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Aleppo grass, Cuba grass,
Egyptian millet, evergreen millet, false guineagrass,
maidencane, meansgrass,
St. Mary’s grass, syriagrass
Sorghum halepense
.
Characteristics. Johnsongrass is a vigorous perennial that spreads by seed or
rhizomes. This native
Mediterranean grass was originally introduced into the US as a hay
or forage crop. It has subsequently
spread throughout the US
to become a major weed of production agriculture, landscaping and many other situations. While normally a good forage crop,
under stress or drought conditions, or due to severe frost, it can form
hydrocyanic acid which accumulates in the plant and then becomes toxic to
grazing animals. Once normally thought of as a warm season grass it has
adapted and now is a problem in most western states.
Identifying
Characteristics. The erect solid stems can reach up to
8 feet tall with very course foliage that may grow to 3 feet long and 1 inch
wide. The midveins of leaves are light colored and
easily snap when folded.
Stems terminate in the flower spike,
which is reddish colored and very open and loosely formed. The spikes have
numerous branches, and begin to droop as they ages. The branches, or spikelets, form at the nodes, with 2 or 3 spikelets at a node.
A heavy infestation can look like a sea of reddish spikes
Johnsongrass
is a troublesome perennial grass weed that reduces yields in corn, grain
sorghum, soybean, cotton and leguminous forages. Originally introduced into
the United States
as a forage crop, it is now an agricultural pest in most states south of the
42nd parallel. Dense johnsongrass infestations severely limited corn
production until the relatively recent introduction of Accent and Beacon
herbicides. Several states have legislation requiring eradication or control
programs steps toward controlling and eradicating the weed on all property
owned or under their control.
Growers now have numerous tools to
manage this weed. As with most other weeds, the key is to implement multiple
control strategies when johnsongrass is first observed and not wait until it
is firmly established.
During 1992 various biotypes of
johnsongrass in Mississippi
were reported to exhibit resistance to Fusilade DX applied at labeled rates.
Resistance appeared in fields that had been treated with Fusilade and Poast
Plus in at least eight of the previous 10 years. Since that time, at least
five counties in Mississippi have johnsongrass
populations confirmed resistant to Fusilade, and there are recent reports of
resistant johnsongrass in Kentucky.
Commonly, resistant populations of johnsongrass are not sensitive to Fusilade
DX or Assure II, no matter what rate is applied. Elevated tolerance to
Poast/Poast Plus has also been noted, but rates three- to fivefold higher
than recommended normally are effective. Field use
rates of Select, another selective grass herbicide, are effective in
controlling resistant johnsongrass
Its ability to produce seed and
rhizomes and spread to uninfested areas contributes to making johnsongrass a
menacing weed. Rhizomes are extensive and are produced in the top 10 inches
of soil but have been found at depths of 5 feet. Johnsongrass readily
reproduces from rhizomes and seed; seedling plants can initiate rhizomes as
few as 19 days following emergence.
A single plant may produce more than
80,000 seeds in a single growing season, and 275 feet of rhizomes. Seeds
shatter easily and fall to the ground beneath plants that produce them.
Instead of germinating uniformly, seeds can remain dormant and produce plants
over several years. Johnsongrass seed can remain viable in the soil for more
than 10 years.
Johnsongrass generally grows in
fertile bottomlands along creek and river banks and in upland fields.
Irrigation ditch banks are also overgrown with johnsongrass, and seed is
carried by the water. In a new location, johnsongrass spreads rapidly and
soon becomes a serious problem. Tillage equipment can spread the rhizomes,
and birds spread the seed.
Agricultural seed, hay and various
livestock feeds sometimes become contaminated with johnsongrass seed. In the
past, watermelons shipped from the South were often bedded in johnsongrass
hay, which was swept out at various dropoff points and contributed to its
spread.
Interfertility with other
sorghum species
Plant breeders, seed dealers and
growers often observe off-type plants or "rogues" of hybrid grain
sorghum. Common rogues are the tall outcrosses to sudangrass, johnsongrass or
other sorghum types. Sorghum outcrosses with johnsongrass as the male parent
often have rhizomes resembling those of johnsongrass. Johnsongrass-grain
sorghum hybrids can become a significant weed threat.
Cultural control
Preventing johnsongrass from becoming
established in new areas is the best available control method, because the
weed spreads in so many ways. Because johnsongrass is a perennial weed,
single cultural control measures or herbicide applications rarely provide
adequate control.
Johnsongrass control programs should
·
Prevent spread of rhizomes from infested to
uninfested areas.
·
Kill or weaken established plants and their
underground rhizome system.
·
Control seedlings originating from shattered seed.
·
Prevent production of seed and its spread to new
areas.
·
Use fall tillage to bring rhizomes to soil surface,
where they may be killed by winter conditions.
