Impacts of grazing management options on pasture and animal productivity in a Heteropogon contortus (black speargrass) pasture in central Queensland. 1. Pasture yield and composition

Abstract

An extensive grazing study was conducted between 1988 and 2001 in a Heteropogon contortus (black speargrass) pasture in central Queensland. The study was designed to measure the effects of stocking rate on native pasture, native pasture with legume oversown, and native pasture with animal diet supplement/spring burning on pasture and animal production. Summer rainfall throughout the study was below the long-term mean. Mean annual pasture utilisation ranged from 13% at 8 ha/steer up to 61% at 2 ha/steer. Increasing stocking rate treatments reduced total pasture yields while total yields in legume oversown treatments were similar to those in native pasture at the same stocking rate. When spring burning was possible, total yields were reduced in the subsequent autumn. Increasing stocking rate in native pasture tended to reduce H. contortus and Bothriochloa bladhii, increased the composition of intermediate species, such as B. decipiens and Chloris divaricata, and also changed the frequencies of a range of minor species. Oversowing legumes resulted in Stylosanthes scabra cv. Seca increasing from <1% of pasture composition in 1988 to 50% in 2000 and was associated with a reduction in H. contortus and changes in the frequencies of some minor species. Stocking rates heavier than 4 ha/steer resulted in annual pasture utilisation greater than 30% and were unsustainable because they reduced total yield and resulted in undesirable changes in species composition. It was concluded that pasture production was sustainable when stocking rates were maintained at 4 ha/steer, which equates to 30% annual pasture utilisation, and through the judicious use of spring burning.

Sustainable grazing management for temporal and spatial variability in north Australian rangelands – a synthesis of the latest evidence and recommendations

“…Although rainfall over the trial period was generally well below average, no major pasture composition change occurred. Nevertheless, some preliminary degradation was recorded indicating that the highest stocking rates were not sustainable (Orr et al 2010).…”

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.…”

“…Fire has previously been found to reduce the herbage mass of A. latifolia, both with (Orr et al 2010a) and without (Orr et al 1991) grazing. Cattle are more likely to graze A. latifolia plants when the herbage mass is low (Phelps 2007), so fire could also increase grazing pressure on this less preferred species post-fire.…”

“…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 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;.…”