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SWEETCLOVERS
Yellow sweetclover (Melilotus officinalis) and white sweetclover
(M. alba)
Also called: HUBAM (actually a cultivar of annual
white sweetclover)
Type: biennial, summer annual or winter annual
legume
Roles: soil builder, fertility source, subsoil aerator,
weed suppressor, erosion preventer
Mix with: small grains, red clover
See charts, pp. 66 to 72, for ranking and management summary.
Within a single season on even marginally fertile soils, this tall-growing
biennial produces abundant biomass and moderate amounts of nitrogen
as it thrusts a taproot and branches deep into subsoil layers. Given
fertile soils and a second season, it lives up to its full potential
for nitrogen and organic matter production. Early in the second year
it provides new top growth to protect the soil surface as its roots
anchor the soil profile. It is the most drought-tolerant of forage
legumes, is quite winter-hardy and can extract from the soil then
release phosphorus, potassium and other micronutrients that are otherwise
unavailable to crops.
Sweetclover thrives in temperate regions wherever summers are mild.
Annual sweetclovers (HUBAM is the most well known) work
best in the Deep South, from Texas to Georgia. There, they establish
more quickly than the biennial types and produce more biomass in
the seeding year in southern regions.
In this chapter, “sweetclover” refers to biennial
types unless otherwise noted.
Sweetclover was the king of green manures and grazing legumes in
the South and later throughout the Midwest in the first half of
this century. Sweetclover is used as a cover crop most commonly
now in the Plains region, with little use in California.
Types
Biennial yellow sweetclover can produce up to 24 inches
of vegetative growth and 2.5 tons dry matter/A in its establishment
year. During the second year, plants may reach 8 feet tall. Root
mass and penetration (to 5 feet) are greatest at the end of dormancy
in early spring, then gradually dissipate through the season (443).
A distinguishing sweetclover feature is bracts of tiny blooms through
much of its second year. White biennial sweetclovers are taller,
more coarsely stemmed, less drought tolerant, and produce less biomass
in both the seeding and second years. White types bloom 10 to 14
days later than yellow, but bloom for a longer season. They reportedly
establish more readily in New York (450).
Tall, stemmy cultivars are better for soil improvement (120,
361, 422).
Both yellow and white sweetclover have cultivars bred for low levels
of coumarin. This compound exists in bound form in the plant and
poses no problem during grazing. However, coumarin can cause internal
injury to cattle when they eat spoiled sweetclover hay or silage.
Annual sweetclover (Melilotus alba var. annua) is not
frost tolerant, but can produce up to 9,000 lb. dry matter/A over
a summer after being oversown into a grain crop or direct seeded
with a spring grain nurse crop. The best-known annual sweetclover
cultivar is HUBAM, a name often used for all annual white sweetclover.
While its taproot is shorter and more slender than that of its biennial
cousins, it still loosens subsoil compaction.
BENEFITS
Nutrient scavenger. Sweetclover appears to have
a greater ability to extract potassium, phosphorus and other soil
nutrients from insoluble minerals than most other cover crops. Root
branches take in minerals from seldom-disturbed soil horizons, nutrients
that become available as the tops and roots decompose (361).
Research in Saskatchewan during a 34-year period showed that phosphorus
(P) availability increased in subsoil layers relative to
surface layers, peaking at an 8-foot depth. Winter wheat
and safflower, with deeper root systems than spring wheat, could
tap the deep P buildup from the legume roots and fallow leaching,
whereas spring wheat could not. The vesicular-arbuscular mycorrhizal
(VAM) fungi associated with legume roots contribute to the increased
P availability associated with sweetclover (69,
70).
N source. A traditional green manure crop in the
upper Midwest before nitrogen fertilizer became widely available,
sweetclover usually produces about 100 lb. N/A, but can produce
up to 200 lb. N/A with good fertility and rainfall. In Ohio, it
contained about 125 lb. N/A by May 15, increasing to 155 lb. by
June 22. Illinois researchers reported more than 290 lb. N/A.
Abundant biomass. If planted in spring and then
given two full seasons, biennial sweetclovers can produce 7,500
to 9,000 lb. dry matter/A (3,000 to 3,500 lb./A in the seeding year,
and 4,500 to 5,500 lb./A the second). Second-year yields may go
as high as 8,500 lb./A.
