Glyphosate herbicide resistance

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). 

Boutsalis, P. (2001). Herbicide Resistance Action Committee (HRAC) Web page http //plantprotection.org/HRAC Bradshaw, L.D., S.R. Padgette, S.L. Kimball, and B.H. Wells (1997). Perspectives on glyphosate resistance. Weed Technol., 11 189-198. Bravin, F., A. Onofri, G. Zanin, and M. Sattin (2004). Is malathion a useful tool to infer the chlorsulfuron-resistance mechanism in mul-tiresistant Italian populations of Lolium spp. 4th International Weed Science Congress, p. 52, S15MT08P00. 

Neve, P, J. Sadler, and S.B. Powles (2004). Multiple herbicide resistance in a glyphosate-resistant rigid ryegrass (Lolium rigidum) population. Weed Sci. 52 920-928. 

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. 

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. 

In the example shown here, selection was made for herbicide resistance. In the upper left-hand corner, glyphosate, the active ingredient in Roundup, is added to the growth media. After several... 

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. 

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. 

The main traits with which GM crops have been modified are herbicide resistance and insect resistance [1]. Herbicide resistance allows for the topical application of broad-spectrum herbicides, such as glyphosate and glufosinate, which would otherwise be detrimental to the crop. Insect resistance has in most cases been achieved by the introduction of insecticidal proteins that naturally occur in the soil bacterium Bacillus thuringiensis, which is also used as a biological pesticide in agriculture, in particular in organic agriculture. 

Finally, weed shifts observed in genetically modified crops, caused by the development of weed resistance to the widely used glyphosate herbicide, will offer market opportunities for herbicides with other modes of action, such as Protox-inhibiting herbicides. 

Herbicides are classified as either selective or broad spectrum. Selective herbicides can be used in-crop to control weeds without significant crop damage. Broad spectrum herbicides such as glyphosate and glufosinate are limited to preplant or post-directed applications. The technology to engineer herbicide resistance has enabled in-crop use of broad spectrum herbicides for improved weed control and yield. 

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]. 

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 three commercially available herbicide resistant traits in cotton glyphosate resistance (Roundup Ready , RR), glufosinate resistance (Liberty Link , LL), and bromoxynil resistance (BXN ). In 2005, herbicide resistance traits were cultivated on over 80% of approximately 14 million total cotton acres in the United States (Fig. 6.1.3) [6]. This acreage represents a combination of herbicide resistance alone or stacked with other traits such as insect control traits. 

Until 2004, the adoption of herbicide resistant traits in com has been slower than in other crops mainly due to trade restrictions in export markets for CM products. Nevertheless, the adoption of herbicide resistant com has resulted in significant grower and environmental benefits. Use of glyphosate and glufosinate resistance traits in 2004 alone resulted in a reduction in crop production costs of 139 million and pesticide use of 18.5 million pounds with numerous positive environmental attributes from increased no-till acres [2]. 

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. 

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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