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BRASSICAS AND MUSTARDS
Type: Annual (usually winter or spring; summer use
possible)
Roles: Prevent erosion, suppress weeds and soilborne
pests, alleviate soil compaction and scavenge nutrients
Mix with: Other brassicas or mustards, small grains
or crimson clover
Species: Brassica napus, Brassica rapa,
Brassica juncea, Brassica hirta, Raphanus sativus,
Sinapsis alba
See charts, pp. 66 to 72, for ranking and management summary.
Nomenclature Note: The cover crops described in this
chapter all belong to the family BRASSICACEAE. Most but not
all of the species belong to the genus Brassica.
In common usage, the various species are sometimes lumped together
as “brassicas” and sometimes distinguished as “brassicas”
vs. “mustards.” In this book, we will use brassicas as
an umbrella term for all species; mustards will be used to distinguish
that subgroup, which has some unique characteristics.
Adaptation Note: This chapter addresses management
of eight different cover crop species with varying degrees of winterhardiness.
Some can be managed as winter or spring annuals. Others are best
planted in late summer for cover crop use but will winterkill. Consult
the information on management, winterhardiness and winter
vs. spring use and the examples throughout the chapter, then
check with local experts for specific adaptation information for
your brassica cover crop of choice.
Brassica and mustard cover crops are known for their rapid fall
growth, great biomass production and nutrient scavenging ability.
However, they are attracting renewed interest primarily because
of their 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 usually have higher concentrations of these chemicals.
Brassicas are increasingly used as winter or rotational cover crops
in vegetable and specialty crop production, such as potatoes and
tree fruits. There is also growing interest in their use in row
crop production, primarily for nutrient capture, nematode trapping,
and biotoxic or biofumigation activity. 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.
BENEFITS
Erosion control and nutrient scavenging.
Brassicas can provide greater than 80% soil coverage when used as
a winter cover crop (176).
Depending on location, planting date and soil fertility, they produce
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 (see also Building
Soil Fertility and Tilth with Cover Crops). 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 or metabolic byproducts 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.
Pest suppression is believed to be the result of glucosinolate
degradation into biologically active sulfur containing compounds
call thiocyanates (152, 320).
To maximize pest suppression, incorporation should occur during
vulnerable lifestages of the pest (446).
The biotoxic activity of brassica and mustard cover crops is low
compared to the activity of commercial fumigants (388).
It varies depending on species, planting date, growth stage when
killed, climate and tillage system. Be sure to consult local expertise
for best results.
Precaution. The use of brassicas for pest management
is in its infancy. Results are inconsistent from year to year and
in different geographic regions. Different species and varieties
contain different amounts of bioactive chemicals. Be sure to consult
local expertise and begin with small test plots on your farm.
Disease
In Washington, a SARE-funded study of brassica green manures in
potato cropping systems compared winter rape (Brassica napus)
and white mustard (Sinapis alba) to no green manure, with
and without herbicides and fungicides. The winter rape system had
a greater proportion of Rhizoctonia -free tubers (64%)
than the white mustard (27%) and no green manure (28%) treatments
in the non-fumigated plots. There was less Verticillium
wilt incidence with winter rape incorporation (7%) than with white
mustard (21%) or no green manure incorporation (22%) in non-fumigated
plots (88).
In Maine, researchers have documented consistent reductions in
Rhizoctonia (canker and black scurf) on potato following
either rapeseed green manure or canola grown for grain (459,
460). They have also observed
significant reductions in powdery scab (caused by Spongospora
subterranea) and common scab (Streptomyces scabiei)
following brassica green manures, especially an Indian mustard (B.
juncea) greenmanure (458,
459).
Nematodes
In Washington state, a series of studies addressed the effect of
various brassica and mustard cover crops on nematodes in potato
systems (260, 266,
353, 283,
284, 285).
The Columbia root-knot nematode (Meloidogyne chitwoodi)
is a major pest in the Pacific Northwest. It is usually treated
with soil fumigants costing $20 million in Washington alone.
Rapeseed, arugula and mustard were studied as alternatives to fumigation.
The brassica cover crops are usually planted in late summer (August)
or early fall and incorporated in spring before planting mustard.
Results are promising, with nematodes reduced up to 80%, but—because
of the very low damage threshold—green manures alone cannot
be recommended for adequate control of Meloidogyne chitwoodi
in potatoes. The current recommended alternative to fumigation is
the use of rapeseed or mustard cover crop plus the application of
MOCAP. This regimen costs about the same as fumigation (2006 prices).
