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SORGHUM-SUDANGRASS HYBRIDS
Sorghum bicolor x S. bicolor var. sudanese
Also called: Sudex, Sudax
Type: summer annual grass
Roles: soil builder, weed and nematode suppressor,
subsoil loosener
Mix with: buckwheat, sesbania, sunnhemp, forage soybeans
or cowpeas
See charts, pp. 66 to 72, for ranking and management summary.
Sorghum-sudangrass hybrids are unrivaled for adding organic matter
to worn-out soils. These tall, fast-growing, heat-loving summer annual
grasses can smother weeds, suppress some nematode species and penetrate
compacted subsoil if mowed once. Seed cost is modest. Followed by
a legume cover crop, sorghum-sudangrass hybrids are a top choice for
renovating overfarmed or compacted fields.
The hybrids are crosses between forage-type sorghums and sundangrass.
Compared with corn, they have less leaf area, more secondary roots
and a waxier leaf surface, traits that help them withstand drought
(361). Like corn, they require
good fertility—and usually supplemental nitrogen—for
best growth. Compared with sudangrass, these hybrids are taller,
coarser and more productive.
Forage-type sorghum plants are larger, leafier and mature
later than grain sorghum plants. Compared with sorghum-sudangrass
hybrids, they are shorter, less drought tolerant, and don’t
regrow as well. Still, forage sorghums as well as most forms of
sudangrass can be used in the same cover-cropping roles as sorghum-sudangrass
hybrids. All sorghum- and sudangrass-related species produce compounds
that inhibit certain plants and nematodes. They are not frost tolerant,
and should be planted after the soil warms in spring or in summer
at least six weeks before first frost.
BENEFITS
Biomass producer. Sorghum-sudangrass grows 5 to
12 feet tall with long, slender leaves, stalks up to one-half inch
in diameter and aggressive root systems. These features combine
to produce ample biomass, usually about 4,000 to 5,000 lb. DM/A.
Up to 18,000 lb. DM/A has been measured with multiple cuttings on
fertile soils with adequate moisture.
Subsoil aerator. Mowing whenever stalks reach 3 to 4 feet
tall increases root mass five to eight times compared with unmowed
stalks, and forces the roots to penetrate deeper.
In addition, tops grow back green and vegetative until frost and
tillering creates up to six new, thicker stalks per plant. A single
mowing on New York muck soils caused roots to burrow 10 to 16 inches
deep compared to 6 to 8 inches deep for unmowed plants. The roots
of mowed plants fractured subsoil compaction with wormhole-like
openings that improved surface drainage. However, four mowings at
shorter heights caused plants to behave more like a grass and significantly
decreased the mass, depth and diameter of roots (277,
450, 451).
Weed suppressor. When sown at higher rates than
normally used for forage crops, sorghum-sudangrass hybrids make
an effective smother crop. Their seedlings, shoots, leaves and roots
secrete allelopathic compounds that suppress many weeds. The main
root exudate, sorgoleone, is strongly active at extremely low concentrations,
comparable to those of some synthetic herbicides (370).
As early as five days after germination, roots begin secreting this
allelochemical, which persists for weeks and has visible effects
on lettuce seedlings even at 10 parts per million (440).
Sorghum-sudangrass hybrids suppress such annual weeds as velvetleaf,
large crabgrass, barnyardgrass (126,
305), green foxtail, smooth
pigweed (190), common ragweed,
redroot pigweed and purslane (316).
They also suppressed pine (214)
and redbud tree seedlings in nursery tests (154).
The residual weed-killing effects of these allelochemicals increased
when sorghum-sudangrass hybrids were treated with the herbicides
sethoxydim, glyphosate or paraquat, in descending order of magnitude
(144).
Nematode and disease fighter. Planting sorghum-sudangrass
hybrids instead of a host crop is a great way to disrupt the life
cycles of many diseases, nematodes and other pests. For example,
when sorghum-sudangrass or sorghum alone were no-tilled into endophyte-infected
fescue pastures in Missouri that had received two herbicide applications,
the disease was controlled nearly 100 percent. No-till reseeding
with endophyte- free fescue completed this cost-effective renovation
that significantly improved the rate of gain of yearling steers
(16).
Renews farmed-out soils. The combination of abundant
biomass production, subsoiling root systems, and weed and nematode
suppression can produce dramatic results.
