Weed resistance

Is weed resistance a significant problem in many crops How can it be combatted ... 

The development of herbicide-resistant weeds has also been an influence on the selection of herbicides used on field corn or soybean. Weed resistance now affects nearly every decision a farmer makes about herbicide selection either a farmer is trying to control resistant weeds or is selecting herbicides that may reduce the possibility of weed populations becoming resistant. The adoption of the imidazolinone- and sulfonylurea-tolerant com hybrids mentioned above was in part a response to the presence of atrazine-tolerant pigweeds or kochia in many fields. However, a recent decrease in die use of imidazolinone and sulfonylurea herbicides can also be attributed to the development of populations of weeds that have become resistant to these herbicides. 

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

Triazines were one of the first family of herbicides where weed resistance was widely recognized and documented in the literature. A simazine-resistant biotype of common groundsel was identified in Washington, United States, in 1968. Since then biotypes of at least 66 triazine-resistant weed species have been reported, mostly in the United States, Canada, and Europe (Heap, 2006). 

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. 

Weed Resistance Based on Enhanced Herbicide Metabolism... 

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. 

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... 

Retzinger, E.J. and C.A. Mallory-Smith (1997). Classification of herbicides by site of action for weed resistance management strategies. Weed Technol., 11 384-393. 

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). 

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. 

Table 11.5 Discovery history of weeds resistant to glyphosatea...

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. 

Chiang, M.Y., C.I. Yuan, and Y.J. Chiang (2004). Weeds resistant to foliar applied herbicides in Taiwan. 4th International Weed Science Congress, p. 61, S17MT08P07. 

Jutsum, A.R. and J.C. Graham (1995). Managing weed resistance The role of the agrochemical industry. Brighton Crop Protection Conference-Weeds, pp. 557-566. 

Nalewaja, J.D. (1999). Cultural practices for weed resistance management. Weed Technol., 13 643-656. 

Padgette, S.R., X. Delannay, L.D. Bradshaw, B.H. Wells, and G. Kishore (1995). Development of glyphosate-tolerant crops and perspectives on the potential for weed resistance to glyphosate, p. 154. In De Prado, R., Garcia-Torres, L., and Jorrin, J., eds., International Symposium on Weed and Crop Resistance to Herbicides. Cardoba, Spain Cardobesa de Impresiones Graficas, SL. Abstract, 92. 

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