SORGHUM (Sorghum bicolor), SORGHUM-SUDANGRASS (S.
bicolor x S. sudanense), SORGHUM x SUDANGRASS, SUDAX, SUDEX
K. Dover, K.-H. Wang & R. McSorley
Last updated January, 2004
Contents:
Problems
and Warnings of Growing Sorghum-sudangrass
Sorghum (Sorghum bicolor (L.) Moench) and
sorghum-sudangrass (S. bicolor x S. sudanense (Piper) Stapf) are
often used as cover crops.
Sorghum-sudangrass (S. bicolor x S. sudanense),
also known as sorghum x sudangrass, sudax, or sudex, is a hybrid between
sorghum (S. bicolor) and sudangrass (S. sudanense). This cover crop has many advantages over
either one of its parents.
Fig. 1 Sorghum bicolor (picture is courtesy
of K.-H. Wang)
Sorghum, currently
classified as S. bicolor (Fig. 1), was formerly known as S. vulgare
Pers. (Purseglove, 1975). For
centuries, sorghum has been used as a grain and forage source. In the United States, sorghum is largely
used as forage. However, the grain is
used as food in many other countries (Magness et al., 1971). This grass can also be used as a source of
sugar, syrup (produced from sweet sorghum types), fiber and grain (Duke,
1983). In addition, various parts of
the plant are reported to have medicinal properties such as being a demulcent,
antiabortive, and diuretic, or it can be an intoxicant or emollient. It has been used in folk medicine against
cancer, epilepsy, and stomachache (Duke and Wain, 1981). However, it is also cyanogenic which means
that it can also be a poison.
The earliest cultivation
of sorghum was in ancient Egypt. Most
of the sorghum grown in the United States is of African origin and is most
popular in the central and southern Midwest, where its excellent drought
tolerance allowed it to flourish in the dry climate (US Grains Council,
2003).
Sorghum can be harvested
either as a grain or forage. The former is of more commercial importance in the
U.S. Approximately 7.3 million acres of
grain sorghum were harvested in 2002 (USDA-NASS, 2003). Grain sorghum (less than 6 feet tall) is
usually shorter than forage sorghum (6-15 feet), and may be planted for
food-used grain or silage, whereas forage sorghum is planted for silage only
(Southern States Cooperative, Inc., 2003).
Forage sorghum matures later in the year than grain sorghum (Sustainable
Agriculture Network, 2003). Although
forage sorghum produced more biomass than sudangrass, it is usually only
harvested once (The Small Farm Resource, 2002).
There are several
varieties of grain sorghum grown throughout the world, and each is put into one
of seven agronomic groups (Table 1).
Table 1. Seven agronomic groups of grain sorghum
(Magness et al., 1971).
|
Agronomic
Group |
Site
of Origin |
Characteristics |
|
Kafir |
South Africa |
Thick stems, large leaves; seeds are medium in size
and may be white, pink, or red; panicles are awnless and cylindrical |
|
Milo |
East Africa |
Wavy leaf blades with yellowish midrib; seeds
are large and may be pink or cream-colored; seed is bearded or awned; more
tolerant than Kafir to drought and heat |
|
Feterita |
Sudan |
Few leaves; stems are thin; seeds are large and
white |
|
Durra |
Mediterranean Area, Middle East, or Near East |
Panicles bearded or hairy, may be closed or
open; seeds are large and flattened |
|
Sballu |
India |
Tall, thin stems; seeds are white, requires a
long growing season because of their maturity |
|
Koaliang |
China, Manchuria, Japan |
Woody stems with few leaves; seeds are brown and
taste bitter |
|
Hegari |
Sudan |
Similar to Kafir, but panicles are more oval and
plants tiller profusely; seeds are chalky white |
Adapted from
Magness et al., 1971.
Sudangrass (Sorghum
sudanense, formerly classified as S. vulgare var. sudanense),
like sorghum, is a summer annual and has no winter hardiness. It has high palatability, and does not
produce toxic compounds that threaten livestock and horses (Mojtahedi et al.,
1993). Some varieties, most notably
‘Trudan 8’, also have nematicidal properties. Therefore, sudangrass can be
planted for forage and nematode management.
Sudangrass usually grows 3-8 feet high and has stems about ¼” in
diameter. It will regrow following each
harvest until cool temperature or lack of moisture (The Small Farm Resource,
2002).
