towards mono cropping.
  2. Little or no cultivation or tillage for weed control or no elimination of weeds that escape herbicide control.
  3. Continuous or repeated use of a single herbicide or several herbicides that have the same mode of action.
  4. High herbicide use rate relative to the amount needed for weed control.
  5. Orchard and vineyard systems.
  6. Roadsides.

Resistance Management

The first step to preventing herbicide resistance is early detection. Scout fields and be on the lookout for patterns that would indicate resistance. Whole fields infested with weeds or strips of weeds does not typically indicate resistance. Patterns of resistance include: patches in fields, patches of dense populations with lessor population radiating out from the central patch and escapes scattered in no particular pattern throughout the field.

How to Prevent or Delay Herbicide Resistance

Weed management strategies that discourage the evolution of herbicide resistance should include the following:

  • Herbicide rotation
  • Crop rotation
    • Plant to a crop having a different season of growth.
    • Plant to a crop having different registered herbicide.
    • Plant to a crop for which there are alternative methods of weed control.
  • Monitoring after herbicide application
    • Check for weedy patches in patterns consistent with application problems.
    • Hand-weed patches that are not in patterns consistent with application problems.
  • Non-chemical control techniques
    • Cultivate.
    • Hand-weed. A 90 percent or greater rate of weed removal reduces the chances that a resistant plant will produce seed.
    • Mulching with both synthetic and organic materials.
    • Solarize the soil.
  • Short-residual herbicides
  • Certified seed
  • Clean equipment
    • Use a power washer or compressed air to remove seeds.

How to Manage Herbicide-Resistant Weeds

To keep herbicide-resistant weeds under control, incorporate these strategies into your weed management plan:

  • Herbicide rotation
  • Fallow tillage
  • Close cultivation
    • Monitor hand weeding crew to insure more than 90 percent removal of weeds in the crop row.
  • Prevention of weed seed spread through use of clean equipment.
    • Enter the field with resistant plants last
    • Use a power washer or compressed air to remove seeds.
  • Monitoring the initial evolution of resistance by recognizing patterns of weed escapes typical of resistant plants.
    • Watch for small weed patches that appear in the same place in the next crop.
    • Watch for weed patches that do not have a regular shape that would indicate an herbicide application problem.
  • Control of weeds suspected of herbicide resistance before they can produce seed.

The potential for herbicide resistance should receive serious and thoughtful attention. As weed management systems change with new herbicides and herbicide resistant crops are introduced, resistance management must be an integral part of the production system. If selection pressure is maintained through the continuous use of the same herbicide, herbicide resistance will soon render it ineffective.

For more information go the University of California Weed Research and Information Center web site at


Ron Vargas
Farm Advisor

Table 1 Most common genera of weeds developing resistance to herbicides worldwide and in California

Number of occurrences
Genus Common name
In California
Amaranthus Pigweed
Chenonpodium Lambsquarters
Conyza Fleabane or horseweed
Lolium Ryegrass
Setaria Foxtail
Avena Wild oat
Echinochloa Barnyardgrass or watergrass
Alopecurus Blackgrass
Senecio Groundsel
Polygonum Knotweed or smartweed
Solanum Nightshade


Table 2 Herbicide resistant weeds in California

Species Common name Area Year Herbicide
Senecio vulgaris Common groundsel Orchard 1981 Triazine (atrazine)
Lolium perenne Perennial ryegrass Roadside 1989 Sulfonylurea (sulfometuron)
Cyperus difformis Smallflower umbrella sedge Rice 1993 Sulfonylurea (bensulfuron)
Sagittaria montevidensis California arrowhead Rice 1993 Sulfonylurea (bensulfuron)
Salsola tragus Russian thistle Roadside 1994 Sulfonylurea (sulfometuron)
Ammania auriculata Redstem Rice 1997 Sulfonylurea (bensulfuron)
Scripus mucronatus Ricefield bulrush Rice 1997 Sulfonylurea (bensulfuron)
Echinochloa phyllopogon Late watergrass Rice 1998 Thiocarbamate (thiobencarb)
Echinochloa phyllopogon Late watergrass Rice 1998 Arylozyphenoxy (fenoxaprop)
Lolium rigidum Rigid ryegrass Orchard 1998 glyphosate

The University of California prohibits discrimination against or harassment of any person employed by or seeking employment with the University on the basis of race, color, national origin, religion, sex, physical or mental disability, medical condition (cancer-related or genetic-characteristics), ancestry, marital status, age, sexual orientation, citizenship, or status as a covered veteran (special disabled veteran, Vietnam-era veteran or any other veteran who served on active duty during or in a campaign or expedition for which a campaign badge has been authorized). University Policy is intended to be consistent with the provisions of applicable State and Federal laws. Inquiries regarding the University's nondiscrimination policies may be directed to the Affirmative Action/Staff Personnel Services Director, University of California, Agriculture and Natural Resources, 1111 Franklin, 6th Floor, Oakland, CA 94607-5200 (510) 987-0096.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture, The University of Arizona.

The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Because labels are subject to frequent change, always consult the label attached to the product before using any pesticide. The user must assume responsibility for proper application and for residues on crops as well as for damage or injury caused by pesticides, whether to crop, person or property.

Any products, services, or organizations that are mentioned, shown, or indirectly implied in this web document do not imply endorsement by The University of Arizona.

Information provided by:
Ron Vargas,, Farm Advisor
University of California Cooperative Extension, Madera & Merced Counties.
Material written May 2001.

Cotton Weed Pubs | Forage Weed Pubs | Grain Weed Pubs | Vegetable Weed Pubs
Home | Weeds Home Page | Citrus Weeds

| Pesticide Use | Alternatives | News | Regulatory | Transgenics | Food Safety
Home | Cotton | Veggies | Forages | Grains | Citrus | Crop x Crop | Photos

document located at:
Copyright © 2001 University of Arizona,
College of Agriculture and Life Sciences