Farming systems in cotton are changing and weed management is again becoming more complex. The rise of herbicide resistance, particularly to glyphosate and the imminent introduction of new herbicide tolerant traits to dicamba and glufosinate has changed the way weeds need to be managed. The project team undertook to pre-empt these changes.

Three new modelling and decision support products were created for industry. The BYGUM decision support tool predicts economic outcomes from summer grass weed management strategies. The tool can be used to create and compare five-year rotations of cotton, grains and fallows, including cover crops, demonstrating the effectiveness, cost and efficiency of herbicide and non-herbicide weed control. BYGUM has been used in workshops and to create extension materials, and has been downloaded by over 300 unique users to date. The Weeds of Australian Cotton ID app allows identification of 50 key weed species in cotton fields. The app uses the Lucid framework and an extensive image library. The app is among the first to include cotyledon shape as a factor for identification, meaning weeds can be identified while still small enough to control effectively. It is free to use and available in Apple and Google app stores for use offline, and via the Identic Lucid Key library for use on desktop and laptop computers.

The Diversity model is a world-first multi-herbicide, multi-species, polygenetic model of herbicide resistance evolution. It determines and quantifies how much diversity is enough, to slow or prevent evolution towards resistance. We used the model to assess the resistance potential of weed management under new triple stack systems, such as Xtendflex® cotton. Our results suggest three key points:

1. That these systems are substantially more diverse than Roundup Ready, and with the right extra tactics can be the basis of long-term effective weed management;

2. Glyphosate, glufosinate and dicamba alone or with minimal extra modes of action are incapable of controlling our existing glyphosate resistant grasses and fleabane—systems are likely to fail due to poorly controlled resistant populations long before new resistances have time to develop;

3. The 2+2+0 strategy is predicted to remain effective, but modelling multiple species at once reminds us that both grasses and broadleaves need multiple effective options in the system.

Genetic exploration of the mechanisms of glyphosate resistance in key weed species led to the discovery of the key role of ploidy and gene copy number in the evolution of glyphosate resistance. Our work on gene expression in fleabane and the genome assembly showed that there are many copies of the target site EPSPS gene in this species. This makes it very hard for fleabane to evolve target site resistance to glyphosate because many copies have to have a mutation, instead, species with many copies of the target site gene have to evolve non-targetsite resistance and this is more difficult for the weed because it usually involves more than one mutation. Feathertop Rhodes grass was found to be diploid with one copy of the EPSPS gene, and this species evolved resistance over 10 times by target site mutations. This understanding explains why it took sowthistle so long to develop resistance to glyphosate, and allows us to make predictions about herbicide resistance evolution in the future. Diploid species with one copy of the target site gene will be more likely to readily evolve resistance to a herbicide than polyploid species or those diploid species with multiple copies of the target site gene.

Work on the population genetics of four key species led to some surprising results. Fleabane is considered a well-dispersed species, but had strong regional genetic structure indicating that wind dispersal may play less of a role than previously expected. Windmill grass and feathertop Rhodes showed very little evidence for outcrossing, but there may have been some admixture in the past. Outcrossing is important because it affects the ability of a weed to develop resistance to multiple modes of action (MOA). Fleabane and sowthistle had evidence for some outcrossing in the genetic data, but we were unable to find experimental evidence for outcrossing in 200 offspring of each species. This highlights how very low levels of outcrossing might still play an important role in the evolution of resistance in species like fleabane that we had previously thought to be only self-pollinating.

In windmill grass and feathertop Rhodes grass, their highly selfing reproductive mode can be used in the fight against herbicide resistance. These species are less able to ‘stack’ resistance to different modes of action.. Our work shows, however, that feathertop has evolved resistance multiple times and how these have spread across the cotton system, and that almost 1/3 of windmill grass populations are now resistant. Each glyphosate resistant individual has the potential to evolve resistance to a second MOA. and our work highlights the importance of controlling glyphosate resistant populations to avoid multiple MOA resistance. Overall, the population genetics work emphasises the importance of the ‘0’ in the 2+2+0 strategy survivor control is essential to prevent the spread of resistance and to avoid multiple MOA resistance.

Studies on the growth and development of awnless barnyard grass, feathertop Rhodes grass, windmill grass, fleabane and sowthistle were conducted. In general, with the summer grasses, plants that emerged at the start of summer grew larger and produced more seed than those emerging later. This is where the focus of control should be for the greatest impact. However, it is important to note that plants emerging later still produce seed and need to be controlled. Sowthistle now has the ability to emerge and grow well throughout the year. Fleabane also appears to be adapting to warmer temperatures, readily producing seed throughout spring, summer and autumn.

The addition of glufosinate and dicamba has the potential to improve control, particularly on the five key species tested. When glufosinate was used, as a double knock partner, effective control was achieved in both glyphosate-resistant and susceptible populations tested. The glufosinate double knock should prove an effective option in Xtendflex® cotton.

Research on cover crops was hampered with dry conditions, and as the result the effects on weed emergence were limited. However, growers have shown cover crops to be an effective option provided they start with a clean crop and ensure that the cover provided is adequate and evenly spread.

The 2+2 and 0 was shown to be an effective management strategy for long-term resistance management. Research also concluded that additional options will provide more effective control in years with more rainfall events and subsequent emergences.