Cover Crops, Brassicas

Brassica and mustard cover crops are known for their rapid fall growth, great biomass production and nutrient scavenging ability. However, they are also attracting interest because of their possible pest management characteristics. Most Brassica species release chemical compounds that may be toxic to soil borne pathogens and pests, such as nematodes, fungi and some weeds. The mustards unusually have higher concentrations of these chemicals.

Brassicas are increasingly used as winter or rotational cover crops in vegetable and specialty crop production. Some Brassicas have a large taproot that can break through plow pans better than the fibrous roots of cereal cover crops or the mustards. Those Brassicas that winterkill decompose very quickly and leave a seedbed that is mellow and easy to plant in.

With a number of different species to consider, you will likely find one or more that can fit your farming system. Don’t expect Brassicas to eliminate your pest problems, however. They are a good tool and an excellent rotation crop, but pest management results are inconsistent. More research is needed to further clarify the variables affecting the release and toxicity of the chemical compounds involved. Since these cover crops share insect pest and diseases with Brassica vegetables and Brassica weeds, care should be taken when building these into your rotation when Brassica crops are important to you crop mix.

Benefits

Erosion control and nutrient scavenging. Brassicas provide excellent soil coverage and up to 8,000 lb. biomass/A. Because of their fast fall growth, Brassicas are well-suited to capture soil nitrogen (N) remaining after crop harvest. The amount of nitrogen captured is mainly related to biomass accumulation and the amount of N available in the soil profile.

Because they immobilize less nitrogen than some cereal cover crops, much of the N taken up can become available for uptake by main crops in early to late spring. Brassicas can root to depths of six feet or more, scavenging nutrients from below the rooting depth of most crops. To maximize biomass production and nutrient scavenging in the fall, Brassicas must be planted earlier than winter cereal cover crops in most regions, making them more difficult to fit into grain production rotations.

Pest Management

All Brassicas have been shown to release biotoxic compounds that exhibit broad activity against bacteria, fungi, insects, nematodes, and weeds. Brassica cover crops are often mowed and incorporated to maximize their natural fumigant potential. This is because the fumigant chemicals are produced only when individual plant cells are ruptured. The timing and method of incorporation seems to be a key factor in how effective they are at suppressing pests.University studies have shown Brassica green manures to suppress Rhizoctonia, Scab, and Verticillium in potatoes and Phtophthora blight in cucurbits. It has also shown efficacy in reducing nematode and weed populations in various cropping systems. The use of Brassica cover crops for disease and insect control is still very much a work in progress. If you have phytophtora in your fields and are interested in exploring this option, please contact us at umassvegetable@umext.umass.edu or by calling (413)-577-3976. 

Deep tillage

Some Brassicas (forage radish, rapeseed, turnip) produce large taproots that can penetrate up to six feet to alleviate soil compaction. This so-called “biodrilling” is most effective when the plants are growing at a time of year when the soil is moist and easier to penetrate.

As the large tap roots decompose, they leave channels open to the surface that increase water infiltration and improve the subsequent growth and soil penetration of crop roots. Smaller roots decompose and leave channels through the plow plan and improve the soil penetration by the roots of subsequent crops.

Most mustards have a fibrous root system, and rooting effects are similar to small grain cover crops in that they do not root so deeply but develop a large root mass more confined to the soil surface profile.

Species 

Rapeseed (or Canola)

Two Brassica species are commonly grown as rapeseed, Brassica napus and Brassica rapa. Rapeseed that has been bred to have low concentrations of both erucic acid and glucosinolates in the seed is called canola, which is a word derived from Canadian Oil. Rapeseed is used as industrial oil while canola is used for a wider range of products including cooking oils and biodiesel.

Annual or spring type rapeseed belongs to the species B. napus, whereas winter-type or biennial rapeseed cultivars belong to the species B. rapa.

Besides their use as an oil crop, these species are also used for forage. If pest suppression is an objective, rapeseed should be used rather than canola since the breakdown products of glucosinolates are thought to be a principal mechanism for pest control with these cover crops. Rapeseed has been shown to have biological activity against plant parasitic nematodes as well as weeds.

Mustard

Mustard is a name that is applied to many different botanical species, including white or yellow mustard (Sinapis alba, sometimes referred to as Brassica hirta), brown or Indian mustard (Brassica juncea)—sometimes erroneously referred to as canola —and black mustard (B. nigra (L.). The glucosinolate content of most mustards is very high compared to the true Brassicas. Mustards have also been shown to suppress growth of weeds.

Because mustards are sensitive to freezing, winterkilling at about 25º F, they are used either as a spring/summer crop or they winter kill except in areas with little freeze danger. Brown and field mustard both can grow to 6 feet tall.

Radish

The true radish or forage radish (Raphanus sativus) does not exist in the wild and has only been known as a cultivated species since ancient times. Cultivars developed for high forage biomass or high oilseed yield are also useful for cover crop purposes. Common types include oilseed and forage radish.

Their rapid fall growth has the potential to capture nitrogen in large amounts and from deep in the soil profile. Above ground dry biomass accumulation reached 8,000 lb./Acre and N accumulation reached 140 lb./Acre in Michigan. Below ground biomass of radishes can be as high as 3,700 lb./Acre. Radishes have been shown to alleviate soil compaction and suppress weeds.