Fig. 2. Sorghum-sudangrass
(S. bicolor x S. sudanense) (picture is
courtesy of R. McSorley).
Sorghum-sudangrass (Fig.
2), also commonly called sudax, sudex, or sorghum x sudangrass (Sudax® is
registered by DeKalb Genetics Corporation, De Kalb, IL) is advantageous over
either parent in that it produces larger quantities of biomass. It resembles sudangrass but is taller and
has larger stems and leaves. Like
sudangrass, the hybrid will regrow after each harvest unless restricted by
environmental conditions (The Small Farm Resource, 2002). Sorghum-sudangrass roots deeply, and may
even help to aerate compacted subsoils (Valenzuela and Smith, 2002; Sustainable
Agriculture Network, 2003). This hybrid
is able to grow in soil with a pH range of 5.5-8.3 and is sometimes used to
reclaim alkaline soil (Valenzuela and Smith, 2002). It is very drought tolerant, has high seedling vigor, and some
varieties have reduced lignin content (such as the brown midrib sorghum x
sudangrass crosses) to increase digestibility for animals and decomposition
rate (Southern States Cooperative, Inc., 2003). However, certain varieties of sorghum-sudangrass have some
disadvantages. For example, brown
midrib varieties have been found to be “environmentally sensitive,” and may
limit their growth in cooler and shorter growing seasons (Casler et al.,
2003). Some sorghum-sudangrass
varieties can be toxic to livestock and horses (Mojtahedi et al., 1993).
Due to its rapid biomass production,
sorghum-sudangrass is recommended as a cover crop to build up organic matter
content in soil. Research in Hawai’i
has determined that dry matter production can be as much as 8,000-10,000
lbs/acre/year. In addition, it generally
does not support populations of some key nematode pests, such as root-knot nematodes
(Halcomb, 2002). In addition,
sorghum-sudangrass is found to have other pest management properties listed
below.
Certain
sorghum-sudangrass varieties are not only poor hosts for root-knot nematodes,
but may also be disease and (insect) pest resistant. According to Monsanto (2003), four of their varieties are
resistant to downy mildew, one to anthracnose, two to maize dwarf mosaic virus,
one to head smut, and nine to greenbug (an aphid, Schizaphis graminum).
Sorghum or some other
crops (including barley, rye, buckwheat, sudangrass, sweet clover, and
sunflower) can be planted to suppress weed growth (Rice, 1984). These crops have traditionally been called
“smother crops” because of their ability to suppress weed growth. The weed-suppressive properties of sorghum
are attributed to competition and its vigorous growth habit. However, some believe that a combination of
competition and allelopathic effects of toxins and other inhibitory substances
produced by sorghum is a better explanation for this weed-smothering effect
(Overland, 1966).
Sorgoleone, a quinone,
is the primary allelopathic chemical found in exudates from sorghum-sudangrass
roots (Almeida Barbosa et al., 2001; Nimbal et al., 1996; Sustainable Agriculture
Network, 2003). This compound is
inhibitory to weeds such as barnyardgrass (Echinocloa crus-galli), large
crabgrass (Digitaria sanguinalis), and velvetleaf (Abutilon
theophrasti) (Nimbal et al., 1996).
In addition, five
phenolic compounds have been identified and quantified in mature plant residues
of sorghum. These compounds are p-coumaric,
syringic, vanillic, ferulic, and p-hydroxybenzoic acids, with p-coumaric
present in the highest amounts (Guenzi and McCalla, 1966). Guenzi and McCalla (1966) noted that since
these acids are mostly bound in the residues, decomposition of the sorghum
residues is necessary to release sufficient amounts of p-hydroxybenzoic
acid to achieve weed suppression. On
the other hand, Fries et al. (1997) found
that soil amendments of p-coumaric and p-hydrobenzoic acid may
even encourage populations of mycorrhizae (a beneficial root-associated fungus)
and enhance crop growth.
A cultivar of sudangrass (‘Trudan 8’) has been shown to suppress the northern
root-knot nematode Meloidogyne hapla infestation in vegetables (Widmer
and Abawi, 2002; Rehiayani and Hafez, 1998).
