Stability and productivity of Stylosanthes pastures in Australia. I. Long-term botanical changes and their implications in grazed Stylosanthes pastures

Abstract

Published and unpublished data from northern Australia on the botanical changes in stylo-based pastures grazed for 5-15 years have been reviewed. Soil available-P status, soil type, grazing management, age of pasture and associated grass species all influenced the level of stylo in the pastures. In general, on soils with >4-5 ppm available P (bicarbonate extraction), the stylo component in oversown native pastures increased over time. On those with low P levels, superphosphate improved stylo growth and hastened stylo dominance. Stylo dominance was achieved in many situations in 5-10 years, especially where Seca was sown. Light grazing may favour spread of stylo, especially Seca, initially, but over time pastures more heavily grazed also became stylo-dominant. Stylos on lighter textured soils within landscapes achieved dominance before heavier textured soils in low-lying areas. There, the vigour of associated grasses, waterlogging and frosting in subtropical areas delayed or prevented stylo dominance. Sown grasses, such as Sabi grass and buffel grass, and the naturalised grass, Bothriochloa pertusa (Bowen strain), unlike the native tufted species, prevented total stylo dominance, and in some cases, prevented the ingress of dicot weeds common to stylo-dominant pastures. Stylo-dominant pastures do not reduce animal gains greatly relative to those on mixed grass-stylo pastures except early in the rainy season. However, stylo-dominant pastures can be ecologically unstable and allow entry of undesirable dicot weeds and annual grasses. They may also provide poor ground cover to protect the soil during early wet season storms. Stylo-dominant pastures have shown increased acidification over time which may influence the soil profile down to 70 cm. This effect is reduced in association with persistent perennial grasses. Options to prevent stylo dominance include suitable burning strategies to manage Seca, grazing management to alleviate severe grazing of the perennial grasses in the early wet season and the use of grazing-tolerant sown grasses. None of these options has been tested widely within a grazing system. The use of superphosphate to improve grass production would be economically viable only on country of potentially high carrying capacity. Choice of perennial grass is important to ensure survival of the associated stylo and the maintenance of good animal production. The value of long-term grazing experiments for assessing the impact of botanical change has been demonstrated.