Fall Weed Scouting

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Fall is an important time to take stock of how effective your weed programs were this year. Success in improving or modifying your weed management program for next season is going to depend on your knowledge of weeds in that field. This fall, when it is fresh in your mind, take note of which weeds were present in the field, how heavy the infestation was, and where those patches were located. Taking notes as you are combining may be the best time to locate these weeds. Also, note size of the weeds. If the weeds are small and did most of their growing after the crop began to dry down, they will not impact yield and they will not produce many seeds that can plague you next year. These weeds were either suppressed by your herbicide program or emerged after your herbicides had been played out. These weeds are of little consequence. On the other hand, note those weeds that are large and competed with the crop all season.

Here are things to consider if a field was weedy at harvest. First, if a weed was not controlled review the label and extension information to be sure that the weed species is supposed to be controlled by the herbicide(s) you used. If the herbicides you used are not effective then you may need to switch or include another herbicide in your program. Also, with all the lack of rain at times this season, poor herbicide performance from your residual herbicides was probably due to the herbicide not being moved into the soil (“not activated”). Finally, if the weed was supposed to be controlled by your program, and the herbicide was a triazine or an ALS-inhibiting herbicide see your county extension agent to discuss the potential of herbicide-resistant weeds. ALS-inhibiting herbicides include Accent, Steadfast, Exceed, Permit, Sceptor, Pursuit, or Harmony GT etc. Finally, with glyphosate, new reports of glyphosate-resistance have shown up in other areas of the US, but if more than one species was not controlled in your field, it is less likely to be a resistance problem.

If perennials are a problem, scouting gives you a chance to locate the patches and identify areas to spot-spray with a post-harvest treatment. In addition, you can plan for next season to help determine if a spot-treatment is appropriate or if the perennials are wide-spread and you need to treat the entire field.

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