This project is a 15 month follow-on from DAN1402 and continues research into herbicide resistant weeds and to identify barriers to adoption of integrated weed management in cotton, particularly the reduced use of residual and layby herbicides. Two Herbicide demonstration sites were established, one at CSD farms (Wee Waa) and the other at the Irrigation Research and Extension Committee [IREC] (Whitton). The addition of pre-emergent or residual herbicides to glyphosate resulted in lower weed numbers and higher yields in those treatments. Glyphosate alone treatments had lower lint yields than the integrated herbicide approach. A weedy control included at IREC, highlights the importance of the “Critical Weed Free Period” in cotton; allowing the weeds to grow unimpeded for six weeks reduced crop yield significantly. This research supports recent scientific publications from Graham Charles.

The project has also reported on the first case of glyphosate and paraquat resistance in two tall fleabane populations in a double knock scenario. In addition to this finding, a hormesis effect was detected when lower doses of paraquat were applied with increases in above ground biomass, plant height and buds per plant.

Weed surveys were conducted during the 2018–19 season. A smaller number of weeds were collected as a result of the dry conditions, however high levels of glyphosate resistance are still being recorded. A total of three populations (out of 11) of Sowthistle are resistant to glyphosate and two populations of Bladder ketmia (narrow) were found to be resistant to glyphosate.

A total of six ryegrass populations were screened; five populations are resistant to glyphosate and all populations are susceptible to both paraquat and ‘double-knock. (glyphosate+paraquat) A total of 23, 16 and 25 populations of Barnyard grass, Windmill grass and Feathertop Rhodes grass were screened with glyphosate, and 52%, 44% and 48% populations were identified as resistant respectively.

A total of 21 and 14 populations of Feathertop Rhodes grass and Barnyard grass were treated with four Group A herbicides including three fops and one dim. Seven and five populations of Feathertop Rhodes grass and Barnyard grass showed resistance to Topik® respectively. Grass populations that are developing-resistance or are resistant to Topik® are also resistant or developing resistance to glyphosate except one population of Barnyard grass (S3) and two populations of Feathertop Rhodes grass (D1 and D12). The importance of not relying on any one herbicide for control of problem weeds is critical for ongoing efficacy of current herbicides. Ecology and biology studies were conducted on emerging weeds collected during the weed surveys. By identifying the patterns of growth and development of these weeds, strategies for their control can be directed at weak links in their lifecycle. Dwarf, Green and Redroot amaranth do not show any seed dormancy. Temperature has significant influences on seed germination of the three tested amaranth weed species. All three species can germinate at a wide range pH solution however; they prefer to geminate in neutral to alkaline conditions. All three species are very sensitive to light and they are photoblastic. Dwarf, Green and Redroot are very sensitive to water stress but Green amaranth is more tolerant than the other two species.

In two populations of Barnyard grass, one Resistant to glyphosate, the other Susceptible, our findings found that herbicide-resistant plants of Barnyard grass will be less fit than wild type in the absence of herbicide (glyphosate), and in the absence of glyphosate, backward selection can occur. This means that without glyphosate applications, the susceptible plants of Barnyard grass, as the fittest population, may eventually dominate in the field. While resistant plants of the same population may dominate when glyphosate is used for control, this depends on glyphosate doses. A research question for the future; is this a possible non herbicide mechanism for controlling a resistant Barnyard grass population? In other words will the fitness penalty be enough to wrest control of resistant populations back or is this an additional tactic to include in an integrated weed control program.