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Chemical - not water - application is cause of pollution

By Don Comis

Environmental crop and
herbicide-use history are more critical to herbicide efficacy and environmental
safety than the timing and amount of irrigation water used, according to Agricultural
Research Service (ARS) scientists.

Dale Shaner and Lori Wiles
made this discovery from ongoing experiments on two irrigated fields at
Colorado State University at Fort Collins, Colorado. In collaboration with
Colorado State colleague Neil Hansen, Shaner and Wiles compared the behavior of
the herbicide atrazine in conventionally tilled corn grown continuously year
after year vs. corn grown in three different crop rotations. They tested
various levels of tillage and irrigation, including no irrigation.

Irrigation Had No Impact

The amount of irrigation,
including a total absence of irrigation, had no impact on the rate of
degradation of atrazine by soil microbes in the top 1 foot of soil. The only
factors that made a difference were prior herbicide use and the choice of crop
sequences, with prior herbicide use the most important factor by far.

This recent fieldwork
confirmed what earlier studies had found: Previous applications of atrazine can
predispose soil to more quickly degrade later applications of the herbicide.
But until now, it was not clear if other factors (such as cropping history and
quantity of irrigation) played a role in the degradation.

There are two consequences
of the more rapid dissipation of atrazine in the plots.

The first consequence is a
loss in weed control. In the plots with the most rapid dissipation, weeds began
to reinfest the plots within four weeks of treatment, while the plots with the
slowest rate of dissipation remained weed-free through the growing season.

The second consequence is
atrazine leached more deeply in the soil where it did not dissipate rapidly.
However, that herbicide did not move below the top 3 inches of the soil where
it was degraded rapidly.

Atrazine Degrades In Delta

Related research in the
Mississippi Delta has found that atrazine undergoes a more rapid degradation in
certain Delta soils. ARS researchers in Stoneville, Mississippi, discovered
that a microbial process, which occurs after a short exposure to this
herbicide, may result in a loss of atrazine’s effectiveness.

In recent years, Mississippi
farmers have shifted away from cotton-only production to a corn-cotton
rotation. In doing so, they’ve turned to atrazine to cut broadleaf weeds.

ARS researcher Bob
Zablotowicz used radio-labeled atrazine to assess the herbicide’s rate of
degradation to carbon dioxide (a process called pesticide mineralization).
Zablotowicz found mineralization was extensive in soils with as few as just one
to three atrazine applications. Mineralization ranged from 45% to 72% in over
30 days, under laboratory conditions. Irrigation, again, was not found to
contribute to atrazine mineralization.  

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