Turnips

Turnips (B. rapa L. var. rapa (L.) Thell) are used for human and animal food because of their edible root. Turnip has been shown to alleviate soil compaction. While they usually do not produce as much biomass as other Brassicas, they provide many macrochannels that facilitate water infiltration. Similar to radish, turnip is unaffected by early frost but will likely be killed by temperatures below 25° F.

Management

Fall-planted Brassica cover crops fit well into vegetable cropping systems following early harvested crops.

Planted in mid to late August, white mustard emerges quickly and produces a large amount of biomass before succumbing to freezing temperatures. As a component of integrated weed management, using Brassica cover crops in vegetable rotations could improve weed control and reduce reliance on herbicides.

Winter-killed forage radish leaves a nearly weed- and residue-free seedbed, excellent for early spring "no-till" seeding of crops such as carrots, lettuce, peas and sweet corn. This approach can save several tillage passes or herbicide applications for weed control in early spring and can take advantage of the early nitrogen release by the forage radish. Soils warm up faster than under heavy residue, and because no seedbed preparation or weed control is needed, the cash crop can be seeded earlier than normal.

Establishment

Most Brassica species grow best on well drained soils with a pH range of 5.5–8.5. Brassicas do not grow well on poorly drained soils, especially during establishment. Winter cover crops should be established as early as possible. A good rule of thumb is to establish Brassicas about 4 weeks prior to the average date of the first 28° F freeze. The minimum soil temperature for planting is 4F; the maximum is 85° F. Winter hardiness. Some Brassicas and most mustards may winterkill, depending on climate and species. Forage radish normally winter kills when air temperatures drop below 23° F for several nights in a row. Winter hardiness is higher for most Brassicas if plants reach a rosette stage between six to eight leaves before the first killing frost. Some winter-type cultivars of rapeseed are able to withstand quite low temperatures (10° F). Late planting will likely result in stand failure and will certainly reduce biomass production and nutrient scavenging. Planting too early, however, may increase winterkill.

Winter vs. spring annual use

Brassica and mustard cover crops can be planted in spring or fall. In our area most species can be managed to winterkill, leaving a mellow seedbed requiring little or no seedbed preparation. For the maximum benefits offered by Brassicas as cover crops, fall planting is usually preferable because planting conditions (soil temperature and moisture) are more reliable and the cover crops produce more dry matter. When using them as bio-fumigants for reducing Phytophthora blight, spring planting may be more appropriate.

Mixtures

Mix with small grains (oats, rye), other Brassicas or legumes (e.g. clover). Brassicas are very competitive and can overwhelm the other species in the mixture. The seeding rate must be adjusted so ensure adequate growth of the companion species. Consult local expertise and start with small plots or experiment with several seeding rates.

Seed and Planting

Because Brassica spp. seed may be scarce, it is best to call seed suppliers a few months prior to planting to check on availability. Brassica seeds in general are relatively small; a small volume of seed goes a long way.

Rapeseed (Canola). Drill 5-10 lb./A no deeper than ¾ in. or broadcast 8–14 lb./A.
Mustard. Drill 5-12 lb./A ¼–¾ in. deep or broadcast 10-15 lb./A.
Radish. Drill 8 to 12 lb/A. ¼–½ in. deep, or broadcast 12-20 lb./A. Plant in late summer or early fall after the daytime average temperature is below 80° F.
Turnip. Drill 4-7 lb./A about ½ in. deep or broadcast 10-12 lb./A. Plant in the fall after the daytime average temperature is below 80° F.

Nutrient Management

Fall planted Brassicas may be able to scavenge adequate nitrogen, but spring planted Brassicas may benefit from supplemental nitrogen and sulfur fertility, especially if there are to be used as a biofumigant. Brassica sulfur (S) nutrition needs and S uptake capacity exceed those of many other plant species, because S is required for oil and glucosinolate production. Glucosinolates are the key to the Brassicas effectiveness as biofumigant. A 7:1 N/S ratio in soils is optimum for growing rape, while N/S ratios ranging from 4:1 to 8:1 work well for Brassica species in general.

Some Brassicas, notably rape, can scavenge P by making insoluble P more available to them via the excretion of organic acids in their root zone.

Brassicas decompose quickly. Decomposition and nutrient turnover from roots (C:N ratios 20- 30) is expected to be slower than that from shoots (C:N ratios 10-20), but overall faster than that of winter rye. A winter-killed radish cover crop releases plant available nitrogen especially early in spring, so it should be followed by an early planted nitrogen demanding crop to avoid leaching losses. Brassica cover crops should NOT be planted in rotation with other Brassica crops such as cabbage, broccoli, and radish because the latter are susceptible to similar diseases. Also, scattered volunteer Brassica may appear in subsequent crops. Controlling Brassica cover crop volunteers that come up in Brassica cash crops would be challenging if not impossible.

-- adapted by Andy Cavanagh from ‘Managing Cover Crops Profitably’, 3rd Edition. Contributors: Guihua Chen, Andy Clark, Amy Kremen, Yvonne Lawley, Andrew Price, Lisa Stocking, Ray Weil