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Termination Methods and Timing

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When and how cover crops are terminated can influence the degree to which certain benefits and challenges are experienced. Farmers must decide termination methods and timing based on their goals, rotation, equipment, other practices, and location.

Termination Methods

Several factors will determine the termination method. When deciding how to terminate, a grower needs to consider the cover crop species, following cash crops, weather conditions, available equipment, cover crop maturity, and the type of management system.

Herbicide Kill

Herbicide use is a common and efficient method for terminating cover crops. However, it’s crucial for farmers to consult labels and local licensed experts to ensure the proper herbicides and rates are applied. This step is essential for the method’s safety and effectiveness, and it helps farmers account for specific cover crop species and growth stages, weather conditions, rotation, and targeted weeds.

Roller Crimping

Cover crops can be terminated mechanically by rolling and crimping. Crimping involves flattening the plant to the soil surface before blades on a cylinder crush the stems of the plant, which will stop the flow of water and nutrients. A specially designed tool called a roller-crimper is used in roller crimping. It consists of a heavy cylindrical drum with blades that crimp the stems of cover crops without cutting them.

Rolling/crimping can be used along with herbicide to lay the cover crop residue across the ground for better coverage or on its own. Growers in conventional and organic no-till systems often find success terminating cover crops with this method, especially during the reproductive stage.

Tillage

Tillage is another mechanical method of terminating cover crops. Farmers already practicing tillage need to ensure their equipment can handle the additional cover crop biomass. To terminate the cover crop successfully, it is crucial to dislodge the roots of the cover crop. This method may hasten the decomposition of cover crop residue into the soil.

Mowing

Mowing a cover crop as close to the ground as possible during the reproductive stage may result in termination. However, tillage often follows this method to produce reliable results and diminish re-growth risk. Flail mowers, using double-edged blades, are usually recommended. They produce finer, more evenly distributed residue that decomposes faster.

Winter Kill

Winter kill is a method of terminating cover crops naturally. During the winter, frost, snow, and freezing temperatures can kill some cover crops without the farmer taking any specific additional action. The freeze causes dehydration of plant cells and cell damage, leading to the eventual death of the plant. Due to warmer temperatures, winter kill should not be considered a reliable termination method in North Carolina.

Termination Timing

There isn’t a ‘one cap fits all’ recommendation on the time to terminate cover crops. Decisions on termination time should be based on the specific location, the specific context, and the objectives for growing cover crops. But below are general considerations that guide the timing of termination.

Termination Timing and Tradeoffs

Delaying termination can allow for additional cover crop growth and biomass, which may enhance several benefits. Farmers looking to maximize biomass often wait to terminate right before or immediately after planting the following cash crop. This is frequently called “planting green” since the grower will plant their cash crop into a living cover.

Growers must understand that the enhanced benefits from increased biomass with delayed termination come with certain risks or tradeoffs they must account for when making their decisions. Soil moisture issues, green bridge concerns, nitrogen immobilization, and planting difficulties are all challenges that may increase with a later termination date.

Soil Moisture Considerations

A living cover crop utilizes soil moisture. In dry conditions leading up to cash crop planting, there is a risk of soil moisture levels being too low for the following cash crop. Check out the Soil Moisture Depletion page to learn more about this challenge. If low soil moisture is a concern, it is generally recommended to terminate a cover crop at least two weeks before planting the following cash crop. However, farmers may terminate even sooner if they are particularly concerned about soil moisture issues.

Green Bridge Considerations

The green bridge refers to cover crops providing habitat for pests to be transferred to the following cash crop. Check out the Green Bridge page to learn more about this challenge. Scouting and monitoring a cover crop as farmers approach the next planting season is recommended to determine if pests are a concern. If there is a concern about pest transfer, growers should consider terminating the cover crop 3 to 4 weeks before planting.

Nitrogen Immobilization Considerations

Nitrogen immobilization refers to the temporary loss of available soil nitrogen resulting from the decomposition of cover crop residue. This issue is primarily related to grass cover crops like cereal rye.

The amount of nitrogen immobilized will depend on the cover crop residue’s carbon-to-nitrogen (C:N) ratio. As C:N ratios increase, more nitrogen will be unavailable for the early development of the following cash crop. Check out the Nitrogen Immobilization page to learn more about this challenge and how it is influenced by C:N ratios.

The C:N ratio of any cover crop species will generally increase as the season progresses and the plant matures. Farmers may elect to terminate a cover crop earlier to avoid increasing C:N ratios and additional nitrogen immobilization.

Planting Difficulties

Farmers in a no-till system will have to plant their cash crop in a field with additional residue following a cover crop. This residue can create challenges for uniform planting and seed-to-soil contact. Several types of residue managers are available to help planters work through additional plant matter. Planting through high levels of cover crop residue may take some practice. That is why it is often recommended to start small and only plant cover crops on a limited number of acres to learn what equipment and settings are needed to plant after a cover crop successfully.

Often, the amount of residue and its level of decay will determine how difficult it is to plant through. Delaying termination may create additional residue that could be a challenge for inexperienced growers. Terminating several weeks before planting could allow the cover crop residue to break down and turn brown. This may be easier to plant through and improve seed-to-soil contact.

Conclusion

Deciding when to terminate a cover crop will be crucial to any management program. Farmers need to assess the risks and benefits of waiting to terminate and determine what works for them. In general, the earlier a cover crop is terminated, the less risk but also the less reward. Growers need to determine their own risk comfort level and plan accordingly. Having a plan is important, but farmers must also be flexible and adaptable to respond to weather, pests, and other factors that may vary from year to year.

Reference

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