These objectives are closely related
and are equally important to the success or failure of a control program. In
limited infestations, it is possible and desirable to use herbicides to kill
the weed and prevent seed production. The critical time to kill johnsongrass
is while the weed is becoming established and before it has spread over the
entire field. For sites with established infestations, a fall application of
Roundup or Touchdown will kill emerged tissue and often developing rhizomes.
Growing early-maturing crops, plowing
immediately after harvest, and tilling as needed are common methods to break
up rhizomes and weaken johnsongrass stands in cultivated areas. Johnsongrass
responds to tillage like most tough perennial weeds. Intensive tillage will
suppress rhizome johnsongrass. Moderate tillage will break and spread
rhizomes and allow johnsongrass to thrive. Johnsongrass will spread relatively
slowly in true no-till regimes; however, given enough time, infestations may
still become severe. Severe infestations often result in pastures converted
to row crops. Uncontrolled johnsongrass in fence rows and ditch banks is a
common source of johnsongrass seed as well as vegetative encroachment into
fields. Spot sprays on fences and ditch banks can eliminate these sources of
seeds and rhizomes.
Alfalfa competes well with
johnsongrass for a limited time but will eventually weaken. Repeated mowing
of an alfalfa-johnsongrass mixture will prevent johnsongrass from producing
seed and will inhibit rhizome development. Intensive grazing and mowing can
be used to reduce a stand of johnsongrass. If grazed or mowed closely for at
least two years, the plants become weak and stunted and the rhizomes become
concentrated near the soil surface. Plowing in late fall exposes the rhizomes
to low winter temperatures and reduces the stand considerably. For infested
areas that cannot be tilled or sprayed, intensive grazing or mowing is
probably the best control.
Johnsongrass control
with herbicides
To reduce johnsongrass infestations
with herbicides, it will be necessary to use an integrated approach
consisting of soil-applied herbicides, postemergence herbicides, crop rotation
and tillage. Weedy plants such as johnsongrass adapt quickly to a cultural
system that does not change from year to year. An example of this type of
system is continuous soybean production such as occurs in many river and
creek bottoms. Crop rotation is important to disrupt the physical environment
that johnsongrass and many other weeds live in and will help to minimize seed
and rhizome production. Herbicide rotation is important to minimize selection
pressure for herbicide-resistant biotypes.
The programs mentioned below are
designed to control other weeds in addition to johnsongrass. However, these
programs are designed for fields where johnsongrass is not adequately
controlled by current methods. For control methods to be effective, growers
must be diligent and apply control techniques as often as necessary.
Corn
In no-till corn production, any herbicide that has postemergence
grass activity will provide some control of seedling johnsongrass. In
conventional-till corn production, Eradicane or another product containing
EPTC (Eradicane) can be applied as a preplant incorporated treatment.
Rhizomes should be cut up into small pieces using several diskings before
treatment. Incorporate Eradicane immediately after application to minimize
volatility losses. However, the burndown or preplant incorporated
applications will not provide adequate season-long control and additional
control measures will be needed.
Postemergence herbicides for
johnsongrass control in corn include Accent, Beacon, Poast HC and Roundup
Ultra. The initial application of Accent or Beacon should be applied to 4- to
10-inch-tall seedling johnsongrass and 8- to 14-inch-tall rhizome
johnsongrass. If regrowth occurs, a second application can be made when
johnsongrass regrowth is 8 to 14 inches tall. Accent can be applied as a
broadcast treatment on corn until it is 24 inches tall. Accent applications
can be made to 24- to 36-inch corn by using drop nozzles. Beacon can be
applied as a broadcast spray to corn that is between 4 and 20 inches tall. Do
not apply Accent or Beacon to corn that has been treated with Counter
insecticide. Consult the label for restrictions with other organophosphate
insecticides and postemergence herbicides.
In SR (sethoxydim-resistant or
Poast-tolerant) corn, Poast Plus or Poast HC should be applied initially to
10- to 25-inch johnsongrass. If regrowth occurs, a sequential treatment can
be applied to 12-inch johnsongrass regrowth. There are no insecticide
restrictions and Poast HC can be applied until corn begins to shed pollen.
When Roundup-Ready corn has received
full federal approval, Roundup Ultra can be applied to johnsongrass up to 24
inches tall. If regrowth occurs, a sequential treatment can be applied to
12-inch johnsongrass regrowth. Consult the label for stage of growth and
insecticide restrictions.
Soybean and cotton
In no-till soybean or cotton production, as in corn production, any
herbicide that has postemergence grass activity will provide some control of
seedling johnsongrass. In conventional-till soybean or cotton production, Treflan and Prowl can be applied preplant
incorporated and will provide some control of seedling johnsongrass.
When incorporating these herbicides, set the implement to run 4 inches deep
and make two passes over the treated area to ensure adequate mixing of the
herbicide with the soil. However, as in corn production, the burndown or preplant
incorporated applications will not provide adequate season-long control, and
additional control measures will be needed.
