This year did not get off to a good start in Indiana.  Rains delayed planting and some fields, especially soybean fields, were sprayed with a burndown herbicide treatment shortly before planting.  Likewise, there were several days of cooler than normal weather for corn growth.  In years where wet and cool conditions prevail, there are usually more hard to control weeds like smartweed present, and sometimes an early post application will not totally control such species, leaving them to regrow.  Herbicides will not always work as well since translocation of the herbicides through the plants is not as great under these conditions and less than ideal control can be obtained.  It may pay to scout fields to see if the herbicide programs have done the job on weeds that they are expected to do, and if not, then a second application should be planned on a timely basic to control these weed.  Older weeds that regrow due to lack of control are much harder to control than younger weeds that emerge after the application is made.  In addition to some of the hard to control weeds not being adequately controlled; cool wet conditions also can cause herbicides to injure emerging crop plants more than they would under wormer growing conditions.  Usually this is temporary crop damage that will grow out of the injury in a short period of time

Soybeans are growing slowly, even those planted early.  Many fields show a good burndown, but new weeds are emerging.  Weather conditions are forecasted to warm-up into the 90’s in the next few days and these weed will grow rapidly.  Be prepared to make early post treatments on these weeds before they get too tall and harden off.  With the growing conditions the way they have been this year, the weeds may likely grow faster than the soybean crop.  Contact herbicides should be used with care, as should any adjuvant or other spray additive that may cause excessive crop burn.

Consult Table 6 of the weed guide for corn http://www.btny.purdue.edu/Pubs/WS/WS-16/CornRainFast.pdf and Table 17 http://www.btny.purdue.edu/Pubs/WS/WS-16/SoybeanRainFast.pdf for soybean to obtain more information on rainfast intervals, maximum growth stage, and spray additives for postemergence herbicides.