Herbicide-Resistant Weeds In GM Crops: Lessons From the North American Experience

GM Crops and Resistant Weeds

The repeated and exclusive use of a herbicide will frequently select resistant individuals within a weed population.

Not a new lesson but one that has been reinforced with significant consequences in herbicide resistant GM crops especially glyphosate resistant Roundup Ready crops.

Diversity in weed management strategies is critical to delay the selection of resistant weed populations.

GM Crops - Background 15 years since the introduction of GM

crops

148 million hectares of GM crops were planted in 2010

> 50% grown in North America 66.8 million ha in USA 8.8 million ha in Canada

Fastest adoption of any agricultural technology.

GM Crops - Background Herbicide-resistant GM crops grown North America

are: Alfalafa (Medicago sativa) Canola (Brassica napus) - > 80% Cotton (Gossypium hirsutum and G. barbadense) - 75% Maize (Zea mays) - 75% Soybean (Glycine max) - 94% Sugarbeet (Beta vulgaris) - 95%

There are other herbicide resistant GM crop species being developed.

NonGM herbicide resistant crops are grown in North America.

GM Crops - Background

Herbicide resistance is the most common GM trait – 61% of ha

Glyphosate resistance (Roundup Ready) most common followed by glufosinate resistance (Liberty Link).

GM crops are being developed with resistance to herbicides with other modes of action such as auxins, PPO inhibitors, HPPD inhibitors.

GM crops have been and are being developed with resistance to more than one herbicide. “stacked traits”

Herbicide-Resistant GM Crops Glyphosate-resistant GM crops are / were

grown because: provided an opportunity for simple and highly

effective weed control crops no longer injured by herbicide

applications opportunity to decrease tillage control of weeds with resistance to other

herbicides most new cultivars contain the trait so difficult

to choose other options

GM Crops and Resistant Weeds Result of abandoning the basic principles of weed

management.

There was no stewardship plan in place to delay or prevent glyphosate resistance when the crops were introduced. total glyphosate system encouraged and rewarded glyphosate rates and number of applications increased over time Roundup Ready crops were not rotated to non Roundup Ready

crops integrated weed management was ignored

Should have been no surprise to anyone that resistance would occur.

GM Crops and Resistant Weeds

GM crops increased the potential for a single herbicide to be used so increased selection pressure for resistant weeds.

Glyphosate resistant weeds were selected.

Worldwide there are 21 glyphosate resistant weed species.

Estimated that > 10 million acres are infested with glyphosate resistant weeds in USA. most associated with glyphosate resistant crops

Glyphosate Resistant Weeds in USA

New York Times 2010

Herbicide-Resistant GM Crops

Benefits are rapidly being lost because of resistance.

Impacts of resistant weeds in agriculture: growers are paying for a product that no

longer provides expected benefits threaten no-tillage or reduced tillage systems

in some areas reduced cropping diversity increased cost of weed control

older herbicide chemistries hand weeding

Glyphosate Resistant Weeds in Glyphosate-Resistant GM Cotton: Lesson From the Field

Glyphosate-Resistant GM Cotton

Glyphosate-resistant cotton was quickly adopted.

Growers were using glyphosate exclusively for weed control.

Production shifted to no-tillage systems. Glyphosate resistant weeds disrupted the

system.

Glyphosate Resistant Weeds in GM Cotton Amaranthus palmeri – Palmer amaranth Conyza canadensis – marestail, horseweed Eluesine indica - goosegrass

Glyphosate-Resistant Weeds in GM Cotton Growers are now using: hand weeding tillage – erosion issues pre-emergence herbicides hooded sprayers to apply herbicides

Resistant Weeds in GM Cotton: Today cotton growers are moving quickly to

the use of glufosinate resistant cotton. Will results be any different? Perhaps because:

glufosinate controls fewer weeds weeds must be smaller so applications will not be

effective all season preemergence or postemergence herbicides will likely

be needed

Perhaps not because: lessons may be ignored higher rates are being allowed may just repeat the glyphosate story

GM Crops With Stacked Traits

Crops with more than 1 herbicide-resistance trait are being introduced. GlyTol-LibertyLink has both glyphosate and glufosinate

resistance Cotton with glyphosate, glufosinate and dicamba resistance

Increase the potential for multiple-resistance in weed species.

May increase the diversity of herbicides used but does not encourage good weed management using nonchemical weed control especially crop rotation.

Herbicide-Resistant GM Crops Resistance issue is similar in glyphosate

resistant soybeans and maize.

Introduction of new generation of GM crops appears to be much the same as first generation.

There needs to be a new approach to the use of these crops.

The crops must be managed at the landscape level not the field level.

Company promotions of GM crops still suggest one pass weed control options and the solution is another resistance trait.

Reflections

Before a herbicide-resistant GM crop is introduced: insure that a management plan is in place to

protect cropping system not a company stewardship plan for protection of

the technology

plan to maintain as many options for weed management as possible

GM Crops and Resistant Weeds The repeated and exclusive use of a herbicide will

frequently select resistant individuals within a weed population.

Not a new lesson but one that has been reinforced with significant consequences in herbicide resistant GM crops especially glyphosate resistant Roundup Ready crops.

Diversity in weed management strategies is critical to delay the selection of resistant weed populations whether crop is resistant or not.

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