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.

Herbicide	Plant Back Interval (U.S.)*	Plant Back Interval (Canada)**
Assure II + Suremix	120 days	None stated
Excel Super	not sold	No longer sold
Poast Ultra	30 days	30 days (no longer being manufactured)
Select + Amigo	30 days	None Stated
Venture	60 days	60 days

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.

	2010		2011
	Time of herbicide application1
Herbicide2	1 DBP	3DBE	1DBP	0 DBE
	Visual Injury (%)3
Assure II + Suremix	0	0	2	10
Excel Super (no longer sold)	0	0	0	9
Poast Ultra + Merge (no longer being manufactured)	0	0	3	5
Select + Amigo	0	0	1	6
Venture L	0	0	0	28

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

	2010		2011
	Time of herbicide application1
Herbicide2	1 DBP	3DBE	1DBP	0 DBE
	Grain Yield (bu/ac)
No herbicide (untreated)	196	195	212	210
Assure II + Suremix	195	199	205	207
Excel Super (no longer sold)	195	191	198	202
Poast Ultra + Merge (no longer being manufactured)	196	198	210	210
Select + Amigo	195	196	212	207
Venture L	196	191	213	195

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