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Cover Crop Termination

COVER CROP

TRAINING MODULE

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Developed by

Dr. Rob Myers, University of Missouri and NCR-SARE

Reviewed by

Dr. Eileen Kladivko (Purdue University) and Dr. Paul Jasa

(University of Nebraska)

Funded by

Walton Family Foundation

These training modules are being made available by free use of other educators through University of Missouri and the Midwest Cover Crops Council, which participated in the WFF project supporting development of these modules and provided many of the technical reviewers.

All photos are by Rob Myers, University of Missouri, unless otherwise noted.

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Cover Crop Termination Approaches

Options:

Winter kill
Herbicide
Roller-crimper
Mowing
Grazing
Tilling

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Terminating with Cold Weather (Winter Kill)

Warm season cover crops typically killed at 28-32 F., depending on the species

Buckwheat
Sunhemp
Cowpeas
Millets
Sunflowers

Some cool season cover crops will winter kill – those that are not very winter hardy – temperature that kills these depends on the species and how abrupt a temperature drop to freezing is in the fall

Spring oats – killed at about 18 F.
Winter oats – killed at about 14 F.
Radishes – killed at about 20 F.
Turnips
Kale
Ethiopian cabbage
Other Brassicas
Some clovers and legumes

buckwheat

Identifying species of cover crops that will winter kill in a given area starts by understanding the biology of the plants. Some cover crop species are considered warm season annuals that will be killed with even a light frost. Several of those are listed above. Of the ones listed as warm season cover crops, most will be killed at around 30-32 F., depending on how long the cold period lasts for. Sunflowers are more cold tolerant, often surviving down to about 28 F.

Cover crops that are classified as cool season annuals do not grow well in the heat of summer, but can tolerate moderately cold temperatures, even down below freezing. Which ones will definitely be killed by the winter conditions in a given area is variable. For areas, that might not get colder than 20-25 F. in a typical winter, cool season cover crops like radishes and oats might overwinter and regrow the next spring. However, in areas that reliably get down to the single F. digits at some point, the above list of cool season cover crops should normally be winter killed. Snow cover will often allow tolerance of somewhat colder temperatures compared to having no snow cover.

The temperatures listed above as typically killing oats and radishes are only averages. They may die at temperatures a few degrees warmer or colder than shown, depending on how suddenly temperatures drop to that level, how cold it has been previously, snow cover, wind speed, length of the cold period, and other factors.

This slide is a good place to talk about how farmers new to cover crop may prefer starting with a cover crop that winter kills, particularly before an early planted crop like corn. As they get a little experience with cover crops, then they may start to feel more comfortable with an over-wintering cover crop that has to be terminated in the spring.

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Terminating with Herbicides

Some cover crops terminated easily with glyphosate, others will require a tank mix of two or more herbicides, such as adding 2,4-D to glyphosate to terminate legumes
Cover crop should be actively growing
Consider soil moisture conditions when deciding on termination timing
If working with cover crop mixes, timing may be particularly important to kill all species at once

For non-organic row crop production, most cover crop that over-winter are terminated with herbicides. Many grain farmers are already doing a “burndown” herbicide to kill winter annual weeds around the time of cash crop planting; that same spring burndown application of herbicide, usually glyphosate, can be used to terminate the cover crop.

The cereal grain cover crops such as cereal rye, triticale and wheat can be terminated just with glyphosate, but legumes usually require the addition of 2,4D or other broadleaf herbicide to kill the legume completely.

If weather conditions are cold and the cover crop is still dormant, don’t spray – the cover crop needs to be actively growing to be killed by a herbicide.

As discussed later in this presentation, deciding when to terminate the cover crop can depend on soil moisture – if it is trending dry, terminate early, if a wet spring, it may be better to terminate later.

Cover crop mixes make careful selection of the herbicide strategy more important, to be sure to kill all the species in the mix.

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Species Specific Herbicide Tips for Termination*

Cereal rye - glyphosate plus Sharpen or plus Select (clethodim) very effective

Winter wheat - glyphosate plus Select very effective

Annual ryegrass – combo of glyphosate and Select is “hands down best way to kill it”

Hairy vetch – Liberty or Gramoxone, or use glyphosate with a broadleaf product

Austrian winter pea – glyphosate plus Sharpen works well

*Suggestions from Dr. Kevin Bradley, University of Missouri Weed Extension Specialist

The above tips on specific herbicide combinations to use for common cover crop species was developed by Dr. Kevin Bradley, University of Missouri weed extension specialist, based on multi-year field research trials.

