Native to the tropical and subtropical Americas, parthenium weed (Parthenium hysterophorus L.) is now a major weed in many regions of the world, including Africa, Asia, the Pacific and Australia. Parthenium weed causes problems that include reductions in crop and pasture yields, acting as an alternative host to crop pests, causing toxicity problems to domesticated livestock and health problems to people. The species is also thought to significantly influence the diversity of native ecosystems. Currently in Queensland, Australia, parthenium weed is mostly a weed of grasslands, affecting beef cattle production by more than ca. $95 million annually in up to 60 million ha.
The effect of parthenium weed upon the species composition, diversity and productivity, was studied within an invaded grassland, where traditional management for parthenium weed was being applied. The greater parthenium weed abundance was found to negative affect the overall species diversity and the native species abundance. Parthenium weed was found being partly replaced by the introduced species green couch (Cynodon dactylon L. (Pers.) 1 year after the application of a selective broad leaf weed herbicide mixture (metsulfuron-methyl as Brush-Off ®; 10 g 100 L-1 water and 2,4-D as Amicide 625; 320 mL 100 L-1) and the removal of grazing and this improvement continued to strengthen over the second year. In this study, this management plan improved the fodder biomass production of the grassland by 80 %; however, it did not improve the species diversity of the grassland and therefore its future resilience to further weed invasion.
The annual capacity of the seed bank formed within the differently managed grassland communities, to recruit new species into the community, and how those recruited species responded to further management, was then evaluated. One year after the application of the selective broad leaf weed herbicide mixture and the removal of grazing, parthenium weed establishment into the above-ground plant community was reduced by 90 %. The resulting lower abundance of parthenium
weed, was found, yet again to negatively affect the abundance of native species. The recruitment of other species into the community was dominated by broadleaved species, showing the advantage these species have to colonize new niches created by the herbicide application. Species with fodder value (grasses) were not recruited first, regardless of the management strategy being applied. However, further reduced grazing pressure and herbicide applications in the subsequent season, did promote the recruitment of a valuable forage grass species (e.g. burr medic; Medicago polymorpha L.).
The role of allelopathy at the early stages of growth was studied in the laboratory as a way of bringing further insight into the inter-specific interactions that are occurring in these grassland communities. Indeed, parthenium weed seedlings were capable of releasing chemicals that could have detrimental effects upon the growth of neighbouring seedlings, and to do this as early as 10 days into their development. The results showed differential species sensitivity to parthenium weed’s allelochemicals with, in general, the Australian native grasses (e.g. curly windmill grass; Enteropogon acicularis L., cotton panic grass; Digitaria brownii L. and weeping grass; Microlaena stipoides L.) to be inhibited more than the introduced ones. This was particularly noticeable with root elongation, where the growth of the native species was reduced by ca. 46 % across all tested species.
Field trials were then undertaken to assess the impacts of a new management strategy: the complementation of biological control with suppressive grassland plant mixtures. The effective suppression of parthenium weed abundance by 78 %, was found to be due, both to the biomass production of one suppressive plant mixture 3 months after sowing and the occurrence of the previously released biological control agent Zygogramma bicolorata Pallister.
Lastly, the inter-specific interactions that took place between seedlings under both the present and a future climate conditions (an elevated atmospheric CO2 concentration and a reduced soil moisture level) were studied. Parthenium weed’s growth was stimulated (30 %) under the elevated atmospheric CO2 concentration, in a dry soil and when in the presence of a high density of high Rhodes grass (Chloris gayana L.), confirming that from an early growth stage, parthenium weed will have a suppressive advantage under the future climate predicted for Queensland. Parthenium weed’s biomass, however was found to decrease as the density of the C4 (Rhodes grass) and C3 (siratro; Macroptilium atropurpureum (DC.) Urb.) species increased under all tested scenarios, with the greatest growth suppression being caused by the higher densities of Rhodes grass, without water stress and at an elevated atmospheric CO2 concentration.
Despite the complex interactions observed, and their possible change under a future climate, this study has shown that it is possible to successfully integrate chemical control of parthenium weed with a reduction in livestock grazing pressure, the use of biological control agents and suppressive plant mixtures, to improve the community’s species composition and potentially increasing the productivity of the grassland.