Waterhemp has been a sporadic weed in Kentucky soybean production since the 1970’s.  However, it was not a major weed problem because metribuzin and linuron were widely used and waterhemp was controlled effectively with these herbicides.  The introduction of imazaquin and imazethapyr in the late 1980’s and their widespread adoption in the 1990’s by Kentucky farmers resulted in ALS-resistant waterhemp in some Kentucky areas.  Since the introduction to glyphosate resistant soybeans in 1996, herbicides with the glyphosate active ingredient have increased steadily due to its efficacy in weed control and low input costs.  In the past few years, waterhemp populations resistant to glyphosate have occurred in soybeans.  The majority of Kentucky soybeans are produced in some type of conservation tillage.  Most Kentucky growers do not desire to return to tillage for soybean production and wish to use glyphosate to control other weeds besides waterhemp.   Waterhemp is known to be resistant to several different mechanisms of action, including EPSPS, ALS, PPO, Triazine, and HPPD.  Kentucky farmers have not used preemergence herbicides for many years and have not used postemergence herbicides that require treating small weeds.  All of these factors have resulted in waterhemp being difficult to control in soybeans that rely exclusively on glyphosate.  For these reasons, waterhemp control research trials were conducted in Union and Hancock Counties in western Kentucky in an attempt to find herbicide combinations to provide season-long control. 

A field in Union Co. in 2010 revealed a waterhemp population not controlled by glyphosate.  ALS herbicides including chlorimuron and imazethapyr controlled an average of 15% of the waterhemp, while PPO herbicides, fomesafen and acifluorfen controlled an average of 46 and 31% of the waterhemp. These herbicides were applied to waterhemp that were between .5 and 1 meter in height.  Seeds from surviving plants were collected at the end of the growing season.  Seeds were scarified and planted; plants were treated with chlorimuron, glyphosate, or fomesafen at 1, 4, and 8 times the labeled rate.  Waterhemp survival decreased as herbicide rate increased.  Percent survival for glyphosate was 47% at 1x, 21% at 4x, and 5% at 8x respectively.  Percent survival for chlorimuron was 68% at 1x, 27% at 4x, and 29% at 8x.  Only one plant survived the fomesafen at the labeled rate, with no survivors at the 4 and 8x rates. 

A field study was established in 2011 in Hancock County.  Herbicides evaluated as pre-emergence treatments were fomesafen plus metolachlor, metribuzin plus metolachlor, sulfentrazone, saflufenacil, and sulfentrazone plus metribuzin.  These same treatments plus fomesafen and glyphosate applied at V3 were also evaluated.  The trial consisted of three replications of plots 3 by 12 meters.  Paraquat was applied to the entire area to control existing weeds. Soybeans were planted on June 1^st and pre-emergence treatments applied on June 3^rd.  Preemergence treatments provided an average of 74.5% control compared to the untreated check.  Preemergence treatments followed by a postemergence treatment provided an average of 97% control 67 days after application of pre-emergence treatments.  Among the treatments applied preemergence, fomesafen plus metolachlor, sulfentrazone plus metribuzin, metribuzin plus metolachlor, and sulfentrazone provided waterhemp control of 91%, 80%, 78%, and 50%, respectively.   Another study in the same field compared flumioxazin plus chlorimuron, sulfentrazone plus chlorimuron, sulfentrazone plus cloransulam, and flumioxazin plus pyroxasulfone followed by glyphosate, glyphosate plus fomesafen, or glyphosate plus fomesafen plus acetochlor.  All treatments provided 90 to 99% waterhemp control.

Pre-emergence treatments provide a longer duration for foliar treatments.  Waterhemp in this study never exceeded 15 centimeters in height which allowed for excellent post application waterhemp control.