Annual ryegrass (a completely different genus and species of plant compared to cereal rye) can pose a particular challenge for spray termination. Detailed recommendations on terminating annual ryegrass can be found in the publication “Annual ryegrass as a cover crop in Midwest corn and soybean production.”

https://www.ryegrass.com/publications/ryegrass-recommendations-2016.pdf

Because there is a risk of developing annual ryegrass plants resistant to glyphosate, it’s important to have a careful approach to terminating the annual ryegrass. In short, the authors of the above publication, based on field experience in the Midwest, recommend annual ryegrass termination be done as follows using glyphosate:

- Spray with a daytime minimum temperature of 55 F (above 60 F is optimum).

- Wait until all the annual ryegrass is actively growing at least 5-7 days for best results.

- If night temperatures drop below 38 F, wait another three days before spraying.

- Soil temperatures should be above 45 F.

Spray at least 4 hours prior to sunset to allow for maximum translocation of the glyphosate within the plant.
Spray on a sunny day rather than a cloudy one.

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Terminating the Cover Crop Mechanically

Main options are roller crimper or mowing, tillage as a last option

For mowing or using a roller crimper, generally need to terminate during reproductive (flowering) growth of the cover crop

Photo credit – Buffett Foundation

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Terminating by Crimping Cover Crop Stems

Some plain rollers can terminate a cover crop by breaking stems, but most often it works better to “crimp” the stems using a roller modified by edged steel vanes that can crush or break the vascular tissue (xylem and phloem) in the plant stems

Photo credit: Edwin Remsburg (SARE image library)

Usually work best if the cover crop has already reached the flowering (reproductive) stage
Often better to roll after planting
If rolling before planting, roll in same direction as planter is traveling

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Roller Crimper Equipment Options

There are now a dozen or more manufacturers selling roller crimpers, some mount in front of the tractor and some in rear, some are single rollers 10-15 feet wide, some are wider folding units with multiple offset and overlapping rollers. There are now even smaller roller-crimpers that can be added to the tool bar of a planter.

Photo credit: Steve Groff

Photo credit: Advance Cover Crops (Dahmer roller)

Roller crimpers attached to a planter

In the last decade, the number of manufacturers offering roller crimpers for sale has increased substantially. Single rollers are often reasonably priced at less than $10,000, but may not cover much ground if only a 10 foot width. This has led to folding designs with multiple rollers able to be transported more easily and also adjust to contours in the ground. However, these folding units can be substantially more expensive.

Roller crimpers are available both for front and rear mounting on tractors. Some rear mounted ones are 3 point hitch, while larger ones maybe be a tow type unit.

The advantage of front-mounted units is that they can crimp the stems before a tractor tire runs over the plants, plus it allows the rolling to be done at the same time and in the same direction as planting, by having the roller in front and planter in back.

Another option for simultaneous planting and rolling is to get add-on rollercrimpers that mount on the planter. These are smaller units but may be suited for certain approaches and situations.

Rear-mounted roller crimpers are normally used as a separate pass from planting, unless they are towed behind a planter. They can be a good choice if the rolling is to be done after the cash crop planting operation. Some organic no-till soybean farmers wait for 1-3 weeks after planting their soybeans before using the roller crimper, with the idea that the cover crop can provide a longer period of weed control, allowing the soybeans to get a head start before the weeds start to emerge. Dr. Erin Silva at University of Wisconsin has done quite a bit of research on this approach and has some videos on the approach.

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Terminating with a Mower

Successful termination with a mower is dependent on timing – for most cover crops, needs to be done after flowering has started; plants that are still in vegetative stage (pre-flowering) will often regrow
Also helps to mow as close

to the ground as feasible

Type of mower can make a

difference, for example, flail

mowers cut residue more

finely but take more power

Photo credit: Johnny’s Selected Seeds

Keys to successful termination of cover crops by mowing are to wait until flowering or past flowering and to mow closely to the ground. Mowing too early or too high off the ground can lead to regrowth of the cover crop.

The type of mower can be important depending on the goals with the mowing. Most often a brush hog type rotary mower is used. Those are inexpensive, durable, and many farmers already have one for general farm maintenance. A flail mower can be used to get the residue chopped more finely, but they tend to be more expensive and require more horsepower than a rotary mower. A sickle bar mower could be used to lay down more intact tall cover crop stems, but might be more likely to tangle up with some cover crop like hairy vetch. As with any management, experimenting on approach is a good idea to optimize for local conditions.