Several brassicas are hosts for plant parasitic nematodes and can
be used as trap crops followed by an application of a synthetic
nematicide. Washington State University nematologist Ekaterini Riga
has been planting arugula in the end of August and incorporating
it in the end of October.
Nematicides are applied two weeks after incorporation, either at
a reduced rate using Telone or the full rate of Mocap and Temik.
Two years of field trials have shown that arugula in combination
with synthetic nematicides reduced M. chitwoodi to economic
thresholds.
Longer crop rotations that include mustards and non-host crops
are also effective for nematode management. For example, a 3-year
rotation of potatoes>corn>wheat provides nearly complete control
of the northern root-knot nematode (Meloidogyne hapla)
compared to methyl bromide and other broad-spectrum nematicides.
However, because the rotation crops are less profitable than potatoes,
they are less commonly used. Not until growers better appreciate
the less tangible long-term cover crop benefits of soil improvement,
nutrient management and pest suppression will such practices be
more widely adopted.
In Wyoming, oilseed radish (Raphanus sativus) and yellow
mustard (Sinapsis alba) reduced the sugar beet cyst nematode
populations by 19-75%, with greater suppression related to greater
amount of cover crop biomass (231).
In Maryland, rapeseed, forage radish and a mustard blend did not
significantly reduce incidence of soybean cyst nematode (which is
closely related to the sugar beet cyst nematode). The same species,
when grown with rye or clover, did reduce incidence of stubby root
nematode (432).
Also in Maryland, in no-till corn on a sandy soil, winterkilled
forage radish increased bacteria -eating nematodes, rye and rapeseed
increased the proportion of fungal feeding nematodes, while nematode
communities without cover crops were intermediate. The Enrichment
Index, which indicates a greater abundance of opportunistic bacteria
–eating nematodes, was 23% higher in soils that had brassica
cover crops than the unweeded control plots.
These samples, taken in November, June (a month after spring cover
crop kill), and August (under no-till corn), suggest that the cover
crops, living or dead, increased bacterial activity and may have
enhanced nitrogen cycling through the food web (432).
Weeds
Like most green manures, brassica cover crops suppress weeds in
the fall with their rapid growth and canopy closure. In spring,
brassica residues can inhibit small seeded annual weeds such as,
pigweed, shepherds purse, green foxtail, kochia, hairy nightshade,
puncturevine, longspine sandbur, and barnyardgrass (293),
although pigweed was not inhibited by yellow mustard (178).
In most cases, early season weed suppression obtained with brassica
cover crops must be supplemented with herbicides or cultivation
to avoid crop yield losses from weed competition later in the season.
As a component of integrated weed management, using brassica cover
crops in vegetable rotations could improve weed control and reduce
reliance on herbicides (39).
In Maine, the density of sixteen weed and crop species was reduced
23 to 34% following incorporation of brassica green manures, and
weed establishment was delayed by 2 days, compared to a fallow treatment.
However, other short-season green manure crops including oat, crimson
clover and buckwheat similarly affected establishment (176).
In Maryland and Pennsylvania, forage radish is planted in late
August and dies with the first hard frost (usually December). The
living cover crop and the decomposing residues suppress winter annual
weeds until April and result in a mellow, weed-free seedbed into
which corn can be no-tilled without any preplant herbicides. Preliminary
data show summer suppression of horseweed but not lambsquarters,
pigweed, or green foxtail (432).
Mustard cover crops have been extremely effective at suppressing
winter weeds in tillage intensive, high value vegetable production
systems in Salinas, California. Mustards work well in tillage intensive
systems because they are relatively easy to incorporate into the
soil prior to planting vegetables. However, the growth and biomass
production by mustards in the winter is not usually as reliable
as that of other cover crops such as cereal rye and legume/cereal
mixtures (45).
Deep tillage. Some brassicas (forage radish, rapeseed,
turnip) produce large taproots that can penetrate up to six feet
to alleviate soil compaction (432).
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.
Their deep rooting also allows these crops to scavenge nutrients
from deep in the soil profile. 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
(446).
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.
Annual or spring type rapeseed belongs to the species B. napus,
whereas winter-type or biennial rapeseed cultivars belong to the
species B. rapa. Rapeseed is used as industrial oil while
canola is used for a wider range of products including cooking oils
and biodiesel.
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 (176,
365).
Due to its rapid fall growth, rapeseed captured as much as 120
lb. of residual nitrogen per acre in Maryland (6).
In Oregon, aboveground biomass accumulation reached 6,000 lb./A
and N accumulation was 80 lb./A.
