Strategic grazing for the control of the invasive wetland weed lippia (Phyla canescens)

Background/Question/Methods Lippia (Phyla canescens) is an invasive weed from South America that is now widespread throughout the Murray-Darling Basin in Australia.  Lippia has invaded internationally significant wetlands often forming a dense ground-layer that excludes co-occurring native species and threatens the integrity of these wetlands. Lippia is unpalatable to grazing stock and in areas where invasion has occurred the loss of productive land causes managers to destock (55 - 100%).  Traditional weed control measures including herbicides and mechanical means, are often impractical and unsuitable, particularly in environmentally sensitive areas and alternative measures need to be devised.  Lippia is primarily dispersed by flood waters and observations indicate that establishment is enhanced by areas of bare ground. The grazing regime is generally set stocked/continuously grazed in these wetlands. We are using strategic grazing to manipulate species composition in order to select for desirable perennial species.  In particular, we addressed whether providing periods of rest from grazing permits native species to establish and out compete lippia in different hydrological zones in several wetland areas.  Small exclosure cages (2 m x 2 m) were used on a fixed and rotational basis to preclude grazing stock, thereby providing a rest period at different stages of the year.

Results/Conclusions Results suggest that maintaining native cover is an effective means of lippia control, with significant reductions in lippia biomass found with increased biomass of co-occurring species. Providing rest from grazing did promote increased growth of native species with significantly larger increases found in a wet year in contrast to a dry year.  The timing of rest that favoured the growth of native species differed between wetland sites, due to climatic differences and species composition (summer vs. winter active species).  Complex interactions between flooding and grazing drive vegetation responses in these ephemeral wetlands, and at this stage maintaining native cover is more closely linked to flood events than grazing management per se. The expansion of irrigated agriculture and construction of dams in the upper catchments has severely altered flow regimes in these wetlands, by reducing the frequency and duration of flooding and this is believed to have favoured the spread of lippia.  Combining strategic grazing or rest periods with environmental flow releases or natural flood events may provide increased community resilience to weed invasions and reduce the spread of lippia in these wetlands.