Postemergence
herbicides for johnsongrass control in soybean and
cotton include Assure II, Bugle, Fusilade DX,
Fusion, Poast Plus/Rezult
G/Conclude G, and Select in regular, non-herbicide-tolerant soybean or
cotton. All of these herbicides provide acceptable control, although Assure
II, Fusilade and Select are the preferred
herbicides for johnsongrass. Roundup Ultra can be
used in Roundup-Ready soybean and cotton. In dense infestations, single
applications of any of these herbicides will provide incomplete control.
Better control is obtained with split applications. It is recommended that
the lower herbicide rates be used on seedling johnsongrass
that is less than 10 inches tall. The higher rates are recommended for use on
rhizome johnsongrass taller than 10 inches. The
recommended approach is to target johnsongrass that
is 10 to 18 inches tall. Smaller johnsongrass has
little leaf area and consequently absorbs limited amounts of herbicide for
complete rhizome control. Regrowth after the first postemergence application indicates that there is a
significant amount of rhizome johnsongrass present
in the field, and a second postemergence
application will be needed. To control regrowth,
apply the higher rate of a postemergence herbicide
to regrowth that is 6 to 12 inches tall. For regrowth applications, it is a good idea to use a
different herbicide than that used for the initial application to avoid
selection of herbicide-resistant johnsongrass
biotypes.
Adjuvants and cultivation in corn, soybean and cotton
Consult the manufacturer's label for the appropriate adjuvant for postemergence herbicides. Allow seven days after postemergence herbicide applications before any
cultivation is practiced. Cultivation too soon after herbicide applications
will reduce johnsongrass control as stems and rhizomes
are broken before the herbicide can be translocated.
Cultivation seven to 10 days after herbicide applications, after the
herbicide has translocated, will further weaken the
herbicide-injured rhizomes and increase johnsongrass
control.
Alfalfa, birdsfoot trefoil, clovers
A preplant incorporated treatment of Balan can be used for control of seedling johnsongrass and other annual grasses in alfalfa. Rhizome
johnsongrass will not be adequately controlled with
Balan alone. Balan must
be incorporated within four hours after application. Variable weed control
may result if incorporation is delayed more than eight hours.
Postemergence
herbicides include Poast/Poast Plus and Select.
These herbicides can be used on new or established seedings
and can also be applied when alfalfa is either dormant or nondormant.
Apply Poast, Poast Plus
or Select to 10- to 25-inch-tall johnsongrass. If regrowth occurs, sequential applications can be made with
similar rates to 12-inch johnsongrass regrowth. Consult the label for the appropriate adjuvant
for the postemergence herbicides.
Grain sorghum
Because johnsongrass is closely related to
other plants in the sorghum family, including grain sorghum (Sorghum
bicolor), current herbicides that control johnsongrass
also control or kill grain sorghum. The best management practice for johnsongrass control in grain sorghum will be to grow an
alternative crop that has more available registered herbicides.
Selective herbicide
applications for johnsongrass control
Selective application equipment can
provide economical johnsongrass control in any of
the above-listed crops. Typical johnsongrass
infestations are distributed unevenly across the field. Chemical costs can be
reduced by using a ropewick applicator or by spot
spraying the infested areas. In a ropewick
applicator, mix 1 part Roundup or Touchdown with 2 parts water. Run the ropewick applicator in two directions across the field so
the johnsongrass is wiped or wicked on both sides.
The ropewick should be safely run above the crop
canopy to avoid contact with crop foliage. Some dripping and crop injury is
inevitable; therefore, it is best to use a commercially available ropewick bar designed to minimize dripping.
Spot spraying can also be an
economical Johnsongrass management tool. In soybean
and cotton, the selective grass herbicides mentioned earlier may be mixed
with water in a 1 to 2 percent solution and spot sprayed with hand wand,
ATV-mounted sprayers, or tractor-mounted sprayers. Roundup or Touchdown spot
sprays can also be used but will cause injury or death to crops that are not
tolerant of glyphosate herbicides (Roundup or
Touchdown).
Johnsongrass control
before harvest will help to reduce rhizome and seed production for subsequent
rotational crops. Roundup is labeled for application to mature corn, cotton,
grain sorghum, soybean, and wheat. Double-crop soybean may particularly
benefit if johnsongrass is sprayed in the preceding
wheat crop. Roundup may be applied to standing soybean that has lost all
green color in the pods as long as the crop is not being grown for seed.
Roundup may be applied to corn at 35 percent grain moisture or less, to
cotton when 60 percent of the bolls are open, to grain sorghum at 30 percent
moisture or less, and to wheat after the hard-dough stage.
Soybean and wheat should not be
harvested for seven to 10 days after application to allow herbicide
translocation. These applications may be of limited utility in soybean and
cotton, as the application time is typically close to frost. Unless herbicide
applications are made at least seven to 10 days before frost, insufficient
translocation will occur and the applications will not be effective
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