Volume 37, 111-118

Tropical Grasslands (2003) Volume 37, 111–118

Influence of seedbed preparation and grazing management on seed production of four tropical legumes in the establishment year

C.K. MCDONALD¹, R.M. JONES² and S.J. COOK³

¹ CSIRO Sustainable Ecosystems, Brisbane
² CSIRO Livestock Industries, Brisbane
³ Wesfarmers Landmark, Dalby

Abstract

Two studies in subcoastal south-east Queensland examined factors affecting the seed production of legumes in the first 15 months after being sown into native speargrass (Heteropogon contortus) pasture. Both experiments were sown to a mixture of legumes: roundleaf cassia (Chamaecrista rotundifolia) cv. Wynn, siratro (Macroptilium atropurpureum) cv. Siratro, shrubby stylo (Stylosanthes scabra) cv. Seca and fine-stem stylo (S. hippocampoides). The first experiment compared the effectiveness of sowing into 5 different seedbeds: complete cultivation, 3 methods of minimum disturbance, and broadcasting seed into undisturbed pasture. There were 2 sowings a year, at the start and in the middle of the growing season, in each of 5 years. Four of the 5 years experienced well below average rainfall. The only species that consistently produced seed in the year of sowing was Wynn cassia, and then only in the fully cultivated seedbed where seed reserves in sowings made early in the growing season averaged about 3000 seeds/m². Seed production in treatments with minimum or zero disturbance was negligible.
The second experiment examined the effect of grazing on the seed production of legumes oversown using a minimum-cultivation technique where legumes were sown in a shallow groove in a herbicide-treated strip in an otherwise undisturbed native pasture. Grazing, at 0.65 head/ha, was imposed immediately after sowing or after 3, 8 or 15 months. Two other treatments examined the effect of a high stocking rate (1.2 head/ha) imposed immediately after sowing and of complete exclusion from stock. As in Experiment 1, Wynn cassia produced by far the most seed, followed by fine-stem stylo. Seca and siratro produced very little seed. Cassia produced seed in all treatments, with seed reserves 18 months after sowing ranging from 250 seeds/m² at the high stocking rate to 770 seeds/m² in the absence of grazing. The implications of the results for successful oversowing of legumes into native pasture are discussed.