Hot-weather producer. Sweetclover has the greatest
warm-weather biomass production of any legume, exceeding even alfalfa.
Soil structure builder. Kansas farmer Bill Granzow
says sweetclover gives his soils higher organic matter, looser structure
and better tilth. See Sweetclover: Good Grazing,
Great Green Manure. HUBAM annual sweetclover also improved soil
quality and increased yield potential in 1996 New York trials (451).
Compaction fighter. Yellow sweetclover has a determinate
taproot root up to 1 foot long with extensive branches that may
penetrate 5 feet to aerate subsoils and lessen the negative effects
of compaction on crops. White types have a strong tap root that
is not determinate.
Drought survivor. Once established, sweetclover
is the most drought tolerant of all cover crops that produce as
much biomass. It is especially resilient in its second year, when
it could do well in a dry spring during which it would be difficult
to establish annual cover crops. The yellow type is less sensitive
to drought and easier to establish in dry soils than the white type.
Attracts beneficial insects. Blossoms attract
honeybees, tachinid flies and large predatory wasps, but not small
wasps.
Widely acclimated. Self-reseeding sweetclover
can be seen growing on nearly barren slopes, road rights-of-way,
mining spoils and soils that have low fertility, moderate salinity
or a pH above 6.0 (183). It
also can tolerate a wide range of environments from sea level to
4,000 feet in altitude, including heavy soil, heat, insects, plant
diseases (120) and as little
as 6 inches of rain per year.
Livestock grazing or hay. If you need emergency
forage, sweetclover has a first-year feed value similar to alfalfa,
with greater volume of lesser quality in the second year.
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YELLOW
SWEETCLOVER
(Melilotus officinalis) |
MANAGEMENT
Establishment & Field Management
Sweetclover does well in the same soils as alfalfa. Loam soils with
near-neutral pH are best. Like alfalfa, it will not thrive on poorly
drained soils. For high yields, sweetclover needs P and K in the
medium to high range. Deficient sulfur may limit its growth (153).
Use an alfalfa /sweetclover inoculant.
In temperate areas of the Corn Belt, drill yellow sweetclover in
pure stands at 8 to 15 lb./A or broadcast 15 to 20 lb./A, using
the higher rate in dry or loose soils or if not incorporating.
In drier areas such as eastern North Dakota, trials of seeding
rates from2 to 20 lb./A showed that just 4 lb./A, broadcast or drilled,
created an adequate sole-crop stand for maximum yield. Recommended
rates in North Dakota are 4 to 6 lb./A drilled with small grains
at small-grain planting, 5 to 8 lb./A broadcast and harrowed (sometimes
in overseeding sunflowers), and 6 to 10 lb./A. broadcast without
incorporating tillage (183).
An excessively dense stand will create spindly stalks that don’t
branch or root to the degree that plants do in normal seedings.
Further, the plants will tend to lodge and lay over, increasing
the risk of diseases. So for maximum effect of subsoil penetration
or snow trapping, go with a lighter seeding rate.
Sweetclover produces 50 percent or more hard seed that can lie
in soil for 20 years without germinating. Commercial seed is scarified
to break this non-porous seedcoat and allow moisture to trigger
germination. If you use unscarified seed, check hardseed count on
the tag and do not count on more than 25 percent germination from
the hardseed portion. The need for scarification to produce an adequate
stand may be over-rated, however. The process had no effect on germination
in six years of field testing in North Dakota —even when planting
70 percent hard seed still in seed pods.
Seed at a depth of 1/4 to 1/2 inch in medium to heavy textured
soils, and 1/2 to 1.0 inch on sandy soils. Seeding too deeply is
a common cause of poor establishment.
Seed annual white sweetclover at 15 to 30 pounds per acre.
Expect 70 to 90 lb. N/A from 4,000 to 5,000 lb. dry matter/A on
well-drained, clay loam soils with neutral to alkaline pH.
A press-wheel drill with a grass seed attachment and a seed agitator
is suitable for planting sweetclover into a firm seedbed. If the
seedbed is too loose to allow the drill to regulate seeding depth,
run the seed spouts from the grass and legume boxes to drop seed
behind the doubledisk opener and in front of the press wheels. Light,
shallow harrowing can safely firm the seedbed and incorporate seed
(183).
