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).

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

Sincerely,

Ron Vargas
Farm Advisor

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

Table 2 Herbicide resistant weeds in California

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