Eradicating barnyard grass in dryland cotton systems

The adoption of the ‘2+2’ fallow management strategy has proven to be very effective in driving down seed bank numbers in dryland Roundup Ready Flex^® cotton systems.

The risk of glyphosate resistance in weeds is widely understood and growers are actively minimising the risk through adherence to Crop Management Plans that include the ‘2+2’ strategy of using at least two non-glyphosate tactics in-crop and two non-glyphosate treatments in the fallow.

Queensland Department of Agriculture, Fisheries and Forestry (DAFF) senior research scientist, Dr Jeff Werth, said barnyard grass was a serious concern in cotton systems because it is known to be prone to glyphosate resistance and renowned as a ‘hard to kill’ weed.

Dr Werth said that while the tactics currently recommended as best practice are very effective there are still questions surrounding the best course of action if individual plants or small patches of weeds were to survive a glyphosate treatment.

Dr Werth believes all survivors should be treated as if they were glyphosate-resistant and that growers can aim to eradicate weeds like barnyard grass.

“The aim is to prevent seed set of all weeds but particularly those that appear to have survived a glyphosate spray,” said Dr Werth.

“If single survivors become a patch of survivors it is important to intensively manage the patch and achieve eradication,” he said. “Research has indicated that fallow management holds the key to effective management of resistant weeds.”

In a long term systems experiment at Hermitage Research Station near Warwick, Dr Werth and his team are studying the efficacy of the ‘2+2’ fallow treatment recommendation and the value of additional eradication tactics in crop and in the fallow on glyphosate resistant barnyard grass.

The additional in-crop eradication tactics being tested in the trial are pre-plant application of metolachlor, double-knock using a shielded sprayer and chipping (table below). In the fallow, chipping of survivors was the only eradication tactic applied due to a dry fallow period.

Non-glyphosate control tactics used in the patch eradication trial. Best management practice is the 2+2 treatment. Four in-crop glyphosate applications were used in Year 1 (cotton) and two in Year 2 (fallow).

Dr Werth said the recommended double knock treatment for barnyard grass plants that have survived glyphosate applications in the fallow is haloxyfop (Group A e.g. Verdict) followed by paraquat (Group L). As paraquat is not a registered option in-crop, monitoring and control of survivors from Group A applications will be essential to prevent Group A resistance.

The study showed a significant reduction in total emergences of barnyard grass achieved when the 2+2 treatment is applied to reduce the reliance on glyphosate.

“As the experiment continues we expect the eradication tactics to become increasingly important once the seed bank levels have been significantly reduced,” said Dr Werth.

Ian Taylor, Cotton Research and Development Corporation (CRDC)General Manager R&D Investment, said dryland cotton growers also have options to use herbicides other than glyphosate throughout the grain crop rotation and may also consider using strategic tillage early in fallow periods where there was a good chance of replenishing rain.

“The Cotton Pest Management Guide has been recently updated and is essential reading for growers,” he said. “The best management practices currently recommended are proven to be effective and their implementation on all cotton fields will protect the long-term efficacy of glyphosate for the industry.”

CRDC and Monsanto Australia are funding this study into survivor management.