The delay in the appearance of resistant weeds is generally attributed to the slower generation time of plants, incomplete selection pressure from most herbicides, soil seed reserves, and the plasticity of weedy plants, all of which keep susceptible individuals in a population and thus delay the evolution of resistance. The appearance of herbicide resistance in plants today is increasing at an exponential rate , mirroring the trends previously seen with insecticide and fungicide resistance. Besides triazine resistance, there are biotypes of 172 weed species expressing resistance to 16 other herbicide classes.The most common mechanism of action or target site of herbicides, the chemical class, and the number of species with biotypes resistant to each herbicide class are summarized in Table 1. In California, herbicide resistance today is most widespread among aquatic weeds in rice production . Many of these weed species have been selected for resistance to the sulfonylurea herbicide bensulfuron. There has also been one report of triazine resistance as well as one report of sulfonylurea resistance in a noncrop area. A roadside survey conducted in 1995 and 1996 found that resistance to sulfonylurea herbicides was common in Russian thistle . Most recently, a rigid ryegrass biotype exhibited resistance to glyphosate in a northern California orchard. Despite these examples, there are few reports to date of herbicide resistance in California, but the problem is significant in the United States and worldwide . Many current and pending registrations in California, however, curing bud involve herbicides that act on branched-chain amino acid synthesis . The use of herbicides in this group has selected many weed species for resistance in the United States and several in California .

In addition, a number of genetically engineered crops that are resistant to specific herbicides—such as Roundup Ready cotton and corn—will soon be available in California. Sole reliance on the herbicide to which these crop varieties are resistant will increase the selection pressure on weeds for resistance to the herbicide used. Herbicide-resistant crops will not be an end-all solution to weed problems, and they will not be a useful tool for weed management if used exclusively. Nationally, an average of six to seven herbicides were registered every year from 1955 to 1975. Since the mid 1970s that number has declined, reaching lows of one to two herbicides per year in the 1980s. Even fewer herbicides are registered for use in California. Re-registration requirements for pesticides have also resulted in the loss of herbicide registrations in many crops. With few new herbicide registrations and a loss of existing compounds, the potential for repeated use of the limited number of herbicides available is increasing, and that increases the potential for selecting resistance in weeds. Weed control programs should include strategies that reduce the likelihood of selection for herbicide-resistant weeds and conserve existing chemical tools.Evolution and natural selection are the processes that have led to the plant species found around the world today. Many plants, particularly weeds, contain a tremendous amount of genetic variation that allows them to survive under a variety of environmental conditions. Most herbicides affect a single specific site of action, and that site is usually under the control of a single gene, or at most a few genes. With a single gene mutation, even minor changes in gene expression can confer resistance by modifying the site where a herbicide has its toxic effect: the site of action. The evolution of a resistant population in a species comes about in response to selection pressure imposed by that herbicide or by another herbicide that shares the same site of action. When a herbicide exerts selection pressure on a population, plants possessing the resistance trait have a distinct advantage. Unlike susceptible plants, resistant plants will survive and reproduce.

Continuous herbicide exposure maintains the selection pressure and thereby rapidly increases the number of resistant plants. Weeds possess traits that promote the evolution of resistance. A high rate of seed production with most seed germinating within a year can accelerate the evolution of resistance. When susceptible plants are removed from the population by the herbicide, prolific seed production by resistant plants rapidly shifts the population toward resistance. High seed production coupled with genetic variation increases the probability that resistance will evolve. Perennial weeds, particularly those with vegetative reproductive tissues, are less likely to evolve resistance than are weeds with an annual life cycle that produce abundant seeds, since generally there is less genetic diversity in perennial weeds that reproduce vegetatively and fewer opportunities for new mutations to be transferred to offspring via seeds. The most common weed genera that contain herbicide-resistant populations are listed in Table 3. All of these genera are dominated by annual species.In the absence of herbicide treatment, weeds resistant to the triazine herbicides are not as fit as are susceptible plants of the same species. This is because the efficiency of photosynthesis is reduced in resistant plants by the alteration of a specific photosynthetic protein that is also the herbicide binding site, so conferring resistance. Since resistant plants are less fit, they reproduce at lower rates and consequently represent a smaller fraction of the number of individuals within a population. In contrast, some resistance traits do not have the same fitness cost. In those cases, resistant individuals often represent a larger fraction of a population . The frequency of the resistance trait within the population is an important factor in determining the rate of selection for resistance among weed species. For example, resistance to triazines evolved after 10 years of continual use of the herbicides. Unlike the triazines, the sulfonylurea herbicides have not been shown to have a significant fitness cost associated with the resistance trait. Resistance to these herbicides took only 4 years to evolve. For weed species with resistance to sulfonylurea herbicides, the initial proportion of resistant plants in a population has been estimated at approximately 1 in 1 million individuals. Thus, if a weed population has a density of 10 plants per m2, one would expect to find one resistant individual in every 10 hectares of infestation. Without multiple control strategies, these resistant individuals are likely to survive long enough to produce resistant seed. Several factors, such as herbicide characteristics, plant characteristics, weed control practices, and production practices, can increase the probability of selection for herbicide resistance. Herbicide factors that contribute to the potential for resistance include a long soil residual activity, a single target site and specific mode of action, and a high effective kill rate for a wide range of weed species. Herbicides with prolonged soil residual activity exert selection pressure for a longer time period since they will kill most of the susceptible plants that germinate over a growing season. A herbicide with a single target site controlled by few genes is more likely to encounter plants with mutations for resistance than is a herbicide with several modes of action. A high effective kill rate rapidly depletes susceptible genes from the population, and the result is a rapid increase in resistance among the progeny of a few initial resistant plants. Although the most common mechanism of herbicide resistance in weeds is an alteration at the site of action, resistance can also result from an enhancement of the plant’s ability to metabolize and detoxify the herbicide. This mechanism is not yet widespread in the United States.

