Glyphosate crop resistance

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. 

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). 

Glyphosate is toxic to plants and free-living microorganisms because it inhibits aromatic amino acid biosynthesis. On the other hand, it is extremely nontoxic to humans and animals because humans derive their amino acids from the diet. Additionally, it is broken down in the soil, so it is non-persistent. The only problem with glyphosate herbicides is that they will kill crop plants as readily as weeds. Recently, genetically engineered crop varieties have been introduced which are resistant to the herbicide, allowing weeds to be killed preferentially. 

Although we will not get to herbicides until later, it is appropriate to mention a very popular herbicide here. Glyphosate (also widely known as Round-Up) is the most widely used pesticide in the United States. In 2001, 40-50 million kilograms of glyphosate was used in the United States. This herbicide is used in conjunction with Round-Up Ready corn and soybeans, crops that have been genetically modified to be resistant to Round-Up. Thus, the entire field can be sprayed, and the weeds will die but the crop will not. 

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). 

Herbicide resistance is currently less of a problem than with the other pesticide groups, although, once it occurs, the effects are profound. Instead, the major interest centres around the control of off-target drift (i.e. application issues), specificity and the use (and side-effects) of certain herbicides with genetically modified crops. The examples listed in Table 8.1 indicate the continuing importance of pre-1980 molecules. For instance, glyphosate has become the most important herbicide, with its recent fall in price, broad spectrum of efficacy, low mammalian toxicity and (where GM crops are acceptable) its compatibility with Roundup ready crops. 

Many of the new transgenic crops (36%) are crops engineered to be resistant to old herbicides.331 These include bromoxynil (11.65) (TD50 111 mg/kg oral, mice) for cotton, and glyphosate for maize, cotton, and soybeans.332... 

In 1995 over 1.7 billion worth of glyphosate was sold. (The total world market for herbicides is estimated to be 14 billion.) This herbicide thus makes up more than 12% of the herbicide market. It has been more than 30 years since the phytotoxic properties of glyphosate were first described, and it is still an herbicide with great unexploited potential through the use of genetically engineered crop plants resistant to it. Whether such techniques are ethically acceptable and favorable for the chemical environment and biodiversity is another question. The debate about this will probably continue for another decade or so. 

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