The residual herbicides commonly used in the production of corn and
soybean are frequently detected in rivers, streams, and reservoirs at
concentrations that exceed drinking water standards in areas where these
crops are extensively grown. When these bodies of water are used as
sources of drinking water this contamination can lead to increased
treatment costs or a need to seek alternative sources of supply.
Additionally, these herbicides can have negative effects on aquatic
ecosystems at concentrations well below their drinking water standards.
When genetically modified, herbicide-tolerant, corn and soybean became
commercially available in the 1990s it became possible to replace some
of the problematic residual herbicides with strongly sorbed, short
half-life, contact herbicides that may be more environmentally benign.
By 2004 almost 90% of the soybean grown in the US was genetically
modified for tolerance to the contact herbicide glyphosate (Roundup),
which is currently the most widely used herbicide in the world.
In a four-year study, researchers at the USDA-ARS’s North Appalachian
Experimental Watershed near Coshocton, OH compared relative losses of
both herbicide types when applied at normal rates to seven small
watersheds planted with Liberty-Linked corn or Roundup Ready soybean. In
their report, published in the March-April issue of the Journal of
Environmental Quality, soil scientists Martin Shipitalo and Lloyd Owens,
and agricultural engineer Rob Malone, noted that losses of contact
herbicides in surface runoff were usually much less than those for the
residual herbicides, as a percentage of the amount of herbicide applied.
Averaged for all soybean crop years, glyphosate loss was about
one-seventh that of metribuzin and one half that of alachlor, residual
herbicides it can replace. Similarly, average loss of the contact
herbicide glufosinate (Liberty) was one-fourth that of atrazine, a
residual corn herbicide it can replace.
More importantly, according to project leader Martin Shipitalo, “The
concentrations of the contact herbicides in the runoff never exceeded
their established or proposed drinking water standards while the
residual herbicides frequently exceeded their standards, particularly in
the first few runoff events after application”. Concentrations of
atrazine in runoff were up to 240 times greater than its drinking water
standard while alachlor concentrations were up to 700 times greater than
its standard. Conversely, the maximum glyphosate concentration noted
was nearly four times less than its standard. Glufosinate currently has
no established standard, but was only detected at low concentrations and
was below its detection limit 80 days after application.
In light of increased economic incentives to grow more corn and
soybean for biofuel production, these results suggest to farmers and the
regulatory community that herbicide losses and concentrations in runoff
can be reduced by planting herbicide-tolerant varieties of these crops
and replacing some of the residual herbicides with the contact
herbicides compared in this study.
The full article is available for no charge until May 30. View the abstract at http://jeq.scijournals.org/cgi/content/abstract/37/2/401.
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