Management options for gamba grass (Andropogon gayanus) in conservation areas of Cape York Peninsula Final report

Andropogon gayanus (gamba grass) is a high-biomass grass native to tropical and subtropical Africa and introduced into Australia as a pasture grass. Under well-managed grazing conditions, gamba grass has proven a useful and palatable addition to tropical cattle pastures. However, it has also become a significant environmental weed and is considered an ecosystem transformer. In recognition of the significant threat posed by gamba grass, it has been listed (along with 4 other invasive grasses) as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). One of the major problems limiting the effective management of gamba grass once 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. There are several current and emerging issues which make a reliance on glyphosate for gamba grass control problematic. Application of glyphosate is logistically difficult in wet and remote areas, it has no residual action and largely relies on follow-up treatments, and there are emerging resistance issues. In addition, there is growing concern that glyphosate may be linked to carcinogenicity, genotoxicity and epidemiological disorders. Alternative herbicides are critical to allow long-term, effective and timely control of gamba grass in the environments encountered on Cape York Peninsula and across northern Australia. The goal of this project was to collate existing knowledge related to control and management of gamba grass and test alternative herbicide options for use in natural areas of Cape York Peninsula. Three herbicides were tested alongside glyphosate in field trials and 10 residual pre-emergence herbicides were tested in pot trials. Neither the field nor pot trials identified a clear suitable alternative to glyphosate that selectively controlled gamba grass with low off-target effects in the contexts in which we tested them. However, there are several herbicides that warrant further testing at a range of additional application rates and in a range of environments (flupropanate, clomazone, oxyfluorfen, imazapyr and indaziflam). In particular, the granular form of flupropanate is worthy of further experimentation because of its portability in the field and flexibility in application, and because it showed the most promising results in the field trials. Ultimately, land managers may need to trade-off significant, short-term, off-target effects for longer term, more effective and permanent control of gamba grass with herbicides.