Gamba grass is a tall tropical pasture grass that was first introduced into the Top End of the Northern Territory over 50 years ago.
While gamba grass can be a beneficial pasture plant on pastoral properties, it is difficult to manage for this purpose.
It has spread from properties where it was sown as a pasture into non-pastoral areas. It is currently found on roadsides, vacant land, reserves and national parks where it threatens the integrity of natural ecosytems. Spread has been accelerated by transportation of seed in hay, road maintenance activities, and the run of above average wet seasons during the mid to late 1990s.
Stands that are not managed by grazing, mowing or slashing can accumulate high dry matter yields, which provide a high fuel load and increase the risk of severe fires.
Gamba grass can be controlled, but in some situations this may take substantial resources. There are a number of methods available to eliminate or control the growth of the grass in the situations where it is currently found.
Gamba grass (Andropogon gayanus cv Kent) is a tall perennial grass that forms large dense tussocks up to 70 cm in diameter. It is a variable species. Plants can have many or few tillers, thick stems and wide leaves or fine stems and thinner leaves. Leaf blades are up to 45 cm long and 1.5 to 3 cm wide, with a strong white midrib. Leaves are covered with fine soft hairs. Foliage height can be up to 180 cm in ungrazed, well-fertilised swards. Flowering stems are erect and up to 4 m high. The "seed" consists of a hairy spikelet, which gives it a fluffy appearance.
Kent gamba grass was derived by natural selection following crossing within and between two lines, one of which came into Australia from Zaria in Nigeria, while the other came from Africa via Brazil.
CSIRO Division of Land Research conducted trials with these 2 lines at Katherine Research Station from 1946. Mr Frank Kent of the Commonwealth Northern Territory Administration (Northern TerritoryA) grew the residual material from the CSIRO trials at Berrimah Experiment Farm in the mid 1950s. It was sown at many sites in a series of pasture species evaluation trials between January 1961 and February 1980. Sites sown included Daly River, Darwin River, Douglas River, Elizabeth River Dairy, Finniss area, Mount Bundey Station, Mudginberri Station,19-Mile Stuart Highway, Tipperary Station Tortilla Flats Research Farm and Wildman River station.
The consensus from these sowings was that establishment was generally poor and that care was needed in establishment. Growth and persistence of the plants that did establish were excellent, and spread was initially gradual.
Kent gamba grass was released as a pasture cultivar through the Northern Territory Herbage Plant Liaison Committee in 1978. It was registered on the Register of Australian Herbage Plant Cultivars in 1986.
Because of difficulties with commercial seed production, quantities of seed were not available until 1983, when approximately 1 tonne of seed was produced. Between 1983 and 1988 areas of Kent were sown on Top End properties each year. It has been sown primarily as a grazed pasture, but has been grown for seed production, hay production including mulching hay, and for making silage.
Currently, it is estimated that there are approximately 7,000 ha of gamba grass in the Top End of the Northern Territory
Gamba grass is mostly found on pastoral properties in an area bounded by the Northern Territory coast in the North and West; the western boundary of Kakadu National Park in the East, and Pine Creek in the South. Stands outside these boundaries are generally small.
The potential distribution of gamba grass is all upland soils in the Northern Territory North of the 600 mm rainfall isohyet, just south of Daly Waters.
Mode of Spread
The light and fluffy seed can be spread by the prevailing winds during the main seeding period in May/June, and later in the dry season, as plants will flower if soil moisture is available. Gamba grass will produce some seed in October/November after early wet season storms. Spread is to the northwest by the southeast winds. Spread by wind is generally gradual.
Seed can also be spread in mulching hay that has been cut late after the seed has matured, by vehicles or machinery eg, graders, mowers and slashers, or by animals. Most spread along roadsides appears to have been by road maintenance machinery.
Recent studies on seed ecology have shown that seed viability declines to less than 1% within 12 months of the seed being shed.
Gamba grass establishes more readily in cultivated paddocks or physically disturbed areas such as road verges, overgrazed pasture areas and previously fertilised paddocks. It establishes well on areas subjected to mowing, slashing or grading, but is not favoured by frequent low mowing. Mounds of soil left by earthmoving equipment are particularly favourable sites for establishment.
