Seed production of Trifolium subterraneum subsp. brachycalycinum as influenced by soil type and grazing

Abstract

Twenty-six strains of Trifolium subterraneum subsp. brachycalycinum were assessed as potential pasture legumes on the 2 major soil types (neutral-slightly acid sandy [sandplain] soils and alkaline mallee soils) near Esperance, W.A. The pH (1 : 5, soil: water, w/v) of the top 10 cm of the sandplain soil was 6.3, and of the mallee soil, 8.2. In ungrazed swards on both soil types, subsp. brachycalycinum buried very few burrs but successfully produced seed in the unburied burrs in the dark, dense canopy of leaves. However, when the swards were grazed up to the start of flowering of the earliest flowering strains, subsp. brachycalycinum produced few burrs and little seed (0-47 kg/ha). By contrast, on the sandplain soils, subsp. subterraneum buried most of its burrs and produced much seed in both ungrazed (390-1050 kg/ha) and grazed (403-987 kg/ha) swards, whereas on the alkaline mallee soils, annual medics (Medicago spp.) produced much seed in unburied burrs in both grazed (506- 1050 kg/ha) and ungrazed (480-730 kg/ha) swards. Grazing may have exposed the developing unburied burrs of subsp. brachycalycinum to light, which is known to inhibit the development of burrs and seed. There was no relationship between seed yield and maturity for strains of subsp. brachycalycinum in ungrazed swards on both soils.