The successful life cycle of the pasture weed giant rats tail grass (Sporobolus pyramidalis)

Abstract

Giant rats tail grass (Sporobolus pyramidalis) is an unpalatable, perennial, tussock grass that has invaded at least 200 000ha of pasture-land in Queensland. This exotic weed from southern Africa has proven to be difficult to control using conventional weed control techniques, with infestations often re-establishing after substantial control efforts. Clearly, a greater knowledge of the life cycle of giant rats tail grass was required to identify its weaknesses and strengths, which could then be targeted or avoided within control strategies.
For this thesis, three field experiments were conducted to observe the response of giant rats tail grass to various pasture management techniques (fire, slashing, fertiliser, cultivation, sown competitive species and herbicides – Chapter 4) and levels of pasture competition (manipulated via sowing competitive species with a range of growth habits and vigour – Chapter 5; by creating artificial canopy gaps and root exclusion tubes in a native pasture – Chapter 6). The impact of these treatments was assessed in relation to the life cycle stages (soil seed bank, seedling, mature plant) and transitions (germination and emergence, survival and growth, seed production) of giant rats tail grass.
Many strengths within the life cycle of giant rats tail grass were identified and characterised (eg. large long-lived soil seed bank, tough persistent seedlings), while only a small number of weaknesses were discovered (eg. seedling emergence and survival of very young seedlings is sensitive to high pasture competition). The results of this thesis have highlighted why giant rats tail grass has become such a problem weed within Queensland’s grazing industry. However, the information gained will allow the strengths of this weed to be addressed within current control strategies (eg. recognizing the need to maintain the pasture in a healthy competitive condition for many years following the removal of giant rats tail grass plants to prevent re-establishment from the long-lived soil seed bank), therefore increasing the likelihood of successful long-term control.
The major strengths identified within the life cycle of giant rats tail grass were: the large (generally 1000-10000 seeds/m2) long-lived (>3years) soil seed bank; the ability of seedlings to germinate and emerge from only a proportion of the soil seed bank whenever conditions are suitable (eg. above-average rainfall seasons); 6-8 week old seedlings which have begun to tiller are tough and able to survive intense pasture competition; the mature plants are resistant to common agronomic manipulations (fire, slashing, fertiliser) and are long-lived (no plant death due to age was identified during 3 years of experiments); the leaf blades of mature plants are tough and therefore avoided by livestock, which selectively graze other species; the high seed production (up to 80000 seeds/m2); and, very high seed viability (generally >90%).
The weaknesses identified within the life cycle of giant rats tail grass included: seedling emergence and early survival is sensitive to plant competition (no seedlings established within a healthy native pasture sward under full competition); the soil seed bank can be significantly depleted by a fire event (a variable 10-90% reduction), however it is generally replenished by the high seed production in the subsequent season; giant rats tail grass plants are sensitive to some herbicide techniques and if all giant rats tail grass plants are selectively removed from a pasture containing an appropriately managed, vigorous competitive species, successful control is possible; and, some vigorous competitive sown pasture species have been identified for use within giant rats tail grass control strategies in south-east Queensland.
A recurring theme throughout the thesis is the importance of a competitive, well-managed pasture sward to minimise gaps within the pasture throughout the year thus preventing giant rats tail grass seedling establishment from the long-lived soil seed bank. Without a vigorous, competitive pasture being present, any attempts to control giant rats tail grass will be futile.