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

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.

How hot? How often? Getting the fire frequency and timing right for optimal management of woody cover and pasture composition in northern Australian grazed tropical savannas. Kidman Springs Fire Experiment 1993–2013

“…Hence, increased rainfall may have driven the observed increases in the herbage mass and ground cover of H. contortus in this study, elsewhere on the Victoria River Research Station (Bastin et al 2003) and in northern Western Australia (Watson and Novelly 2012). The faster transition from arid short grass to H. contortus seen inside grazing exclosures (Bastin et al 2003), reflects the sensitivity of density and plant survival of H. contortus to grazing (Orr et al 2010a(Orr et al , 2010b and suggests grazing levels are likely to have mediated the rate of compositional changes under the current higher rainfall regime.…”

“…Burning has previously been found to promote H. contortus, but more so if livestock were removed or reduced following fire (Orr et al 1991(Orr et al , 2010a(Orr et al , 2010bFurley et al 2008). Burning and light grazing increases recruitment and density of H. contortus (Orr et al 2010b), enhancing soil seedbanks and germination (Campbell 1996).…”

“…Burning has previously been found to promote H. contortus, but more so if livestock were removed or reduced following fire (Orr et al 1991(Orr et al , 2010a(Orr et al , 2010bFurley et al 2008). Burning and light grazing increases recruitment and density of H. contortus (Orr et al 2010b), enhancing soil seedbanks and germination (Campbell 1996). However, heavy grazing reduces the herbage mass of H. contortus (Orr et al 2010a), both through removal of recent growth by grazing, but also lower growth due to decreased survival, density, basal area, seedling recruitment and soil seedbanks (Orr et al 2010a(Orr et al , 2010b.…”

“…Burning and light grazing increases recruitment and density of H. contortus (Orr et al 2010b), enhancing soil seedbanks and germination (Campbell 1996). However, heavy grazing reduces the herbage mass of H. contortus (Orr et al 2010a), both through removal of recent growth by grazing, but also lower growth due to decreased survival, density, basal area, seedling recruitment and soil seedbanks (Orr et al 2010a(Orr et al , 2010b. Heteropogon contortus is more preferred by cattle when its herbage mass is low (Hendricksen et al 2010), and when it is burnt (Paton and Rickert 1989;.…”

Can extension programs improve grazing management in rangelands: a case study in Australia

“…Decisions about herd management and stocking pressures, particularly in drought years, have long-run resource condition implications (Ash et al 1995;O'Reagain et al 2011).Overgrazing, in different formats, can degrade pastures and land condition and cause other effects such as erosion. Although overgrazing can be simply the effect of higher stocking rates, it can also occur in more subtle ways, such as on localised areas of fragile vegetation or soils, in waterways and frontage areas in wet seasons, or through failure to destock appropriately in drought years (Orr et al 2010;Orr and O'Reagain 2011;O'Reagain et al 2011). It may also relate to management styles, where techniques such as pasture resting, wet season spelling for frontage areas or more varied access to water can limit impacts of stock on soils (McKeon et al 2004;Orr and Phelps 2013;Moravek and Hall 2014) The challenge in any production enterprise is to identify the optimal point of production where profit is maximised.…”