Excerpt from 'Linkages between resistance problems, resistance management and genetically modified crops?' a summary of a lecture at a PAN Europe meeting. Full text and links: http://www.biotech-info.net/linkages.html
Genetically modified crops are grown for resistance management purposes!
The databank presents a summary table of herbicide resistant weeds worldwide. In this summary 58 weed species and biotypes resistant to ALS inhibitor herbicides (such as imidazolinones, sulfonylureas, triazolopyrimidines) in 16 countries have climbed to position two, next to 61 triazine-resistant weeds in 22 countries; in November 1999 they took the lead. ALS inhibitor resistant weeds are most problematic in cereal, corn/soybean (!!), and rice production. It is likely that they will present farmers with greater problems in the next five years than triazine-resistant weeds have caused in the past 25 years.
By analysing and compiling databank information on the first proven occurence of weeds in soybean fields being resistant to ALS inhibitor herbicides in US states (Arkansas, Iowa, Minnesota, Ohio and others), I am able to conclude that these resistant biotypes should cause substantial problems for weed control. Consequently it follows that, in the USA, Roundup Ready soybeans are introduced and planted for weed resistance management purposes. Farmers switch from the overused herbicidal mode of action B (ALS inhibitor herbicides which resulted in resistant weeds within a few years) to the herbicidal mode of action G which is glyphosate, a chemical whose resistance risk is comparatively moderate.
But by doing only one rotational step, i.e. replacing imazethapyr, imazaquin, imazapyr, chlorimuron-ethyl and other ALS inhibitor herbicides in soybean with glyphosate sprayed on to genetically modified Roundup Ready soybean varieties, US farmers will hardly solve the problem. They will indeed spray their acres quickly into the next weed resistance catastrophy. Glyphosate resistant weeds will be selected easily from the increased glyphosate usage as many fields with a no-tillage history already have often a long field history of glyphosate usage. In the least, to my opinion, glyphosate resistant weeds will develop because of out-crossing transgenes.
It can be shown that in the USA and in China genetically engineered Bt-cotton is also introduced and "needed" for resistance management purposes.
full summary: http://www.biotech-info.net/linkages.html