Herbicide Resistant Horseweed Found

 

Horseweed is difficult to pull and owing makes the problem worse. It grows 8 to 10 feet tall, competing with everything around it for water, nutrients and sun. Even worse, each plant produces 150,000 to 200,000 seeds

Horseweed, also known as mare?EUR??,,????'???s tail and by its botanical name Conyza canadensis, is now growing unchecked on irrigation canal banks, vacant lots, orchard and vineyard floors, roadsides and gardens in California. Apparently certain biotypes of horseweed have evolved that are resistant to the most commonly used herbicide ?EUR??,,????'??? glyphosate, the active ingredient in 55 brand-name and generic herbicides the most common of which is Roundup. According to the California Department of Food and Agriculture, 5.7 million pounds of glyphosate were used by the agricultural industry in 2003. Continuous use of glyphosate has, in effect, selected for horseweed that is resistant to the chemical.

The scientists believe that another weed, hairy fleabane, may also be evolving glyphosate resistance, a phenomenon that has been confirmed in hairy fleabane in only two other areas worldwide ?EUR??,,????'??? one in Spain and the other in South Africa. Land managers who notice a great number of horseweed or hairy fleabane should use a diversity of methods to bring them under control, making sure the weeds do not go to seed. Hand pulling and pre-emergent herbicides will control the pest.