How to remove a poor stand of corn prior to replanting

In any given year, as much as 32,000 acres of Ontario corn is re-planted due to uneconomical plant stands. A two year study conducted at the University of Guelph demonstrated that either Assure II + Sure Mix, Excel Super or Select + Amigo could be applied the day before replanting corn to effectively control a poor stand of glyphosate (Roundup Ready) or glufosinate (Liberty Link) tolerant corn.  None of the herbicides tested negatively affected the emergence, growth and yield of the replanted corn. The risk of crop injury increased when the herbicides were applied after replanting and was greatest when made closest to corn emergence.

Deciding whether or not it makes economic sense to replant

There is an Ontario based re-plant calculators that can assist with determining whether to re-plant corn.  The first was developed by Greg Stewart  (Maizex), Dave Hooker and Ken Janovicek (University of Guelph)

Getting rid of the poor stand to re-plant

The majority of corn hybrids planted in Ontario are tolerant to non-selective herbicides that contain the active ingredient glyphosate (e.g. Roundup Weathermax) and glufosinate (e.g. Liberty). As such these herbicides are ineffective at removing poor corn stands prior to re-planting. If a poor stand is not removed prior to re-planting, it becomes a weed with significant yield implications to the re-planted corn crop. A group of herbicides known as the “graminicides” are used in soybeans to control emerged volunteer corn and annual grassy weeds but offer no residual weed control. These herbicides would therefore appear to be suitable options for removing poor corn stands prior to re-planting. However, significant discrepancies exist between the plant back back restrictions listed on American herbicide labels compared to their Canadian equivalent (Table 1). This has caused uncertainty as to whether or not there may be negative implications for crop injury and yield if such herbicides were to be used to remove poor corn stands prior to replanting.

Table 1. Herbicide rotational, or “plant back” intervals listed on the U.S. label compared to the Canadian label.

HerbicidePlant Back Interval (U.S.)*Plant Back Interval (Canada)**
Assure II + Suremix120 daysNone stated
Excel Super not soldNo longer sold
Poast Ultra30 days30 days (no longer being manufactured)
Select + Amigo30 daysNone Stated
Venture60 days60 days
* based on label search at http://www.cdms.net/LabelsMsds/LMDefault.aspx?t=  **based on label search at http://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php

Results: University of Guelph Study (funded by: Grain Farmers of Ontario)

To download research summary poster, click:  graminicide corn poster (1)

  1. No injury or yield loss resulted when the herbicide treatments were applied 1 day before replanting and 3 days before emergence of the replanted crop and at twice the labelled rate required to control volunteer corn (Tables 2 and 3).
  2. Crop injury resulted when herbicide treatments were applied at the time of emergence of the replant crop but did not reduce grain yield (Tables  3).
  3. When applied at the time of replant corn emergence, crop injury was greatest with Venture, followed by Assure II, Excel Super, Poast Ultra and Select.
  4. The Venture and Poast Ultra labels contain plant back restrictions that would prohibit their use for eliminating poor stands and replanting corn.

Table 2. Effect of herbicide treatment and time of application on corn injury.

 20102011
 Time of herbicide application1
Herbicide21 DBP3DBE1DBP0 DBE
 Visual Injury (%)3
Assure II + Suremix00210
Excel Super (no longer sold)0009
Poast Ultra + Merge (no longer being manufactured)0035
Select + Amigo0016
Venture L00028
 1DAP = days before planting, DBE = days before emergence  
All herbicides were applied at  twice the rate labelled for control of  volunteer corn
3  Visual injury was taken at 4 and 6 days after emergence in 2010 and 2011, respectively.

Table 3. Effect of herbicide treatment and time of application on corn yield.

 20102011
 Time of herbicide application1
Herbicide21 DBP3DBE1DBP0 DBE
 Grain Yield (bu/ac)
No herbicide (untreated)196195212210
Assure II + Suremix195199205207
Excel Super (no longer sold)195191198202
Poast Ultra + Merge (no longer being manufactured)196198210210
Select + Amigo195196212207
Venture L196191213195
 1DAP = days before planting, DBE = days before emergence  
All herbicides were applied at  twice the rate labelled for control of  volunteer corn
venture injury on corn - applied day before emergence
Injury to the replanted corn crop as a result of Venture being applied after replanting to eliminate the initial poor stand of corn

Acknowledgements: Thanks to Dr. Clarence Swanton and Kevin Chandler.