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Population dynamics of volunteer annual grass species as affected by spring pasture management

A.S. Beattie, R.P. Thompson, P. Michell and E.M. Bond

Department of Primary Industry, PO Box 180, Kings Meadows TAS 7249

In the temperate pasture zone of south-eastern Australia replacement of the sown perennial grass species with volunteer annual grass weeds (Vulpia spp., Bromus spp., Poa annua, and barley grasses, Hordeum spp.) is commonly observed. At Oatlands, Tasmania, the population dynamics of an old pasture's grass component was studied.\

Methods

Management treatments were: (i) lax grazing at 12.5 single suckled ewes/ha; (ii) early cutting the forage as silage in November; (iii) late cutting the forage as hay in December; (iv) higher grazing intensity at 20 single suckled ewes/ha. Density of grass species on each of the treatments was determined by counting tillers in 4, 9 cm diameter pre-treatment, (19 October 1989) and 15, 11 cm diameter post-treatment (30 October 1990) cores. Treatments were imposed from 19 October 1989 to 13 December 1989 while from December 1989 through to the October 1990 the treatment sites were treated similarly.

Results and discussion

Ryegrass and Vulpia tiller densities were similar before the treatments were imposed (Table 1). Total density declined from 1989 to 1990 reflecting climatic conditions of the 1989/90 autumn break. Early cutting decreased Vulpia spp. component (P<0.01) of pasture while late cutting increased the amount of Bromus spp. (P<.001) and barley grass (P<.001). There was a trend for early cutting to reduce Bromus spp. component and late cutting to increase Vulpia spp. content. Cutting early and removes seed heads earlier before drying reducing the amount of viable seed and seed shattering. Cutting late allows maturation of seed and seed fall. Higher grazing intensity treatment did not significantly reduce the amount of annuals. Cameron and Cannon (1) noted decreases in Vulpia spp., Bromus spp. and barley grass to trace proportions over five years through increasing grazing intensity.

Table 1.Grass species tiller densities before and after various spring managements.

Use of pasture management to control weeds requires investigation; practices such as early cutting could be used in combination with reduced herbicide application. Removing the dominant annual grass in this experiment left a large proportion of bare ground through the summer allowing invasion by weeds unless the desired species is re-introduced. Investigation is proceeding into the success of chemical free-direct drilling desired perennial species into these pastures where annuals are removed through pasture management.

Reference

Cameron, I.H. and Cannon, D.J. 1970. Proc. XI Int. Grassld Cong. pp. 640-643.

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