By Alyssa Essman and Mark Loux, Ohio State University Extension, Weed Science Specialist
The 2019 growing season came and went and left many fields in a state of disarray heading into 2020. Many growers that were unable to plant decided to use cover crops, to reduce soil erosion and provide some weed suppression during the extended fallow period. Terminating these cover crops using the right methods at the right time will be critical to ensure timely planting and prevent the cover crops from competing with cash crops. The three main methods of cover crop termination are natural (species that winter kill), chemical, and mechanical. Cover crops may also be bailed, grazed, or harvested as silage. Most species require some sort of management decision for termination. Cover crop species, growth stage, weather, and cover cropping goals should all be considered when planning termination method and timing. These decisions require a balance between growing the cover long enough to maximize benefits and terminating in time to prevent potential penalties to the following cash crop.
Summer and fall planted cover crops that die naturally over the winter in Ohio include: oats, sorghum-sudangrass, tillage and oilseed radish, turnips, and winter pea (if planted after August).
The use of these species can simplify spring management. However, they provide a shorter period of soil protection, especially if planted after a late harvest. For this reason, they are often included in some sort of species mixture with other grass or legume species that over winter to provide weed suppression and soil protection in the spring. Producers have also started to experiment with interseeding and broadcast seeding in the late summer or early fall. Doing so can maximize benefits from winter killed species and species mixes, and avoid the risk of not being able to plant cover crops in the event of a late harvest.
Termination with herbicides is reliable if applied at the appropriate herbicide rate and growth stage. Refer to the label for rate and surfactant recommendations. Glyphosate has proven to be an effective means of control. In general, grass species including wheat, barley, rye, oats, and annual ryegrass can be controlled with glyphosate alone or mixes that include glyphosate. Recent research shows that for these grass cover crop species, glyphosate alone or glyphosate plus 2,4-D, saflufenacil, or clethodim was most effective (Whalen et al. 2019a). Use caution when considering annual ryegrass as a cover crop species, as it is especially aggressive and can quickly turn weedy and escape chemical control. Systemic herbicides, such as glyphosate, are most successful when applied on warm, sunny days when plants are actively growing. Termination treatments should be applied before the boot stage of grasses. Control of grasses with paraquat and glufosinate often declines 4 weeks after application, relative to the longer-term effectiveness of glyphosate (Pittman et al. 2019). It is often recommended that cereal rye be terminated 10 to 14 days before corn planting to reduce the effects of allelopathy on the germinating corn crop. Soybeans are generally less susceptible to this effect from cereal rye, and some growers have had success planting into a growing stand of rye. As with all new practices, it is important to start experimenting on a small scale and work with your crop insurance agent regarding termination requirements.
Non-selective contact herbicides such as paraquat and glufosinate can also be used in the termination of cover crops, and can be beneficial in particularly cool springs. The key to successful termination with contact herbicides is complete coverage and including other herbicides in the mix that improve effectiveness. Legume species such as Austrian pea, crimson clover, and hairy vetch are best controlled using a mix of actives that include either paraquat or glyphosate (Pittman et al. 2019). Hairy vetch and crimson clover can also be controlled with 2,4-D. Recent multi-state research on cover crop termination shows that chemical applications which include glyphosate are more effective compared to applications that included paraquat or glufosinate. This study showed that for broadleaf cover crop species, glyphosate, paraquat, or glufosinate applied with either 2,4-D or dicamba were most effective (Whalen et al. 2019a). When making your spray plan keep in mind that some antagonism can occur if glyphosate and glufosinate are applied together, and also for grasses where a growth regulator herbicide is added to glyphosate. A follow up POST treatment of growth regulator herbicide and/or glyphosate in corn and certain soybean trait systems can complete control of covers that partially survive a burndown treatment.
Residual herbicides can be integrated into cover crop termination applications to reduce additional field passes. Preplant and POST termination applications that include a residual can provide effective cover crop termination and residual weed control. There is generally more weed control benefit from including the residual in pre-plant applications compared with inclusion in the POST application. Timing of termination and levels of biomass are often determined by the goals of the cover crop, and will impact the amount of residual product that reaches the soil. If high biomass is the goal, then later termination and inclusion of the residual with the POST application is recommended (Whalen et al. 2019b).
Recommendations for using herbicides to terminate a cover crop:
- Generally easy to kill
- Glyphosate up to 18 inches
- Base rate: 0.75 lb ae (22 oz PowerMax)
- Increase the rate on taller rye
- Antagonism with residual herbicides possible, increase rates or apply separately
- Gramoxone can be effective
- Use high rates on tall plants
- Coverage is essential, 20 GPA
- More effective with atrazine or 28%
- Tougher to kill than cereal rye, more issues with antagonism, weather, and rate
- Glyphosate up to 18 inches
- 1 to 1.5 lb ae (33 to 44 oz PowerMax)
- Increase rate on taller wheat, possible antagonism with residual herbicides
- 28% a concern, most effective when applied alone in water
- Most easily controlled when plants are small
- Gramoxone not consistently effective
- Ryegrass should be less than 6 inches tall
- Control is faster in warm weather, cold weather slows herbicide activity
- Glyphosate is most effective
- 5 lbs ae/A minimum (44 oz PowerMax)
- Can use a higher rate if plants are large or in cold weather
- Can add Select, Assure II
- Gramoxone is variable, possibly high cost
- Terminate small plants at high rates
- More effective with atrazine
- 20 GPA is preferable, aim for medium spray droplets
Hairy vetch, winter pea
- Fairly easy to kill, large vetch especially
- Glyphosate plus 2,4-D or dicamba
- Glyphosate: 0.75 to 1.1 lb ae (22 to 33 oz PowerMax)
- Gramoxone is effective on larger hairy vetch
- Add 2,4-D and/or atrazine
- Not necessarily easy to kill
- Glyphosate plus 2,4-D or dicamba
- Glyphosate: 1.1 to 1.5 lb ae (24 to 44 oz PowerMax)
- Clopyralid is very effective on these species
- Surestart, TripleFlex, Hornet, Stinger
- Gramoxone is generally not a good choice
- Can kill larger crimson clover with 2,4-D
Mechanical means of cover crop termination include tillage, rolling/crimping, or mowing. Tillage from field cultivators can terminate a cover crop by burying the plant residue and cutting the roots. Vertical tillage is a less effective termination option, and many types of tillage may require multiple passes to achieve the desired level of control. Strip-tillage can be performed to break up residue and increase soil warming in the row. Termination via tillage speeds up the breakdown of residue and incorporates it into the soil. In general, this method of termination can negate some of the benefits associated with using cover crops.
A roller-crimper can be used to control a number of cover crop species, but doing so at the right stage based on species is critical for complete termination. Cereal rye can be rolled after pollen shed to form a dense mat of residue.
This can be an effective option, as this residue is capable of choking out weeds and conserving soil moisture in the hot summer months. Hairy vetch can be rolled in full bloom just before corn planting. This termination method might be suitable for organic operations looking for less soil-intensive means of weed control. The use of multi-species mixes can complicate termination via roller-crimping, as the different species often require termination at different times due to varying maturation rates. In these instances it is best to roller-crimp according to the latest maturing species. Mowing is generally less effective than tillage or roller-crimping, with often unpredictable effectiveness and regrowth of some species.
As you plan for the 2020 season and decide on methods to terminate your cover crop, contact your crop insurance agent to know your options and requirements.