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Silvergrass residue effects on wheat

J.E. Pratley

Charles Sturt University, Wagga Wagga, NSW 2650

Silvergrass (Vulpia spp.) has emerged as one of the major weeds of cereals and pastures in southern Australia. It is particularly difficult to control in direct drilled cereal crops (1, 2), where minimal soil disturbance is effected. Large yield losses usually result. A pot experiment showed that the presence of silvergrass residues caused large reductions in wheat production probably due to the presence of a toxic principle.

Method

Silvergrass residues were collected from the field in Wagga Wagga at various stages of degradation - in December, immediately following senescence; in May, before sowing following a dry summer/autumn period; and in winter.

A series of pots was sown at 1 cm with 20 seeds of wheat (cv. Vulcan) and chopped residue placed evenly on the surface at rates equivalent to nil, 0.5, 1.0 and 1.5 t ha-1. Four replications were used in a randomised block design. Pots were lightly and frequently watered to maintain the residues in moist condition but to avoid drainage of leachate from the pots.

Results and discussion

There was a slight but non-significant reduction in germination rate with the May stubbles, the overall rate being 96%. However, from about the 3-leaf stage, seedlings in the pots treated with May residues showed signs of stress with a proportion dying within the next three weeks. Plants which survived, recovered to normality. The effects were greater with residue burdens of 1.0 and 1.5 t/ha (Figure 1) and were also evident, but to a lesser extent, at the higher rates of the residues collected in August.

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Figure 1. Effect of different residue burdens of Vulpia spp. at different degradation states on wheat growth.

These results indicate an allelopathic effect of silvergrass residues on wheat. This is likely to be of greater significance in direct drilled crops where residues remain on the soil surface. Such effects are also likely to be greater after a dry autumn as the toxin will remain in the residues until leached during the break of seasons close to sowing. Wet autumns are likely to promote the leaching and subsequent breakdown of the toxic principle before sowing occurs, thereby lessening the effect. Further research is needed to identify the toxic principle and to establish management practices to minimise its impact.

1. Forcella, F. (1984). Australian Weeds, 3(1):305.

2. Pratley, J.E. (1985). Proc.3rd Aust.Agron.Conf., Hobart, 385.

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