On a low-producing muck field in New York where onion yields had
fallen to less than a third of the local average, a single year
of a dense planting of sorghum-sudangrass hybrid restored the soil
to a condition close to that of newly cleared land (217).
Widely adapted. Sorghum-sudangrass hybrids can
be grown throughout the U.S. wherever rainfall is adequate and soil
temperature reaches 65° F to 70° F at least two months before
frost. Once established, they can withstand drought by going nearly
dormant. Sorghum-sudangrass hybrids tolerate pH as high as 9.0,
and are often used in rotation with barley to reclaim alkaline soil
(421). They tolerate pH as
low as 5.0.
Quick forage. Sorghum-sudangrass is prized as
summer forage. It can provide quick cover to prevent weeds or erosion
where legume forages have been winterkilled or flooded out. Use
care because these hybrids and other sorghums can produce prussic
acid poisoning in livestock. Grazing poses the most risk to livestock
when plants are young (up to 24 inches tall), drought stressed or
killed by frost. Toxicity danger varies between cultivars.
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SORGHUM-SUDANGRASS
(Sorghum bicolor X S. bicolor var. sudanese |
MANAGEMENT
Establishment
Plant sorghum-sudangrass when soils are warm and moist,
usually at least two weeks after the prime corn-planting date for
your area. It will tolerate low-fertility, moderate acidity and
high alkalinity, but prefers good fertility and near-neutral pH
(361). Standard biomass production
usually requires 75 to 100 lb. N/A.
With sufficient surface moisture, broadcast 40 to 50 lb./A, or
drill 35 to 40 lb./A as deep as 2 inches to reach moist soil. These
rates provide a quicker canopy to smother weeds than lower rates
used for forage production, but they require mowing or grazing to
prevent lodging. Herbicide treatment or a pass with a mechanical
weeder may be necessary if germination is spotty or perennial weeds
are a problem. New York onfarm tests show that a stale seedbed method—
tilling, then retilling to kill the first flush of weeds just before
planting—provides effective weed control.
Warm season mixtures. Plant sorghum-sudangrass
in cover crop mixtures with buckwheat or with the legumes sesbania
(Sesbania exaltata), sunnhemp (Crotolaria juncea),
forage soybeans (Glycine max) or cowpeas (Vigna unguiculata).
Broadcast these large-seeded cover crops with the sorghum-sudangrass,
then incorporate about 1 inch deep. Fast-germinating buckwheat helps
suppress early weeds. Sorghum-sudangrass supports the sprawling
sesbania, forage soybeans and cowpeas. Sunnhemp has an upright habit,
but could compete well for light if matched with a sorghum-sudangrass
cultivar of a similar height.
Field Management
Plants grow very tall (up to 12 feet), produce tons of
dry matter and become woody as they mature. This can result in an
unmanageable amount of tough residue that interferes with early
planting the following spring (277).
Mowing or grazing when stalks are 3 to 4 feet tall encourages tillering
and deeper root growth, and keeps regrowth vegetative and less fibrous
until frost. For mid-summer cuttings, leave at least 6 inches of
stubble to ensure good regrowth and continued weed suppression.
Delayed planting within seven weeks of frost makes mowing unnecessary
and still allows for good growth before winterkilling (277,
361).
Disking while plants are still vegetative will speed decomposition.
Make several passes with a heavy disk or combination tillage tool
to handle the dense root masses (277).
Sicklebar mowing or flail chopping before tillage will reduce the
number of field operations required to incorporate the crop and
speed decomposition. Sicklebar mowers cut more cleanly but leave
the stalks whole. Using a front-mounted flail chopper avoids the
problem of skips where tractor tires flatten the plants, putting
them out of reach of a rear-mounted chopper.
Any operations that decrease the residue size shortens the period
during which the decomposing residue will tie up soil nitrogen and
hinder early planted crops in spring. Even when mowed, residue will
become tough and slower to break down if left on the surface.
Flail chopping after frost or killing the cover crop with herbicide
will create a suitable mulch for no-till planting, preserving soil
life and soil structure in non-compacted fields.
Pest Management
Weeds. Use sorghum-sudangrass to help control
nutsedge infestations, suggests Cornell Extension IPM vegetable
specialist John Mishanec. Allow the nutsedge to grow until it’s
about 4 to 5 inches tall but before nutlets form, about mid-June
in New York. Kill the nutsedge with herbicide, then plant the weed-smothering
hybrid.
