There are only two main types of GM crops commercially available: herbicide resistant crops - the best known being Roundup Ready - and pest resistant Bt crops. The point of both is to make weed and pest management simpler.

But research recently published in the journal Nature Biotechnology predicts Bt crop failures in India within the very near future. According to the lead researcher, "it wouldn't be surprising to find Bt-cotton crop failures in some parts of India, starting with Gujarat in a couple of years from now."
http://mumbai.indymedia.org/en/2005/02/210116.shtml

In the case of GM herbicide reistant crops, which constitute about 80% of all the GM crops grown worldwide, a leading international expert on weed control recently noted that the adoption of Roundup Ready crops across vast slices of the United States is already inexorably leading to herbicide-resistant weeds on a massive scale. He predicts the U.S. is likely to soon lead the world in the weed resistance stakes.
http://www.lobbywatch.org/archive2.asp?arcid=4892

Below TWN draw attention to a comprehensive study published in the journal Pest Management Science which looks further at the problems of herbicide-resistant crops and weed resistance to herbicides.
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THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
15 March 2005

Dear Friends and colleagues,

RE: Herbicide-resistant crops and weed resistance to herbicides

The adoption of transgenic herbicide-resistant crops has increased dramatically in the last decade. Most of the increase in hectares of transgenic crops planted is attributable to herbicide-resistant (especially glyphosate-resistant) soybean, maize, canola and cotton.

In a comprehensive study published in the journal Pest Management Science, Michael Owen and Ian Zelaya of Iowa State University found that there are many risks associated with the production of GM and herbicide-resistant crops including problems with grain contamination, segregation and introgression of herbicide-resistant traits, marketplace acceptance and an increased reliance on herbicides for weed control.

In particular, the widespread adoption of herbicide-resistant cultivars, particularly glyphosate-resistant crops, has dramatically impacted weed communities. Weed population shifts to naturally resistant species and to species with inherent biological characteristics that make the populations difficult to manage (e.g. delayed emergence), as well as the evolution of herbicide-resistant weed populations are real. Another problem is the emergence of the herbicide-resistant crops themselves as volunteer weeds. The speed at which these changes have occurred is cause for significant concern.

The abstract of the paper is reproduced below. The full paper is available in Pest Management Science 61:301 311 (2005)

With best wishes,
Lim Li Lin and Chee Yoke Heong
Third World Network
121-S Jalan Utama
10450 Penang
Malaysia
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
Website: www.twnside.org.sg

Ref: Doc. TWN/Biosafety/2005/A

Herbicide-resistant crops and weed resistance to herbicides
Micheal DK Owen and Ian A Zelaya
Iowa State University, Ames, IA 50011-1011, USA
Pest Management Science 61:301 311 (2005)

Abstract: The adoption of genetically modified (GM) crops has increased dramatically during the last 3 years, and currently over 52 million hectares of GM crops are planted world-wide. Approximately 41 million hectares of GM crops planted are herbicide-resistant crops, which includes an estimated 33.3 million hectares of herbicide-resistant soybean. Herbicide-resistant maize, canola, cotton and soybean accounted for 77% of the GM crop hectares in 2001. However, sugarbeet, wheat, and as many as 14 other crops have transgenic herbicide-resistant cultivars that may be commercially available in the near future. There are many risks associated with the production of GM and herbicide-resistant crops, including problems with grain contamination, segregation and introgression of herbicide-resistant traits, marketplace acceptance and an increased reliance on herbicides for weed control. The latter issue is represented in the occurrence of weed population shifts, the evolution of herbicide-resistant weed populations and herbicide-resistant crops becoming volunteer weeds. Another issue is the ecological impact that simple weed management programs based on herbicide-resistant crops have on weed communities. Asiatic dayflower (Commelina cumminus L) common lambsquarters (Chenopodium album L) and wild buckwheat (Polygonum convolvulus L) are reported to be increasing in prominence in some agroecosystems due to the simple and significant selection pressure brought to bear by herbicide-resistant crops and the concomitant use of the herbicide. Finally, evolution of herbicide-resistant weed populations attributable to the herbicide-resistant crop/herbicide program has been observed. Examples of herbicide-resistant weeds include populations of horseweed (Conyza canadensis (L) Cronq) resistant to N-(phosphonomethyl) glycine (glyphosate). An important question is whether or not these problems represent significant economic issues for future agriculture.