Cotton. Click the image for more information
Delta Soils Found to Alter Herbicide's
September 12, 2006
Why do soils in certain parts of
the Mississippi Delta cause atrazine to degrade too fast? Agricultural Research
Service (ARS) scientists have found that
a microbial process, which occurs after a short exposure to this herbicide, may
result in a loss of atrazine's effectiveness.
Scientists at the ARS
Weed Science Research Unit in Stoneville, Miss., have been working with
corn growers to figure out why this is happening. In recent years, as
Mississippi farmers have shifted away from cotton-only production to a
corn-cotton rotation, they've turned to atrazineone of the most widely
applied herbicides in North Americato curb broadleaf weeds.
Zablotowicz and his team at Stoneville collected samples of soils with
known management histories from 21 sites in Leflore, Sunflower and Washington
counties in Mississippi. Then they used radio-labeled atrazine to assess the
herbicide's rate of degradation to carbon dioxidea process called
pesticide mineralizationin the samples. Mineralization is a term for
complete breakdown of an organic compound to simple inorganic components like
carbon dioxide, water and ammonium.
The researchers found mineralization was extensive in soils with as few as
just one to three atrazine applications. Cumulative mineralization ranged from
45 to 72 percent over 30 days, under laboratory conditions.
Their findings, recently published in Weed
Science, suggest that microbial populations capable of accelerating
atrazine degradation have developed in Mississippi Delta soils after a very
short exposure to the herbicide. This may be due in part to the Delta region's
very mild winters and abundant rainfall that allow continual survival of
atrazine-degrading microbes once they appear in the soil.
Most well-drained Mississippi Delta soils have historically been used solely
for cotton production, bringing growers about $600 million annually in revenue
from about 1.1 million acres. But long-term monoculture production can
eventually degrade the soil and cause increased invasive pest problems,
including weeds, insects and nematodes.
Recently, corn has become a desirable rotational crop, since it requires
lower inputs and can provide a good return with less risk than cotton. Efforts
now are needed to find a suitable replacement for atrazine in production areas
where its effectiveness has been significantly reduced.
ARS is the U.S. Department of
Agriculture's principal scientific research agency.