Waterhemp “Officially” Glyphosate Resistant

Chris Boerboom, Ext. Weed Scientist

Aug 10, 2006

Dr. Kevin Bradley and Travis Legleiter, University of Missouri, have officially declared a population of common waterhemp in northwest Missouri as glyphosate resistant.  This is the same case that I mentioned at meetings last winter.  At that time, glyphosate resistance had only been tested in greenhouse studies and the results were highly suggestive that the waterhemp was resistant.  Now, Kevin and Travis have conducted studies in the fields with the resistant population.  Even at extremely high rates of glyphosate in their field trials, they are observing poor control of this population (one photo shows waterhemp surviving an application of 88 oz/a of Roundup Original Max).  With these new results, they are officially declaring this waterhemp population glyphosate resistant.    

Other herbicides are being evaluated to control the glyphosate-resistant waterhemp.  In soybean, the preemergence herbicides like Spartan, Valor, Boundary, Dual II Magnum, and Intrro are providing good initial control of the resistant waterhemp and conventional postemergence herbicides are also being tested.  In corn, many preemergence herbicides like Lumax, Harness Xtra, etc. are also providing excellent initial glyphosate-resistant waterhemp control and they are obtaining good control with postemergence applications of Distinct and Callisto. 

The management history in these fields was continuous soybean where glyphosate was the only herbicide used since 1996.  Missouri may be somewhat unique in the higher frequency of continuous soybean than other Midwest states.  In their case, continuous soybean likely means continuous Roundup Ready soybean and glyphosate-only weed management programs.  The development of a glyphosate resistant weed is a logical outcome of this high selection pressure and they suspect that other cases of glyphosate-resistant waterhemp are inevitable. 

Although Wisconsin has few fields of continuous soybean, we have the potential for growing continuous Roundup Ready crops (ie RR soybean and RR corn) and could rely solely on glyphosate for weed control.  I predict the same outcome of glyphosate-resistance if this is done. 

For waterhemp management, I believe Roundup Ready soybean became available just at the right time for Wisconsin soybean growers.  Waterhemp was spreading rapidly in Wisconsin in the mid-1990’s and was a serious problem because it was ALS resistant.  With Roundup Ready soybean, glyphosate became a new and excellent tool to control waterhemp and limited the spread and impact of waterhemp.  This is a weed where glyphosate has high value and we do not want resistance to develop.  Rotating glyphosate with herbicides that have other modes of action is strongly encouraged to prevent or delay the development of glyphosate resistance.