When Pre-Applied Herbicides Fail In Cotton, Soybean

We have several reports of non-activated pre applied herbicides in corn needing post applications to clean up weeds.

Published on: Apr 30, 2012

By Larry Steckel

The environmental conditions that allowed all the early planting are working against us from a weed management stand point.  We have many starting to plant soybeans and cotton into soil that is dry.  The hit and mostly miss showers of the last couple weeks do not bode well for getting pre applied herbicides activated in these crops.  We have several reports of non-activated pre applied herbicides in corn needing post applications to clean up weeds.  In corn this is a problem.  In cotton and soybean fields infested with glyphosate-resistant Palmer amaranth this is a huge issue.    What can be done to help minimize this risk?

When Pre-Applied Herbicides Fail In Cotton, Soybean
When Pre-Applied Herbicides Fail In Cotton, Soybean

First, it is critical that the cotton and soybean are Palmer amaranth free at planting.  We just had a report today of a, to be planted, cotton field in Hardeman county with a very thick stand of 2 to 3' tall Palmer amaranth (picture right).  The Palmer in this field had 12" long flowering branches with viable seed!  Palmer this thick and tall cannot easily be either tilled or sprayed in order to remove the Palmer prior to planting.  It will require sequential burndown treatments or a burndown followed by tillage in order to start clean.  The bottom line is the cotton must have a head start on the pigweed not the other way around to have a chance to make a crop.

Second, if you have irrigation in these fields do not hesitate to cut them on and water in the pre applied herbicide.  A half inch of precipitation will enable a pre applied herbicide work every time.

Third, consider your post-emergence spraying capacity.  If the worst case scenario occurs and the pre-applied herbicide does not get activated and Palmer emerges with the crop; do you have the spraying equipment and labor to apply a post emergence application timely?  These large modern air seeders and cotton planters can plant many acres of soybean or cotton in a few days.  In Roundup Ready or conventional soybean can you spray a fomesafen product prior to pigweed reaching 3" tall across those same acres in about a 3 day window? If not, you may want to slow the planting process down.    In Liberty Link cotton or soybean the post emergence window is wider to control pigweed but still for best results it must be timely.  If pre-applied herbicides fail to buy us anytime this spring, the ability to spray post emergence applications timely will make all the difference.

(Steckel is a weed specialist with the University of Tennessee.)