Gamba grass is a tall tussock pasture grass, which has established along roads and on vacant disturbed country. Its spread, which is assisted by grading, slashing, mowing and transport of late cut hay containing seed, has been accelerated by the record or near record wet seasons during the last 5 years.
In ungrazed situations it can grow unchecked to 4 or 5 metres tall and produce a high fuel load, which is a fire hazard. If burnt the result is intense fires, which threaten properties and the environmental values of the Top End. It can also restrict vision along roads, requiring extra maintenance, making road maintenance more expensive.
There were many reports from Department of Business, Industry and Resource Development staff, Local Government councils and industry that gamba grass was difficult to control and kill.
Myths of controlling gamba grass
Six litres per hectare of glyphosate is needed to kill gamba grass. It is too costly to kill gamba grass with Glyphosate.
You can only kill gamba grass when it is young. If it's over one metre high the herbicide will not kill it.
You can only spray gamba grass from November to March, because after March the plants are too big and old to take up the herbicide.
Research conducted by Pasture Development Section and Weeds Branch during the 1998/99 wet season showed that gamba grass can be killed at any time during the wet season, November to May, by spraying with glyphosate (1%, 10ml per lire of water) plus a sticker (wetting agent) such as LI-700 (0.1%, 1ml per litre of water). The sticker is essential to ensure that the glyphosate gets into the plant.
A significant number of plants can be killed with a relatively small amount of herbicide. Generally, 300 to 800 young plants " up to 30 cm high " can be killed with a 15 litre capacity backpack sprayer. This uses 150ml glyphosate costing $1.25 and 15ml LI-700 costing 20c.
At the end of the wet season when the plants are tall and flowering, the number killed with the same amount of herbicide is about 100 large plants.
We have observed a number of instances where tussocks have not been killed because only part of the tussock was sprayed. The whole tussock needs to be covered with the glyphosate herbicide to ensure a kill.
Spraying before November or after June may not kill the whole tussock, as some of the tillers may be dormant and not growing, but it will reduce the number of plants present, their yield and seed production.
Manual Control - Mattock
Manual removal has proven to be an effective method for control of isolated plants and small groups of plants. Depending on the size of the plants, 100 to 200 plants can be dug out by one person in about half an hour.
Current (1999) Demonstration
The Northern Territory Fire Service burned the area on the 03/06/1999.
Treatments were applied on 09/07/1999.
Each plot is 2 m x 15 m with a 1 m buffer strip between plots.
Plants were between 10 and 20 cm high weeks after burning.
Today, six weeks later, all three rates have been effective.
Gamba Grass Description
Gamba grass (Andropogon gayanus cv Kent) is a tall perennial grass, which forms large dense tussocks up to 70 cm in diameter.
Leaf blades are long, linear, up to 45 cm long and 1.5 to 3 cm wide, with a strong white midrib. Leaves are pubescent (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, more commonly 60 - 70 cm.
The "seed" consists of a hairy spikelet, which gives it a fluffy appearance. There are approximately 450,000 per kg. The caryopses contained in the "seed" are small, 2-3 mm long, and 1 mm wide, about 890,000 per kg, light brown to brownish black in colour.
Climate and Soils
Gamba grass is a native of tropical Africa and is adapted to areas with a 3-6 month dry season and an annual rainfall of over 600 mm.
Control Demonstration on Major Roads
As part of the Department's response to community concerns about the effect of gamba grass on fire regimes and biodiversity, in 1998, Weeds Branch commenced a survey to map gamba grass presence on some of the major roads in the Top End.
This survey and mapping by Pasture Development Section and Weeds Branch has continued during 1999.
Control on roads
In late 1997, Extension Officer Les Huth and John Brock of the Litchfield Shire sprayed an area of gamba grass near Wishart Siding to demonstrate control methods.
Since April 1998, staff of Pasture Development Section, Edmund Pickering, Barbara Ross, Greg Hore and myself, have conducted a survey and control demonstration on the Stuart Highway south of Adelaide River and between Livingstone Road and Coomalie Creek, and on the Arnhem Highway east of the Adelaide River. The process involves travelling along the roads, recording where the grass is present, removing single plants and small groups, and noting the location of larger groups which need to be sprayed. The objective of this exercise is to demonstrate that the grass can be controlled and its distribution reduced.
Other Agricultural Development staff, Graham Schultz and Barry Lemcke, have contributed to the effort to control Kent Gamba grass along the roads by treating with herbicides or manually removing isolated plants and small groups of plants each time they travel out of Darwin.
Specialist Weed Control, the Weed Control contractors for the Roads Division of the Department of Transport and Works have controlled substantial amounts of gamba grass around road furniture on highways and roads during the last wet season.
Parks and Wildlife staff have removed plants inside Litchfield National Park.
Progress to Date
The combined efforts of a small number of people have virtually eliminated gamba grass on the Stuart Highway south of Emerald Springs, and reduced it to a small number of locations on the Arnhem Highway east of the Adelaide River and on the Stuart Highway between Livingstone Road and Coomalie Creek.
It is expected that, by the middle of next year, a number of roads will be free of gamba grass or have the distribution along the road reduced to a small number of locations. Apart from those listed above, the roads expected to be free of gamba grass are Cox Peninsula Road, Dorat Road, Oolloo Road and the Stuart Highway between Adelaide River and Emerald Springs.
If we have more people taking the time to control even small amounts of gamba grass, the task will become easier for everyone, and the threat it presents will be removed. This is a case where every little bit does count!
Arthur Cameron Principal
Thank you for your participation.