Sudangrass contains a compound in the cell cytoplasm call dhurrin, a
cyanoglucoside.
As ‘Trudan 8’ decomposes, an enzyme degrades the dhurrin and releases
hydrogen cyanide (Adewusi, 1990). Other
products of this degradation, such as nitriles or isothiocyanaates, may also
have nematicidal properties (Donkin et al., 1995). It is noted that soil
amended with all parts of sudangrass resulted in lower reproduction of M.
hapla on lettuce than soil amended with only roots of sudangrass (Viaene
and Abawi, 1998). In a greenhouse
study, soil incorporated with leaves of sudangrass reduced root gall on lettuce
(up to 54%) (Widmer and Abawi, 2003). There was also a correlation between
amount of free cyanide in the soil and root gall reduction. However, in the same experiment,
incorporation of white clover and flax reduced galling by 45%, and 53% respectively
(Widmer and Abawi, 2002).
In a greenhouse tests, sorghum-sudangrass ‘SX-17’ did not support
reproduction of Meloidogyne incognita (races 1 and 3), M. arenaria (race
1) or M. javanica (McSorley et al., 1994a). No egg masses were found on ‘SX-17’ in any of the tests. Another study by McSorley et al. (1994b)
suggests that sorghum-sudangrass (here, ‘SX-17’) could be a beneficial crop for
use in a rotation for the control of nematodes
(Meloidogyne spp.) and yield improvement of subsequent vegetable crops.
In an Oregon potato trial in which sudangrass and sorghum-sudangrass
residues were incorporated into the soil, the sudangrass cultivar ‘Trudan 8’
and sorghum-sudangrass hybrids ‘Sordan 79’ and ‘SS-222’ reduced populations of Meloidogyne
spp. These are poor root-knot nematode
hosts and chemicals present in the leaves were found to be nematicidal
(Mojtahedi et al., 1993).
Another experiment compared nematode populations in several varieties of
corn, a variety of sorghum (‘FS25E’), and a sorghum x sudangrass hybrid
(‘SX-17’). Both ‘FS25E’ and ‘SX-17’
maintained low populations of Meloidogyne incognita, whereas corn (which
is more susceptible) supported higher populations of the root-knot
nematode. However, no variety
significantly reduced populations of Paratrichodorus minor (stubby root
nematode), Pratylenchus scribneri (lesion nematode), or Criconemella
spp. (ring nematode) (McSorley and Gallaher, 1991).
It has been noted, however, that sudangrass can harbor large populations
of a lesion nematode (Pratylenchus penetrans) (Marks and Townshend,
1973). A more recent study conducted by
Thies et al. (1995) found that although forage sorghum, sudangrass and
sorghum-sudangrass are all hosts for P. penetrans, these crops are less
suitable hosts than are other forage crop species such as white clover, oat,
and rye. Sting nematode (Belonolaimus
spp.) populations were found to be high enough to reduce yields in
cool-season vegetables when planted after sorghum-sudangrass (Rhoades, 1980),
and populations of Meloidogyne arenaria (root-knot nematode) juveniles
in a sorghum-soybean rotation were elevated above numbers found in either a
soybean monoculture or a corn-soybean rotation (Rodriguez-Kabana et al.,
1991). Also, Paratrichodorus minor
(stubby-root nematode) populations built up when sorghum-sudangrass was used in
a rotation (McSorley et al., 1994b).
The nematode species persist in the site and nematode susceptibility of
the subsequent crop(s) should be evaluated before deciding on a sorghum or sorghum-sudangrass
cover crop.
Varieties of sorghum, sudangrass or sorghum-sudangrass suppressive to
nematodes are summarized in Table 2, whereas varieties that enhanced nematodes
are shown in Table 3. Table 4 showed
nematodes considered to be key pests of sorghum, sudangrass or
sorghum-sudangrass in several countries (Sharma and McDonald, 1990).
Table 2.
