Following the introduction of transgenic, herbicide tolerant varieties, the Australian

cotton industry has developed glyphosate-centric weed management systems which

are less reliant on residual herbicides and non-herbicide methods of weed control,

such as cultivation and hand hoeing, than has been the case in the past. These

systems have contributed to improved crop yields, while reducing the

environmentally negative aspects of cotton production. However, farming systems

which rely too heavily on a single weed management tool are not sustainable, with

inevitable shifts to weed species that are tolerant of the management tool. Where the

single management tool is a herbicide, such as glyphosate, selection of herbicide

resistant weed species is also likely.

The primary aim of this project was to develop “intelligent” weed management

systems for transgenic cotton to optimize herbicide use, achieving better yields,

while maintaining the value of glyphosate to the farming system by minimizing the

selection pressure for species shift and glyphosate resistant weeds. This was

achieved using a series of field experiments to develop a weed control threshold

based on the critical period for weed control concept.

Information on the weed control threshold and sampling techniques has been

extended to the industry through articles in the Australian CottonGrower and in

WEEDpak on the Cotton CRC website. Ongoing work is looking at developing a

simplified sampling technique for the threshold using an electronic sensor to

estimate weed biomass and hence weed competitiveness.

Information on weed growth, development and seed production has also been

produced to provide guidance for the control of weed populations which are below

threshold, ensuring that weeds are controlled before they set seed.

The 2nd aim of the project was to provide cotton growers with information to allow

them to assess the likely effects of herbicide damage on a cotton crop in terms of

final yield and maturity, and subsequently to make better informed management

decisions for herbicide damaged crops.

A series of field experiments explored the effects of a range of phenoxy and other

herbicides on post-damage crop growth and development, including leaf, square

and boll production, crop maturity and final yield. Herbicides have been applied at

varying rates and stages of crop development. This information has been published

in WEEDpak on the Cotton CRC website, along with information assisting cotton

growers to compare the post-damage symptoms for a range of herbicides.

Experiments have also explored post-damage crop management options, but to date

have found no options which improve crop recovery.

These outcomes significantly progress the science of weed management in the

Australian cotton industry, providing guidelines for best practices for weeds.