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How Weed Resistance Develops
Ron Meyer, Area
Extension Agent - Golden Plains Area
Date: 1/19/2012
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Weed control has always
been a critical challenge for crop producers because weeds compete with crops
for light, water and nutrients. Herbicides used in modern agricultural systems
enable farmers to manage most weeds across vast acreages, efficiently and up
until current times, reliably.
However, weed adaptations can occur in
response to herbicide use and other management decisions. Changes in weed
populations begin when a small number of plants within a species, called a
“biotype,” have a distinct genetic makeup that allows them to
tolerate a particular herbicide application. Multiple weed biotypes can and do
exist in a single field.
As a grower continues to use a particular
herbicide without any other herbicide modes of action, or doesn’t use any
other cultural practices, the resistant biotype continues to survive and
produce seed. Subsequent populations of the resistant biotype will continue to
increase until they are the dominant weed in the field.
Weed scientists
cannot predict exactly which weed species will have biotypes resistant to
certain herbicides. Prediction can be difficult due to complex biology and
environmental interactions. Scientists have found that there are particular
weed characteristics that can facilitate development of herbicide resistance.
These include:
* large amount of seeds produced per plant
* high
levels of germination of those seeds
* several weed flushes per season
*
high frequency of resistant genes within a weed population
Both company
and university weed scientists have also identified specific common factors
that are often present in areas where glyphosate resistance has developed.
These factors are:
* Limited or no crop rotation
* Limited or no
tillage practices
* A high dependency on glyphosate alone or a limited use
of other herbicides
* Reduced rates of glyphosate
Confusion about
what is or is not weed resistance is common. Herbicides are not known to
directly cause genetic mutations in weeds that lead to resistance. However,
herbicide resistant biotypes may already exist in native weed populations. When
a herbicide is applied over and over again, some of these biotypes survive,
mature and produce seed. If a farmer relies on only one herbicide with the same
mechanism of action, again, the percentage of the resistant biotypes in the
population is likely to increase. This is referred to as herbicide selection
pressure.
Strategies that address herbicide resistance issues include
crop rotation and employing herbicides with multiple modes of action. Crop
rotation includes exiting the corn-on-corn rotation. Wheat, sunflower, or
soybeans are acceptable options for cropping choices. Keep in mind that with
crop rotations, producers must also employ alternative herbicides to achieve
acceptable resistant weed control. Recent plant testing in the area has
identified Kochia as showing resistance to glyphosate applications in some
fields. Therefore, if glyphosate resistance Kochia is the issue, also employ
either pre-emergent or post-emergent herbicides which are not glyphosate type
products that will control Kochia. Depending on the crop, there are numerous
herbicides available that will continue to control this weed. However, the
first step with Best Management Practices is to employ them.
As a result
of employing Best Management Practices for resistant weeds a number of things
happen. Fields are cleaner, the herbicide usefulness will be extended and
producers will be more profitable.
Source: © 2011 Monsanto
Company