Herbicide glyphosate resistant crops

Kleter, G.A., Harris, C.A., Stephenson, G.R. and Unsworth, J. (2008) Comparison of herbicide regimes and the associated potential environmental effects of glyphosate-resistant crops vs. what they replace in Europe. Pestic Manage Sci, 64, 479-488. 

Throughout western Canada and the central Great Plains of North America, volunteer wheat is becoming a more serious problem (Leeson el al., 2005). This may become a special concern if the volunteer wheat is glyphosate-resistant (Harker el al., 2005). There are also many examples of integration of traits from weeds into crops, and there is some evidence of spread from herbicide-resistant crops into weeds (Gressel, 2002). 

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. 

The advantage for the farmer is that he needs only one product, instead of several different selective (and more expensive) herbicides. Roundup ready soybeans were launched in 1996 and today 50 percent of the soybean crop in the United States is derived from roundup ready seeds. Other glyphosate-resistant transgenic crops introduced by Monsanto are maize and oil seed rape. Competing companies also developed herbicide-resistant plants or plants genetically modified to be protected against certain pests, but none has achieved a commercial breakthrough, mainly because of political reasons. 

One of the most important characteristics contributing to the value of herbicides has been plant selectivity. The fact that some plants (weeds) are killed by herbicides while others (crops) are naturally resistant has been very important to selective weed control in crop production. A few herbicides that have been useful without natural or biological resistance, such as glyphosate and paraquat, have found great success in crop production due to the use of other methods of selective weed control (e.g., directed applications, applying prior to crop emergence or applying to dormant perennials). 

This strategy has led to commercial development of herbicide resistance for glu-fosinate, glyphosate and bromoxynil. Glufosinate and glyphosate resistance will be discussed in detail in later sections of this chapter (see also Chapter 6.2). Bro-moxynil s herbicide activity is due to inhibition of electron transport in photosystem II. Crops engineered with bromoxynil nitrilase metabolize the herbicide to a non-phytotoxic compound [5]. 

This strategy was used to engineer resistance against glyphosate, and imidazoli-nones and sulfonylureas that inhibit acetolactate synthase (ALS), a key enzyme in the biosynthesis of branched chain amino acids. ALS resistant crops have primarily been generated through selection for an herbicide insensitive ALS allele from natural or mutagenized cell or plant populations [3]. 

There are no commercial herbicide resistant crops that function by increased expression of the protein target, although some level of plant resistance has been reported for glyphosate, glufosinate, some DPEs and inhibitors of hydroxy-phenylpyruvate dioxygenase. Similarly, cellular sequestration of the herbicide from the target has been reported with some DPEs, auxins and photosystem I inhibitors, but none have been developed commercially [3]. 

This section includes data for herbicide resistant crops generated by both selection and biotechnology processes. The first commercially available herbicide resistant crop in the United States was imidazolinone resistant corn introduced in 1992. This was followed by glyphosate resistant soybean and canola in 1996. 

There are several other commercially available herbicide resistant crops. The imidazolinone resistant trait is also available in rice, wheat, sunflower, and lentil. Glufosinate resistance is available in rice and glyphosate resistance was recently commercialized in alfalfa in 2005. 

Over the years, Roundup hzs developed into the commercially most successful herbicide ever, superior by far to all other plant protection agents. [30] A major contributor to this blockbuster story was the timely development of genetically modified (GM), f o nd p -resistant crops, especially soya and maize. Thereby, the loss in profit from the patent expiration of glyphosate in 2000 was not only compensated, Monsanto even managed to extend the life cycle of Roundup by tying it to its new and prosperously growing seed business. 

Current research into glyphosate formulations has driven the technology to high loading, low viscosity, low irritancy and inexpensive formulations. With the proliferation of herbicide resistant crops, the formulation must also be nonphytotoxic to the crop it is applied to. 

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