GENET archive


2-Plants: Linkages between resistance problems, resistance management and GE crops?

----------------------------- GENET-news -----------------------------

TITLE:  Linkages between resistance problems, resistance management
        and genetically modified crops?
SOURCE: PAN Germany, by Crescentia Freudling
          summary of lecture at PAN Europe meeting 1999
DATE:   March 2000

-------------------- archive: --------------------

Linkage among resistance problems, resistance management and 
genetically modified crops?
(presented at the PAN Europe Meeting 1999 - Pesticide Reduction, Time 
for Action, 30.9. - 2.10.1999 in Hamburg)

The issue of pesticide resistance is linked to pesticide reduction 
through the fact that insecticide and herbicide resistance in insects 
or weeds commonly correlate with higher pesticide usage and higher 
control costs.

The many discussions of Bt resistance management plans (i.e. high 
toxin production in the Bt plants and the percentage of non-Bt plants 
grown in refuge fields) by public researchers and NGO persons 
generally neglect the vast knowledge and practical experience of 
resistance management strategies gained from sprayed insecticides, 
fungicides and herbicides. It is a fact that until now, with 
pesticides, there have been many more resistance management failures 
than successes.

The risk factors highly influencing the selection of herbicide 
resistant weeds are known and published: no crop rotation, no 
ploughing, only chemical weed control, the herbicides used with only 
one biochemical mode of action. Therefore a combination of "three Rs" 
is recommended to prevent weeds becoming herbicide resistant or to 
solve herbicide resistance problems. The rotation of crops, rotation 
of cultural practices (ploughing included) and a rotation of the 
modes of action of the used herbicides are advised.

Information on resistant weeds: new glyphosate resistant weeds

There is a very informative and freely accessible documentation of 
herbicide resistant weeds worldwide ( It seems 
this databank is neither widely known nor used by NGOs. Namely, two 
more cases of glyphosate resistant weed species, proven and published 
in 1999, have gone undetected: a glyphosate resistant rigid ryegrass 
(Lolium rigidum) in California/USA and a glyphosate resistant 
goosegrass (Eleusine indica) in Malaysia (at the homepage of please select "mode of action", then choose 
"glycines" and you will find the examples). In all likelyhood these 
two resistant weeds have been selected in no-tillage field systems.

Genetically modified crops are grown for resistance management 

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 

- The link between resistance problems and the newly grown 
genetically modified crops should be adressed seriously and properly. 
- The US agriculture is no model! To quote experts of the herbicide 
industry: "From experience, we can conclude that rotation of 
herbicides alone is not enough to prevent the development of 
resistance." The USA should introduce integrated weed resistance 
management practices immediately! 
- All interest groups, NGOs included, should become involved in 
acquiring knowledge on resistance management and resistance 
prevention because resistance problems, IPM measures, the quantity of 
pesticide usage and pesticide dependency are closely interlinked! 

A more detailed version of these arguments (with six tabels and 
reference literature) is available on request from the author or the 
PAN Germany bureau!

Crescentia Freudling

PAN Germany
Fax: +49-40-3907520


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