Review of Stability and Productivity of Native Pastures Oversown with Tropical Legumes NAP Occasional Publication 1

Summary

Native pastures oversown with introduced legumes are an important component of beef production systems in northern Australia. These pastures can give greater animal production than native pastures but there are increasing concerns about the stability and productivity of these systems. It is important for these issues to be further investigated and in September 1996 the MRC commenced a review to document current information on the issues, clearly outline the major issues and problems, identify and outline appropriate management practices/strategies based on current knowledge, identify critical deficiencies in the current knowledge and practices and outline potential actions and R&D activities to overcome them.

The results of the review are contained in this report. The impacts of oversown pastures on soil, vegetation and animal production are reviewed. Most oversown legume pastures have been developed by sowing the seed on the soil surface with no soil disturbance after burning or heavily grazing the existing native pasture. Establishment is usually slow and the legumes frequently contribute little to the sward in the initial years. However in most situations the legume increases with time and in many cases comes to dominate the sward. This may result in reduced biodiversity but this area requires more research to adequately assess the impacts.

Animal production is higher from legume-based pastures than native pastures with most advantage occurring during the late wet and early dry seasons. Legume-based pastures can increase soil fertility but serious declines in soil pH have been measured under tropical legume pastures bringing into question the long term productivity of these systems. The population biology of tropical legumes and grasses is discussed along with factors (seasonal conditions, soil type, fertiliser, animal supplements, grazing pressure, fire, associate grass, tree clearing, legume species/cultivar, geographic location, plant competition, spatial distribution) affecting the composition and stability oflegume-based pastures. Practices available to manage botanical composition in legume augmented pastures are burning to promote perennial grasses and manage legume populations, grazing management to favour the grass or the legume, sowing a grazing tolerant grass when sowing stylos, and choosing soils with a high phosphorus level or applying superphosphate.

Knowledge gaps are listed and recommendations made for future research and development (a detailed inventory of existing commercial pastures; a grazing trial to examine combinations oflevels of pasture utilisation, pasture spelling, fire and sown grasses on the stability and productivity of pastures which are currently legume dominant; suitable information systems be developed to capture knowledge of the performance and management of pasture species, and to make this widely available to users; continued monitoring of fertility decline and soil pH changes).