Delta Soils Found to Alter Herbicide's Effectiveness

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 Southern 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 atrazine—one of the most widely applied herbicides in North America—to curb broadleaf weeds.

Research leader Bob 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 dioxide—a process called pesticide mineralization—in 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.