In the Canadian Northern Plains, dribble the seed through drill
box hoses directly in front of the presswheels for quick and easy
establishment (32).
If your press-wheel drill has no legume box or grass-seed attachment,
you can mix the legume and small grain seed, but mix seed often
due to settling. Reduce competition between the crops by seeding
a part of the companion crop first, then seed a mix of the clover
seed and the balance of the grain seed at right angles (183).
Spring seeding provides yellow sweetclover ample time to develop
an extensive root system and store high levels of nutrients and
carbohydrates necessary for over-wintering and robust spring growth.
It grows slowly the first 60 days (153).
Where weeds would be controlled by mowing, no-till spring seeding
in small grain stubble works well.
Broadcast seeding for pure sweetclover stands works in higher rainfall
areas in early spring where soil moisture is adequate for seven
to 10 days after planting. No-till seeding works well in small grain
stubble.
Frostseeding into winter grains allows a harvest of at least one
crop during the life cycle of the sweetclover and helps control
weeds while the sweetclover establishes. Apply sweetclover seed
before rapid stem elongation of the grain. Cut grain rate about
one-third when planting the crops together.
Sweetclover spring seeded with oats exhibited poor regrowth after
oat harvest in two years of a Wisconsin study. To establish a sweetclover
cover crop in this way, the researchers found sweetclover did not
fare well in years when the combine head had to be run low to pick
up lodged oats. When oats remained upright (sacrificing some straw
for a higher cut), sweetclover grew adequately (402).
You can plow down spring-planted yellow sweetclover in late fall
of the planting year to cash in early on up to half its N contribution
and a bit less than half its biomass.
Plant biennial sweetclover through late summer where winters are
mild, north through Zone 6.Plant at least six weeks before frost
so roots can develop enough to avoid winter heaving. In the Northern
Plains into Canada, it should be planted by late August.
First-year management. Seeding year harvest or
clipping is usually discouraged, because the energy for first-year
regrowth comes directly from photosynthesis (provided by the few
remaining leaves), not root reserves (361,
402).
Top growth peaks in late summer as the plant’s main taproot
continues to grow and thicken. Second-year growth comes from crown
buds that form about an inch below the soil surface. Avoid mowing
or grazing of sweetclover in the six- to seven-week period prior
to frost when it is building final winter reserves. Root production
practically doubles between Oct.1 and freeze-up.
Sweetclover establishes well when sown with winter grains in fall,
but it can outgrow the grain in a wet season and complicate harvest.
Second-year management. After it breaks winter
dormancy, sweetclover adds explosive and vigorous growth. Stems
can reach 8 feet before flowering, but if left to mature, the stems
become woody and difficult to manage. Plants may grow extremely
tall in a “sweetclover year” with high rainfall and
moderate temperatures.
Nearly all growth the second year is topgrowth, and it seems to
come at the expense of root mass. From March to August in Ohio,
records show topgrowth increasing tenfold while root production
decreased by 75 percent (443).
All crown buds initiate growth in spring. If you want regrowth after
cutting, leave plenty of stem buds on 6 to 12 inches of stubble.
You increase the risk of killing the sweetclover plant by mowing
heavier stands, at shorter heights, and/or at later growth stages,
especially after bloom (183).
Before it breaks dormancy, sweetclover can withstand flooding for
about 10 days without significant stand loss. Once it starts growing,
however, flooding will kill the plants (183).
Killing
For best results ahead of a summer crop or fallow, kill sweetclover
in the second year after seeding when stalks are 6 to 10 inches
tall (183, 361).
It can be killed by mowing, cultivating or disking once it reaches
late bloom stage (32). Killing
sweetclover before bud stage has several benefits: 80 percent of
the potential N is present; N release is quick because the plant
is still quite vegetative with a high N percentage in young stalks
and roots; and moisture loss is halted without reducing N contribution.
Sweetclover may regrow from healthy crowns if incorporated before
the end of dormancy. For optimum full-season organic matter contribution,
mow prior to blossom stage whenever sweetclover reaches 12 to 24
inches high before final incorporation or termination (361).
