Tropical Grasslands (1991) Volume 25, 104110
Sustaining productive pastures in the tropics
5. Maintaining productive sown grass pastures
R.J.K. MYERS1 and G.B. ROBBINS2
1Division of Tropical Crops and Pastures, CSIRO, Brisbane; and
2Queensland Department of Primary Industries 'Brian Pastures' Research Station, Gayndah; Queensland, Australia.
Sown grass pastures in the tropics and subtropics are initially productive, but productivity declines with age, a process commonly referred to as rundown. Run-down is often associated with loss of desirable species. Nitrogen (N) deficiency is the major causal factor.
The initially high production of sown pastures is a transient consequence of increased available N and water that accumulates during fallow, and the run-down condition is the normal equilibrium. The severe N deficiency in soils with apparently adequate total N is due to progressive immobilisation of N and limited mineralisation of humic material.
To increase the productivity of run-down pastures, external N must be supplied or mineralisation of N from either soil organic matter or plant residues must be enhanced. Most potential solutions require either external inputs or exploitation of finite soil reserves.
Potential management options under investigation that may increase productivity by increasing N supply include rotations of pastures with annual crops, sowing pasture legumes, fertilising with N, optimising grazing management, renovating by cultivation, establishing earthworm populations, establishing beneficial shade trees, and changing to stoloniferous grass types. Another more simple solution is to accept that productivity run-down will inevitably occur and to reduce stocking rate if gain per head is to be maintained.