by Ron Vargas,
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:
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:
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:
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:
How to Manage Herbicide-Resistant Weeds
To keep herbicide-resistant weeds under control, incorporate these strategies into your weed management plan:
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.
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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.
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Information provided by:
Ron Vargas, email@example.com, Farm Advisor
University of California Cooperative Extension, Madera & Merced Counties.
Material written May 2001.
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