• Cover crops are in important management tool in the effort to reduce nutrient losses from agricultural fields.
• Grass, legume, and brassica cover crop species each provide unique additional benefits in the cash crop rotation.
• Species selection should be driven by the specific goals of the farmer.
• A single species cover crop may provide a narrower spectrum of benefits, but can be a simpler way to get started with cover crops.
• Cover crop mixtures generally provide a more diverse array of benefits to meet goals, but may be more complex to manage.
• Successful integration of cover crops requires intensive management.

Cover crops are expected to be a prominent management tool in the effort to reduce nutrient losses from agricultural fields.  But cover crops can serve many additional beneficial functions in the cash crop rotation, depending upon the species or mix selected.  When determining which cover crops to plant, a farmer should give careful consideration to the goals for the cover crop.  Some example goals may be: 

• Reducing soil erosion and nutrient loss
• Scavenging and temporarily holding excess soluble nutrients
• Generating biomass to improve organic matter and soil health
• Suppressing weeds or pathogens
• Providing nitrogen for the following cash crop
• Diversifying the crop rotation and breaking disease cycles

Cover crops require intensive management for successful integration into your farming system, just like the cash crop.  Management factors include species and variety selection, seeding rate, seeding method, timing of establishment, timing and method of termination, residue handling, and pest issues.  Failure to actively manage these factors can lead to reduced yields in the subsequent cash crop and significant additional costs.

In this article, we will review the benefits and management considerations of a few select cover crop species in use across the Midwest.

Grasses
Benefits:  Grasses are generally good at protecting against soil erosion, scavenging nutrients, and improving soil condition, with penetration of fibrous roots into the soil.  Some grasses may help suppress weed growth.  Grasses are good companion covers to aid in establishment of legumes or brassicas.  Grasses contribute large amounts of mulch to help protect the soil and conserve moisture in no-till systems.  Some winter cereals, like cereal rye, tolerate late seeding and fit well into corn-soybean rotations.  Winter cereals will overwinter to generate more biomass in the spring.  Spring cereals, like oats, planted in the fall will self-terminate in the winter, avoiding herbicide burndown costs and releasing nutrients more quickly the following year.  Grasses can provide excellent grazing forage, provided that herbicide label restrictions on plant-back intervals have been followed.

Management Challenges:  Some grasses, particularly annual ryegrass, can become weed problems.  Some annual ryegrass populations in the Southern US have developed resistance to glyphosate.  Cereals and grasses effectively immobilize nutrients, but with a high carbon-nitrogen ratio, subsequent nutrient release may be delayed.  Small grain cover crops may harbor certain pest insects, like black cutworm and slugs.  Timely termination 2-3 weeks before planting can be a challenge in a wet, cool spring.  Dead grass residues tend to keep soils cooler and wetter, which can delay planting.  Planting delays have recently led to experimentation with 'green-planting' techniques, where the cash crop is planted into green standing cover that is terminated a few days after planting.  

Legumes
Benefits:  Legumes are noted for having roots that penetrate deep into the soil, if established early enough and allowed to grow long enough.  Most legumes will contribute nitrogen for the next cash crop.  With a lower carbon-nitrogen ratio, the legume residue usually breaks down more quickly and releases nutrients at more optimal timing than grasses.

Management Challenges:  With the exception of crimson clover, many legumes are slow to establish in the fall.  Clovers have traditionally been seeded into or planted after harvest of winter wheat.  In corn-soybean rotations, legumes must be inter-seeded into the standing cash crop in order to promote adequate fall growth.  Some legumes, like hairy vetch, can be competitive and difficult to terminate in the spring.

Brassicas
Benefits:  The common brassicas used as cover crops in the Midwest include daikon radishes (also known as oilseed radishes or tillage radishes), turnips, rapeseed, and canola.  Tillage radishes establish rapidly in the fall and produce a large taproot that penetrates through soil hardpans.  Radishes are good companions with grass species, help scavenge excess nutrients, and readily winter kill.  When the radishes die, they leave open channels into the soil to improve infiltration and cash crop root penetration.   Radishes readily release sequestered nutrients.  Some research has shown that the by-products of brassica decomposition may suppress nematode populations.  Some brassicas, like turnips, are well-suited for winter grazing forage.

Management Challenges:  Radishes may emit odors when decomposing, which neighbors may find offensive.  Some soil experts are concerned that the channels, left in the soil where radishes are grown, may inadvertently serve as direct conduits for nutrients to leave the field via macro-pore water flow.

