School of Botany - Theses
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ItemThe ecology of Agrostis spp. (bent grass) invasion into temperate pastures( 1996)The grass, commonly called 'bent grass', has been rated by meat producers and consultants working in industry as one of the worst weeds of perennial grasses in temperate pastures in Victoria. Bent grass in Victorian pastures has traditionally been identified as Agrostis capillaris. However it has become clear that Agrostis castellana rather than A. capillaris is predominantly found in Victorian pastures. Management techniques to reduce the proportion of A. castellana in pastures must be modified based on the assumption that we are dealing with A. capillaris. The vegetative morphology of A. castellana populations sampled from diverse locations within Victoria was similar. A. castellana spreads vigorously by rhizomes which will reduce the success of presently-accepted management techniques (e.g. mob stocking) but will enhance the success of techniques that focus on reducing the rhizomatous capability of A. castellana. Flowering times differed between A. castellana populations such that genotypes of populations from drier climates completed flowering earlier or within a shorter time than genotypes from other populations. Techniques which focus on preventing floral development (e.g. spray-topping) will require refinement for different locations in Victoria. The cost of renovation combined with limited success to reduce the proportion of Agrostis spp. in the long term (> 10 years) has reduced the adoption of pasture renovation in some areas. Agrostis spp. regenerated from seedlings after pasture renovation, at variance with general conceptions. Thus pasture renovation techniques must reduce the regenerative ability of Agrostis spp. from seed. Agrostis castellana regenerates from viable rhizome fragments remaining after cultivation, in addition to seedling regeneration. Rhizomes of A. castellana displayed apical dominance in that lateral buds on rhizomes remain dormant. Presently-accepted pasture renovation does not take into account the fact that apical dominance in rhizomes ensures continuative regeneration. The technique of pasture renovation must ensure that the regenerative ability of bent grass is severely reduced, and this was achieved when rhizomes were broken into small 1-node fragments, buried beyond 75 mm and a rapidly establishing pasture sown. The dominance of bent grass in pastures indicates that bent grass competes successfully with other pasture species under current agricultural practices in Victoria. The mechanism of competition under A. castellana-dominant pastures supported the resource competition theory in that a greater proportion of the nitrogen absorbed under A. castellana pasture remained in the roots than in shoots, reducing the availability of nitrogen to be re-cycled in faeces and urine by the animal. Thus a greater proportion of nitrogen was returned by plant death and decomposition in A. castellana-dominant pasture than in L. perenne-dominant pasture. Decomposing shoots, roots and rhizomes are slow to release mineral nitrogen which is confirmed by reduced gross mineralization and reduced concentrations of nitrate in soil solution under A. castellana-dominant compared with L. perenne-dominant pasture. N uptake by A. castellana was as nitrate, in preference to ammonium, further depleting the concentration of nitrate in soil solution. When L. perenne grows in association with A. castellana, plant nitrate requirements are unlikely to be met and L. perenne pasture may then degrade to A. castellana-dominance. The likelihood of A. castellana dominance in L. perenne pastures may be reduced by improving litter quality and rates of mineralization.