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Two serious weed species in the Northern Territory are Sida acuta, a perennial weed of improved pastures, and Pennisetum pedicellatum, an annual grass that infests grain sorghum crops. In P. pedicellatum the customary successive ploughing carried out to kill seedlings in the early wet season may not give satisfactory control of the weed, while with S. acuta there appears to be a long-term dormant seed pool which can give rise to new generations of seedlings, Investigation into the germination requirements of the 2 species showed that both are dormant at seed fall and require high alternating temperatures to remove an after-ripening requirement. In addition S. acuta needs a further period at high temperature to fracture its impermeable seed coat. Almost all P. pedicellatum seed is non-dormant at the end of the dry season, but that of S. acuta still retains 30% hard seed after one dry season. In the field P. pedicellatum germinated faster than S. acuta, with 80% of the total wet season germination occurring with the first rain. In S. acuta germination was spread over the first 2 months of the season. Ploughing immediately after the break of the wet season killed seedlings of P. pedicellatum, but could bury viable dormant seed. Once buried, this store of dormant seed could remain viable and germinate when disturbed at crop planting. At the end of the wet season no viable seed of P. pedicellatum remained in the soil, but there could be a considerable amount of hard S. acuta seed. The results suggest that control of P. pedicellatum could be best accomplished by the use of minimum tillage procedures, with little ground disturbance to bury the seed, while oversowing with a vigorous perennial pasture species such as calopo (Calopognium mucunoides) could lead to suppression of S. acuta. |