Mowing or grazing at bloom can kill the plants.
In dryland areas, the optimum termination date for a green manure
varies with moisture conditions. In a spring wheat>fallow rotation
in Saskatchewan, sweetclover incorporated in mid-June of a dry year
provided 80 percent more N the following spring than it did when
incorporated in early July or mid-July— even though it yielded
up to a third less biomass at the June date. Mineralization from
sweetclover usually peaks about a year after it is killed. The potential
rate of N release decreases as plants mature and is affected by
soil moisture content (147).
In this study, the differences in N release were consistent in
years of normal precipitation, but were less pronounced. Little
N mineralization occurred in the incorporation year. Nitrogen addition
peaked in the following year, and has been shown to continue over
seven years following yellow sweetclover (147).
In northern spring wheat areas of North Dakota, yellow sweetclover
is usually terminated in early June just at the onset of bloom,
when it reaches 2 to 3 feet tall. This point is a compromise between
cover crop gain (in dry matter and N) and water consumption. A quick
kill from tillage or haying is more expensive and labor intensive
than chemical desiccation, but it stops moisture-robbing transpiration
more quickly (153).
Grazing is another way to manage second-year sweetclover before
incorporation. Start early in the season with a high stocking rate
of cattle to stay ahead of rapid growth. Bloat potential is slightly
less than with alfalfa (153).
Sweetclover: Good Grazing, Great Green
Manure
Bill Granzow taps biennial yellow sweetclover
to enhance soil tilth, control erosion and prevent subsoil
from becoming compacted. He uses common varieties, either
from the elevator or one his father originally bought from
a neighbor.
Granzow, of Herington, Kan., produces no-till
grain and runs cattle in an area midway between Wichita and
Manhattan in the east-central part of the state. Granzow overseeds
sweetclover into winter wheat in December or January at 12
to 15 lb./A using a rotary broadcaster mounted on his pickup.
Sometimes he asks the local grain cooperative to mix the seed
with his urea fertilizer for the wheat. There’s no extra
charge for seed application. Alternately, Granzow plants sweetclover
at the same rate with March seeded oats.
Yellow sweetclover has overgrown Granzow’s
wheat only when the wheat stand is thin and abnormally heavy
rains delay harvest. The minimal problem is even rarer in
oats, he says.
He uses yellow sweetclover with the companion
wheat crop in four possible ways, depending on what the field
needs or what other value he wants to maximize. For each,
he lets the clover grow untouched after wheat harvest for
the duration of the seeding year. He used to disk the sweetclover
at least twice to kill it. Now 100% no-till, he sprays with
Roundup and “a little bit of 2, 4-D.” Second-year
options include:
Grazing/green manure. Turn in livestock
when the clover reaches 4 inches tall, let them graze for
several weeks, spray to kill, then plant grain sorghum within
a couple of days. He feeds an anti-bloat medication to keep
cattle healthy on the lush legume forage.
Quick green manure. Spray after it has
grown 3 to 4 inches, then plant sorghum. This method contributes
about 60 pounds of N to the soil. He knocks back persistent
re-growing sweetclover crowns in the sorghum by adding 2,
4-D or Banvel to the postemerge herbicide mix.
Green manure/fallow. Kill at mid- to full
bloom, leave fallow over summer, then plant wheat again
in fall. This method provides about 120 lb. N/A, according
to estimates from Kansas State University.
Seed crop. He windrows the plants when
about 50 percent of the seedpods have turned black, then
runs the stalks through his combine. To remove all of the
hulls, he runs the seed through the combine a second or
third time.
Despite the heavy growth in the second year,
yellow sweetclover matures and dies back naturally. If the
residue is heavy, he sets the drill a bit deeper for planting.
He rates fall sweetclover hay from the seeding
year as “acceptable forage.” He’s aware
that moldy sweetclover hay contains coumarin, a compound that
can kill cattle, but he’s never encountered the problem.
Second-year yellow sweetclover makes silage at initial to
mid-bloom stage with 16 percent protein on a dry matter basis.
“Mixed with grass hay or other silage,
it makes an excellent feed,” he says, adding value to
its cover crop benefits and giving him farming flexibility.
