A new generation of selective herbicides has 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.

These new 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. However, for herbicides to remain effective, attention must be given to herbicide resistance.

Weed resistance to herbicides is not new, but has not gained the attention that fungicide and insecticide resistance have received.

The first report of herbicide resistance occurred in 1960 with the discovery of Trazine 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. 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.

Resistance gain likely

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 sugar beets, 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.

Weeds contain a tremendous amount of genetic variation that allows them to survive under a variety of environmental conditions and therefore the development of a resistant species comes through selection pressure imposed by the continuous use of a herbicide. Long residual pre-emergence herbicides or repeated application of post emergence 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:

  • Annual growth habit.

  • High seed production.

  • Relatively rapid turnover of the seed bank due to high percentage of seed germination each year (i.e., little seed dormancy).

  • Several reproductive generations per growing season.

  • Extreme susceptibility to a particular herbicide.

  • High frequency of resistant gene(s), (e.g. Lolium rigidum).

    In contrast, weed species less likely to develop resistance generally have:

  • A slower generation time.

  • Incomplete selection pressure for most herbicides.

  • Ability to adapt to changing environment.

  • Lower fitness for resistant biotypes.

  • Extended seed dormancy in the soil.

These factors increase the number of susceptible biotypes in the population.

Characteristics

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

  • A single site of action

  • Broad spectrum control.

  • 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:

  • Shift away from multi crop rotations towards mono cropping.

  • Little or no cultivation or tillage for weed control or no elimination of weeds that escape herbicide control.

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

  • High herbicide use rate relative to the amount needed for weed control.

  • Orchard and vineyard systems.

  • Roadsides.

    The first step to preventing herbicide resistance is early detection. Scout fields and be on the lookout for patterns that would indicate resistance. Entire 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 lesser population radiating out from the central patch and escapes scattered in no particular pattern throughout the field.

    Preventing or delaying herbicide resistance involves weed management strategies that discourage the evolution of herbicide resistance. It should include herbicide rotation and crop rotation by:

  • Planting a crop having a different season of growth.

  • Planting a crop having different registered herbicide.

  • Planting to a crop for which there are alternative methods of weed control.

    Monitor after herbicide application by:

  • Checking for weedy patches in patterns consistent with application problems.

  • Hand-weeding patches that are not in patterns consistent with application problems.

Non-chemical control

Using non-chemical control techniques:

  • Cultivation.

  • Hand-weeding. 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.

  • Soil solarization.

    Also use short-residual herbicides and certified seed. And, clean equipment using a power washer or compressed air to remove seeds.

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

  • Herbicide rotation. Go to www.wric.ucdavis.edu to guide your herbicide rotation decisions.

  • Use fallow tillage and close cultivation.

  • Monitor hand-weeding crew to insure more than 90 percent removal of weeds in the crop row.

  • Prevent weed seed spread through use of clean equipment.

  • Enter the field with resistant plants last

  • Monitor 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 a herbicide application problem.

  • Control weeds suspected of herbicide resistance before they can produce seed.

As weed management systems change with new herbicides and herbicide resistant crops are introduced, resistant 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.

Roundup-resistant ryegrass identified

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 one-pound a.i. or one quart per acre use rate has no visible effect. Some effects are noted with three pounds a.i. of gylphosate, with considerable chlorosis and desiccation occurring at five pounds. a.i. per acre. It appears between 5 and 10 pounds a.i. per acre is needed to completely control this population.