To extend weed suppressive effects into the second season, select
a cultivar known for weed suppression and leave roots undisturbed
when the stalks are mowed or grazed (440).
Beneficial habitat. Some related sorghum cultivars
harbor beneficial insects such as seven-spotted lady beetles (Coccinella
septempunctata) and lacewings (Chrysopa carnea) (421).
Nematodes. Sorghum-sudangrass hybrids and other
sorghum-related crops and cultivars suppress some species of nematodes.
Specific cultivars vary in their effectiveness on different races
of nematodes. These high-biomass-producing crops have a general
suppressive effect due to their organic matter contributions. But
they also produce natural nematicidal compounds that chemically
suppress some nematodes, many studies show.
Timing of cutting and tillage is very important to the effectiveness
of nematode suppression. The cover crop needs to be tilled before
frost while it is still green. Otherwise, the nematicidal effect
is lost. For maximum suppression of soilborne diseases, cut or chopped
sudangrass must be well incorporated immediately (308).
For suppressing root-knot nematodes in Idaho potato fields, rapeseed
has proven slightly more effective and more dependable than sorghum-sudangrass
hybrids (394).
In an Oregon potato trial, TRUDAN 8 sudangrass controlled Columbia
root-knot nematodes (Meloidogoyne Chitwoodi), a serious
pest of many vegetable crops. Control extended throughout the zone
of residue incorporation. The cover crop’s effect prevented
upward migration of the nematodes into the zone for six weeks, working
as well as the nematicide ethoprop. Both treatments allowed infection
later in the season (285).
In the study, TRUDAN 8 sudangrass and the sorghum-sudangrass hybrid
cultivars SORDAN 79 and SS-222 all reduced populations of root-knot
nematodes. These cultivars are poor nematode hosts and their leaves—not
roots—have a nematicidal effect. TRUDAN 8 should be used if
the crop will be grazed due to its lower potential for prussic acid
poisoning. The sorghum-sudangrass cultivars are useful if the cover
crop is intended for anti-nematicidal effects only (285).
In other Oregon and Washington trials, the cover crop suppression
required supplemental chemical nematicide to produce profitable
levels of U.S. No. 1 potatoes (285).
These same sudangrass and sorghum-sudangrass hybrid cultivars failed
to show any significant nematicidal effects in a later experiment
in Wisconsin potato fields (249).
When faced with infestations of the nematodes Meloidogoyne
incognita and M. arenaria, Oswego, N.Y., onion grower
Dan Dunsmoor found that a well-incorporated sorghum-sudangrass cover
crop was more effective than fumigation. Further, the nematicidal
effect continued into the next season, while the conditions a year
after fumigation seemed worse than before the application. He reports
that the sorghum-sudangrass cover crop also controls onion maggot,
thrips and Botrytis leaf blight (217).
Insect pests. Chinch bug (Blissus leucopterus),
sorghum midge (Contarinia sorghicola), corn leaf aphid
(Rhopalosiphum maidis), corn earworm (Heliothis zea),
greenbugs (Schizaphis graminum) and sorghum webworm (Celama
sorghiella) sometimes attack sorghum-sudangrass hybrids. Early
planting helps control the first two pests, and may reduce damage
from webworms. Some cultivars and hybrids are resistant to chinch
bugs and some biotypes of greenbugs (361).
In Georgia, some hybrids hosted corn leaf aphid, greenbug, southern
green stinkbugs (Nezara viridula) and leaffooted bug (Leptoglossus
phyllopus).
Summer Covers Relieve Compaction
A summer planting of sudangrass was the best
single-season cover crop for relieving soil compaction in
vegetable fields, a team of Cornell researchers found. Yellow
mustard, HUBAM annual white sweetclover and perennial ryegrass
also were effective to some extent in the multi-year study.
But sudangrass has proven the most promising so far,”
says project coordinator David Wolfe. It has shown the fastest
root growth.”
“Sudangrass is best managed with one mowing
during the season,” Wolfe adds. Mowing promotes tillering
and a deep, penetrating root system. Mowing also makes it
easier to incorporate the large amount of biomass produced
by this crop. With its high C:N ratio, it adds to soil organic
matter.
Farmers and researchers have long known that
alfalfa’s deep root system is a great compaction-buster.
