Organic Broadcaster

Cover crop mixes offer benefits as well as drawbacks

By Jody Padgham, MOSES

Cover crops have become a hot topic for a wide diversity of farming systems as their benefits in nutrient cycling, weed management, erosion prevention, and as a source of nitrogen and habitat for beneficial insects become better known.

A big part of the cover crop conversation is which of a broad diversity of cover crop species to use when. Each will provide one to several functions, and fit into specific crop timing windows. Various calculators and tables have been created to help producers make cover crop choices. Several are listed on the MOSES Soil, Cover Crops and Systems webpage.

One newer approach to using cover crops is to mix species together to create a “cover crop cocktail.” Generally made up of three to eight species of cool and warm season forbs and grasses, including legumes and brassicas, mixes are becoming available from seed suppliers—some even offer custom mixing. Research has been undertaken to determine how various mixes work in different systems.

But, according to Allen Philo, specialty crops consultant at Midwestern BioAg, attention placed on finding the “perfect” mix can divert from the core purpose of cover crops.

“We need to use cover crops to feed soil life,” Philo claimed. “While there are lots of other benefits, they are all less critical.” Green growing matter, as well as roots and the sugars they exude, are critical for feeding soil microbes. “Soil microbes must be fed so that they can repair damage in the soil environment” Philo pointed out, listing that tillage and heavy rain, among other things, damage soil.

“Good farming is balancing the necessary destruction we do to the soil with production techniques that will repair it,” he said. “The major difference between a natural meadow and a farm field is the amount of ground cover and root mass, and thus the amount of energy available to feed the soil microbes.”

Philo explained that while the tillage used in organic systems will disturb soil, the alternative of chemicals is often more harmful to the soil. “Farming causes disturbance one way or the other,” he said. “We can choose which way, and then work to counteract it.” Cover crops are a great way to do this.

Focusing on cover crops that you can manage within your production system is key. “Having something growing on the soil is what is most important,” he said. “Plant what you can manage effectively.”

Harriet Behar, MOSES senior organic specialist, backs this up. While she is very fond of specific cover crops for particular purposes, she agrees that the presence of a cover crop is what is most important.

“You never want bare ground,” she said. “Nature abhors a vacuum. Weeds will come in, and create problems. You can have soil erosion. It is better to get some kind of green cover crop growing that can be incorporated in.”

Both Philo and Behar emphasized that when planning cover crops it is critical to consider your overall goals—not just this year, but long-term. Those managing pasture-based systems have much different considerations than those with row crops or diversified vegetables or orchards. For example, while introducing large amounts of high C:N organic matter on a yearly basis may be effective into a pasture system, doing the same in an annual cropping system may tie up nitrogen.

And, note that different cover crops perform specific functions. Some are good for suppressing weeds, others support beneficial insects, others help with nitrogen retention or supply, and all will contribute biomass and organic matter. For instance, with a primary goal of reducing a weed such as nut sedge, Behar plants Japanese millet, and uses sorghum Sudan grass to out-compete Canada thistle.

“Plan your cover crop to match what you are trying to accomplish,” she said. “Then remember why you planted it, and terminate it when you’ve reached your goal.”

Mixes will bring a broader diversity of functions, but will generally decrease the impact of any one function. Research at Penn State (“Using Cover Crop Mixtures to Achieve Multiple Goals”) found that, if well designed to produce balanced biomass, mixes won’t be as good at any single function as a monoculture, but can buffer the variation of impacts over a long-term average.

When choosing cover crops to mix together, there are a few things to consider. The Penn State team recommends that you:

• Identify the different functions you want the cover crops to play (e.g., building organic matter, weed suppression, nitrogen release, nitrogen fixation, fall or spring forage production, beneficial insect habitat).
• Identify the planting window (spring, fall, mid-season).
• Note when and how to terminate your cover crop—winterkill or incorporation.
• Fine-tune the mix to fit your specific farm characteristics, microclimate and nitrogen availability.

There are some challenges associated with planting cover crop mixes. In some conditions, one plant will out-compete others in a mix. The Penn State researchers found that spring mixes planted after corn were soon dominated by cereal rye, and that canola planted into late season mixes tends to dominate.

The topic of how to plant diverse mixes also comes up, since covers have seeds that range from hard and small to soft and grassy. The experts recommend mixing seeds in a grain drill and planting at a three-quarter-inch depth, separating seeds into large and small sizes and putting them into two different seed boxes, or broadcasting by hand in smaller amounts. Large operations have found success with aerial applications, especially useful for interseeding between rows of cash crops.

Termination is again an issue, as some cover crops will winterkill, and others must be rolled and crimped, mowed, or incorporated. If you are incorporating a young stand this will not be an issue, but seed set and creating a subsequent seed bed will be an issue for crops, such as canola, buckwheat, clovers and vetches, if they are not terminated at the right time.

Behar and her husband own and manage a diversified organic vegetable system in Gays Mills, Wis., and use a broad diversity of cover crops, including mixes, for many different purposes. They also love to use cover crops to interseed between rows of their cash crop vegetables. They do this for weed control, for water retention, to protect the soil, to create clean beds for crops like squash, melons and pumpkins, and for the soil microorganism and organic matter benefits the covers provide.

Below are a few of Behar’s favorite mixes and her growing tips:

• Field peas, barley and wheat—a common pre-mix known as “succotash.” Incorporate once the peas start blooming and the grain heads out. Great source of nitrogen.
• Perennial rye and hairy vetch—great for vegetable growers, but not for small grain producers as the hairy vetch will reseed a bit. Plant in August or September, overwinter and till in early in the spring when green.
• If you have a wet spring, plant whatever cover crop seeds you have around. Till in as soon as you can get in the field, then plant a late cash crop. This provides fertility, and keeps the weeds at bay.
• Chickling vetch, buckwheat and oats—seeded mid-season into sweet corn at last cultivation (mid-July).
• Oats and crimson clover in mid- to late August to overwinter. The oats will winterkill. The legume will last through and bloom in early spring when it can be tilled in before your cash crop.
• Anything planted after your final crop in the fall to protect the soil through the winter.

Behar encouraged using creativity when using cover crops. “There is no one way that is right,” she added. Think about what windows of opportunity your rotations allow, and plan rotations with 2-3 week windows where covers could be planted. Identify what you want the cover crops to accomplish, and incorporate seeds that you have on hand. In addition, you must do advance planning, as seed for some late-season plantings, such as buckwheat and field peas, will be hard to find in the summer and fall.

Behar even suggested planting things out of their regular seasonal window to utilize seed that you may already have that won’t last into the next season, and get the ground covered. “Even if the ground will only be open for a few weeks, throw some seed in, and then till in the 2-4 inch shoots,” she said. This adds organic matter and will feed the soil microorganisms. “The younger, green, vegetative stuff will break down quicker, and you can come in and plant your cash crop sooner.” As Philo pointed out, the vegetative material is what the soil microorganisms thrive on.

“Everyone is harvesting, tilling and weeding, taking away nutrients,” Behar noted. “Cover crops help to balance those activities.” Philo agreed.

“Cover crops are one of the great, cost-effective tools that farmers can use to repair soils and restore the microbiology,” he said. “But, remember that there is much that we don’t understand about the complexity of the soil micro community, and so there will be no one perfect way to use cover crops.”

Jody Padgham is the financial director for MOSES.

From the March | April 2016 Issue

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