Improving gamba grass control on Cape York Peninsula

Gamba grass (Andropogon gayanus) was introduced to Australia as a pasture grass from Africa. It has become a significant environmental weed and is considered an ecosystem transformer, posing multiple threats to savanna ecosystems of northern Australia. Of particular importance is its capacity to increase fuel loads, resulting in intense fires that may subsequently lead to permanent transformation of the structure and diversity of the ecosystems it invades. Despite its listing as a key threatening process under the Commonwealth Environment Protection and Biodiversity Conservation Act 1999, gamba grass has continued to spread through the Northern Territory, Queensland and Western Australia.

One of the major problems limiting the effective management of gamba grass once it’s established as an environmental weed is the lack of registered herbicides for use in natural systems and conservation areas. Glyphosate is the primary herbicide in use in northern Australia and the only herbicide registered for use on protected land. However, non-selective herbicides such as glyphosate have significant off-target impacts on native vegetation. Other herbicides have been trialled with varying success as soil and/or foliar applications. However, none have consistently proven effective at recommended rates of application or if they are effective, they also have negative impacts on non-target species.

There are numerous logistical issues with the widespread use of glyphosate in the north – application relies on large, heavy and expensive spray equipment as well as access to clean water which can be difficult to manage in remote areas. The herbicide must be applied during the hot and rainy growing season which often limits or prevents access. Additionally, glyphosate has no residual action and its effectiveness relies on follow-up treatments, and there are emerging resistance issues. Alternative herbicides are critical as insurance against future limitations on the use of glyphosate, to combat resistance, and to allow effective, timely control in the ecosystems of Cape York Peninsula and across northern Australia.

This project sought to determine whether there was a suitable alternative to glyphosate that selectively controls gamba grass with low off-target effects on native vegetation.

This project sought to identify alternative herbicide options for gamba grass by:

• reviewing gamba grass biology related to its effective control and management in northern Australia
• collaborating with land managers and gamba experts to understand successes and challenges with existing on-ground management of gamba grass
• conducting field trials in southern Cape York Peninsula to test the effectiveness of glyphosate and three additional herbicides on gamba grass
• conducting pot trials to test the tolerance of gamba grass, co-occurring native plant species, plus one widely distributed exotic grass, Bothriochloa pertusa (Indian blue grass), to a range of residual pre-emergence herbicides to identify herbicides that selectively control gamba grass as it germinates while minimally impacting at least a few native plant species
• identifying herbicides that are worthy of further trials at various application rates and in different environmental contexts.

Gamba grass can spread quickly, threatening assets such as protected areas. Gamba data from ALA 2018. Protected area data from CAPAD 2016.

• Among the tested herbicides, field trial results do not indicate a clear suitable alternative to glyphosate for gamba grass control.
• A single field application of glyphosate was found to have limited effectiveness at reducing cover and suppressing flowering and recruitment into the second growing season post-treatment.
• The herbicides flupropanate, terbacil and sulfometuron were largely ineffective at reducing gamba cover, suppressing flowering or gamba recruitment in the field.
• In the pot trials, the most effective herbicides for suppression of gamba germination were clomazone, oxyflurorfen, imazapyr and indaziflam. All had large off-target effects on native species.
• Granular flupropanate shows some promise for small, remote infestations as it does not require water on application, has a relatively long window of application, and has few off-target effects if applied directly to a tussock. Further testing is required to identify optimal rates of application.
• Land managers may need to trade off short-term off-target effects for more effective and permanent control of gamba grass.
• Suppressing all grass establishment for a period to deplete the short-lived gamba seed bank prior to allowing natural site restoration may also be an effective strategy for new infestations that are inaccessible in the wet season.

Various pots at 13 weeks after treatment. Photo: Queensland Department of Agriculture and Fisheries.

The herbicide rates and applications used in our field trials were designed to be consistent with the rates and methods used by local land managers who are largely controlling a low-density gamba infestation on foot, once per year. There are several potential reasons why the treatments used here in the field did not result in longer-term reductions in gamba cover. One application is clearly not sufficient to provide effective control by any of the herbicides we tested, and follow-up applications over multiple years are likely to be required. Higher rates of application are likely to be more successful in reducing cover; however, they are also likely to have more off-target effects.

The pot trials identified some herbicides which were not effective in suppressing gamba grass and also had low off-target effects at the rates applied. For these herbicides, higher rates could be trialled to determine if there is an optimum application rate which will selectively control gamba germination while minimising off-target effects. In contrast, the herbicides which did effectively suppress gamba germination also had high off-target effects on native species germination.

Although neither the field nor pot trials identified suitable herbicides that selectively controlled gamba grass with low off-target effects, there are several herbicides that warrant further testing at a range of application rates and in a range of environments. These include flupropanate (liquid and granular), clomazone, oxyfluorfen, imazapyr and indaziflam.

Further tests could also include a broader range of environments and co-occurring native species to identify when and where particular herbicides are most effective.

Currently, integrated management remains the best option for managing and eradicating gamba incursions and infestations. This includes various combinations of restricting seed movement, herbicide application and other management practices such as grazing, burning and slashing. Success of integrated management is often context-specific; however, increasing our understanding of how herbicides behave in combination with other management practices will be critical to controlling the spread of gamba grass in northern Australia.

Gamba grass can form dense stands up to 4 m tall, such as this one in the Northern Territory. Photo: NESP Northern Australia Hub.