R. HEIDA¹, and R.M. JONES²

¹Agricultural University, Department of Field Crops and Grassland Science, Haarweg 333, 6709 RZ Wageningen, The Netherlands
²CSIRO Division of Tropical Crops and Pastures, 306 Carmody Road, St. Lucia 4067, Australia

Abstract

Seed reserves of barrel medic (Medicago truncatula) and snail medic (Medicago scutellata) in the topsoil of a fertile brigalow clay soil were measured in 1985 at Narayen in subcoastal south-east Queensland. Measurements were made on 2 grazed crop-pasture rotations where medic was sown every fourth year, on an ungrazed crop-pasture rotation sown to medic every year and on a 10-year-old permanent pasture. Seed reserves to a depth of 10 cm were highest under permanent pasture (14,200 seeds/m²) followed by the ungrazed ley (8600/m²), a 3-year pasture: 1 year-wheat rotation (6400/m²) and a 2-year pasture: 2 year-wheat rotation (2300/m²). In the surface 10 cm of soil under permanent pasture, 96% of medic seed was in the top 5 cm of soil compared with 77% in ley pastures. The percentage of seed recovered in pods was highest in the top 5 cm of soil (70–80%) and lowest at 15–20 cm (< 5%). Seedling emergence after adequate germinating rain in early winter accounted for less than 10% of the reserves of medic seed. The possibility of low densities of medic seedlings limiting medic productivity is discussed