| [Image: Table 1] | | [Image: Table 2] | | | | | | |
Richard Bengtson and Magdi Selim
Minimizing the levels of herbicides in surface water and groundwater is of major concern nationally and within the agricultural community. Little work has been carried out on correlating application of herbicides used in sugarcane production with water quality impairment. Moreover, a Louisiana Department of Environmental Quality (DEQ) report from 1990 indicated that most water bodies in Louisiana are impaired to some degree. To address the need for more knowledge on sugarcane production and water quality, researchers at the LSU AgCenter initiated a study in 1992, as part of a DEQ sugarcane demonstration project. The goal was to compare losses of applied herbicides (atrazine and metribuzin) in surface runoff water on sugarcane fields under different management practices. The study consisted of three treatments over six growing seasons. The site chosen was at the LSU AgCenter’s St. Gabriel Research Station. A sump equipped with an electric pump was installed on the low side of each of six research plots. Runoff was measured with a water meter and sampled with an automatic water sampler. Sample collection was triggered automatically when runoff was detected.
Atrazine and metribuzin (sold as Sencor to farmers) are two herbicides used in sugarcane production. Atrazine has been used extensively for more than 40 years and is perhaps one of the most widely applied herbicides in the world. The lifetime health advisory for atrazine in drinking water is 3 parts per billion (ppb). Metribuzin is a commonly used herbicide with a lifetime health advisory level in drinking water of 200 ppb.
Three Treatments
To optimize the benefit from this work, the amounts of herbicides applied to the plots were above and below those recommended for use as the accepted cultural practice for sugarcane in Louisiana. The high management treatment consisted of herbicides applied full broadcast at a rate 1.8 pounds per acre for atrazine and 2.0 pounds per acre for metribuzin. The standard management treatment consisted of herbicides applied in a 36-inch band over the row at a rate 0.9 pound per acre for atrazine and 1.0 pound per acre for metribuzin. This is the cultural practice used by the Louisiana sugarcane industry. The low management treatment consisted of herbicides applied in a 24-inch band over the row at a rate 0.6 pound per acre for atrazine and 0.7 pound for acre for metribuzin. For layby treatments, herbicides were applied full broadcast at a rate 1.8 pounds per acre for atrazine and 2.0 pounds per acre for metribuzin.
Average Rainfall
From 1994 to 1999, the average annual rainfall was 57.07 inches, which was close to the normal of 56.87 inches. The annual rainfall ranged from a high of 70.48 inches in 1997, to a low of 46.98 in 1999. The average annual surface runoff was 22.08, 21.98 and 20.47 inches for the high, standard and low treatments, respectively.
Atrazine was applied to the plots on Jan. 6, 1994, and Dec. 20, 1994. The average annual atrazine losses were 2.24, 0.82 and 0.48 ounces per acre (Table 1) for the high, standard and low treatments, respectively. This was a loss of 7.8 percent, 5.7 percent and 5.0 percent, respectively, of the amount of active ingredient applied. The three treatments were significantly different. Reducing the amount of atrazine applied and banding the herbicide on the top of the cane row significantly reduced the amount lost in the surface runoff.
Metribuzin was applied to the plots in March 18, 1994; May 2, 1995; and May 8, 1997. The average annual metribuzin losses were 1.10, 0.46 and 0.14 ounces per acre (Table 2) for the high, standard and low treatments, respectively. This was a loss of 3.5 percent, 2.9 percent and 1.2 percent, respectively, of the amount of active ingredient applied. The three treatments were significantly different. Reducing the amount of metribuzin applied and banding on the top of the cane row significantly reduced the amount lost in the surface runoff. The 24-inch band treatment produced satisfactory weed control.
Major Rainfall
On the morning of June 17, 1997, broadcast treatments of atrazine and metribuzin were applied to designated plots for layby weed control. Two hours after application, it started raining. It rained 8.05 inches in 6 hours and 11.40 inches in 24 hours. This was an unusual rainfall expected to occur only once every 75 years. It caused runoff from the area of more than 10 inches. The total metribuzin lost in 44 days from the broadcast treatment was 16.43 ounces per acre (51 percent of the applied active ingredient). The load from June 17 was 16.29 ounces per acre, which accounted for 99 percent of the total loss. The largest metribuzin concentration of 431 ppb was detected in the first runoff.
The total atrazine lost for the 44-day period was 1.26 ounces per acre (4.5 percent of the applied active ingredient). The load from June 17 was 1.14 ounces per acre, which accounted for 88 percent of the total loss. The largest atrazine concentration of 85 ppb was detected 21 days after application. The atrazine concentration detected in the first runoff (30 ppb) was 93 percent less than the metribuzin concentration the same day. This enormous loss of metribuzin caused concentrations to diminish rapidly in the following events. Metribuzin concentrations were reduced to levels below the maximum concentration limit (MCL) of 200 ppb within 26 days after application, and atrazine concentrations were detected above its MCL of 3 ppb until 44 days after application.
The 75-year return period storm that occurred within hours after pesticide application had the timing and high runoff volumes for major surface runoff. Since both atrazine and metribuzin are poorly bound to soils, their potential loss in surface runoff is high.
Summary
The atrazine losses were significantly different among treatments. The atrazine losses from the broadcast method used in the high treatment were 273 percent higher than for the 36-inch band used in the standard treatment. The herbicide falling between the rows from the broadcast method washed off with the surface runoff. Applying atrazine in 24-inch bands (low management treatment) reduced atrazine losses by 58 percent compared to the 36-inch bands (standard management practice).
The metribuzin losses were significantly different among treatments. The metribuzin losses from the broadcast method (high management treatment) were 239 percent higher than for the 36-inch bands (standard management treatment). The 24-inch bands used in the low management treatment reduced metribuzin losses by 30 percent from the 36-inch bands.
Atrazine is less vulnerable than metribuzin to losses in high volume runoff that occurs shortly after application. Although atrazine concentrations (30 ppb) in the 75-year return period storm were high, its low water solubility kept levels far below the 431 ppb for metribuzin. The atrazine loss from the broadcast layby treatment was 92 percent less than for metribuzin.
Conclusions
Sugarcane growers have adopted the management practice of spraying herbicide in 36-inch bands, which reduces herbicide loss by 87 percent from the broadcast method. Growers have also started using metribuzin instead of atrazine. The concentrations of metribuzin in runoff water averaged 40 ppb, which was 80 percent less than the 200 ppb MCL. Use of metribuzin controls weeds and keeps the herbicide loss in runoff water within U.S. Environmental Protection Agency (EPA) standards. |