Bowing to demands from Washington, the Colombian government has agreed to test a granular herbicide to kill coca crops, despite public warnings from the chemical's American manufacturer against its use in Colombia.

In the United States, the herbicide, tebuthiuron, is used mostly to control weeds on railroad beds and under high-voltage lines far away from food crops and people.The Environmental Protection Agency requires a warning label on the chemical that says it could contaminate groundwater, a side effect Colombian environmental officials fear could prevent peasants from growing food where coca once grew.

U.S. officials have decided to concentrate more heavily on treating illegal drug crops with chemicals, particularly in parts of southern Colombia under the control of leftist guerrillas. Those guerrillas have fired on aircraft attempting to spray herbicides on coca crops. But tebuthiuron can be dropped instead of sprayed, making the task easier under such conditions.

Washington has lobbied Andean governments to accept tebuthiuron for more than a decade, even though the chemical's manufacturer, Dow AgroSciences, a subsidiary of Dow Chemical Co., strongly opposes its use in Colombia.

Dow, which faced years of lawsuits and public protest over the use of its Agent Orange defoliant during the Vietnam war, said that if approached, it would refuse to sell tebuthiuron for use in Colombia. However, American officials note Dow's patent on the chemical has expired, allowing others to make it legally.

Critics in Colombia, including Eduardo Verano, the nation's environmental minister, say the health effects of tebuthiuron on farming areas are unknown, and its use will only increase deforestation by pushing coca growers deeper into forests.