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A Hot Afternoon Helps Weeds Resist Herbicide

By Don Comis
August 14, 2000

A few hours’ difference in herbicide application time on a hot afternoon can mean the difference between success and failure for cotton farmers trying to rid their fields of major weeds like pigweed.

James R. Mahan, an Agricultural Research Service plant physiologist, and Peter Dotray, a weed scientist at both Texas Tech and Texas A&M universities in Lubbock, have found that when the herbicide Staple was sprayed on pigweed on an afternoon when the temperature climbed above 93 degrees Fahrenheit, pigweed was barely affected. But just six feet away, pigweed that was sprayed in the cooler morning was almost totally killed.

Staple is important in the Cotton Belt because it is the only herbicide that farmers can spray over the top of cotton plants without harming cotton.

Staple works by inhibiting a key plant enzyme. Mahan, Dotray and student Ginger Light took a close look at the enzyme in test tubes and found it to be most vulnerable to Staple at temperatures between 68 and 93 degrees Fahrenheit—its “Thermal Application Range.”

Two years of field studies confirmed the lab results.

Mahan and Dotray recommend that farmers check the five-day forecast before they spray Staple, to see which will be the coolest days. Farmers might consider stopping the application before the day gets too warm. The temperature at spraying time has the greatest effect on how well Staple works, even though it takes the compound two weeks to kill weeds.

The experiment grew out of Mahan’s discovery that key plant enzymes did best within a narrow temperature range, causing plants to grow best at those temperatures. The scientists expect several other weeds to have similar thermal application ranges.

ARS is the chief research agency of the U.S. Department of Agriculture.

Scientific contact: James R. Mahan, ARS Plant Stress and Germplasm Research Unit, Lubbock, Texas, phone (806) 749-5560, fax (806) 723-5272,