Managing grazing lands in Queensland

Abstract

http://www.qld.gov.au/dsitia/assets/soil/managing-grazing-lands-in-qld.pdf Managing grazing lands in Queensland 3 Land and pasture degradation Land degradation has had an enormous impact on the produc tivity and biodiversity of some of the state’s grazing lands. Forms of degradation include: • soil erosion by both water and wind • invasion by exotic weeds including succulents, vines and woody species • thickening of native species • reduction in pasture quality through selective grazing or overgrazing • salinity. The introduction of cattle with a greater ability to fora ge under adverse conditions, combined with the use of drought supplements, has had a negative affect on land condition. That is, when a drought breaks increased numbers of stock feed on the growing grasses. Erosion by water begins when raindrops fall on bare soil. Raindrop splash and the dislodgement of soil particles cause the surface to seal, and runoff begins. Lighter textured , bare soil surfaces are also susceptible to wind erosion as winds move much faster over a soil surface without vegetativ e cover. If all of the topsoil is removed, the land is referred to as being ‘scalded’, with the term ‘c lay pan’ also used to describe such land. Erosion reduces productivity by dislodging topsoils that are often very shallow and contain the majority of available nutrients and organic matter in the soil profile. In arid, low nutrient landscapes, many of the available nutrients can be lost when dust particles are lifted by a dust storm. Eroded soils are subject to more extreme temperature ranges and have lower porosity and microbial activity. A downward spiral of degradation begins when high runoff rates result in reduced entr y of water into the soil causing less plant growth. Land degradation can have a si gnificant affect on natural biodiversity by reducing habitat quality for most living things—whether above or below ground level, or in water. Nutrients are made available to pastures via the process of mineralisation and the breakdown of organic matter. Fertility decline, including a decline in soil carbon levels in pasture lands is a comm on occurrence especially where land is subject to continued hea vy grazing pressure and erosion. Virtually no fertilisers have ever been used in the rangela nds of Queensland because thei r use is not considered to be economic. In many areas a positive response to fertilisers would be likely in good seasons when soil moisture is not limiting plant growth. However, the high cost of purchasing and distributing fertiliser in areas with an unpredictable climate generally rules out any opportunity for its use. Once land condition deteriorates, its ability to grow usef ul pasture declines and it becomes more susceptible to extreme weather events. In GLM workshops graziers are provi ded with information to help them classify land into four different conditions that vary from condition A to condition D. Conditi on A has a good coverage of desirable pastures species, few weeds, good soil condition and no problems from woodland thickening. Condition D land can have problems such as a general lack of desirable past ures, severe erosion resulting in a hostile environment for plant growth and thickets of woody plants over most of the area Table 1 provides an example for the Poplar box with sandalwood understorey land type in the Maranoa Balonne GLM Region. For the same land type, land in very good condition (GLM Condition A) produces more pasture and has a higher safe carrying capacity than land that has been subjected to significant degradation (GLM Condition C) Table 1 How land condition affects safe carrying capacity Data applicable to the Poplar box with sandalwood understorey land type in the Maranoa Balonne GLM Region GLM condition A GLM condition C Average dry matter production kg/ha/year (as calculated by the GRASP * Model) 3000 1350 Average amount of pasture available assuming a 20% utilisation rate (kg) 600 270 Pasture consumed by a 450 kg steer in one year (kg) 3650 3650 Number of hectares required for one 450 kg steer 6.1 13.5 Safe carrying capacity for a 500 ha paddock (450 kg steers) 82 37 * GRASP is a pasture growth model for tropical and sub-tr opical grasses which can be calibrated for a broad range of pasture communities, soil types and climatic conditions. Sustainable stocking rates and favourable seasons may, over time, allow land to improve its condition and to store increased levels of soil carbon. However, land in condition D will, in most cases, have lost all of its topsoil and preferred grass species. It may be incapable of being brought back to a better land condition. Gully erosion occurs in drainage lines or other areas wher e runoff has concentrated. It may occur as an isolated event or over large areas, especially where there are soils with highly erodible subsoil. Stream bank erosion is a common problem in most Queen sland river systems. River frontages are particularly subject to erosion in areas where stock congregate. The movement of sediment, nutrients and organic matter may adversely affect water quality in streams and the coastal environment. Soils with highly erodible, dispersible sub-soils contribute significantly to turbid runoff. Dispersible soils are structurally unstable and when e xposed to water readily break down into their constituent particles of sand, silt and clay. Individual clay particles ar e very fine and remain suspended in water rather than settling out. This leads to a turbid or m uddy appearance in dams and watercourses.