Starting with a clean slate has some positive psychological benefits, and in the world of soybeans, a clean slate without glyphosate-resistant waterhemp is even more heady. As you prepare for the 2012 soybean crop, there are some events you cannot control, but there are also some pro-active management strategies you can take to win the battle and the war with waterhemp.

Our waterhemp worries have been a function of strong reliance upon a weed control program dependent solely upon glyphosate, and now typical evolutionary abilities of waterhemp have allowed it to become immune to glyphosate toxicity. A trio of Illinois weed specialists has published a prescription for farmers to control waterhemp and other problem weeds with a return to herbicides that have residual activity in the soil.

Step 1 is selecting the most appropriate soil-applied herbicide or herbicide pre-mix, which has good control over most of the weeds that have an impact in your field. Pay close attention to the amount of active ingredient that is available, since it varies widely, and that means a shorter duration for controlling waterhemp in some cases.

Step 2 focuses on the timing of the application, since an application 14 days before planting will control weeds longer after planting than one applied three weeks before planting. The label is important and should be consulted to determine the length of time between application and planting. 

Step 3 is the rate of application, which should be controlled by soil texture, organic matter and timing. However higher application rates may control weeds longer into the growing season. But that does not mean season- long control. 

Step 4 is determined by the soil moisture. If they are not moved into the soil with the help of precipitation, they need to be physically incorporated. Spraying an herbicide on a dry soil surface may be a waste of a good herbicide. And it may take a half to an inch of rain to move the herbicide into the weed development zone. If that does not happen in a timely manner, mechanical incorporation will be beneficial.

Step 5 requires you to be aware of the potential for crop injury, and burning of soybean tissue.  While your soybeans may be tolerant of a glyphosate shower, they may be burned by other products under certain conditions. That may happen with adequate soil moisture and low humidity. That may also happen if the application is made immediately before or after the crop is planted, and there may be a pocket of herbicide at the point of germination. 

The weed specialists offer a chart with water hemp control 30 and 60 days after treatment with a variety of soil-applied residual herbicides. Those include Authority, Valor, IntRRO, Outlook, Dual II Magnum, Linex, Sencor, Prowl H2O, Atrazine, and Pursuit, applied at the recommended rates. The percent of waterhemp control ranges from 6 percent to 95 percent 30 days after treatment, and 3 percent to 91 percent 60 days after treatment.

Summary:
With the growing resistance of waterhemp and some other weed species to glyphosate, the use of a soil-applied residual herbicide become an important tool for protecting soybean yields. A five step program can help by focusing on the herbicide selection, timing and rate of application, amount of soil moisture and being aware of potential crop injury.