Tropical pasture systems are typically dominated by C4 grasses growing on nitrogen (N) deficient soils. Under these conditions, N2-fixing legumes should have a competitive advantage, yet low legume contents are often reported in these systems. This work investigates whether below-ground competition for phosphorus (P) is limiting the ability of legumes to compete in swards of C4 grasses when grown in a sand matrix. The external P requirement of a subtropical legume (butterfly pea, Clitoria ternatea L.) and a C4 grass (buffel grass, Cenchrus ciliaris L.) were initially determined in a P-response experiment. Four rates of P (4.6–78.2 mg P kg–1 of Colwell P) were subsequently selected to investigate the growth response of the butterfly pea when grown with and without competition from a sward of N-deficient buffel grass. Shoot dry matter was determined over successive cuts and P uptake determined at the final harvest at 72 days. Buffel grass dominated the mixed swards and reduced the shoot dry matter production of the butterfly pea by >50% relative to the pure swards. A significant difference in the soil P response curve and shoot P uptake of butterfly pea was not detected between pure swards and those with competition from buffel grass. The ability of C4 grasses to acquire and convert resources (i.e. light, water and nutrients) more efficiently into shoot dry matter is likely to be a major factor resulting in grass-dominated pastures in tropical systems.
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Vol. 64 • No. 10