Establishment and spread are generally slow in native vegetation where the soil surface has not been disturbed. This is shown on road verges where gamba grass has been spread along roads by maintenance operations, but has not spread into native vegetation from the disturbed zone over periods as long as 20 years.
In some instances gamba grass has established a long way from known stands.
On coastal properties the upland native pastures cannot support the number of stock during the wet season that can be carried on the floodplains during the dry season.
Pastoralists in the Top End plant gamba grass on upland areas as trial results and industry experience has shown that it will feed and fatten livestock at high stocking rates during the wet season. Fertilised, well-managed gamba grass pastures can carry up to 40 times more cattle than the native pastures of these areas.
Eight hectares of native pastures are needed to sustain a steer which will gain 60-90 kg during the wet season. Well-managed gamba grass pastures will carry up to 5 head per hectare and increase their liveweight by 25 kg per head per month during the five months from December to April.
Sowing areas to gamba grass has the additional advantage of giving pastoralists more flexibility in managing areas available for wet season grazing and allows them to reduce stocking pressure on native pastures.
The main problem is the fire hazard created by poorly managed stands of the grass.
Gamba grass has become established on roadsides and non-pastoral properties in the Top End. It is generally not grazed, mowed or slashed in these situations and can produce high fuel loads which pose a risk to property and firefighters.
It has the potential to reduce biodiversity in woodland situations in the Top End by competing with native vegetation and creating increasingly intense fires. There are indications that these fires will impact on the tree overstorey, even species that are relatively fire resistant.
Gamba grass was sown on the leasehold pastoral properties of Mudginberri, Wildman River and Camp Creek that later became National Parks. This change in land use and management has resulted in a pasture plant becoming a weed in these National Parks.
It is a grass that requires a high level of management for it to remain productive as a pasture and not to become a weed in other areas. Pastoralists should manage the grass to contain it within pastoral areas. Where gamba grass has spread to non-pastoral areas, costs can be incurred for damage (increased fire intensity, loss of biodiversity, aesthetic loss) or control.
DBIRD is not recommending new sowings of gamba grass.
Gamba grass should not be used on small rural blocks with insufficient stock numbers and lack of stocking flexibility to adequately control the growth of the grass. The aim is to control the grass growth during the wet season to prevent it becoming tall, stemmy and rank in the dry season. The fire hazard it creates can be more acute on small blocks.
On pastoral properties where gamba grass is currently used, only a small proportion of the pastures on the property should be sown to allow stock numbers to be concentrated on it during the wet season. Paddocks should be kept to a size that can be heavily stocked with all available stock on the property, should that be required.
Paddocks should be sited so that the prevailing southeasterly winds during the May-July period, when the bulk of the seed is produced, do not carry seed to adjacent properties. The adjacent property may not be engaged in livestock production and Kent gamba grass may be regarded as a weed. A buffer strip of 1 km should be left between the area sown and adjacent properties.
Points which should be considered are:
Unwanted gamba grass can be controlled in a number of ways. The most suitable control method depends on the amount of grass present, the situation and the time of the year. Gamba grass can be present as isolated plants, small groups of plants or as areas or stands of plants with different densities, from thin to thick.
Control may require a combination of mechanical, herbicide and land management techniques.
There are three aspects to control: (1) control of growth to reduce dry matter accumulation and fire fuel load; (2) control of seed production and spread and (3) eradication of the grass from an area. The third aspect achieves all three. In some cases, eradication may not be possible. For eradication to be successful, follow up control is essential. Check the area for 2 or 3 wet seasons following initial treatment to eliminate new seedlings.
All control operations should be towards, rather than away from areas of gamba grass, and machinery should be thoroughly cleaned after working in areas of seeding gamba grass. Cleaning of machinery is essential to prevent spread of gamba grass by seed
Control strategies for different situations and timings are listed below.
(a) Isolated plants and small groups of plants
Elimination is the best option for these plants. They can be removed at any time of the year by hand, sprayed with glyphosate using a backpack sprayer while actively growing, by scraping or grading late in the wet season before the plants have set seed or treated with Velpar using a spot gun during the wet season. The area should not be disturbed early in the wet season when it may encourage establishment from seed, or when the plants are seeding.
