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Drift of sulfonylurea herbicides
by Bob Hartzler

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December 2, 1996-Between courses at the annual Thanksgiving gathering at my wife's aunt's house, I picked up the December issue of Organic Gardening magazine. Inside was an article which basically condemned DuPont for problems associated with the sulfonylurea herbicides. The article reported that drift of chlorsulfuron (Glean, used for weed control in wheat) caused dramatic yield losses in several crops without visual injury symptoms. The author concluded that the sulfonylurea herbicides, and other ALS-inhibitor herbicides, posed a serious threat to gardeners due to their high level of activity.

The article was based on the following article: Fletcher, J.S. et al. 1996. Potential impact of low levels of chlorsulfuron and other herbicides on growth and yield of nontarget plants. Environmental Toxicology and Chemistry 15:1189-1196. The authors evaluated the impact of Glean, atrazine, Roundup and 2,4-D applied during reproductive stages on the growth and yield of smartweed, canola, sunflower and soybeans in greenhouse studies. The herbicides were applied either at 0.004 or 0.008 times the label rate to simulate drift.

The researchers found that atrazine and glyphosate did not influence yield or growth of any of the test plants; 2,4-D did not effect canola, smartweed, or soybean, but it did reduce sunflower yield by 4% and height by 58%. Chlorsulfuron reduced seed yield by up to 90, 82, 85 and 100% in canola, smartweed, soybean and sunflower, respectively. Interestingly, the height of canola and soybean were reduced by less than 20% even with the severe yield loss.

Fortunately we don't have to worry about drift of Glean onto soybeans in Iowa, but what implications does this paper have for persons involved in weed management. First, we have stressed the importance of making timely herbicide applications and avoiding applications during reproductive stages of crop growth. Although this research evaluated an ALS-inhibitor not used in corn or soybeans, the research suggests that ALS products may pose a greater risk of yield impacts when applied during flowering than herbicides with other modes of action. Although I wouldn't expect to see the dramatic yield losses reported in this paper, observations made in growers' fields have documented the risk of late applications. The paper also reinforces the need to take appropriate precautions to minimize drift and problems associated with off-target movement.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Copyright 1996-2003, Iowa State University, all rights reserved  

Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.