Glyphosate is a post-emergence, broad-spectrum chemical herbicide found in commercial products marketed under several names. It is widely used on weeds in no-till food crop management such as soybeans and corn, and has few to no latency effects in the soil.
While the number of glyphosate-resistant weed types is relatively low (as Chris Boerboom at the University of Wisconsin and Michael Owen at Iowa State University discuss in their work "Facts About Glyphosate-Resistant Weeds"), the number of weeds becoming resistant to the herbicide is increasing quickly.
Rigid ryegrass was first reported as glyphosate resistant in Australia in 1997, and in California in 1998. Rigid ryegrass is often found in wheat crops and is resistant to a broad range of herbicides approved for use on wheat crops, making it difficult to control.
Horseweed (or Marestail, as it is also known) was first reported as glyphosate resistant in 2000; resistance incidence has since been reported in 14 states and impact several million acres of crop land sweeping from Delaware to Illinois.
Waterhemp was first reported as glyphosate resistant in 2005 in Missouri. It grows widely in corn and soybean fields and is already resistant to a range of other herbicides, making it challenging to control.