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The effect of wheat stubble on the emergence of burr medic

M.A. Ewing1 and C.K. Revell2

1 School of Agriculture, University of Western Australia, Nedlands 6009
Department of Agriculture, Dryland Research Institute, Merredin, W.A. 6415

Large quantities of cereal stubble have often been observed to reduce the emergence and subsequent dry matter production of legume pastures. Possible mechanisms include a reduction in the rate of hardseed softening, physical impedence and/or the presence of phytotoxic residues leached from the decomposing stubble. In 1986 an experiment was carried out to measure the effect of varing quantities of wheat stubble on the emergence of a first year spineless burr medic pasture (Medicago polymorpha var brevispina). The design enabled separation of the direct effect of stubble from its effect on the rate of seed softening.


An area of Serena burr medic was sown into dry soil before the break of the season at a rate of 30 kg/ha (potential plant density 610 plants/m2). Plots 3.3 m x 3.3 m were pegged and stubble residues spread by hand at rates equivalent to 1, 2, 4 and 8 t/ha. Plant establishment counts were recorded four weeks after the initial germinating rains. Seed yields were measured at the end of the growing season.

Results and discussion

At the two lowest rates of stubble there was little reduction in medic seedling establishment. At the two higher rates, medic numbers were reduced but only at the 8 t/ha rate was this reflected in reduced seed production. Observations of these plants showed a substantially longer hypocotyl, perhaps predisposing them to a greater incidence of disease and insect attack.

Table 1. Effect of stubble residues on the emergence and seed production of Serena burr medic.

Since plots were established with newly sown seed and stubble was applied after seeding, plant establishment counts were not confounded by effects of stubble residues on seed softening over summer. The reductions in seedling establishment observed with high rates of stubble are therefore the result of direct chemical or physical effects on plants emerging after the break of the season. These effects however only become important at stubble levels rarely encountered in the field and then only when stubble is concentrated in header trails. Further studies need to be carried out to determine the effect of stubble on changes to the rate of hardseed softening in annual medics.

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