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Comparing Mowing vs. Crimping

Sorghum-sudangrass rolled on the left, and mowed on the right

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Terminating through Grazing

Basic idea is to simulate mowing by using grazing livestock

As with mechanical mowing, works best if cover crop is already flowering
Plants need to be grazed hard, removing high percentage of biomass
Good rotational strategy with high density stock is critical
Need to avoid just trampling plants, or they may regrow

There has been interest in using grazing livestock to terminate cover crops, but not many farmers are doing it yet because doing it effectively requires very careful timing and management.

Several management points are outlined above. A key one is to not allow the livestock to simply knock down and trample the cover crop – that is likely to lead to some regrowth as opposed to biting off every single plant close to the ground. Having a high stocking density and moving the animals daily will help achieve more complete termination.

Although using grazing as the sole tool for termination can be tricky, grazing cover crop in the spring with livestock can be a boon to soil health and when combined with another termination approach, can work well. For example the field could be grazed in mid-to-late April and then terminated a couple of weeks later by herbicide or mechanical means.

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Terminating through Tillage

Terminating cover crops with tillage should generally be avoided, especially for farmers who are already in a no-till or strip-till system

Photo credit: Edwin Remsburg (SARE image library)

If tilling, be sure to dislodge the roots of the cover crop to ensure complete termination
If voles or slugs are an issue, light tillage may be warranted in affected areas
Tillage termination is most common for organic farmers and some types of horticultural operations

In past decades, the most common method of cover crop termination was to use tillage. Often, legumes such as red clover or sweet yellow clover were frost seeded into winter wheat, say in March, then the clover grew well after wheat harvest through the summer, fall and again the next spring. In mid-spring, the clover was moldboard plowed before corn planting. This approach was called “green manuring” and dates back centuries. Even farmers like Thomas Jefferson and George Washington used that green manure approach.

Today, through modern soil health research, we understand we are better off in minimizing soil disturbance from tillage. Using herbicides, roller crimpers, or even mowers allows us to avoid the need for tillage termination. There may still be a few situations where tillage termination of cover crops is viable, such as small horticulture operations incorporating a legume and not wanting to spray, or maybe a situation where some light tillage can aid with control of voles or slugs. Still, tillage termination should be the last choice considered compared to other methods that leave intact the vital macropores and soil structure of the rootzone.

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Cover Crop�Termination Timing

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Terminating the Cover Crop Early

Photo credit – Rob Myers

Advantages

Minimizes amount of cover crop residue to deal with in planting the cash crop
May reduce the chance of some pest problems
Can prevent the soil from being dried out by the cover crop
Often a more comfortable approach for first time cover crop users

Caution

Avoid terminating a high biomass cover crop 10-14 days before cash crop planting; may end up making a wet mat that prevents soil from drying out

The focus in this slide and the next one is either terminating early or terminating late. There can be challenges in terminating a high biomass cover crop, like rye that is 2-4 feet tall, then attempting to plant into it 10-14 days after planting. By that point, if rains have come, the dead rye may be forming a dense wet mat of residue that prevents the soil from drying out and substantially delays planting.

As outlined above, early termination is recommended in a dry spring and can be a more comfortable approach for farmers just getting started with cover crops.

The issue of pests is a complicated one. Some pests may be less likely with early termination, others may be less likely if the cover crop is allowed to grow longer, but overall, there’s somewhat less risk of a serious pest problem when early cover crop termination is done. (However, don’t give the perception that late terminated cover crops WILL have pest issues, it’s more a matter of occasionally there MIGHT be a pest problem that requires spraying or some other management step with late termination). More research continues to be done on how termination timing relates to issues with insects, slugs, and plant pathogens.

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Terminating Cover Crops Late, Including “Planting Green”

Photo credit – Dave Robinson

Advantages

Allowing the cover crop to grow longer maximizes cover crop benefits for soil organic matter, erosion reduction, weed control, soil health, etc.
Longer period of cover crop growth can help draw down moisture in a wet spring and enable earlier cash crop planting
Sometimes planting into a still standing and living cover crop (planting green) is easier than into a thick mat of dead cover crop residue

Caution

If spring conditions are trending dry, best to go ahead and terminate the cover crop earlier to avoid drying out soil

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Summary

If using herbicides to terminate a cover crop, match the herbicide choice to the species of cover crops being sprayed
For farmers seeking to minimize or eliminate herbicides, mechanical termination options are available, just have to pay more attention to the growth stage of the cover crop
For first time cover crop users, sometimes cover crops that winter kill are a more comfortable way of trying out cover crops
Likewise, for an overwinter cover crop, early termination may be an easier strategy for a cover crop novice
For more experienced cover crop users, there can be advantages to delayed cover crop termination, including using a “planting green” approach