Will Herbicide Carryover Hurt Fall-Seeded Cover Crops?

Corn and soybean herbicide carryover can impact fall cover crop stand establishment.

Published on: Aug 22, 2012

The question about whether corn or soybean herbicide programs will pose a problem for seeding fall cover crops arises again, particularly in areas of severe drought where corn is harvested earlier than normal and the desire to plant a cover/forage crop is strong. So here's a best-guess assessment from Bill Curran and Dwight Lingenfelter, Penn State Extension weed specialists.

If you look at the rotation crop restrictions for corn and soybean herbicides in the Agronomy Guide (Tables 2.2-17 and 2.4-15), you'll see that many products limit rotation to alfalfa and/or clover as well as some of the small grains. This is a good place to start when thinking about rotation to fall cover crops.

Will Herbicide Carryover Hurt Fall-Seeded Cover Crops?
Will Herbicide Carryover Hurt Fall-Seeded Cover Crops?

However, these tables are inadequate. Cash crop rotation restrictions center concern for herbicide residues accumulating in forage or feed, not carryover injury. If the crop is not going to be harvested and consumed by livestock or humans, then the primary concern is carryover injury and achieving an acceptable stand that provides the benefits of a fall or winter cover.

While few cover crops are listed in these Agronomy Guide tables or on herbicide labels, look for close plant relatives to get an idea of how certain species may succeed. For example, there's no listing for the legume hairy vetch or the mustard daikon radish. But by looking at the alfalfa or clover restrictions or at annual mustards such as canola you can "guesstimate" which herbicides may cause potential injury to related cover crop species.

Where the greatest concern is
In general, products with a four month or less rotation restriction for the species of interest, close relative, or sensitive species (i.e. clovers) should pose little problem.

Corn products of concern for fall cover crops are atrazine or simazine, particularly for legumes, brassicas, or annual ryegrass. Both persist longer on higher pH soils (pH 7 or greater) or soils recently limed, so watch surface pH in particular.

Herbicides with shorter half-lives (time required for 50% of the active ingredient to dissipate) are always less of a concern. In addition, species sensitivity can play a role if only a small amount of residue is necessary to cause injury.

This past summer, Curran and Lingenfelter did a demonstration at the Landisville research farm using a log sprayer and herbicide rate to simulate herbicide half-life. Cover crops were seeded and herbicides applied in mid-May. The estimated half-life for atrazine is 60 days.

In that demonstration, the sorghum-sudangrass was mostly unaffected. All the other cover crop stands were still reduced at the 1/4X rate (0.375 pounds of atrazine per acre) which would be equivalent to two half-lives or 120 days with some species still being affected at 1/16X (0.094 pound acre) which would in theory be equivalent to 240 days or 8 months after application. Alfalfa and crimson clover were particularly sensitive to  atrazine with a 60% reduction in stand even at the lowest rate.

So, look at what herbicide programs you're using and the potential for carryover injury to fall cover crops before seeding something this fall.

 

Typical herbicide rates, sensitive species, half- life, and potential for carryover injury to sensitive crops (carryover potential based on half lives and soil availability).

Herbicide

Typical rate/acre

Sensitive species

Half life (days)

Carryover potential

Accent

2/3 oz

brdlfs+grass

21

Low

Assure/Targa

8 oz

grass

60

Low

Atrazine

1 lb

brdlfs+grass

60

Moderate

Authority

4 oz

brdlfs

32-302

Moderate

Balance Pro

2 fl oz

brdlfs+grass

50-120

Moderate

Callisto

6 fl oz

brdlfs

5-32

Moderate

Classic

2 oz

brdlfs

40

Moderate

Dual II Mag

1.67 pt

brdlfs+grass

15-50

Low

FirstRate

0.33 oz

brdlfs

8-33

Low

Harmony

1/8 oz

brdlfs

12

Low

Harness

2 pt

brdlfs+grass

10-20

Low

Impact

0.75 fl oz

brdlfs+grass

14

Low

Laudis

3 fl oz

brdlfs+grass

14

Low

Outlook

16 fl oz

brdlfs+grass

20

Low

Peak

1 oz

brdlfs

9-152

Moderate

Permit

2/3 oz

brdlfs

9-27

Low

Prowl H2O

3 pt

brdlfs+grass

44

Low

Pursuit

4 fl oz

brdlfs+grass

60-90

Moderate

Raptor

5 fl oz

brdlfs+grass

20-30

Low

Reflex

1.5 pt

brdlfs

100

Moderate

Resolve

2 oz

brdlfs+grass

2-4

Low

Select

10 oz

grass

3

None

Sencor

0.33 lb

brdlfs+grass

14-60

Low

Sharpen

3 fl oz

brdlfs

7-35

Low

Simazine

1 lb

brdlfs+grass

60

Moderate

Stinger

5 oz

brdlfs

40

Moderate

Valor

2.5 oz

brdlfs

12-20

Low