Volume 27, 65

Tropical Grasslands (1993) Volume 27, 65–74

Population dynamics of Siratro and shrubby stylo in south-east Queensland as affected by phosphorus, soil type, stocking rate and rainfall

R.M. JONES, P.C. KERRIDGE and R.W. McLEAN

Division of Tropical Crops and Pastures, CSIRO, St Lucia, Queensland, Australia

Abstract

The effects of P application and stocking rate on the persistence of Siratro (Macroptilium atropurpureum cv. Siratro) and shrubby stylo (Stylosanthes scabra cv. Fitzroy) were monitored in 2 experiments over 9 years on 2 soil types in subcoastal, subtropical Queensland. Experiment 1 examined the effects of 2 stocking rates (0.8 and 1.2 steers/ha) and annual rates of P application, ranging from 0–15 kg/ha P, on Siratro. Experiment 2 compared the response of Siratro and Fitzroy to a restricted range of P and S treatments.
Phosphorus application increased the yield of both legumes, but had only a negligible effect on the survival of the original plants. However, P application to Siratro resulted in the soil seed bank of the treatment receiving 15 kg/ha P (P 15) being some 5 times higher than in the treatment receiving no P (P0).
This in turn led to more recruitment of Siratro in the P-fertilised plots and a 3-fold higher plant density in the P15 treatment compared with the P0 treatment at the end of the experiment. However, the density of Siratro decreased with time as recruitment of new plants did not compensate for death of older plants. The lower than median rainfall may have adversely affected Siratro recruitment. There was very little effect of stocking rate or soil type (red earth or yellow podzolic) on Siratro persistence.
Fitzroy density increased with time whereas Siratro density declined. This is attributed to the higher seed set of Fitzroy which lead to reserves of soil seed being more than 100 times those of Siratro after 9 years and to the successful recruitment of stylo plants from seedling strikes. Appreciable quantities of viable seed of Fitzroy were excreted in cattle dung.