COVER CROP SPECIES FOR CORN-SOY ROTATIONS
Farmers who are new to cover crops may want to consider planting single species to keep management simple.  Research published in 2017 demonstrated that monoculture cover crops performed comparably to multi-species mixtures in all measured parameters, except fall soil cover. 

Cereal Rye.  This winter cereal has become the anchor cover crop species across the Midwest.  Fibrous root system provides excellent soil stabilization.  Strong nutrient scavenging to protect water quality.  Can be established later in the fall and very winter hardy.  Requires good seed-soil contact for germination.  Direct-seeding or broadcast-seeding with light vertical tillage provide best results.  Aerial seeding success is highly dependent on favorable weather (rainfall).  Rye and rye mixtures produce the highest above-ground biomass, carbon, and nitrogen, while sequestering the most soil nitrate.  Easier to terminate than annual ryegrass.  Can immobilize N in corn crops if not terminated 3 weeks ahead of the corn crop.           

Oats.  Reliable, low-cost spring cereal that usually winter-kills.  Fast-growing and good fall nutrient scavenger.  Great companion for legume establishment.  Possible allelopathic weed suppression.  Establishes easily via broadcast seeding with light incorporation.  Seed about 40 days before killing frost for adequate fall growth. 

Annual Ryegrass.  This cool season annual is also known as Italian ryegrass.  Establishes and covers well.  Requires minimal seed to soil contact, which makes it suitable for broadcast and aerial seeding.  Excellent nutrient scavenger.  Deep root system has been shown to effectively 'mine' sub-soil nutrients and bring them to the surface for the next crop.  Annual ryegrass can be challenging to terminate and manage and can become a weed problem if allowed to set seed.

Tillage (oilseed) Radish.  Brassica with deep taproot to penetrate clay hardpans, alleviate compaction, and improve internal drainage.  Good nutrient scavenger.  Can be aerially seeded or broadcast prior to harvest of previous cash crop.  May emit unpleasant odors during decomposition.

Crimson Clover.  Legume that establishes more easily, grows more rapidly in the fall, and is capable of fixing large amounts of nitrogen, depending on when established and terminated.  Well suited to aerial and broadcast seeding, but should be established as early as possible in standing crop. 

COMMON COVER CROP MIXTURES FOR CORN-SOY ROTATIONS
Cover crop mixtures typically offer a wider array of benefits to meet goals.  Blends may help mitigate weather-related risks for establishment, ensuring that at least one species survives.  One crop may serve as a companion or nurse crop to aid establishment of the slower growing species and improve fall cover. 

Soybean Mix:Cereal Rye (93%) and Radish (7%).  An outstanding combination of soil stabilization, nutrient scavenging, and soil tilth improvement ahead of soybeans.  Establishes best when direct-seeded, but some have had luck broadcasting the seed into standing corn.  Grows fairly rapidly in the fall.  Timely spring termination is necessary before the rye sets seed to avoid excessive nitrogen immobilization. 

Corn Mix:Oats (90%) and Radish (10%).  Excellent for soil stabilization, nutrient scavenging, and deep soil penetration.  Establishes rapidly in the fall and normally winter kills, saving the grower the cost and time required to terminate the cover crop in the spring, which makes this mix well-suited for fields going to corn.  This combination does not contribute as much residue as other cover crops, reducing risk of nitrogen tie-up. 

Quick Cover:Annual ryegrass (65%) and Radish (35%).  Combines extensive fibrous root system of ryegrass with radish tap root for deep soil penetration and outstanding nutrient scavenging.  Broadcast-seed into standing crop at least 60 days before a killing frost.  Mix grows rapidly in the fall.  Annual ryegrass requires intensive management in the spring and must be terminated before heading. 

The Midwest Cover Crops Council offer an online cover crop decision tool, which may aid in species selection:  http://mccc.msu.edu/covercroptool/covercroptool.php

Your local FS Member Company provides a complete catalog of individual cover crop species, pre-mix blends, and even custom blends to fit your goals.  Varieties are specifically selected for superior performance to competitive brands and bin-run seed.  Your FS Crop Specialist can provide the expertise needed to help you successfully implement cover crops on your farm.  Click here to view the FS Cover Crop Management guide.

Image 1: Ryegrass and Radish