Updated in 2007 by Andy Clark |
Pest Management
Sweetclover is a rather poor competitor in its establishment year,
making it difficult to establish pure sweetclover in a field with
significant weed pressure. Once established, it provides effective
weed control during the first fall and spring of fallow, whether
or not it is harvested for hay, incorporated or left on the soil
surface (33).
Sweetclover residue is said to be allelopathic against kochia,
Russia thistle, dandelion, perennial sowthistle, stinkweed and green
foxtail. Repeated mowing of yellow sweetclover that is then left
to mature is reported to have eradicated Canada thistle. Letting
sweetclover bloom and go to seed dries out soil throughout the profile,
depleting the root reserves of weeds.
Sweetclover weevil (Sitonia cylindricollis) is a major
pest in some areas, destroying stands by defoliating newly emerged
seedlings. Long rotations can reduce damage, an important factor
for organic farmers who depend on sweetclover fertility and soil
improvements. In the worst years of an apparent 12 to 15-year weevil
cycle in his area, “every sweetclover plant across the countryside
is destroyed,” according to organic farmer David Podoll, Fullerton,
N.D. “Then the weevil population crashes, followed by a few
years where they’re not a problem, then they begin to rebuild.”
Cultural practices have not helped change the cycle, but planting
early with a non-competitive nurse crop (flax or small grains) gives
sweetclover plants the best chance to survive weevil foraging, Podoll
says. Further research is needed to develop management techniques
to control the weevil.
In a three-year Michigan trial of crop rotations to decrease economic
losses to nematodes, a yellow sweetclover (YSC)>YSC>potato
sequence out-yielded other combinations of rye, corn, sorghum-sudangrass
and alfalfa. Two years of clover or alfalfa followed by potatoes
led to a yield response equivalent to application of a nematicide
for control of premature potato vine death (78).
Legume-supplied N coupled with an overall nutrient balance and enhanced
cation exchange capacity from the cover crop are thought to be involved
in suppressing nematode damage (271).
Crop Systems
In the moderately dry regions of the central and northern Great
Plains, “green fallow” systems with water-efficient
legumes can be substituted for bare-ground or stubble mulch fallow.
In fallow years, no cash crop is planted with the intent of recovering
soil moisture, breaking disease or weed cycles and maximizing yields
of following cash crops. The retained residue of “brown”
fallow lessens the erosion and evaporation of tillage intensive
“black fallow,” but “green fallow” offers
even more benefits in terms of soil biological life, biodiversity,
beneficial insect habitat, possible harvestable crops and alternate
forages.
Rapeseed (Brassica campestris) is a summer annual cash
crop in the dryland West that can serve as a nurse crop for sweetclover.
A Saskatchewan study of seeding rates showed optimum clover yield
came when sweetclover was sown at 9 lb./A and rapeseed was sown
at 4.5 lb./A. The mixture allows an adequate stand of sweetclover
that provides soil protection after the low-residue rapeseed (255).
Sole-cropped oilseed species (rapeseed, sunflower, crambe and safflower)
require herbicides for weed control. Many of these materials are
compatible with legumes, offering a post-emergent weed-control option
if the covers do not adequately suppress weeds. The covers greatly
reduce the erosion potential after oilseed crops, which leave little
residue over winter (153).
Interplanting works with tall crops. A Wisconsin
researcher reported success drilling sweetclover between the rows
when corn was 6 to 12 inches tall. Overseeding sweetclover into
sweet corn works even better due to greater light penetration.
Soil water availability at cover crop planting and depletion during
growth are always a concern in semi-arid regions. The potential
benefits must be balanced against possible negative effects on the
cash crop.
Sweetclover overseeded into sunflowers at last cultivation succeed
about half the time, North Dakota trials show. Dry conditions or
poor seed to-soil contact were the main reasons for not getting
a stand. A heavier seeding rate or earlier planting will tend to
increase stand. Band-seeding sweetclover over the row with an insecticide
box at sunflower planting proved more successful in the trial. The
method also permits between-row cultivation (153).
Even though legume green manures in another North Dakota study
used about 2.8 inches (rainfall equivalent) more water than fallow,
they led to a 1-inch equivalent increase over fallow in soil water
content in the top 3 inches of soil the following spring (14).