But alfalfa does not establish easily on wet compacted fields,
and most vegetable growers can’t afford to remove land
from production for two to three years to grow it, notes Wolfe.
Many also lack the equipment to subsoil their fields, which
is often only a temporary solution, at best. That ’s
why Wolfe geared his study to identify cover crops that can
produce results in a single season. In the case of heat-loving
sudangrass, it also may be possible to squeeze a spring or
fall cash crop into the rotation while still growing the cover
during summer.
Heavy equipment, frequent tillage and lack of
organic matter contribute to compaction problems for vegetable
growers in the Northeast, where frequent rains often force
growers into the fields when soils are wet. Compacted soils
slow root development, hinder nutrient uptake, stunt plants,
delay maturity and can worsen pest and disease problems (451).
For example, the Cornell researchers found that slow-growing
cabbages direct-seeded into compacted soils were vulnerable
to flea beetle infestations (450).
Brassica cover crops such as yellow mustard
were solid challengers to sudangrass as a compaction reliever,
but it was sometimes difficult to establish these crops in
the test. We still have a lot to learn about how best to grow
brassicas and fit them into rotations with vegetables,”
Wolfe says.
Wolfe and his team assessed the cover crops’
effectiveness by measuring yields of subsequent crops and
conducting a host of soil quality measurements, including
infiltration rates, water-holding capacity, aggregate stability
and organic matter levels.
For more information, contact David Wolfe, 607-255-7888;
dww5@cornell.edu.
Updated in 2007 by Andy Clark |
Crop Systems
There are several strategies for reducing nitrogen tie-up from residue:
 Interplant
a legume with the sorghum-sudangrass hybrid.
Plant
a legume cover crop after the sorghum-sudangrass hybrid, in either
late summer or the following spring.
Apply
nitrogen fertilizer or some other N source at incorporation and
leave the land fallow for a few months when soil is not frozen
to allow decomposition of the residue.
If you kill the cover crop early enough in fall, the residue will
partially break down before cold temperatures slow biological action
(361). Where possible, use
sorghum-sudangrass ahead of later planted crops to allow more time
in spring for residue to decompose.
Planting sorghum-sudangrass every third year on New York potato
and onion farms will rejuvenate soil, suppress weeds and may suppress
soil pathogens and nematodes. Working a legume into the rotation
will further build soil health and add nitrogen. Sorghum-sudangrass
hybrids can provide needed soil structure benefits wherever intensive
systems cause compaction and loss of soil organic matter reserves.
See Summer Covers Relieve Compaction.
Grown as a summer cover crop that is cut once and then suppressed
or killed, sorghum-sudangrass can reduce weeds in fall-planted alfalfa.
Sorghum-sudangrass suppressed alfalfa root growth significantly
in a Virginia greenhouse study (144),
but no effect was observed on alfalfa germination when alfalfa was
no-till planted into killed or living sorghum-sudangrass (145).
In Colorado, sorghum-sudangrass increased irrigated potato tuber
quality and total marketable yield compared. It also increased nutrient
uptake efficiency on the sandy, high pH soils. In this system, the
sorghum-sudangrass is grown with limited irrigation, but with enough
water so that the biomass could be harvested for hay or incorporated
as green manure (112, 113).
In California, some table grape growers use sorghum-sudangrass
to add organic matter and to reduce the reflection of light and
heat from the soil, reducing sunburn to the grapes.
COMPARATIVE NOTES
Sorghum-sudangrass hybrids can produce more organic matter per
acre, and at a lower seed cost, than any major cover crop grown
in the U.S. Incorporated sorghum-sudangrass residue reduces N availability
to young crops more than oat residue but less than wheat residue
(389).
Cultivars. When comparing sorghum-sudangrass cultivars,
consider traits such as biomass yield potential, tillering and regrowth
ability, disease resistance, insect resistance (especially if greenbugs
are a problem) and tolerance to iron deficiency chlorosis.
If you plan to graze the cover crop, select sorghum-sudangrass
hybrids and related crops with lower levels of dhurrin, the compound
responsible for prussic acid poisoning. For maximum weed control,
choose types high in sorgoleone, the root exudate that suppresses
weeds. Sterile cultivars are best where escapes could be a problem,
especially where crossing with johnsongrass (Sorghum halpense)
is possible.
Seed sources. See Seed
Suppliers.
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