Dry Weather and Corn Weed Control

Many corn fields were sprayed with preemergence herbicide followed by 7 to 10 days without rain to activate them.  Widespread field observations and reports indicate that the dry weather conditions in May have resulted in inadequate weed control in numerous corn fields throughout the region.  After any herbicide application is made you should check those fields to make certain that you have controlled the weeds.  If you have not checked your corn field since it has been sprayed, now is the time to do so.

For most preemergence herbicides to do their job they must be present in the soil solution where the weed seeds germinate.  Most weed seeds will germinate in the top one inch of the soil.  Depending on the herbicide used, it requires about ½ inch of rainfall to sufficiently move the herbicide to the soil depth require for effective weed control.  This is the main reason for so many failed preemergence herbicide treatments this season.

If you have applied a preemergence herbicide to corn it is time to scout for any possible weed escapes.  If any of these fields have emerged weeds that are breaking through, here is your list of options:

1. You can wait and see if the recent rainfall will provide any “reach back” activity.  Reach back is a term used to describe the potential for a preemergence herbicide to kill small, emerged weeds.  It involves the herbicide being moved into soil solution and taken up by the roots of the weeds.  This type of herbicide activity is highly inconsistent but does exist.  If it does not look like you will be getting any reach back activity, you need to take action and apply a postemergence herbicide before it is too late.  Once the broadleaf weeds and annual grasses get much bigger than one inch tall, the chance for any reach back is gone. There is a higher likelihood of reach back activity on broadleaf weeds than grasses and nutsedge.  Not all preemergence herbicides will be able to provide control in this manner.  If you have applied products such as atrazine, Acuron, Lumax, Lexar, atrazine plus acetamide premixes (i.e. Bicep Lite II Magnum, Charger Max ATZ Lite, Keystone LA NXT, Harness Xtra, Degree Xtra), Resicore, Verdict, Dual, Outlook, Harness, Capreno there is a chance for this to occur.  Do not expect any reach back from Prowl (or other dinitroanilines).
2. You can decide to take action now and make a postemergence herbicide application.
3. • If it is conventional corn, there are many good choices available to control most any emerged weed in a corn field.  Proper weed identification is necessary when choosing the herbicide.  If you are dealing with emerged grasses it is very important to know what grass species are present.
   • If it is glyphosate tolerant (Roundup Ready) corn, apply glyphosate and consider adding an herbicide with soil residual activity to provide season long weed control.  This does not allow you to delay the timing of application.  Remember, to avoid yield loss from early season weed competition, you still must apply postemergence herbicides when the weeds are no more than 4 inches tall.  Unless the label prohibits reduced rates, it is not necessary to use the full rate of the soil residual herbicide(s) at this time.