The Australian Cotton Industry is now in the midst of dealing with both glyphosate resistant and tolerant weeds in the cotton system. Glyphosate, the world’s most important herbicide continues to be relied on as the primary source of weed control. The sustainability of this herbicide is dependent upon the ability to incorporate it into a diverse system and prevent the introduction and spread of glyphosate resistant weeds in the farming system.

Previous research has demonstrated how glyphosate resistant and tolerant weeds can be managed in cotton systems. This information has been packaged into management guides in WEEDpak for three species: flaxleaf fleabane, feathertop Rhodes grass and glyphosate-resistant awnless barnyard grass. This information is designed to help growers understand the ecology of these weeds as well as their management in crop and fallow. As more information is compiled from current research, these management packages will continue to be updated.

An updated version of the online glyphosate resistance risk assessment tool has been developed. This new version enables users to determine their resistance risk for grass and broadleaf weeds independently. Users can also compare different strategies for resistance prevention/management with greater efficiency.

An in-depth understanding of triggers for emergence of a number of key species, in particular awnless barnyard grass, feathertop Rhodes grass, fleabane and sowthistle has been gained. This research has shown that the smaller seeded summer grasses and asteraceae (fleabane and ) can emerge within 2-3 days of rainfall events of 10 mm or greater. This has contributed to their success in glyphosate based farming systems, and it important that fields are monitored soon after rainfall event so the control measures can be conducted in a timely manner.

Studies have been conducted on how to manage survivors of glyphosate application. This is an important part of the CMP for Roundup Flex cotton, and weed management in general. Results from trials on fleabane and awnless barnyard grass have shown that double knock applications with either paraquat or glufosinate as the second knockdown herbicide are most effective. Particularly as these plants are often larger, and well past optimal spraying time.

Minimising the numbers of weed seeds entering the seed bank is critical for sustainable weed management. Preliminary experiments on reducing seed viability have shown that this is achievable when plants have past optimal spraying time. Results have shown the glufosinate was most effective on awnless barnyard grass and feathertop Rhodes grass, and 2,4-D+picloram was most effective on fleabane. However, fleabane and feathertop Rhodes grass in particular are able to regrow and produce viable seeds. At this stage research indicates that an attempt to reduce seed viability is a short-term option until plants can be subsequently controlled with more robust methods.

Field and glasshouse trials that provide data to a spatial simulation model has been developed to determine the feasibility of eradicating patches of glyphosate-resistant awnless barnyard grass. Results to this point have shown that species such as awnless barnyard grass that are predominately selfing, have a relatively short seed bank life and are not transported by wind can be effectively managed at a patch level. Long-term management strategies that include two non-glyphosate tactics in crop and fallow have been successful in significantly reducing the seed bank and the number of emergences in the short-term. As the trial continues, the added benefits of extra eradication tactics will be determined for their effect on driving down the seed bank.