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Seed bank dynamics of Vuilipa in pastures

C.E. Jones, R.D.B. Whalley, J.V. Lovett' and S. McIntyre

Departments of Agronomy and Soil Science, Botany, 'Ecosystem Management, University of New England, Armidale NSW 2351

Vulpia spp. are cool-season annual grasses which originated from the Mediterranean region and have successfully invaded large tracts of land in southern and eastern Australia. They are considered undesirable components of pastures due to relatively low dry matter production and their capacity to produce large numbers of sharp-pointed, awned seeds which are both a contaminant of wool and a cause of eye injury to sheep. Autumn sowings of improved pasture on the Northern Tablelands of NSW can fail completely in the presence of high populations of Vulpia spp., most commonly mixed stands of V. bromoides and V. myuros. Effective management of these weeds requires an understanding of factors affecting induction and/or enforcement of seed dormancy, longevity of seed in soil, seedling recruitment and seed production of established plants.

Methods

Dynamics of the soil seed bank of Vulpia spp. was assessed during 1990 and 1991 by (i) recording emergence within finely meshed grazing exclosures which prevented the ingress of both wind-borne and animal-borne seeds, (ii) collecting soil cores both before and after Vulpia spp. flowering and monitoring emergence in the glasshouse, (iii) establishing artificial banks of known numbers of viable seeds of V. myuros and V. bromoides under different levels of pasture cover and (iv) collecting data from natural field populations under a range of grazing, mowing and spraying regimes.

Results and discussion

Emergence of seedlings of Vulpia spp. within paddock exclosures and from soil cores demonstrated that the soil seed bank could persist for at least three years, even when adequate soil moisture had been available for germination in previous autumn and winter periods. The residual seed bank in both 1990 and 1991 was approximately 1%. This figure appears low but is of biological significance. Vulpia spp. plant populations in the paddocks sampled in 1990 ranged from 386 to 35,952/m2, with an average population of 7,655/m2. If all seed production was completely prevented from these plants for one year, recruitment from the residual seed bank in the following year could give rise to between 4 and 360 plants/m2 with an average of 77 plants/m2. It is likely that from these levels populations could return to their original densities within two years of treatment.

Control methods have little chance of success in the long term unless they are aimed at reducing the seed bank to negligible levels and preventing its replenishment. An ungrazed sward of warm season perennial native grasses (4 t/ha dry matter) during autumn 1991 was found to inhibit V. myuros emergence by 80% and V. bromoides emergence by 61%. During the winter months, pasture cover continued to have an inhibitory effect on the emergence of V. myuros but had no effect on V. bromoides. However, V. bromoides plants which emerged in March 1990 produced 53 times more seed than did plants which emerged in July of the same year. Since Vulpia are annual species they are reliant on seed production for regeneration and if delayed seedling recruitment reduces the capacity of a given population to produce seed then this is of significance for population control. If heavy grazing prior to seeding in spring can further reduce reproductive capacity then effective long-term management of these grass weeds may be achievable. Vulpia spp. populations of 20 plants/m2 were recorded in a paddock at the Newholme Field Laboratory, near Armidale, NSW, which had been grazed heavily in spring for three years in succession. Average populations in equivalent areas of adjoining paddocks from which stock had been excluded in spring were 13,160 and 10,200 plants/m2. It would appear that the use of heavy grazing in spring to prevent seed bank replenishment and light or nil grazing in autumn to inhibit early seedling recruitment could be a viable means of reducing the seed bank and controlling population densities of Vulpia spp. Larger scale field trials are currently underway to evaluate these strategies.

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