The drought provided a lot of headaches with low yields and reduced revenue, but it may also provide many agronomic headaches for months to come.  First of all there is herbicide carryover, since the lack of moisture was not present to activate your 2012 herbicides.  But the small kernels of corn went through your combine like water and now have provided a remarkably healthy cover crop.  You not only lost the revenue from grain left in the field, but now have to do something with it before you plant wheat.  But it was a stacked gene hybrid, and how do you kill it?

Cleaning up all of the problems from the drought could provide a winter full of challenges, but in some cases you don’t have the luxury of time.  Purdue weed specialists Travis Legleiter and Bill Johnson  provide some headache remedies.

Herbicide Carryover

The lack of activation moisture for herbicides not only poses a hidden problem for fall wheat seeding, but potentially for spring-planting in 2013.  The Purdue weed staff says a typical year would allow the rotational restrictions to play out in a timely manner, but this year’s lack of moisture has reduced the dissipation rate.  Their biggest concern is for atrazine carryover and potential injury for wheat.  While labels vary, the majority of atrazine pre-mix labels range from 14 to15 months.  Lumax allows wheat planting 4.5 months following application, so Lumax users may have escaped the problem.

Legleiter and Johnson also point to fomesafen applied as a post-emergent weed control for soybeans.  That is found in Reflex, Flexstar, Dawn, and Rhythm, which allows wheat four months after application, but a late application could pose a problem, particularly if there was limited rainfall.

The specialists say some areas where there has been significant rain should benefit from diminished problems, particularly for next spring.  But they warn, “ Producers who have not had these fall rainfalls should still be wary of atrazine and or HPPD inhibitor (Callisto, Laudis, Corvus, and Impact) carryover into soybean, especially in high pH and/or high clay content soils. Producers should also be aware of potential imidazolinone chemistry (Scepter and Pursuit) carryover into next year’s corn crop.”

Testing for Herbicide Carryover

If you have concerns about an herbicide carryover, conduct a bioassay or have your soil analyzed for herbicide residues, are their suggestions.  The bioassay is accomplished by planting a susceptible crop in the suspect soil and having it tested, and compared to a control crop in the non-suspect soil.  The concern is that soil types change across a field and reactions can vary per soil type.  The other option is have soil samples analyzed by a commercial laboratory.  Both tests should be done in the late fall or early spring to allow for the maximum degradation of the herbicide.

Managing Volunteer Corn

If your combine was not set for collecting smaller kernels created by the drought, many of them went out the back with the stover, and have performed well in germination tests throughout the field.  But now you are planning to take advantage of the high market prices and the crop insurance guarantee for 2013 wheat, and have a lush green field of volunteer corn in the way.  Normally it could be burned down with either Round up or Liberty, but your 2012 corn had both glyphosate and gluphosinate tolerance and its offspring carry those genes as well.

Johnson and Legleiter suggest two alternatives. 

  1. Fields containing volunteers following a RR/LL crop can only be controlled with applications of gramaxone or by tillage. Grass herbicides cannot be used to control volunteer corn prior to wheat planting as rotational restriction to wheat vary from 30 to 120 days. If choosing to apply Gramaxone, be aware that applications can result in variable control.
  2. Fields containing heavy volunteer corn populations that are glyphosate and glufosinate tolerant would be best managed with tillage prior to wheat planting.


The 2012 drought has continued to provide challenges well after the diminished harvest.  The lack of moisture means a reduced activation of many herbicides, therefore many have not degraded and will impact both fall-planted wheat, and spring planted crops next year.  To know whether you have a problem a bioassay can be conducted on a test crop planted into the soil or soil could be tested for residue.  Another problem is the emergence of volunteer corn that may contain genetic tolerance for Roundup and Liberty, meaning it can only be managed with a heavy dose of gramoxone or tillage.

Source: FarmGate blog