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Cover Crops

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Introduction

Cover crops can be a valuable tool for organic producers seeking to enhance crop diversity and leverage the numerous agronomic and environmental benefits. Each cover crop species has different benefits based on plant characteristics. For example, legumes such as crimson clover or hairy vetch can be incorporated for increased nitrogen contributions, and grasses such as cereal rye or millet can produce large amounts of biomass quickly for weed suppression and erosion control. Cover crops can help break disease and pest cycles. If you know your field faces pressure from a specific disease or pest it is important to select a cover crop which will break, not encourage, this cycle.

When choosing a cover crop species, producers should take into consideration: 

– Current weed, pest, and disease pressure(s)

-Crop rotation schedule

-Management goals

-Soil and climate

Each producer has management preferences and operational constraints; it is important to pick a cover crop species or species mixture that fits into the operation’s needs. Each species has different timing requirements for planting and termination. Timely termination of cover crops will be an important factor in successfully incorporating cover crops into an operation. SARE has a publication called Managing Cover Crops Profitably, which is a comprehensive guide for producers on how to consider incorporating cover crops into their operation, including profiles on specific crops and regional recommendations. 

Another new tool producers can use to evaluate the right cover crops for their operation is the Cover Crop Species Selector Tool. This online tool can help producers see which cover crop species fit their management goals and production environment. This platform has detailed information about ideal planting dates, termination timing, and growth habit of many different cover crop species.

In North Carolina organic commodity production systems, winter cover crops are often used in rotation with summer cash crops. These cover crops are primarily legumes or grasses that are planted in the fall and grow through winter until early spring. 

Examples of Cover Crops in Organic Rotations

A producer may use a rotation of Austrian winter pea in winter, butternut squash and sweetpotato in summer, wheat or crimson clover the next winter, and sorghum-sudangrass or buckwheat as a summer cover the next summer between summer cash crops.

A producer may use a cereal rye, crimson clover, Austrian winter pea, or triticale before and after a summer cover crop of corn or soybean in an organic corn, soybean rotation.

For more in-depth information on examples of organic rotations visit ‘Chapter 2: Organic Crop Production Systems’ of the Organic Commodities Production Guide.

Leguminous Winter Annual Cover Crops

Common winter annual legume cover crops in North Carolina include hairy vetch, crimson clover, and Austrian winter pea. Leguminous cover crops can contribute nitrogen to organic systems and reduce the need for other organic fertilizer inputs. Nitrogen contributions of leguminous cover crops are determined by the species, biomass of the cover crop, and the protein content of the cover crop. Biomass and protein content are influenced by cover crop planting date and development stage at time of termination. The termination method can also influence nitrogen availability in the soil based on the rate of cover crop decomposition. NC State researchers, as part of the Precision Sustainable Agriculture network, have developed a calculator to estimate the nitrogen contributions of different cover crop species called CC-NCALC.

Termination for Leguminous Cover Crops

In addition to getting a good stand, terminating at the right time to maximize cover crop biomass and plant available nitrogen is key to getting the most out of leguminous cover crops. Often leguminous cover crops will have the most biomass as they enter their reproductive stages.

Hairy vetch increases in nitrogen content as it matures through its reproductive stages, however hairy vetch can become difficult to terminate as its growth habit becomes weedy and matted. Termination of hairy vetch should balance maximizing nitrogen and ease of incorporation (Managing Cover Crops Profitably). 

 Both hairy vetch and crimson clover produce hard seed, which can lay dormant through the spring and summer and germinate in the fall. If allowed to set seed, hairy vetch can become a volunteer and may produce challenges in following winter cash crops such as wheat. To minimize this, terminating earlier in the reproductive phase (~50% bloom) is ideal (Managing Cover Crops Profitably).

Crimson clover is less challenging as a volunteer. Producers can terminate crimson clover when it is in late bloom for maximum nitrogen content (Managing Cover Crops Profitably). Some producers may even encourage crimson clover to set seed and re-establish itself the following winter. 

