Commercialized Herbicide Resistant Crops

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. 

Since then, the cultivation of herbicide resistant crops has grown globally with multiple herbicide resistant traits available in many large-acre crops (Table 6.1.1). The acreages of herbicide resistant traits in wheat and rice are insignificant and therefore not included in the table. The United States cultivates the greatest acreage of herbicide resistant crops and will be the primary focus of our discussions. 

Crop Herbicide resistance Trade name Company 

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The development of herbicide-resistant crop varieties as undertaken by American Cyanamid is an evolutionary process that proceeds through a series of decision points. Initially, the market potential for herbicide resistance in the crop is evaluated. After a project is assessed as worthwhile, a scheme to make the crop resistant is established and implemented. Once the scheme proves successful and resistance is introduced into the crop of interest, the trait is characterized to assess commercial utility. A method for delivering resistant crops to the marketplace is then identified. Finally, through close cooperation between the seed company and the chemical company, the herbicide-resistant crop becomes available commercially. 

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. 

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. 

Cloned herbicide resistant ALS genes have been used to transform both homologous and heterologous plant species. ALS genes can be modified in vitro in order to achieve selective resistance toward broad or narrow classes of inhibitors. The modified genes can be introduced into a variety of commercial crops. 

Herbicide resistance in weeds can be useful, and has already been commercially transferred to crops by conventional breeding. 

Herbicide resistance is at present one of the most commonly used traits in commercial cultivars of GM crop plants [1], Since this resistance operates via the production of an enzyme to break down the herbicide and bees lack such substrates, they are extremely unlikely to be harmed by these plants. Bioassays with the purified products of such genes have not been carried out, but some experiments with GM plants have (see Chapter 15). 

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]. 

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. 

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