Some winter-type cultivars are able to withstand quite low temperatures
(10° F) (352). This makes
rapeseed one of the most versatile cruciferous cover crops, because
it can be used either as a spring- or summer-seeded cover crop or
a fall-seeded winter cover crop. Rapeseed grows 3 to 5 feet tall.
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.) (231).
The glucosinolate content of most mustards is very high compared
to the true Brassicas.
In the Salinas Valley, California, mustard biomass reached 8,500
lb./A. Nitrogen content on high residual N vegetable ground reached
328 lb. N/A (388, 422).
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.
In Washington, a wheat/mustard-potato system shows promise for
reducing or eliminating the soil fumigant metam sodium. White mustard
and oriental mustard both suppressed potato early dying (Verticillium
dahliae) and resulted in tuber yields equivalent to fumigated
soils, while also improving infiltration, all at a cost savings
of about $66/acre (see www.plantmanagementnet
work.org/pub/cm/research/2003/mustard/).
Mustards have also been shown to suppress growth of weeds (See
“Weeds”
and 39, 176,
365).
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 (170 lb./Acre in
Maryland (234). Above ground
dry biomass accumulation reached 8,000 lb./Acre and N accumulation
reached 140 lb./Acre in Michigan (304).
Below ground biomass of radishes can be as high as 3,700 lb./Acre.
Oilseed radish is less affected by frost than forage radish, but
may be killed by heavy frost below 25° F. Radish grows about
2–3 feet tall.
Radishes have been shown to alleviate soil compaction and suppress
weeds (177, 446).
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
(359). Similar to radish, turnip
is unaffected by early frost but will likely be killed by temperatures
below 25° F.
In an Alabama study of 50 cultivars belonging to the genera Brassica,
Raphanus, and Sinapis, forage and oilseed radish
cultivars produced the largest amount of biomass in central and
south Alabama, whereas winter-type rapeseed cultivars had the highest
production in North Alabama (425).
Some brassicas are also used as vegetables (greens).
Cultivated varieties of Brassica rapa include bok choy
(chinensis group), mizuna (nipposinica group),
flowering cabbage (parachinensis group), chinese cabbage
(pekinensis group) and turnip (rapa group). Varieties
of Brassica napus include Canadian turnip, kale, rutabaga,
rape, swede, swedish turnip and yellow turnip. Collard, another
vegetable, is a cabbage, B. oleracea var. acephala,
and B. juncea is consumed as mustard greens.
A grower in Maryland reported harvesting the larger roots of forage
radish (cultivar DAIKON) cover crop to sell as a vegetable. In California,
broccoli reduced the incidence of lettuce drop caused by Sclerotinia
minor (175).
Mustard Mix Manages Nematodes in Potato/Wheat
System
Looking for a green manure crop to maintain
soil quality in his intensive potato/wheat rotation, Dale
Gies not only improved infiltration and irrigation efficiency,
he also found biofumigation, a new concept in pest management.
Farming 750 irrigated acres with two sons and
a son-in-law in the Columbia basin of Grant County, Wash.,
Gies started growing green manure crops in 1990 because he
wanted to improve his soils for future generations. Since
then, he has reduced his use of soil fumigants thanks to the
biocidal properties of Brassica cover crops. In particular,
Gies is most excited about results using a mixture of white
or oriental mustard and arugula (Eruca sativa), also
a brassica, to manage nematodes and potato early dying disease.
“We use the mustards to augment other
good management practices,” Gies cautions. “Don’t
expect a silver bullet that will solve your pest problems
with one use.”
Controlling nematodes is essential to quality
potato production, both for the domestic and the international
market. Farmers typically manage root knot nematodes (Meloidogyne
chitwoodi) and fungal diseases with pesticides, such
as Metam sodium, a fumigant used routinely to control early
dying disease (Verticillium dahliae), that cost that
up to $500 per acre. Farmers are especially vulnerable to
early dying disease if their rotations contain fewer than
three years between potato crops.
However, with potato prices dropping, potato
farmers in Washington and elsewhere started looking for ways
to reduce costs. Gies contacted Andy McGuire at Washington
State University Extension for help documenting the results
he was seeing with brassicas. With research funding from SARE,
McGuire confirmed that the mustards improved infiltration.
He also showed that white mustard was as effective as metam
sodium in controlling potato early dying disease.
“The findings suggest that mustard green
manures may be a viable alternative to the fumigant metam
sodium in some potato cropping systems,” says McGuire.
“The practice can also improve water infiltration rates
and provide substantial savings to farmers. Until more research
is done, however, mustard cover crops should be used to enhance,
not eliminate, chemical control of nematodes.”
Researchers have found that mustards can also
suppress common root rot (Aphanomyces euteiches)
and the northern root-knot nematode (Meloidogyne hapla).
