By Kristy Borrelli
Cover crops can provide multiple benefits. For example, they can improve soil health, supply nutrients to cash crops, suppress weeds, help manage insect pests, produce forage, support pollinators and beneficial insects, and reduce water and air pollution. However, not all cover crop species provide the same benefits. How can you best reap the multiple benefits of cover cropping with many species to choose from? Plant mixtures to multiply and diversify your cover crop benefits.
Top 7 Reasons Farmers Use a Cover Crop Mixture
Results from interviews with 47 farmers about why they planted cover crops on a total of 110 fields.
- Reduces erosion
- Increases soil organic matter
- Increases yields for the following cash crop
- Controls weeds
- Reduces compaction
- Scavenges nitrogen
- Provides nitrogen for the upcoming cash crop
Tailoring a Cover Crop Mixture to Meet Farm Management Objectives
Every farm is different. Even within one farm, management objectives for a given field and crop will vary based on weather, site location and history, crop rotations, and many other factors. The design of a cover crop mixture must, therefore, take into account the current and future management objectives for each field. Whether the goal is breaking up a plow pan, overcoming low fertility, knocking out a pernicious weed, or a combination of services, different cover crop mixtures and specific management approaches will be needed. The starting point for developing mixtures is understanding the strengths and weaknesses of individual cover crop species.
Cover Crop Strengths and Weaknesses
Individual species of cover crops often excel at providing one or two functions while also having specific drawbacks. For example, forage radish (Raphanus sativus var. longipinnatus) can suppress weeds and reduce compaction in the fall, but because it winter-kills, it does not provide a living root system or residue cover in the spring. Red clover (Trifolium pratense) captures nitrogen from the atmosphere, but it may not suppress weeds when seeded in the heat of summer without a companion species. Cereal rye (Secale cereale) can stop nitrogen from leaching, but it may deprive the following cash crop of nitrogen. Meeting multiple objectives while avoiding basic pitfalls may require combining several species. Tables 1 and 2 on pages 16-19 list many of the common cover crop species used in the Northeastern United States, their relative ability to provide different services, known drawbacks, and recommended planting date windows.
Building a Complementary Mix
The success of a cover crop mixture depends on each species in the mix providing the desired services in the appropriate balance with other species in the mix. Achieving this balance can be difficult because certain species are highly competitive, causing the desired services of the less competitive species to go unrealized. Often, these services are tied to a cover crop's biomass production or the density of certain plant parts, such as taproots or flowers. For instance, legume cover crops with greater biomass and nitrogen content will supply more nitrogen, and a greater flower density in a cover crop stand will attract more pollinators. However, more is not always better. In some cases, excessive biomass production by species in a mixture can lead to challenges for cover crop termination and incorporation and for planting the following crop, as well as reduce the efficacy of other species in the mix. Balancing the services provided by a mixture requires selecting species that are complementary in their growth periods, growth forms, nitrogen acquisition strategies, and resources for pollinators and beneficial insects.
How to Make a Mixture
The following steps can help build a mixture that meets farmer objectives, avoids pitfalls, and takes advantage of the power of cover crop diversity. (Refer to Tables 1 and 2 as needed.) The finer points in this process, such as choosing complementary species and seeding rates, are discussed below. is not always better. In some cases, excessive biomass production by species in a mixture can lead to challenges for cover crop termination and incorporation and for planting the following crop, as well as reduce the efficacy of other species in the mix. Balancing the services provided by a mixture requires selecting species that are complementary in their growth periods, growth forms, nitrogen acquisition strategies, and resources for pollinators and beneficial insects.
- Identify the top three desired cover crop services for a specific field or farm.
- Identify the cover crop planting date and termination date, which will define the cover crop growth window available in your rotation.
- Pick a core set of cover crops that excel at providing the desired services (two or three species) and that are adapted to the growth window available.
- If possible, select species with complementary growth periods, growth forms, and nutrient acquisition strategies.
- Identify the drawbacks from these cover crops, or "missing services" that they don't provide. If any have severe drawbacks, drop that species from the mix.
- Make a list of cover crops that could account for the drawbacks or provide the missing services. Choose one or two that fit with your planting window and, ideally, are complementary with the core cover crop species identified above.
- Determine the appropriate seeding mix and planting method, being sure to keep extremely competitive species at a low seeding rate.
- Get planting!
- Watch the cover crop growth and be sure to terminate at the right time. Pay special attention to cereal grasses that are about to become lignified or plants going to seed that could become weedy.
- Observe the results and make adjustments in the species mix and seeding rates to achieve even biomass representation from all the species.
Complementary Growth Periods
Different cover crop species can have a variety of temporal growth periods. Some species have narrow or restricted seeding windows in the fall to achieve successful establishment. Some species will winter-kill due to cold temperatures, while others will require termination early in the spring to avoid excessive growth, and others yet will require delaying termination later into spring to allow for sufficient growth. Navigating the maze of complementary growth periods is a sure way to hone down the list of cover crop species that will perform well together in a mix.
Click here to see more...