Weeds resistance to triazines

Basis of Crop Selectivity and Weed Resistance to Triazine Herbicides... 

For example, kochia is normally very sensitive to atrazine and can often be controlled with l.lkg/ha (llb/A) along railroads and on farm lands. However, Burnside et al. (1979) reported that after 13 years of atrazine use, Union Pacific personnel were applying up to 15 kg/ha (13 lb/A) with poor control. These high rates are no longer allowed for atrazine uses. The use of mixtures of triazines and herbicides with alternate modes of action has been an effective management strategy to control weeds resistant to triazines. 

Biotypes of weed species are resistant to virtually all classes of herbicides previously used for their control. Most weeds resistant to triazine herbicides have appeared after the triazines alone were used for 8-10 years of consecutive treatments, sometimes much longer (Eleftherohorinos et al., 2000 Gressel, 2002). Weed biotypes resistant to the ALS inhibitors have often been reported after only 3 to 5 years of repeated use, and in some cases after only 1 or 2 years (Kendig and Barrentine, 1995 Jeffers et al., 1996 Lovell et al., 1996b Sprague et al., 1997a Hall et al., 1998). 

Development of weed resistance to triazine herbicides has necessitated alternative weed control strategies. Rotation of triazines with other herbicides and combining triazines with other chemistries has been effective in ornamentals and Christmas tree plantations (Van Himme, 1989). Supplemental treatments in Christmas trees have been effective, using directed sprays of phenoxy herbicides or glyphosate before resistant weeds mature and produce seeds. Weed resistance from triazines is not a problem in most forest settings where the herbicide is applied only once or twice in a rotation, since 1 or 2 years of herbaceous weed control normally ensures survival and eventual dominance by conifers. When the conifer canopy closes, virtually all herbaceous vegetation is shaded out for some decades hence resistant weeds, if present, fail to survive from one generation to the next. 

Past experience has shown that weeds resistant to triazines can be managed or confined within a reasonable limit. In the U.S., the total area of land or crops infested with triazine resistant weeds is still relatively small and does not seem to be expanding rapidly. In most areas of the U.S. where triazine resistant weed populations have evolved, it has not been necessary or desirable to discontinue the use of the triazine herbicide of choice, due to the many triazine susceptible weeds that are usually prevalent. In a few cases, the resistant biotypes have even disappeared (e.g., some triazine resistant biotypes of Setaria sp. in Nebraska). However, in other countries, especially when resistance strategies were not followed, resistant biotypes quickly became serious problems. 

Triazine herbicides provide selective weed control in crops such as corn, sorghum, and sugarcane. In addition, some members of the triazine family are used for weed control in orchards, horticultural, and perennial crops, etc. A unique selective use of triazine herbicides is in triazine-tolerant rapeseed. Although triazine herbicides provide control of a wide variety of grass and broadleaf weeds, the long-term, widespread, and repetitive use of triazine herbicides in crop and noncrop situations has led to the selection of many triazine-resistant weeds. The physiological and biochemical basis of triazine selectivity between crops and weeds and of resistance to triazine herbicides in weeds is well understood. 

Weed resistance to the triazine herbicides was first identified in the late 1960s, with a biotype of common groundsel that was resistant to simazine (Ryan, 1970). Since then, resistance to triazine herbicides has been reported in many weed species (Holt and LeBaron, 1990 LeBaron and McFarland, 1990 Gronwald, 1994). Most cases of triazine resistance have been reported in the US, Canada, and Europe, where triazine herbicides have been used extensively in corn monocultures (LeBaron and McFarland, 1990 Stephenson et al., 1990 LeBaron, 1991). Most of the. v-triazinc-resistant weed species have been selected against atrazine and usually show a high level of cross-resistance to other. v-triazine herbicides. In most cases, these weeds also show a low level of resistance to as-triazinones (e.g., metribuzin). Triazine-resistant weeds are often less vigorous than nonresistant weeds, which facilitates their management. 

In other weed biotypes, resistance to triazine herbicides is likely conferred by rapid metabolism of the herbicides to inactive compounds. A chlorotoluron-resistant biotype of blackgrass (slender foxtail) was cross-resistant to various other groups of herbicides, including triazines (Kemp et al., 1990). The mechanism of chlorotoluron resistance was Cyt P450-based enhanced oxidative metabolism through /V-demethylation and ring-methyl hydroxylation (Moss and Cussans, 1991). Consequently, it is likely that resistance to triazines in this blackgrass biotype is also due to enhanced herbicide detoxification. 

