A report out of Delaware indicates that marestail (horseweed) that is resistant to glyphosate has been confirmed in three separate fields -- and may be present in as many as six fields in Delaware and others in New Jersey and Maryland. Glyphosate is the active ingredient in Roundup and Touchdown herbicides.

The three fields have similar herbicide histories, according to Mark VanGessel, a weed scientist at the University of Delaware. Roundup had been used in combination with other herbicides as a burndown over a number of years. Over the last three years, each field has had a burndown and an over-the-top application of Roundup or Touchdown with no other herbicides in the rotation.

The three fields are at least 60 miles apart, VanGessel says. In one field sprayed with the recommended rate of glyphosate last season, VanGessel saw dead marestail right next to marestail that looked like nothing had happened to it.

"It certainly looked like resistance," VanGessel says. "Marestail control was random throughout the field, which ruled out sprayer problems or applicator error. With almost ideal weather conditions early in 2000, we also ruled out environmental or stress factors."

VanGessel says they applied 10 times the recommended rate of Roundup and Touchdown on the uncontrolled marestail and only stunted the weeds.