Nematode-suppressive varieties of cover crops.
|
Plant
and Variety |
Nematode
Suppressed |
Method
of Suppression |
Reference |
|
Sudangrass |
|
|
|
|
‘Trudan 8’ |
Meloidogyne hapla |
Incorporation of leaves into the soil reduced galling by 54% |
Widmer and Abawi, 2002 |
|
|
Meloidogyne chitwoodi |
Poor reproduction in roots |
Mojtahedi et al., 1993 |
|
‘Trudex 9’ |
M. chitwoodi |
Poor reproduction in roots |
Motjahedi et al., 1993 |
|
‘Piper’ |
M. chitwoodi |
Poor reproduction in roots |
Motjahedi et al., 1993 |
|
‘332’ |
M. chitwoodi |
Poor reproduction in roots |
Motjahedi et al., 1993 |
|
Sorghum-sudangrass |
|
|
|
|
‘SX-17’ |
M. incognita (Races 1 and 3), M. javanica, M. arenaria
(Race 1) |
No reproduction occurred in roots; reduction in galling |
McSorley et al., 1994a |
|
|
Meloidogyne spp. |
No reproduction occurred in roots; reduction in galling |
McSorley et al., 1994b |
|
|
Meloidogyne incognita |
Reduced populations, although reproduction did occur |
McSorley and Gallaher, 1991 |
|
‘ST6E’ |
Meloidogyne javanica |
Poor or non-host |
Sipes and Arakaki, 1997 |
|
‘Sordan 79’ |
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
|
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
‘SS-222’ |
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
|
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
‘Bravo II’ |
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
|
M. chitwoodi |
Poor reproduction in the roots |
Mojtahedi et al., 1993 |
|
Sorghum |
|
|
|
|
(No specific variety) |
Heterodera glycines |
Poor host |
Rodriguez-Kabana et al., 1991 |
|
Forage sorghum |
|
|
|
|
‘FS25E’ |
Meloidogyne incognita |
Poor host; Reduced populations, although reproduction did occur |
McSorley and Gallaher, 1991; Gallaher et al., 1991 |
|
‘BR64’ |
Meloidogyne incognita |
Poor host |
Gallaher et al., 1991 |
Table 3.
Nematode-enhancing varieties of cover crops.
|
Cover
Crop |
Nematode
Enhanced |
Method
of Enhancement |
Reference |
|
Sudangrass |
|
|
|
|
(No specific variety) |
Pratylenchus penetrans |
Good host (although less suitable as a host than other forage,
including white clover, oat and rye) |
Marks and Townshend, 1973 |
|
(No specific variety) |
Meloidogyne incognita |
Supports reproduction |
Johnson et al., 1977 |
|
‘Piper’ |
Meloidogyne chitwoodi |
Good host; reproduction occurred in roots |
Mojtahedi et al., 1993 |
|
‘332’ |
M. chitwoodi |
Good host; reproduction occurred in roots |
|
|
Sorghum-sudangrass |
Belonolaimus spp. |
Good host |
Rhoades, 1980 |
|
(No specific variety) |
Paratrichodorus minor |
Good host |
McSorley et al., 1994; McSorley and Gallaher, 1991 |
|
(No specific variety) |
Pratylenchus penetrans |
Good host (though less suitable as a host than other forage, including
white clover, oat and rye) |
Thies et al., 1995 |
|
(No specific variety) |
Pratylenchus scribneri |
Good host |
McSorley and Gallaher, 1991 |
|
(No specific variety) |
Criconemella spp. |
Good host |
McSorley and Gallaher, 1991 |
|
‘P855F’ |
M. chitwoodi |
Good host; reproduction occurred in roots |
Mojtahedi et al., 1993 |
|
‘P877F’ |
M. chitwoodi |
Good host; reproduction occurred in roots |
Mojtahedi et al., 1993 |
|
Sorghum |
|
|
|
|
(No specific variety) |
Meloidogyne arenaria |
Reproduction occurred (when used in a sorghum-soybean rotation) |
Rodrigues-Kabana et al., 1991 |
|
Forage sorghum |
|
|
|
|
(No specific variety) |
Pratylenchus penetrans |
Good host (though less suitable as a host than other forage, including
white clover, oat and rye) |
Thies et al., 1995 |
|
(No specific variety) |
Pratylenchus scribneri |
Good host |
McSorley and Gallaher, 1991 |
|
(No specific variety) |
Paratrichodorus minor |
Good host |
McSorley and Gallaher, 1991 |
|
‘FS25E’ |
Criconemella spp. |
Good host |
McSorley and Gallaher, 1991 |
|
‘BR64’ |
Criconemella spp. |
Good host |
Gallaher et al., 1991 |