Waterhemp is one of the most common weed species found in Midwest field crops. A member of the pigweed (Amaranth) family, this summer annual is capable of producing over a million seeds per plant while significantly reducing crop yields.

Glyphosate-resistant waterhemp has been confirmed in 12 states. In Missouri, over two-thirds of the waterhemp populations sampled from soybean fields were found to be resistant to the herbicide glyphosate.

In the October-December 2013 issue of Weed Technology, a study sought to identify factors that could serve as predictors of glyphosate resistance in future populations of this weed. Waterhemp seed samples were collected from 144 soybean fields in 54 Missouri counties in 2008 and 2009 and served as the basis for the conclusions. Landowners from each survey location were contacted to determine a 5-year history of crop rotation and herbicide use.

In this study, 94% of the glyphosate-resistant waterhemp populations had three field aspects in common: 

1) Soybeans were the only crop in consecutive years. 
2) Glyphosate was the only herbicide used for three or more years. 
3) The field contained waterhemp showing obvious signs of having survived the previous herbicide application. 

These factors provide the best indication that glyphosate-resistant waterhemp is likely to occur in the future.

Full text of the article, “A Survey of Glyphosate-Resistant Waterhemp in Missouri Soybean Fields and Prediction of Glyphosate Resistance in Future Waterhemp Populations Based on In-Field Observations and Management Practices,” Weed Technology, Vol. 27, No. 4, October-December 2013, is available at http://www.wssajournals.org/doi/full/10.1614/WT-D-13-00042.1.