Austrian winter peas should be killed after full bloom to provide maximum nitrogen (Managing Cover Crops Profitably).

Other Considerations for Leguminous Cover Crops

Timing of planting and planting methods are key to getting a good stand, which  is critical to maximize cover crop benefits .It is also important to note that if soils are deficient in phosphorus or potassium, supplemental fertilization may be needed to get a healthy stand.

For more information on management recommendations for hairy vetch, crimson clover, and Austrian winter pea; visit NC State’s SoilFacts publication ‘Winter Annual Cover Crops

Cereal Winter Annual Cover Crops

Common winter annual cereals include cereal rye, triticale, and oats. Small grains as cover crops are a valuable tool for erosion control and weed suppression. While legumes are nutrient sources, cereal cover crops are nutrient scavengers. Small grains provide nutrient-scavenging services, which can help reduce nutrient leaching during the winter and protect surface water and groundwater quality. When managing a cereal, producers should try to maximize biomass and nitrogen available to the following cash crop. Cereal cover crops may tie up plant available nitrogen in soil as they go to maturity, so timely termination is important. Unlike leguminous cover crops, some fertilizer may be needed for adequate establishment and biomass production. 

These cover crops can also help break disease cycles. For example, cereal rye is a non-host to root-knot nematodes and provides habitat for beneficial predatory insects (Managing Cover Crops Profitability). Cereal rye, triticale, barley, and oats are not inhabited by Hessian Fly. 

Some cover crops can be hosts to pests. Cereal rye can host wire-worms and cut-worms in no-till rotations. If wire worm or cut worm pressure exists in your field cereal rye might not be the right fit (Managing Cover Crops Profitability).

Termination of Cereal Cover Crops

Unlike with the termination of leguminous cover crops, much of the termination of cereal cover crops is focused on minimizing the loss of N to the cover crop as it transitions from vegetative to reproductive growth. A tradeoff with the termination of small grains is maximizing biomass without tying up too much nitrogen.

In cereals, N immobilization is directly related to maturity. Cereal rye rapidly grows in spring and should be killed before it reaches maturity; N accumulated within cereal rye biomass is slow to decompose and does not readily mineralize so it will not be largely available to summer cover crop. Furthermore, the cover crop residue has a high C:N ratio, meaning that it can also tie up nitrogen applied in the spring for the cash crop. 

Triticale is a cross between cereal rye and wheat, its spring growth is not as fast as cereal rye and it does not put on as much biomass as cereal rye but it matures faster than wheat and puts on more biomass. Generally triticale will be terminated later than cereal rye.

Oats should be terminated soon after their vegetative growth ends and their reproductive growth begins, this should be at the milk or soft dough stage.

Winter Cover Crop Mixtures

Producers may consider using both a winter-annual legume and cereal species mixture to get benefits of nitrogen contributions and significant biomass. When planning to use a mix of cover crops it is important to take into account the seed size, germination timing, growth habits, and termination timing of your cover crop mix. Certain cover crop species benefit from being in a mix. For example, hairy vetch biomass production can be enhanced when grown in a mix with cereal rye or triticale. Hairy vetch likes to climb and uses the grasses as a trellis, which can also help reduce the amount of matting and potential disease seen in a hairy vetch monoculture. It’s important to understand that growing a legume-cereal cover crop mixture will change the final biomass C:N ratio. Compared to a pure legume cover monoculture, the C:N ratio of a mix will likely be higher, which can draw out nitrogen mineralization later in the season. This may pose an issue depending on timing of crop nitrogen demand.

There are many species mixes on the market, some contain multiple species of legumes, forbes, brassicas, and cereals in a single mix. 