One green fallow option is planting yellow sweetclover with spring
barley or spring peas. This is challenging, however, because barley
can be overly competitive while herbicide compatibility is a concern
with the peas.
Further north into the Canadian Great Plains, sweetclover depleted
soil moisture by September of year 1, but by May of year 2, soil
moisture was greater due to snow trapping, increased infiltration
and reduced evaporation (32).
Fred Kirschenmann of Windsor, N.D., controls spring weed flushes
on his fallow after sunflowers with an initial shallow chisel plowing
then a rod weeder pass or two before planting sweetclover with a
nurse crop of buckwheat or oats (or millet, if there is less soil
moisture). He harvests buckwheat, hoping for a 900 lb./A yield,
then lets the clover grow and overwinter. In early summer, when
he begins to see yellow blossoms, he disks the cover, lets it dry,
then runs a wide-blade sweep plow just below the surface to cut
apart the crowns. The biomass contribution of the sweetclover fallow
builds up organic matter, he says, in contrast to the black-fallow
route of burning up organic matter to release N. Preventing humus
depletion holds back the dreaded kochia weed.
In temperate areas you can overseed spring broccoli with HUBAM
annual sweetclover, let the cover grow during summer, then till
it in before planting a fall crop. Alternatively, you can allow
it to winterkill for a thick, lasting mulch.
In Pennsylvania, Eric and Anne Nordell seed sweetclover after early
vegetables (in June or July) and allow it to grow throughout the
summer. It puts down a deep taproot before winter, fixes nitrogen
and may bring nutrients to the soil surface from deep in the soil
profile. See Full-Year
Covers Tackle Tough Weeds.
Other Options
First-year forage has the same palatability and feeding value as
alfalfa, although harvest can reduce second-year vigor. Second-year
forage is of lower quality and becomes less palatable as plants
mature, but may total 2 to 3 tons per acre (120).
Growers report seed yield of 200 to 400 lb./A in North Dakota .Minimize
shattering of seedpods by swathing sweetclover when 30 to 60 percent
of its pods are brown or black. Pollinating insects are required
for good seed yield (183).
Hard seed that escapes harvest will remain in the soil seed bank,
but organic farmer Rich Mazour of Deweese, Neb., sees that as a
plus. A 20- to 30-percent stand in his native grass pastures comes
on early each spring, giving his cattle early grazing. Once warm-season
grasses start to grow, they keep the clover in check. In tilled
fields, sweep cultivators and residue-management tillage implements
take care of sweetclovers with other tap-rooted “resident
vegetation,” Mazour says.
COMPARATIVE NOTES
Sweetclover and other deep-rooted biennial and perennial legumes
are not suited for the most severely drought-prone soils, as their
excessive soil moisture use will depress yield of subsequent wheat
crops for years to come (163).
When planting sweetclover after wheat harvest, weeds can become
a problem. An organic farmer in northeastern Kansas reports that
to kill cocklebur, he has to mow lower than the sweetclover can
tolerate. Annual alfalfa can tolerate low mowing (205).
After 90 days’ growth in a North Dakota dryland legume comparison,
a June planting of yellow sweetclover produced dry matter and N
comparable to alfalfa and lespedeza (Lespedeza stipulacea Maxim).
Subclover, fava beans (Vicia faba) and field peas had the
best overall N-fixing efficiency in the dryland setting because
of quick early season growth and good water use efficiency (331).
Cultivars. Yellow cultivars include MADRID, which
is noted for its good vigor and production, and its relative resistance
to fall freezes. GOLDTOP has excellent seedling vigor, matures two
weeks later, provides larger yields of higher quality forage and
has a larger seed than MADRID (361).
Yellow common and YUKON joined GOLDTOP and MADRID—all high-coumarin
types—as the highest yielding cultivars in a six-year North
Dakota test (269).
Leading white biennial cultivars are DENTA, POLARA and ARCTIC.
POLARA and ARCTIC are adapted to very cold winters. Best for grazing
are the lower-producing, low-coumarin cultivars DENTA and POLARA
(white) and NORGOLD (yellow).
Seed sources. See Seed
Suppliers.
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