In summary, triazine resistance in weeds is most commonly due to a target site alteration that confers a very high level of resistance to. y-triazinc herbicides. Although a Ser264 to Gly mutation in the D1 protein is most common, additional alterations have been identified that confer resistance to triazines and other classes of PS II inhibitors. Enhanced herbicide metabolism plays a major role in conferring resistance in only a few weed biotypes. In these biotypes, the pattern of resistance may be broader, with some cross-resistance to av-trazinones, uracils, heterocyclic ureas and phenyl ureas. The level and pattern of resistance to various herbicides in these biotypes depend, presumably, on the activity and specificity of the enzyme(s) responsible for the enhanced herbicide metabolism. 

The levels of infestations or seriousness of triazine resistance within each species varies gready. The author is aware of 19 cases where the resistant weed is no longer present or cannot be identified as resistant to triazine herbicides. In other cases, the current status is unknown. Several triazine-resistant biotypes are likely to be of little or no agronomic importance within a geographical area. 

As had been predicted by many weed scientists who understood the biological cause and nature of herbicide-resistant weeds, resistance to the ALS inhibitors and other newer herbicides developed more rapidly than resistance to the triazine herbicides. 

LeBaron, H.M (1998). The role of triazines in managing weeds resistant to other herbicides. In L.G. Ballentine, J.E. McFarland, and D.S. Hackett, eds., Triazine Herbicides Risk Assessment. ACS Symposium Series 683. Washington, DC American Chemical Society, pp. 9-23... 

Maertens, K.D., C.L. Sprague, P.J. Tranel, and R.A. Hines (2004). Amaranthus hyridus populations resistant to triazine and acetolactate synthase-inhibiting herbicides. Weed Res., 44 21-26. 

Heap (1999) accurately predicted that due to the economic importance of ALS and ACCase inhibitor herbicides worldwide and the ease with which weeds become resistant to them, it is likely that the weeds resistant to these herbicides will present farmers with greater problems in the next 5 years than triazine-resistant weeds have in the past 25 years. 

Triazines are Important Tools to Manage Weeds Resistant to Other Herbicides 145... 

Modem herbicides have revolutionized the efficient production of most agricultural crops, and they will continue to be essential in feeding our present and future population. Atrazine and the triazine herbicides are critical in the management of weeds resistant to alternative herbicides. We must continue to develop management strategies for triazines and other herbicides as essential tools for weed control in agricultural production. 

Other costs which were identified but not included in many of the studies, primarily because of the difficulty in quantifying their effect, are costs from increased weed resistance to herbicides replacing the triazines direct labor and management costs of developing and using new weed control practices and increased erosion damage costs, such as siltation of lakes, subsequent water recreation reduction, and lower land productivity. The economic impact due to weeds becoming resistant to the triazines was found to be minor. 

Biotypes of at least 40 broadleaf and 15 grass weed species are known to have evolved resistance to triazine herbicides somewhere in the world. Only 21 of these resistant biotypes have been found in the U.S., and one or more of these resistant biotypes have invaded 31 states, four provinces of Canada, and 18 other countries. The confirmed triazine resistant weed species and their distribution by years are summarized in Tables III and IV from data collected in our recent worldwide survey. Details of this extensive survey, mostly conducted in 1988, have not yet been published. Data in the summary Tables III, IV and V are published here for the first time. 

Table VIII shows some interesting trends in the first occurrences of herbicide resistant biotypes. Not only are herbicide resistant weeds appearing after fewer repeat annual applications of some of the newer herbicides (e.g., 4 to 5 years of treatment with sulfonylures herbicides), but there seem to be more species that have potential for resistance, as shown by the 26 new cases in 1988. In addition, the resistant biotypes are more fit and competitive than most biotypes resistant to triazine herbicides.

Problems and Model If the development of herbicide resistance continues, the lifespan of several herbicides will probably be shortened, leaving farmers to face several impossible weed control problems. Moreover, weeds resistant to herbicides are costly because alternative herbicides are not always available. In some cases, supplementary treatments must be applied in addition to the standard treatment that remains efficient for controlling numerous "ordinary" weeds. Of the 3 million hectares of corn grown in France, 1.2 million hectares are now subjected to a postemergence treatment (e.g., pyridate) specifically directed against triazine resistant weeds. Thus, this so-called "remedial treatment" is becoming a standard treatment that is carried out after the preemergence atrazine treatment. This creates a four-fold increase in the cost of weed control in these corn fields. 

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