A grazing study conducted between 1979 and 1983 assessed the seasonal trends of ewe productivity in Astrebla grassland in south- western Queensland. This study was designed originally to compare productivity on two pastures with different compositions, however, these differences in pastures composition were not achieved. Large differences in liveweight, wool growth and reproductive performance occurred between years in response to differences in pasture growth resulting from large variation in the seasonal incidence of rainfall. Rainfall effective for plant growth, both forbs and grasses, resulted in a high quality diet which resulted, in turn, in increased sheep productivity. Reproductive performance was particularly sensitive to the quality of the ewes diet around the time of lambing through the effect of diet quality on lamb survival and growth rate. It is suggested that the effect of rainfall on reproductive performance is pasture.
The basal area, yield and plant populations of Astrebla spp. were monitored under grazing and exclosure in Astrebla grasslands between 1975 and 1986. This study was undertaken to develop an understanding of how Astrebla spp. respond to grazing, particularly in relation to the high variability of rainfall. Basal area and yield of Astrebla spp. varied widely between years in response to summer rainfall, with few differences between exclosure and grazing. The failure of summer rainfall resulted in increased utilization of Astrebla spp. but this effect was partially offset in some years by the growth of forb species following winter rainfall. Grazing stimulated the recruitment of Astrebla spp. seedlings by increasing the density of inflorescences which, in turn, increased seed production. Survival of seedling cohorts depended on adequate summer rainfall and grazing tended to enhance cohort survival. There was a greater plant flux under grazing than under exclosure. It was concluded that rainfall is the major factor influencing the biomass and plant population dynamics of Astrebla spp. and that grazing up to about 30% utilization is not detrimental to Astrebla spp.
A field study was conducted to examine the process by which grazing promotes seed production in Astrebla spp. (predominantly A, lappacea). To simulate grazing, plots within an area of lightly grazed Astrebla grasslands were clipped once, at a range of severities, at the end of the dry season. Grazing pressures ranging from 10% to 90% utilization of A. lappacea were simulated by the clipping treatments. Increasing the severity of clipping gave increased seed production, by increasing the density of inflorescences borne on secondary tillers. In contrast, inflorescence density on main tillers was unaffected by clipping. We concluded that grazing promotes seed production in Astrebla spp. through its effect on inflorescence density on secondary tillers.
Recruitment of Astrebla spp. (Mitchell grass) may be restricted by high densities of the summer, annual grass Iseilema spp. (Flinders grass). This paper reports a glasshouse study undertaken to assess the competitive ability of A. lappacea in relation to that of Iseilema spp. and also that of the summer, annual forb Hibiscus trionum (Bladder ketmia). Astrebla lappacea was equally competitive with H. trionum but more competitive than Iseilema sp. However, it is concluded that, in the field, the recruitment of Astrebla spp, may be restricted by high densities of Iseilema spp. It is unlikely that the density of summer forb species are sufficient to restrict Astrebla spp.
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