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() Dissipation of three sulfonylurea herbicides in fortified farm dugouts (ponds).

Cessna, A.1, 2, Donald, D.3, Bailey, J.4, Waiser, M.4, Headley, J.4, 1 Agriculture and Agri-Food Canada, Lethbridge, Alberta, Canada2 National Water Research Institute, Saskatoon, Saskatchewan, Canada3 Environment Canada, Regina, Saskatchewan, Canada4 National Water Research Institute, Saskatoon, Saskatchewan, Canada

ABSTRACT- Sulfonylurea herbicides are used to control broad-leaved weeds in a variety of crops. With low mammalian toxicity, worldwide use is increasing due to their high herbicidal activity at very low application rates (2 to 75 g ha-1). There is concern that this high phytotoxicity may result in negative environmental impacts on sensitive ecosystems such as prairie wetlands which, in Canada, are widely dispersed within agricultural areas where sulfonylurea and other herbicides are extensively used. There is limited information on the persistence of sulfonylurea herbicides in prairie wetlands or their potential to affect the flora and fauna within these ecosystems. On June 26, 2003, three sulfonylurea herbicides (thifensulfuron-methyl, metsulfuron-methyl and ethametsulfuron-methyl) commonly applied in the prairie region of Saskatchewan were each applied to one of three farm dugouts at 1 to 4.5 L-1. Target fortification levels in each dugout were equivalent to that resulting in a theoretical wetland containing a half metre of water and oversprayed at the recommended application rate. A fourth unfortified dugout was used to provide control water samples. Water samples (500 mL) were collected on the day prior to application (day 0) and on days 2, 3, 4, 6, 8, 11; then every 5 d until July 22; every 10 d until August 20 and every 20 d until October 20. Winter samples were also collected monthly through the ice from November to February and following snow melt runoff on May 06, 2004. Following solid-phase extraction, the water sample extracts were analysed by HPLC and the sulfonylurea herbicides quantitated by positive ion electrospray tandem mass spectrometry. Mean recoveries from distilled water fortified at 10 ng L-1 ranged from 71.2 to 97.5% with RSDs less than 7% (n = 4). Analytical results indicated that the sulfonylurea herbicides were much more persistent in dugout water than conventional herbicides (eg., 2,4-D, MCPA) and that the order of decreasing persistence was metsulfuron-methyl > ethametsulfuron-methyl > thifensulfuron-methyl. This order of persistence in the dugouts reflects the maximum recropping intervals recommended for these herbicides: 48 months, 22 months, and no restrictions the year after treatment, respectively. The methyl ester linkage in all three herbicides was resistant to hydrolysis.

Key words: farm dugouts, sulfonylurea herbicides, fortification, dissipation

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