Previous research has highlighted that glyphosate resistant and tolerant weeds are increasing in cotton farming systems. This situation is not unique to cotton systems, indeed broadacre agriculture in Australia is dealing with a range of herbicide resistant weeds across all farming systems. Glyphosate tolerant cotton has been widely adopted in Australia since its introduction in the early 2000’s. A consequence of this technology was a shift to a simplified system of weed control dominated by multiple applications of glyphosate in crop.

A series of comprehensive weed surveys were conducted across cotton farming systems in 2015, 2016 and 2017 seasons. The focus of the surveys was five common weeds identified as the most problematic in cotton farming systems: feathertop Rhodes grass, windmill grass, awnless barnyard grass, sowthistle and fleabane.

The samples were germinated in a glasshouse and screened against the commercial rate of glyphosate. The testing process confirmed very high levels of resistance in fleabane (>95%) windmill grass (>90%) and awnless barnyard grass (>65%). The level of resistance in sowthistle and feathertop Rhodes grass was lower, however in excess of 25% of populations tested as resistant. Group A resistance was also confirmed in grass populations collected during the surveys.

The additional information from the resistance testing was a catalyst for updating the Herbicide Resistance Management Strategy (HRMS). The new strategy includes information on herbicide resistance for: Group M, L, I and A herbicides. A resistance risk assessment tool has also been incorporated into the HRMS table. Feedback from consultants and growers has been positive, especially with the traffic light approach in the table. The new HRMS has been included in the 2018 Cotton Pest Management Guide. In addition, all tables within the Weed Management section have been updated and consolidated into a more user friendly format.

Since the start of the project in 2014 and the extension of herbicide resistance results there has been an increase in the use pattern of pre-emergent and residual herbicides. This is a positive outcome for the industry and further reinforces the need to provide growers with additional options for weed control other than glyphosate. A research officer was appointed to the project in February 2017 further increasing the capacity of weed research in the cotton industry.

A study tour to the US cotton industry in September 2017 reinforced to researchers, growers and industry the importance of maintaining a diverse approach to weed control in Australian cotton farming systems. The reliance on herbicides alone for weed control has resulted in widespread resistance developing in the US, especially to glyphosate. The Australian industry based HRMS (2 + 2 & NO survivors) is essential to maintaining the efficacy of glyphosate for the cotton industry. The importance of non-glyphosate tactics in-crop and in fallow, and controlling any survivors is paramount to the long term sustainability of our cotton farming system.

Phenology studies confirmed that susceptible windmill grass populations accumulated higher levels of shikimate than resistant phenotypes. This work suggests that resistance is likely to be target site based (not confirmed). Low levels of paraquat applied to fleabane populations showed increased growth rates for resistant populations compared to susceptible types (hormesis). One population of fleabane has tested R to paraquat and this is being communicated through future WEEDsmart and Spotlight articles. Dose response experiments on barnyard grass showed that even when exposing plants to high rates of glyphosate they were able to survive by reducing tiller numbers, shortened growth habit and a reduction in seeds per spikelet.

The pupae busting experiment at ACRI has been updated to include a range of integrated weed management tactics to differentiate previous trial treatments. Early indications are of a weed species shift between the control (W3) treatments and the introduction of pre-emergent herbicides.

Eric was appointed as the Weed Management Technical Lead with CottonInfo in February 2017. Eric has also assumed the role as the co-ordinator of the Herbicide Technical Panel. The Technical Lead has responsibility for the Weed component of the Annual Operating Plan for CottonInfo Regional Extension Officers’s. A series of weed management case studies was completed in the Summer of 2018 and published on the WeedSmart web site and in industry publications. Eric has participated in the filming of three videos on Herbicide resistance management and integrated weed management.

Industry engagement at workshops, conferences and field days were a key component of the project. A pre-emergent demonstration trial at Whitton NSW highlighted the effectiveness of adding a pre-emergent herbicide especially in the presence of high populations of grass weeds.

The Situation paper “Weeds, Herbicides and Traits in Australian Cotton” is in final draft form and awaiting feedback from reviewers.