Impacts of grazing management options on pasture and animal productivity in a Heteropogon contortus (black speargrass) pasture in central Queensland. 2. Population dynamics of Heteropogon contortus and Stylosanthes scabra cv. Seca
D. M. Orr A C , M. C. Yee A , M. T. Rutherford A and C. J. Paton BA Department of Employment, Economic Development and Innovation, PO Box 6014, Rockhampton Mail Centre, Qld 4702, Australia.
B Department of Employment, Economic Development and Innovation, PO Box 308, Roma, Qld 4455, Australia.
C Corresponding author. Email: david.orr@deedi.qld.gov.au
Crop and Pasture Science 61(3) 255-267 https://doi.org/10.1071/CP09194
Submitted: 3 July 2009 Accepted: 11 December 2009 Published: 9 March 2010
Abstract
The dynamics of Heteropogon contortus and Stylosanthes scabra cv. Seca populations were studied in a subset of treatments in an extensive grazing study conducted in central Queensland between 1988 and 2001. These treatments were 4 stocking rates in native pasture and 2 of these stocking rates in legume oversown and supplement/spring burning treatments. For the 1999–2000 summer, population data for H. contortus in 5 of these native pasture and supplement/burning treatments were compared with those for an additional burnt treatment. Seasonal rainfall throughout this study was below the long-term mean and mean annual pasture utilisation ranged from 24 to 61%. Increasing stocking rate from 5 to 2 ha/steer in native pasture reduced H. contortus plant density. Increasing stocking rate reduced seedling recruitment as a result of its effect on soil seedbanks. Seedling recruitment was the major determinant of change in plant density, although some individual H. contortus plants did survive throughout the study. Burning in spring 1999, particularly at light stocking rate, promoted seedling recruitment above that in both unburnt native and legume oversown pasture and resulted in increased H. contortus plant density. In the legume oversown treatments, S. scabra cv. Seca density increased rapidly from 15 plants/m2 in 1988 to 140 plants/m2 in 2001 following a lag phase between 1988 and 1993. This increased S. scabra density was associated with an eventual decline in H. contortus plant density through reduced seedling recruitment. It was concluded that H. contortus population density is sustainable at stocking rates of 4 and 5 ha/steer (30% pasture utilisation) and that spring burning at light stocking rate can promote H. contortus populations. Increasing densities of S. scabra need to be managed to prevent its dominance.
Additional keywords: stocking rate, legume oversowing, burning.
Acknowledgments
Special acknowledgment is made to the Neill-Ballantine family (Galloway Plains Pastoral Co.) for their ready co-operation and for the long-term use of land on which the study was based. Many people contributed to the overall conduct of this grazing study during its 13-year duration and it is impossible to comprehensively list all these people here. We would especially acknowledge the large number of Department of Primary Industries and Fisheries and Department of Natural Resources and Water staff who provided major contributions to this study’s success. A Producer Advisory Group readily gave constructive advice throughout the study. Funding was provided by Meat and Livestock Australia.
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