Volume 27, 261

Tropical Grasslands (1993) Volume 27, 261–275

Tropical pasture establishment
1. A systems perspective of establishment illustrated by legume oversowing in the subtropics

D. GRAMSHAW¹, G.M. McKEON¹ and R.L. CLEM²

¹Department of Primary Industries, Brisbane, and
²Biloela, Queensland, Australia

Abstract

A systems schema of pasture establishment is presented to provide a generic framework for understanding and analysing establishment processes, problems and outcomes. The system represents the establishment continuum by 8 phases; the first 4 deal with seed supply phases and their influence on seed properties before sowing, the other 4 with post-sowing field establishment sequences which determine the capacity of the developing sown species to contribute future production or resource-maintenance benefits.
Key biological, environmental and management variables operating in each phase and likely to influence establishment outcomes are illustrated through a case study of low input, oversown shrubby stylo (Stylosanthes scabra), a particularly useful example in which a species with high levels of hardseededness is sown into a difficult establishment environment. Aspects highlighted, which in some instances include the presentation of predictive relationships, are: the variation in hardseededness of legume seeds from different geographical sources or production environments, which is shown to be closely related to seed moisture content; the possible modification of hardseededness through different methods of seed harvesting and processing or by deliberate, physical modification of seeds before sowing; post-sowing softening of hard seed in response to surface temperature thresholds, and the increases in potential speed of germination related to small falls of rain before a significant field germination event; a dominant influence of water availability on germination and early seedling establishment; a demonstrated relationship between seedling death rates and soil water deficits in the root zone which becomes more acute under conditions of high evaporative demand and where there is competition from other plants; and the impact of enforced juvenility of seedlings, caused by competition, in delaying reproduction and increasing plant deaths when moderate frosting occurs.
A probabilistic assessment of the risks associated with oversowing is derived from a series of experimental field sowings of shrubby stylo in the Queensland subtropics. This quantifies the quite low levels and high risks of establishment associated with low input methods, and the appreciable improvement and moderation of these, respectively, when competition from other plants is controlled and when the best seed treatments are used to reduce hardseededness. An economic analysis of a number of establishment scenarios also illustrates the considerable financial penalty incurred with low input oversowing methods when pastures develop slowly or there is an establishment failure and the pasture is resown.