The introduction of glyphosate tolerant cotton has significantly improved the

flexibility and management of a number of problem weeds in cotton systems.

However, reliance on glyphosate poses risks to the industry in term of glyphosate

resistance and species shift. The aims of this project were to identify these risks, and

determine strategies to prevent and mitigate the potential for resistance evolution.

Field surveys identified fleabane as the most common weed now in both irrigated

and dryland system. Sowthistle has also increased in prevalence, and bladder

ketmia and peachvine remained common. The continued reliance on glyphosate has

favoured small seeded, and glyphosate tolerant species. Fleabane is both of these,

with populations confirmed resistant in grains systems in Queensland and NSW.

When species were assessed for their resistance risk, fleabane, liverseed grass,

feathertop Rhodes grass, sowthistle and barnyard grass were determined to have

high risk ratings. Management practices were also determined to rely heavily on

glyphosate and therefore be high risk in summer fallows, and dryland glyphosate

tolerant and conventional cotton. Situations were these high risk species are present

in high risk cropping phases need particular attention.

The confirmation of a glyphosate resistance barnyard grass population in a dryland

glyphosate tolerant cotton system means resistance is now a reality for the cotton

industry. However, experiments have shown that resistant populations can be

managed with other herbicide options currently available. However, the options for

fleabane management in cotton are still limited. Although some selective residual

herbicides are showing promise, the majority of fleabane control tactics can only be

used in other phases of the cotton rotation.

An online glyphosate resistance tool has been developed. This tool allows growers

to assess their individual glyphosate resistance risks, and how they can adjust their

practices to reduce their risks. It also provides researchers with current information

on weed species present and practices used across the industry. This tool will be

extremely useful in tailoring future research and extension efforts.

Simulations from the expanded glyphosate resistance model have shown that

glyphosate resistance can be prevented and managed in glyphosate-tolerant cotton

farming systems. However, for strategies to be successful, some effort is required.

Simulations have shown the importance of controlling survivors of glyphosate

applications, using effective glyphosate alternatives in fallows, and combining

several effective glyphosate alternatives in crop, and these are the key to the

prevention and management of glyphosate resistance.