Two types of mustard commonly used in the Columbia
Basin are white mustard (Sinapis alba, also called
Brassica hirta or yellow mustard), and Oriental mustard
(Brassica juncea, also called Indian or brown mustard).
Blends of the two are often planted as green manures. Fall
incorporation seems to be best to control nematodes and soil-borne
diseases, and Oriental mustard may be better at it than white
mustard.
Gies plants a mix of mustards and NEMAT, an
arugula variety developed in Italy for nematode suppression.
The arugula attracts nematodes but they cannot reproduce on
its roots, so nematode populations reduce, according to Washington
State University researcher Ekaterini Riga.
Riga’s greenhouse studies showed that
arugula reduced Columbia root knot nematode (Meloidogyne
chitwoodi) populations compared to the control or other
green manure treatments. Subsequent field trial in 2005 and
2006 showed that arugula in combination with half the recommended
rate of Telone (another fumigant) or full rates of Mocap and
Temik reduced root knot nematode populations from 700 nematodes
per gram of soil to zero. The combination also improved potato
yield and tuber quality and it is still affordable by the
growers.
“Arugula acts both as a green manure and
a nematode trap crop,” says Riga.
“It contains chemicals with high biocidal
activity that mimic synthetic fumigants. Since nematodes are
attracted to the roots of Arugula, it can be managed as a
trap crop.”
What causes brassicas to have biocidal properties?
Researchers are keying in on the presence of glucosinolates
in mustards. When the crop is incorporated into the soil,
the breakdown of glucosinolates produces other chemicals that
act against pests. Those secondary chemicals behave like the
active chemical in commercial fumigants like metam sodium.
More research is needed to better determine
site- and species-specific brassica cover crop effects on
pests. It seems to be working for Dale Gies, however, “whose
short season fresh market potato system probably functions
differently than processing potatoes” according to WSU’s
Andy McGuire. To stay updated on cover crop work in Washington
State, see www.grant-adams.wsu.edu/agriculture/covercrops/green_manures/.
For Gies, however, “Tying the whole system
together makes it work economically, and it improves the soil.”
—Andy Clark |
AGRONOMIC SYSTEMS
Brassicas must be planted earlier than small grain cover crops
for maximum benefits, making it difficult to integrate them into
cash grain rotations.
Broadcasting seeding (including aerial seeding) into standing crops
of corn or soybean has been successful in some regions (235).
See also After
25 Years, Improvements Keep Coming. Brassica growth does not
normally interfere with soybean harvest, although could be a problem
if soybean harvest is delayed. The shading by the crop canopy results
in less cover crop biomass and especially less root growth, so this
option is not recommended where the brassica cover crop is intended
for compaction alleviation.
In a Maryland SARE-funded project, dairy farmers planted forage
radish immediately after corn silage harvest. With a good stand
of forage radish, which winterkills, corn can be planted in early
spring without tillage or herbicides, resulting in considerable
savings. The N released by the decomposed forage radish residues
increased corn yield boost in most years. This practice is particularly
useful when manure is fall-applied to corn silage fields. (For more
information see SARE project report LNE03-192 http://www.sare.org/MySare/ProjectReport.aspx?do=viewProj&pn=LNE03-192).
Precaution: Brassica cover crops may be susceptible
to carry-over from broadleaf herbicides applied to the previous
grain crop.
Vegetable Systems. Fall-planted brassica cover
crops fit well into vegetable cropping systems following early harvested
crops. White mustard and brown mustard have become popular fall-planted
cover crops in the potato producing regions of the Columbia Basin
of eastern Washington.
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 (39).
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.
|
Rape or Canola
(Brassica rapa) |
MANAGEMENT
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 45° F; 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) (352).
Late planting will likely result in stand failure and will certainly
reduce biomass production and nutrient scavenging. Planting too
early, however, may increase winterkill in northern zones (166).
In Washington (Zone 6), canola and rapeseed usually overwinter,
mustards do not. Recent work with arugula (Eruca sativa)
shows that it does overwinter and may provide similar benefits as
the mustards (430).
In Michigan, mustards are planted in mid-August, and winterkill
with the first hard frost, usually in October. When possible, plant
another winter cover crop such as rye or leave strips of untilled
brassica cover crop rather than leave the soil without growing cover
over the winter (391).
In Maine, all brassica and mustards used as cover crops winterkill
(166).
Winter vs. spring annual use
Brassica and mustard cover crops can be planted in spring or fall.
Some 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.
