Genetically Modified Herbicide Resistant Crops

Claire A. Cajacob, Paul C. C. Feng, Steven E. Reiser, and Stephen R. Padgette 

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. 

In 2005, GM crops were cultivated in 21 countries with 71% of those acres being accounted for by herbicide resistant traits in soybean, corn, canola, and cotton. This percentage increases to 82% if one includes herbicide resistance trait acres that are stacked with other biotechnology traits. Globally, GM herbicide resistant soybean, cotton, canola, and corn were grown on 134.4 (60%), 12.1 (14%), 11.4 (18%), and 24.5 (7%) million acres, respectively. (Fig. 6.1.1) [1]. 

The growing global use of GM crops has had several positive agronomic, economic, and environmental impacts. In the United States in 2004 alone, the use of GM crops reduced pesticide use by 62 million pounds, with 55.5 million pounds of that accounted for by the use of herbicide resistant crops. GM crops also produced significant environmental benefits. In addition to reduced pesticide use, increased no-till practices have reduced water runoff, greenhouse gas emissions, 

Modem Crop Protection Compounds. Edited by W. Kramer and U. Sdiirmer Copyright 2007 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31496-6 

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Chapter 5, entitled Safeners for Herbicides , demonstrates the progress in this research field, bringing out new compounds that create highly competitive products for the farmers out of only partly selective herbicides having a very broad weed spectrum and very low application rates ( the chemical answer to genetically modified herbicide resistant crops ). 

I 6 Genetically Modified Herbicide Resistant Crops Spray atomization... 

Nevertheless, more and more genetically modified varieties (mainly insect- and herbicide-resistant crops) become listed in the Common EU Catalogue of varieties, indicating that the cultivation of those GM varieties could accelerate in the near future. GM varieties need to be authorised in accordance with Directive 2001/18/EC (former 90/220/EEC) before they are included in this Common Catalogue and potentially marketed in the EU. By doing so, this directive intends to provide a freedom of choice between GM and non-GM products, not only for... 

In the herbicide market, more than USD 250 milHon of the sales of herbicides in the USA were transferred in 1998 from stand alone herbicides to herbicides that could be combined with a genetically modified crop. This redistribution of herbicides puts traditional agrochemical businesses at risk. Companies where herbicides account for more than 50 percent of the total revenues and that have a high market share in the USA are already suffering. The biggest short term losers are players that used to have strong sales in those areas (like soybean and com) where herbicide-resistant crops have been particularly successful. 

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. 

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. 

Cotton has been genetically modified to tolerate herbicides or to resist insect pests. In the US, over three quarters of the cotton crop is GM. In theory, cotton engineered to make its own pesticides should need fewer chemical sprays. However, different insect pests are taking over and the main cotton pests are becoming resistant to the GM cotton, meaning that farmers still have to spray their crops just as often. 

U.S., 90% of soybeans are genetically transformed to be herbicide-resistant 80% of cotton is transformed for resistance to herbicides and, through incorporation of Bacillus thuriengensis, to various insect pests and 50% of field corn is similarly modified for both herbicide and insect resistance [4], None of the crops are used directly for human food, although food use is made of corn and cottonseed oil from genetically modified corn and cotton and the bulk of the transformed soybeans and corn, and cottonseed meal, are fed to animals which enter the human food supply. End-user and consumer nonacceptance of genetically modified foods continues in many quarters. 

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. 

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