Ron Vargas (UCCE Farm Advisor, Madera and Merced Counties); and

Steve Wright (UCCE Farm Advisor, Tulare County)


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

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 Trazine resistant common groundsel. Since that time 287 weed biotypes around the world have

evolved resistance to herbicides. Both hairy fleabane and buckhorn plantain are resistant to glyphosate in South

Africa. Hairy fleabane has become difficult to control with glyphosate in our California production systems,

indicating possible development of resistance. Reports of poor or ineffective control of lambsquarter in our

Roundup Ready cotton systems have surfaced in the last two years. And just recently, Roundup resistance

horseweed (marestail) has been confirmed in the eastern U.S.

In California the greatest herbicide resistance problems have occurred in aquatic weeds in rice production in the

Sacramento Valley. Many of these weeds species have been selected for resistance to the sulfonylurea herbicide

bensulfuron (Londax). Rigid ryegrass (Lolium rigidum) has exhibited resistance to glyphosate (Roundup) in

northern California. 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 cotton and corn, BXN (bromoxynil tolerant) cotton and the soon

to be released, Roundup Ready alfalfa, may promote the sole reliance on one particular herbicide that will

increase the selection pressure on weeds for resistance.

Definition of Resistance

“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 and reproduce with an 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.

June 20, 2004

Cotton Field Check (Herbicide Resistance Management – Vargas & Wright) page 2 of 5

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.

Factors Leading to the Development of Herbicide Resistance

Most weed species contain a tremendous amount of genetic variation that allows them to survive under a variety

of environmental conditions. The development of herbicide-resistant weed species is brought about through

selection pressure imposed by repeated, often nearly continuous use of a 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).

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

1. A single site of action for the herbicide material

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 (orchard and vineyard systems).

2. Reduced or no till productions systems.

3. Continuous or repeated use of a single herbicide or several herbicides that have the same mode of

action (transgenic herbicide tolerant crops).

4. High and/or low herbicide use rate relative to the amount needed for weed control.

Resistant 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


Cotton Field Check (Herbicide Resistance Management – Vargas & Wright) page 3 of 5

How to Prevent or Delay Herbicide Resistance

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

Herbicide Rotation

Crop Rotation

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