In Maryland, rapeseed and forage radish were more successful as
winter- rather than spring annual cover crops. The early spring
planted brassicas achieved about half the quantities of biomass
and did not root as deeply, before bolting in spring (432).
In Michigan, mustards can be planted in spring following corn or
potatoes or in fall into wheat residue or after snap beans. Fall
seedings need about 900 growing-degree-days to produce acceptable
biomass, which is usually incorporated at first frost (usually October).
Spring seeding is less reliable due to cool soil temperatures, and
its use is limited mostly to late-planted vegetable crops (391).
In Maine, brassicas are either planted in late summer after the
cash crop and winterkill, or they are spring-seeded for a summer
cover crop (166).
Rapeseed planted in late spring to summer has been used with some
success in the mid-Atlantic region to produce high biomass for incorporation
to biofumigate soil for nematodes and diseases prior to planting
strawberries and fruit trees.
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.
Washington farmers use mixtures of white and brown mustard, usually
with a greater proportion of brown mustard.
In Maryland and Pennsylvania, farmers and researchers seed the
small grain and forage radish in separate drill rows rather than
mixing the seed. This is done by taping closed alternate holes in
the two seeding boxes of a grain drill with both small seed and
large seed boxes. Two rows of oats between each row of forage radish
has also proven successful (432).
Rye (sown at 48 lb./A) can be grown successfully as a mixture with
winter-killing forage radish (13 lb./A).
Killing
Brassica cover crops that do not winterkill can be terminated in
spring by spraying with an appropriate herbicide, mowing, and/or
incorporating above-ground biomass by tillage before the cover crop
has reached full flower. Rolling may also be used to kill these
covers if they are in flower.
Rapeseed has proved difficult to kill with glyphosate, requiring
a higher than normal rate of application—at least 1 quart/acre
of glyphosate— and possibly multiple applications. Radish,
mustard, and turnip can be killed using a full rate of paraquat,
multiple applications of glyphosate, or glyphosate plus 1pt/acre
2, 4-D.
In Alabama and Georgia, brassica cover crops were reportedly harder
to chemically kill than winter cereals. Timely management and multiple
herbicide applications may be necessary for successful termination.
If not completely killed, rapeseed volunteers can be a problem in
the subsequent crop. Always check herbicide rotation restrictions
before applying.
Another no-till method for terminating mature brassicas is flail
mowing. Be sure to evenly distribute residue to facilitate planting
operations and reduce allelopathic risk for cash crop. As mentioned
above, many producers incorporate brassica residues using conventional
tillage methods to enhance soil biotoxic activity especially in
plasticulture systems.
Brassica pest suppression may be more effective if the cover crop
is incorporated.
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
Brassicas and mustards need adequate nitrogen and sulfur fertility.
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. 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.
Ensuring sufficient N supply to brassicas during establishment
will enhance their N uptake and early growth. 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 (168).
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
(432).
COMPARATIVE NOTES
Canola is more prone to insect problems than mustards, probably
because of its lower concentration of glucosinolates.
In the Salinas Valley, which has much milder summer and winter
temperatures than the Central Valley of California, brassica cover
crops are generally less tolerant of suboptimal conditions (i.e.
abnormally low winter temperatures, low soil nitrogen, and waterlogging),
and hence are more likely to produce a nonuniform stand than other
common cover crops (45).
Precautions. The use of brassicas for pest management
is in its infancy. Results are inconsistent from year to year and
in different geographic regions. Be sure to consult local expertise
and begin with small test plots on your farm.
Biotoxic activity can stunt cash crop growth, thus avoid direct
planting into just-killed green residue.
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.
Black mustard (Brassica nigra) is hardseeded and could
cause weed problems in subsequent crops (39).
Rapeseed contains erucic acid and glucosinolates, naturally occurring
internal toxicants. These compounds are anti-nutritional and are
a concern when feeding to livestock. Human consumption of brassicas
has been linked to reducing incidence of cancer. All canola cultivars
have been improved through plant breeding to contain less than 2%
erucic acid.
Winter rape is a host for root lesion nematode. In a SARE funded
study in Washington, root lesion nematode populations were 3.8 times
higher in the winter rape treatment than in the white mustard and
no green manure treatments after green manure incorporation in unfumigated
plots. However, populations in the unfumigated winter rape treatment
were below the economic threshold both years of the study. For more
information, go to www.sare.org/projects/
and search for SW95-021. See also SW02-037).
Rapeseed may provide overwintering sites for harlequin bug in Maryland
(432).
Contributors: Guihua Chen, Andy Clark, Amy Kremen,
Yvonne Lawley, Andrew Price, Lisa Stocking, Ray Weil
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