My weed science colleagues at the University of Missouri have many years of experience in managing waterhemp, including waterhemp biotypes that are glyphosate, ALS-, and PPO-inhibitor resistant. This winter they published a new bulletin focused on waterhemp management in corn and soybean titled Management of Glyhosate-Resistant Waterhemp in Corn and Soybean, bulletin IPM1030 (http://extension.missouri.edu/explorepdf/agguides/pests/ipm1030.pdf). A few remarks from this bulletin are certainly worth considering for those in Wisconsin who also have waterhemp in their fields.   

 Key waterhemp facts

 

The Missouri weed scientists note many non-glyphosate herbicides are available to control glyphosate resistant waterhemp including preemergence and postemergence products. Preemergence herbicides include the grass herbicides (i.e. Dual, Harness/Surpass, Outlook, etc.), their atrazine premixes (i.e. Bicep Lite, Keystone LA, G-Max Lite, etc), and mesotrione products (i.e. Camix, Lumax). Post herbicides include atrazine, dicamba-based products (i.e. Banvel, Clarity, Status, etc.), HPPD herbicides (Callisto, Impact, Laudis), and Ignite. Of course, post treatments must be applied at the correct waterhemp size to be effective.

 

Preemergence herbicides are key in managing glyphosate-resistant waterhemp in soybean and are more effective than postemergence PPO-inhibiting herbicides. In populations that are glyphosate, ALS, and PPO resistant, they state “there are essentially no postemergence tank mix options for control”. Row cultivation is the only option to control resistant waterhemp plants that escape treatment. Ignite is an option in LibertyLink soybeans, but the best program is still a preemergence herbicide followed by Ignite before waterhemp exceeds 4 inches.  

 

The following table lists the soybean herbicides with excellent or good ratings from the bulletin.

Preemergence herbicides		Postemergence herbicides
Authority Assist	Excellent	Cobra/Phoenix	Good/Excellent
AuthorityFirst/Sonic	Excellent	Flexstar	Good/Excellent
Authority MTZ	Excellent	Ultra Blazer	Good/Excellent
Boundary	Excellent	Ignite	Good
Prefix	Excellent
Dual II Magnum	Good/Excellent
Enlite	Good
Outlook	Good
Gangster	Good
Valor SX	Good

                                       

While we do not have any confirmed glyphosate-resistant waterhemp in Wisconsin, we should maintain a diversity of herbicide use to prevent its development. Using alternate herbicides in corn fields with waterhemp is one strategy. Another strategy to consider is the use of preemergence herbicides in soybean fields that have a history of waterhemp.

 

Pictures: Pigweed seedhead top; waterhemp seedheads bottom.