Dearl E. Sanders and James L. Griffin

Atrazine is a herbicide commonly used for the control of broadleaf weeds in corn, grain sorghum, sugarcane and turfgrass. Although widely used in Louisiana since the early 1960s, atrazine has recently become the center of controversy in south central Louisiana. Even when used according to label instructions, a small amount of atrazine can move from a treated field into runoff water – usually associated with a heavy rainfall shortly after application.

As with all pesticides, the U.S. Environmental Protection Agency (EPA) has established maximum allowable levels of atrazine residues in food and water. Recently published EPA guidelines indicate no problems with atrazine residue in the national food supply; however, problems with excessive levels have been identified in 34 of the nation’s community water systems, including Iberville Water District #3 in Iberville Parish in Louisiana.

EPA has established the maximum allowable level of atrazine at 12 parts per billion (ppb) as a quarterly average in finished drinking water. Finished drinking water is defined as the water leaving a community water system after all sedimentation, filtration and chemical treatments have been completed. During the past 10 years of intensive monitoring of the national water supply, EPA has identified only 34 systems in which the 12 ppb limit has been exceeded. The majority are in the Midwest.

An area defined by EPA as the Upper Terrebonne Basin comprising most of the parishes of Point Coupee, West Baton Rouge and Iberville contains approximately 50,000 acres of sugarcane and varying acreages of corn. Both crops are historically significant users of atrazine. Because of constraints by a series of levees and canals, all of the drainage from the Upper Terrebonne Basin – including runoff from sugarcane and corn fields – leaves the area via the Intracoastal Waterway in lower Iberville Parish. The Iberville Water District #3 established a water system in the early 1990s and chose for its sole source of water the Intracoastal Waterway. Monitoring by several governmental agencies has shown atrazine levels exceeding the 12 ppb limit at the Iberville Water District #3 water treatment plant during February and March for several years.

Since 1997, LSU AgCenter researchers have studied herbicide runoff from sugarcane and corn fields. Monitoring of runoff indicates that between 80 percent and 90 percent of the atrazine that leaves a treated field during a growing season does so after the first rainfall event sufficient to cause runoff. This corresponds with results of monitoring efforts conducted at the water treatment plant where high levels of atrazine have been recorded following heavy rains in February, followed by declines through March and, finally, low or nondetectable levels through the summer and fall.

EPA has continued reregistration of atrazine since 2001. An agreement between EPA and Syngenta Crop Protection (the primary atrazine registrant) in 2003 states that unfinished or “raw” water will be monitored on a weekly basis for five years beginning in 2004 for seven systems, including Iberville Water District #3. If the atrazine levels there exceed 37.5 ppb as a rolling 90-day average in the unfinished water during the five-year period, then all uses of atrazine will be banned in the Upper Terrebonne Basin.

Anticipating the possibility of a partial or complete atrazine ban, LSU AgCenter researchers in 1997 began projects aimed at either reducing atrazine runoff or replacing atrazine with other weed control strategies in the basin. Corn acreage in the basin fluctuates greatly, based primarily on annual grain prices and federal government grain price supports. Corn acreage has steadily declined since 1997, corresponding to a decline in use of atrazine.

Studies indicate that while runoff from corn fields contributes to the overall atrazine problem, it is only a lesser part of the problem because runoff from corn fields usually occurs in April. This is after the peak levels have passed at the water plant. Additional studies have shown that alternative weed control strategies in corn can reduce or even eliminate the use of atrazine without sacrificing weed control. Unfortunately, all of the alternative programs investigated were significantly more expensive than the programs using atrazine.

The peaks in atrazine levels in February and March at the Iberville Water District #3 water plant appear more related to atrazine use in sugarcane, the major crop grown in the Upper Terrebonne Basin. LSU AgCenter researchers have found that sugarcane residue remaining on fields from harvest of the previous year’s crop can reduce both herbicide and soil loss from fields. Research efforts in sugarcane have led to the development of best management practices (BMPs) to reduce the potential for herbicide loss from sugarcane fields. As in corn, research in sugarcane has concentrated on weed control alternatives to atrazine. Currently labeled herbicides, including Dupont K4, Spartan, Sencor and Direx, can serve as alternatives to atrazine; however, these herbicides are more expensive than atrazine. Because of concerns over atrazine, the AgCenter emphasis has been to educate growers on efficient use of atrazine and the alternatives available.

Should atrazine levels exceed the newly established 90-day average in unfinished water at the Iberville Water District #3 site, the implications are significant. Atrazine use will be banned in the basin. If this occurs, both corn and sugarcane could still be produced in the basin but at an increased cost to growers. Alternative weed control programs will be more expensive, reducing the net return for the grower.

Acknowledgments: Richard Bengston, Reed Lencse, Magdi Selim and Ron Strahan for their research contributions and the Louisiana Soybean and Grain Research and Promotion Board and the American Sugar Cane League for their funding support.

(This article was published in the spring 2004 issue of Louisiana Agriculture.)