Weed biotypes herbicide-resistant

It should be possible to assess the potential and the reason for decreased fitness in resistant biotypes in cases where the mechanism of resistance is known (2). Table I lists the herbicides or classes of herbicides to which resistance has been found in field situations and for which the mechanism of resistance is known or suspected. (A complete list of weeds with herbicide resistant biotypes as of December, 1988 is presented in the chapter by LeBaron and McFarland in this book.)... 

Triazlne-reslstant weeds have appeared In fields where trlazine herbicides have been continuously used. At least 38 resistant weed species have been Identified (2). The weed biotypes are resistant to a number of trlazine herbicides, including atrazlne and terbutryn, and have a characteristic, altered spectrum of sensitivity to other herbicides (1). Resistance in Amaranthus hybridus (J) and Solanum nigrum ( ) has been attributed to a mutation in the chloroplast psbA gene. The frequency of appearance of resistant blotypes has been calculated to be very low C ). It was therefore proposed that a nuclear-encoded chloroplast mutator gene was involved (1). 

Pfister, K. and C.J. Amtzen (1979). The mode of action of photosystem II-specific inhibitors in herbicide-resistant weed biotypes. Z. Naturforsch. Sect. C Biosci., 34 996-1009. 

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 high efficacy of triazine herbicides and their repetitive use in crops and noncrop situations has resulted in the selection of weeds that are resistant to these herbicides or are not well controlled at the lower rates now being used. In most instances, triazine resistance is due to an alteration in the herbicide-binding site in PS II. Despite the widespread occurrence of triazine resistance, these herbicides are still widely used, even in fields in which triazine-resistant biotypes are known to occur. The rate of increase in the selection for triazine-resistant weed species depends in part on the integration of alternative weed control strategies, in addition to the use of triazine herbicides, for control of these weed species. Due to their resistance mechanism, many triazine-resistant weeds are less competitive than their susceptible counterparts. 

Excellent progress has been made in the understanding of the cause, nature, genetics, mechanism and solutions of herbicide-resistant weeds since the first triazine-resistant common groundsel was reported more than 35 years ago. Resistance management programs have been extremely successful in controlling most weeds that have developed resistance to the triazine herbicides. However, research is critical to better understand the rapid increase and spread of many new weed biotypes resistant to several classes of herbicides. 

Clay, D.V. and C. Underwood (1989). The identification of triazine- and paraquat-resistant weed biotypes and their response to other herbicides. In Cavalloro, R., and Noye, G., eds, Proc. E. C. Experts, Grays, Tollose, Denmark, November 15-17, 1988. Luxembourg Office for Official Publications of the European Communities, pp. 47-55. 

Lopez-Garcia, M.C., C. Zaragoza, and R. DePrado (1996). Survey and agronomic importance of atrazine resistant weed biotypes in Argon, (Spain). In R. Deprado, J. Jorrin, L. Garcia Torres, and G. Marshall, eds., International Symposium on Weed and Crop Resistance to Herbicides. April 3-6, 1995. Cordoba, Spain Graficas TYPO, S.L, pp. 251-252. 

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). 

Weeds are rapidly becoming resistant to some of the newer herbicides, and it is important to realize the consequences. For example, in Missouri, Bader et al. (1995) conducted a study on corn-soybean rotations in which they used only ALS-inhibitor herbicides, that is, imazethapyr in soybean and primisulfuron-methyl in corn. Within 4 years, a common waterhemp biotype resistant to 5-fold higher rates of ALS inhibitors was flourishing. Greenhouse tests confirmed that ALS-resistant common waterhemp biotypes were present in several plots in the experiment. Within the same state, Bader et al. (1994) reported one case of atrazine-resistant common waterhemp, which developed where a farmer grew continuous com and used only atrazine for more than 10 years. Table 11.1 shows trends in the numbers of herbicide-resistant weeds. 

In Table 11.2, a listing of known herbicide-resistant biotypes is given according to the type of weeds (dicots and monocots), and herbicide class. 

Table 11.1 Number of weed species that have biotypes with resistance to major classes of herbicides over time3...

Table 11.2 Occurrence of resistant weed biotypes to different herbicide groups up to 2006...

Table 11.3 The number of herbicide-resistant weed biotypes in each of 52 countries in 2006...

Although the ALS inhibitor herbicides have been used for approximately 20 years, the number of resistant weed biotypes for this group now exceeds those for all other types of herbicides. Singh and Shaner (1995) and Boutsalis (2001) reported that a total of five chemical families or herbicide classes are commercially marketed as inhibitors of ALS, and that these herbicides comprise more than 50 active ingredients for selective use in many different crops. They include sulfonylureas, imidazolinones, triazolopyrimidines, sulfonylamino-carbonyl-triazolinones, and pyrimidinyl (thio)benzoates. 

Christopher et al. (1992) reported that a chlorsulfuron-resistant rigid ryegrass in Australia was resistant to most other sulfonylurea and imidazolinone ALS inhibitors. However, a common cocklebur biotype resistant to several imida-zolinone herbicides was not resistant to sulfonylurea herbicides (Saari et al., 1994). It is, therefore, difficult to generalize as to patterns of resistance within the five classes of ALS inhibitors. Weed biotypes resistant to one herbicide will usually show some level of resistance to most herbicides within the same class, and may in addition show some resistance to ALS inhibitors in other classes. 

Multiple-resistance mechanisms, defined as resistance due to more than one mode of action or class of herbicide, have been reported in several ALS-resistant weed biotypes - including false cleavers, wild oat, common waterhemp, kochia, rigid ryegrass in Australia (Powles and Matthews, 1992 Preston and Mallory-Smith, 2001), and wild radish (Walsh etal, 2004a). 

Tierney, M.J. and R.E. Talbert (1996). Competition of herbicide-resistant and -susceptible common cocklebur (Xanthium strumarium) biotypes. Proceedings South Weed Science Society, p. 147. 

In certain situations it is possible to overcome herbicide metabolism-based resistance by adding an ingredient that will block detoxification of the herbicide in the resistant weed. One example is with propanil-resistant Echinochloa colona in rice in Latin America. The addition of piperophos, an organophosphate insecticide that inhibits the aryl acylamidase activity that confers resistance on the weed biotype [11]. This combination, based on an undo standing of the resistance mechanism, has beat approved for use on resistant... 

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