Stability of the Townsville stylo-annual grass pasture ecosystem on cleared Tippera clay loam

Abstract

Competition was measured between the annual self-seeding legume Townsville stylo (Stylosanthes humilis) and each of the annual grass weeds Digitaria ciliaris, Brachiaria pubrigera, Echinochloa crus-galli and Brachyachne convergens. Digitaria was the most competitive of the grasses, all of which were stronger competitors than the legume in the main period of growth. The results of the competition experiments assisted in interpretation of nine years' observations of vegetation pattern in a Stylosanthes pasture sown in a cleared woodland on Tippera clay loam at Katherine, N.T. An apparently stable situation was reached in the eighth year, when the legume occupied 14 per cent, palatable grasses (primarily annuals) 3 per cent and unpalatable grasses (Brachyachne, Aristida) and bare ground 83 per cent of the pasture area, the corresponding dry matter yields being 12, 35 and 53 per cent respectively of the total pasture yield. The Townsville stylo-annual grass pasture ecosystem is basically unstable in cleared and ploughed woodland where it will persist only under careful grazing management. It is suggested that, for the situation studied, Stylosanthes humilis should be replaced by Stylosanthes hamata, since in other experiments it has proved more competitive than Stylosanthes humilis. The implication of the results for further land development and experimentation in the Katherine area is briefly discussed.