(b) Stands of plants
It may not be possible to eradicate larger stands of gamba grass in some cases, but control can be achieved. If the stand is too large or the plants are among trees where access is prevented, it may be difficult to eradicate the stand in the short term. Where the stand is too large to eradicate, slashing or mowing during the wet season can control the growth of the plants. This will reduce dry matter and seed yields. Slashing twice during the wet season will greatly reduce dry matter yields, while slashing after mid March will reduce seed set. All slashing should be carried out before the grass has set seed in May/June each year. Outlying isolated plants or small groups of plants should be removed to reduce the rate of spread and contain the grass to the area currently occupied.
Where the stand can be accessed, a number of control options are available. These include; scraping or grading the stands late in the wet season before the plants have set seed; spraying with glyphosate while the plants are actively growing during the wet season; ploughing out the areas early in the wet season on cleared agricultural land or slashing or mowing to reduce yield.
Burning late in the wet season or early in the dry season reduces the standing fuel load and reduces the risk of a hot fire late in the dry season. Burning at this time requires the previous seasonís dry rank growth as the grass is too green to burn.
Some of the control techniques create disturbed areas which may allow the growth of gamba grass and weed seedlings. Consideration should be given to replacing the gamba grass with other species.
Tussocks can be dug out with a mattock or a hoe. Their root system is extensive but mostly shallow, which makes them easy to remove.
Removing a tussock with a mattock
Note the extensive shallow root system
The plants should be sprayed any time during the wet season with glyphosate (36% a.i.) at a dilution at 1:100 L of water when they are actively growing. The addition of an anionic wetting agent to the spray mix enhances the effect. Plants should be sprayed before the end of May to prevent them seeding. If the plants are tall, rank and dry, the mature growth can be removed by grazing, cutting, mowing, slashing or burning to encourage fresh regrowth that can then be sprayed.
Spraying a small group of plants with a
Dead gamba grass killed by spraying with glyphosate
Isolated plants can be killed with a 4 ml shot of Velpar L(R) or an 8 ml shot at a dilution of 1:1 L of water, applied to the base in the centre of the tussock.(c) Ploughing
Areas of gamba grass can be ploughed out. The area may need to be ploughed more than once. This is best done early in the wet season.
(d) Grading or scraping
Grading or scraping out of the ground should only be done late in the wet season prior to seeding when seed from the previous wet season has all germinated. This is not recommended at the start of the wet season, when disturbance may promote the establishment of new gamba grass or weed seedlings and lead to erosion.(e) Slashing, cutting or mowing
Slashing or mowing during the wet season will slow grass growth, reduce the fuel load at the end of the wet season and reduce seed set, but generally will not kill the tussocks. Mechanical treatments should be carried out either before the grass has set seed or after all seed has dropped, as seeds may be trapped on machinery and drop off in other areas. If seed has not been shed when slashed or mowed, machinery should be cleaned before proceeding to other areas.
Burning generally does not kill the tussocks. Burning of dry rank growth must be carried out with adequate firebreaks and fire control equipment. Fires can be extremely hot and the smoke plume can carry smouldering embers long distances. It is best to burn early in the dry season ie April/May when the plants still have a high proportion of green leaf. This gives a cooler fire and reduces the risk of a late hot fire by removing much of the fuel load.
Basal regrowth following a severe fire
Care should be taken when grading firebreaks, fencelines and roadsides to leave a level surface.(g) Grazing
Grazing at recommended stocking rates will not eliminate Kent gamba grass, but it will ensure that the grass is short, not a fire risk at the end of the wet season and seed production is reduced. Prolonged grazing at excessive stocking rates will kill tussocks. It will also eliminate all other vegetation, greatly reduce animal productivity and leave bare ground susceptible to erosion.
Natural Resources Division
A. G. CAMERON
Principal Agronomist Pastures DBIRD
|DBIRD:||I. Miller, G. Flanagan, G.
Schultz, W. Mollah, V. Hristova, K. Levey,|
B. Lemcke, J. Pitt, B. Ross
|PWCNorthern Territory:||C. Wilson, D. Liddle, W Panton, T. Flores, A. Bielby|
|Parks North:||P. Barrow|
|Northern TerritoryFRS:||D. Pettit|
Valid from 14 April 2000
Care has been taken in the production of this Information Sheet, however it is provided as general information only and specific professional advice should be sought on your particular situation.
The Northern Territory of Australia disclaims all liability, whether for negligence or otherwise, for any loss, expense, damage or injury caused by any use or reliance on this information.