Stacked-trait GMOs
Herbicide-resistant weeds have more than doubled since 2009 to infest
approximately 70 million acres of American farmland –an area larger than the
states of Ohio and Illinois combined. 20
years after the introduction of genetically engineered (GE) Roundup Ready corn and soybeans, more tools are needed to maintain productivity.
However, use of Roundup (glyphosate) steadily increased, even as more and more weeds became resistant. In 2007, as much as 185 million pounds of glyphosate was used by U.S. farmers, double the amount used six years ago, according to Environmental Protection Agency (EPA) data.
2,4-D and Dicambra are
herbicides that are already used to “burn down” the weeds in the autumn, and as
pre-emergent herbicides as a prophylactic in the spring, before planting. But up until now these more-toxic herbicides
could not be used during the growing season, as glyphosate can on GE corn and
soybeans. More tools were needed to maintain yields.
Enter Dow's Enlist Duo
Dow recently secured regulatory approval to roll out Enlist
Duo in 2015, a stacked-trait GE for corn and soybean cultivars. Stacked traits have already been used to
enhance herbicide-resistant Roundup Ready soybeans with Bt, a natural
pesticide. There are also stacked trait
soybeans that contain transgenes to produce oils that are less susceptible to
rancidity, and commands a premium price on the market.
The new GE crops will be resistant to both glyphosate and 2,4-D,
allowing farmers to kill glyphosate-resistant weeds during the growing
season.
2,4-D is a plant hormone that kills broadleaf plants (but
not grasses like corn, or wheat) by overstimulating growth. In contrast, glyphosate works by inhibiting a
crucial plant enzyme that is not present in animals. Both are widely used in both residential
(lawns and gardening) and commercial (farm) settings.
Resistance Will Develop
Agronomists predict
that resistance to 2,4-D will develop as rapidly as resistance to
glyphosate, because farmers will spur evolution by using the same herbicide on
the plants in the same fields, successively selecting for anything with
resistance. USDA and EPA have vowed to
better manage the technology, but compliance with integrated pest management
strategies is voluntary.
2,4D has been known to drift off fields and kill nearby
woodlots, fruit trees, and organic crops, so Dow has changed the chemical to
reduce volatility and designed special nozzles to better control
application. EPA is “imposing first-time
ever restrictions to manage injury to sensitive crops. The EPA has put in place restrictions to
avoid pesticide drift, including a 30-foot in-field “no spray” buffer zone
around the application area, no pesticide application when the wind speed is
over 15 miles per hour, and only ground applications (with the special nozzles)
are permited.
Another first for the GE crops is
that the EPA is also imposing requirements to reduce potential for developing
resistant weeds, such as mandating extensive surveying and reporting to EPA and
grower education and remediation plans.
EPA will reevaluate after 6 years, and may impose new restrictions at
that point.
Resistant Superweeds
Some of the most common resistant weeds are: Marestail, Giant Ragweed,
Volunteer Corn, Common Ragweed, Lambs quarter, Agronomists idenitify resistant weeds based on the fact “that most... soybeans are RoundupReady, and that if weeds are still in the soybean field at
the end of the season, then there must have been a failure of the system (i.e. spraying
herbicides didn’t control them)."
"Experience with the Enlist system indicates that even
without a fall herbicide treatment, multiple in season application of 2,4D seem
to control marestail well. Doing so will
probably result in the development of resistance to 2,4-D in marestail, though,
since this is the type of approach that led to glyphosate resistance – multiple
applications of the same herbicide for control of the same weed." -Mark Loux,
OSU Extension Herbicide Specialist
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