Impacts of level of utilisation by grazing on an Astrebla (Mitchell grass) grassland in north-western Queensland between 1984 and 2010. 1. Herbage mass and population dynamics of Astrebla spp.

Abstract

Managing large variations in herbage production, resulting from highly variable seasonal rainfall, provides a major challenge for the sustainable management of Astrebla (Mitchell grass) grasslands in Australia. A grazing study with sheep was conducted between 1984 and 2010 on an Astrebla grassland in northern Queensland to describe the effects of a range of levels of utilisation of the herbage at the end of the summer growing season (April–May in northern Australia) on the sustainability of these grasslands. In unreplicated paddocks, sheep numbers were adjusted annually to achieve 0, 10, 20, 30, 50 and 80% utilisation of the herbage mass at the end of the summer over the ensuing 12 months. Higher levels of utilisation reduced both total and Astrebla spp. herbage mass because of the effects of higher utilisation on Astrebla spp. and this effect was accentuated by drought. The tussock density of Astrebla spp. varied widely among years but with few treatment differences until 2005 when density was reduced at the 50% level of utilisation. A major change in density resulted from a large recruitment of Astrebla spp. in 1989 that influenced its density for the remainder of the study. Basal area of the tussocks fluctuated among years, with increases due to rainfall and decreases during droughts. Seasonal rainfall was more influential than level of utilisation in changes to the basal area of perennial grasses. Drought resulted in the death of Astrebla spp. tussocks and this effect was accentuated at higher levels of utilisation. A series of three grazing exclosures were used to examine the recovery of the density and basal area of Astrebla spp. after it had been reduced by 80% utilisation over the preceding 9 years. This recovery study indicated that, although grazing exclusion was useful in the recovery of Astrebla spp., above-average rainfall was the major factor driving increases in the basal area of perennial grasses. Spring values of the Southern Oscillation Index and associated rainfall probabilities were considered to have potential for understanding the dynamics of Astrebla spp. It was concluded that Astrebla grassland remained sustainable after 26 years when grazed at up to 30% utilisation, while, at 50% utilisation, they became unsustainable after 20 years. Results from this study emphasised the need to maintain the population of Astrebla spp. tussocks.

Management of Grazing Intensity in the Semi‐Arid Rangelands of Southern Australia: Effects on Soil and Biodiversity

“…did occur at this location, it was not common (Table S3). In addition, there may be limited opportunities for root biomass accumulation for Astrebla spp., as this species is contingent on receiving large amounts of warm season rainfall (Orr & Phelps, ). At this location, low‐growing, dominant, shorter‐lived perennial shrubs (e.g.…”

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

“…There is substantial evidence that low to moderate pasture utilisation rates maintain or improve land condition (McKeon et al 2009). For example in a 26 year study on Astrebla grasslands, pasture condition was maintained at a 30 % utilisation rate of dry season standing forage while 50 % utilisation proved unsustainable with a marked decline in pasture condition after 20 years (Orr and Phelps 2013). There is however, a lack of direct empirical evidence showing that stocking at LTCC is more profitable in the longer term than heavy stocking.…”

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

“…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. In a beef grazing enterprise, this challenge can be summarised to the choice regarding the amount of grazing pressure that is applied to the available pasture resources.…”

“…In a beef grazing enterprise, this challenge can be summarised to the choice regarding the amount of grazing pressure that is applied to the available pasture resources. 2 If the stocking rate is too low then beef production is not maximised, although if the stocking rate is too high, then per animal performance falls with detrimental effects on beef production (Scanlan et al 1994;MacLeod and McIvor 2008;Orr and Phelps 2013;Scanlan, et al 2013). Between these extremes, producers have to tradeoff production to optimise performance, where variations in intensity often have only marginal impacts on productivity and profit in agricultural systems (Pannell 2006).…”

“…Land in better condition generally responds more quickly and produces more grass per rainfall event than land in poorer condition, thus 2 The available pasture resource is the amount available for domestic animals after consumption by wildlife, white ants or decomposition is accounted for. Economics of extension programs in grazing systems The Rangeland Journal leading to greater production and higher profits (Orr and O'Reagain 2011;Orr and Phelps 2013;Koci et al 2020). This can be shown conceptually in Fig.…”