Roller Crimping

While conventional producers have the option of herbicides for cover crop termination, organic producers must terminate cover crops mechanically either through tillage or through roller crimping. It is important to effectively terminate your cover crop to ensure successful summer planting and prevent competition later in the season from surviving cover crops. Additionally, the timing of termination will be important in determining the potential nitrogen contribution, in-season nutrient availability for the summer crop, and the amount of time that the ground is bare before planting if the cover crop is terminated with conventional tillage.

For producers looking to reduce tillage on their operation, roller crimping may be a good option. Roller crimping terminates the cover crop by putting force on the stem of the cover crop, ‘crimping’ it, and disrupting the plant’s ability to transport water and nutrients (Rolled Cover Crop Mulches). When using a roller crimper, the growth stage at which the cover crop will have an important effect on the efficacy of termination.

Winter cover crops should be rolled and crimped after the cover crop completes vegetative growth, attempting to terminate during vegetative growth can lead to insufficient termination. When choosing at what point in reproductive growth to terminate, producers should consider how they can avoid the cover crop going to viable seed while maximizing biomass and nitrogen.

Generally cereals should be rolled at their soft dough stage, this timing will vary by cereal species with cereal rye maturing earlier than triticale and wheat. Leguminous cover crops can be more challenging to terminate with a roller crimper due to their growth habit and structure. Crimson clover and Austrian winter pea should be terminated at 100% flowering and hairy vetch should be terminated at 50% flowering.

For more information on roller crimping in an organic system, Chapter 11 of the Organic Commodities Production Guide, ‘Rolled Cover Crop Mulches for Organic Corn and Soybean’ has further information.

Mowing and Mowing/Tillage Combination

Another method for terminating cover crops is mowing. Two common types of mowers are flail mowers and bush-hog or rotary mowers. Flail mowers have the advantage of spreading residue evenly on the field. Similar to roller crimping, the timing of mowing will be important to ensure effective termination. If cover crops are terminated too early then regrowth can occur. Mowing is often used in combination with tillage to fully terminate the cover crop and incorporate residue into the soil. Another advantage of mowers is that they are able to make a high-biomass cover crop such as rye easier to incorporate with tillage.

Summer Cover Crops

While less commonly used, there are also summer cover crops producers may plant. Farmers may opt to use these warm-season cover crops if they produce a fall grain or vegetable crop or simply want to let their land rest prior to the following year’s summer crop.

Similar to winter cover crops, most summer cover crops grown are cereals or legumes. Common cereal summer crops for North Carolina include buckwheat, sorghum-sudangrass, German millet, pearl millet, and japanese millet. Common leguminous short-season cover crops include cowpea, soybean, velvetbean, and sunhemp. For additional information on summer cover crops, visit the page, Summer Cover Crops, part of NC State’s Horticulture Information Leaflets.

Other Considerations for Cover Crops in Organic Systems

The National Organic Program encourages the use of cover crops as a means to improve soil health and reduce weed pressure. North Carolina organic producers have a good diversity of cover crops to choose from to incorporate into their rotations.

While most cover crop seed isn’t treated, keep in mind that cover crop seed must fit organic standards in order for land to remain organically certified. Seed treatments or genetically modified crops are not compliant. The seed supplier page on the Organic Commodities portal has a list of seed suppliers who sell organic seed, including cover crops.

There may be opportunities for cost-share programs if you are looking to adopt cover crops in your operation. The NRCS Organic Initiative has cost-share opportunities for organic producers looking to add cover crops to their rotation. The North Carolina Department of Agriculture also has a cost-share program for cover crops.

Additional Resources

Extension Publications

Winter Annual Cover Crops NC State Extension Publication

Summer Annual Cover Crops NC State Extension Publication

Managing Cover Crops Profitably

Managing Cover Crops in Organic No-Till

Cover Crop Chapter in North Carolina Organic Commodities Production Guide 

SARE Termination of Cover Crops Publication

Online Tools for Cover Crop Management

Cover Crop Species Selector Tool

Cover Crop Nitrogen Calculator (CC-NCALC)

Cover Crop Economic Decision Support Tool (CC-ECON)

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