EFFICACY OF PRE-EMERGENT HERBICIDES ON FRESH TRASH BLANKETS – RESULTS ON LATE-HARVESTED RATOONS

WEED MANAGEMENT in ratoons in green-cane trash blanket (GCTB) systems often relies on the use of knockdown herbicides, sometimes together with pre-emergent herbicides such as hexazinone, atrazine and diuron, applied before the out-of-hand stage. Pre-emergent herbicides may have an adverse environmental impact if applied just before or during the wet season. However, application of pre-emergent herbicides on a GCTB just after harvest is not commonly considered as an option by growers due to doubts regarding the chemicals’ efficacy with thick trash., Trials were implemented in the 2008–9 season in the Mackay district on grower farms in late-harvest ratoons to evaluate the suitability of this strategy. In each trial, the efficacies of three pre-emergent herbicides were compared when the products were applied on bare soil or on a GCTB. Cane yield was also assessed when pre-emergent herbicides were applied just after harvest compared with the application of knockdowns followed by a pre-emergent herbicide just before the out-of-hand stage. Changes in weed composition were studied in the untreated plots, comparing bare soil and variable amounts of trash. Results show that: more than 6 t/ha of trash on the ground provided significant weed control; each weed species responded differently to different amounts of trash on the ground; there was no difference in efficacy of the pre-emergent herbicides when applied on trash or bare soil; application of pre-emergent herbicides generated significantly higher yield than no treatment, but only if those fields were rapidly infested by weeds.