Like target site changes, selection for enhanced metabolism can also occur in response to repeated applications of the same herbicide or of a group of herbicides that are vulnerable to the same detoxification enzymes. For example, enhanced metabolism is thought to confer resistance to picolinic acid herbicides in yellow starthistle in eastern Washington. Weed biotypes with enhanced metabolism have a much lower level of resistance than weeds expressing resistance through site of action changes. Selection for weeds with enhanced metabolism is more rapid when a herbicide is used continuously at or below the low recommended rate. This allows a gradual increase of the weed biotypes that are more able to metabolize the compound. The most likely way to cause evolution of resistant weed populations is to exert selection pressure on weeds with the same herbicides over several generations. By using long–soil-residual herbicides, cannabis drying rack the same herbicide continuously, or a rotation of herbicides that target the same site, you apply selection pressure for resistance over several generations. The continuous planting of the same crop within and between growing seasons reduces your options for rotating to herbicides with different target sites. For example, crop rotation with California rice is difficult, so fields are planted continuously to the same crop. The herbicide bensulfuron was registered for rice in California in 1989. It was highly effective on most rice weeds. Few alternative control techniques were used in rice, so Londax was used extensively for several years. Resistance evolved quickly, and now at least four weed species are resistant to Londax . The limited number of herbicides registered for many minor crops restricts the grower’s ability to rotate among compounds with different sites of action. This often leads to continuous use of one or a few herbicides and increases the probability that herbicide resistance will evolve among weed populations in those fields. Resistance has not yet become a problem in California’s minor crop production areas, however. This is probably because of the extensive use of hand labor, cultivation, and frequent rotation among a number of crops for which herbicides with different target sites are registered. While hand labor and cultivation continue as effective methods for preventing resistance, the herbicide rotation that has accompanied crop rotation may become ineffective as herbicides that target branched-chain amino acid synthesis are being registered for several of the nationally minor crops grown in California, including tomatoes and sugar beet. In addition, ALS inhibiting herbicides have been registered for cotton, corn, and alfalfa. The risk that weeds will evolve resistance to these herbicides will increase if ALS inhibiting herbicides are used continuously in several crops within a rotation. The exclusive use of herbicides for weed control can rapidly select for resistance when other control practices such as tillage or hand hoeing are not also used to control herbicide-resistant weeds. In general, non-chemical methods will not select between susceptible and resistant plants, so they should be used whenever possible. Resistance also evolves more quickly in lower-value solid-seeded crops grown on large acreages, since cultivation and hand-weeding of these crops may not be feasible. Farmers who grow crops over large areas tend to rely heavily on herbicides for weed control. Any management action that reduces the selection pressure for resistance will reduce the rate of resistance evolution. Crop rotation is one of the best tools for preventing resistance. Rotation to another crop allows the grower to use both chemical and non-chemical control methods. Manipulation of planting time, the competitiveness of the crop, cultivation techniques, hand weeding, and applications of herbicides with different target sites all are possible in a crop rotation system. Farmers and Pest Control Advisors in California use many of these methods to control weeds. Probably because of these characteristics of California agriculture, few weed species have evolved herbicide resistance in this state. As highly effective herbicides with the same target site become registered in California in multiple crops of a rotation, however, the risk increases that resistance will evolve.Herbicides with different chemistries and trade names but with a target site in common can reduce the effectiveness of herbicide rotation. Some common crop rotations include cotton, corn, tomato, sugarbeet, and alfalfa. All of these crops now have registered herbicides that target the same site . As noted earlier, biotypes resistant to these herbicides may have no fitness cost associated with resistance and there may be high numbers of resistant individuals in a population. Weed species will evolve resistance rapidly unless farmers rotate to herbicides with different target sites. Herbicide-resistant crops represent a new technology whose use is increasingly widespread. In many cases, farmers who grow these crops will rely more heavily on a single herbicide. Such a strategy will likely select for weed biotypes that are resistant to that herbicide or mode of action. Tank mixing, rotating herbicides, rotating to varieties without the resistance trait, and integrating non-chemical control options within the weed management program will reduce the potential that weed biotypes will evolve resistance.