Dr. Bill McCloskey
Extension Weed Specialist

•   Selection pressure
•   Shifting populations
•   Tolerance vs resistance
•   Understanding herbicides
•   Current state of herbicide resistance
•   Factors contributing to resistance
•   Identifying resistance

•   Where do the opportunistic species or weedy invaders of bare ground or empty space come from?
–   They are species that have always been part of the natural environment.
–   Natural ecosystem processes have always resulted in disturbed sites or habitats.
–   retreating ice sheets following glacial ice ages
–   stream banks
–   alluvial flood plains (e.g., Mississippi River)
–   volcanic deposits (e.g., Mount St. Helens)
–   meteorite impact craters and surrounding areas
–   severe wild fires

1.
Germination requirements met in many environments
2.
Discontinuous germination and seed longevity
3.
Rapid growth through vegetative phase to flowering
4.
Continuous seed production for as long as growing conditions permit
5.
Very high seed output in favorable environments
6.
Production of some seed in a wide range of environmental conditions; tolerance and plasticity
7.
Adaptations for short-distance dispersal and longdistance dispersal
8.
If perennial, vigorous vegetative reproduction or regeneration from fragments

•   When humans first began to domesticate plants, they created disturbed sites.
•   As agricultural technology became more powerful
–   the scale and intensity of disturbance increased and
–   invasion by weedy plants increased.
•   Natural selection resulted in weed species that evolved simultaneously with crop plants as humans domesticated plants.
•   Weed populations change in response to the weed control practices used in agro-ecosystems and natural ecosystems.

•   High genetic diversity in weed populations
•   Weed populations in a field usually consist of a mixture of species
–   Relative proportion of individual species is dynamic and can vary over time in response to management practices
–   Repeated use of a single control tactic can lead to weed populations dominated by species not controlled by that practice

•   Earliest changes in weed populations were shifts in weed species composition (i.e., weed shifts) caused by changes in agricultural practices
•   In Denmark in the 60 years after 1911, there was an increase in abundance of annual weed species and concomitant decrease in herbaceous perennial weeds.
Adoption of tractors and greater soil disturbance resulting from tractor drawn tillage implements compared to horse drawn tillage implements.

•   Example: changing from disking to mowing resident weeds:
–   Favors the growth of grasses and prostrate broadleaf weeds such as puncturevine.
–   Decreases population of upright growing broadleaves such as pigweeds.

•   The use of 2,4-D in cereal grain crops caused a shift in weed species:
–   Grass weeds tolerant to 2,4-D increased, and
–   Broadleaf weeds susceptible to 2,4-D decreased in abundance.
•   2,4-D is an example of the use of a selective herbicide.
–   Shift to tolerant or resistant species

– Repeated use of trifluralin, suppression of summer annual grasses and increase in purple nutsedge (8-4-2008)

– Repeated use of trifluralin, suppression of summer annual grasses and increase in purple nutsedge (9-5-2008)

– Repeated use of trifluralin, suppression of summer annual grasses and increase in purple nutsedge (2-19-2009)

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•   From 1996 through 2011
1.372 billion hectares of HT crops have been grown:
–   406 million of HT corn
–   829 million acres of HT soybeans
–   135 million acres of HT cotton
•   Between 1996 and 2011:
–   HR crops increased herbicide use by 527 million pounds
–   Bt crops decreased insecticide use by 123 million pounds
70
60
50
40
30
20
10
HT Corn HT Soybeans HT Cotton %HT
90
80
0
1996 2001
Year
2006 2011
Benbrook CM. 2012. Impacts of genetically engineered crops on pesticide use in the U.S. – the first sixteen years. Environmental
Sciences Europe 24:24. http://www.enveurope.com/content/21/1/24

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#
Other causes of herbicide failure have been ruled out.
#
The same herbicide or herbicides with the same mode of action have been used year after year.
#
One weed species that is normally controlled is
NOT controlled while other weed species are controlled.
#
Healthy weeds are mixed with killed weeds
(same species)
#
A single-species weed patch of uncontrolled plants is spreading.

•   Weed Management exerts selection pressure
–   Tolerance, species shifts & resistance
•   Understand and increase the diversity of herbicide modes of action
•   Use other weed management tactics to reduce selection pressure