The Australian cotton industry is considered a global leader in sustainable agriculture. However, herbicide resistance and problem weeds threaten the productivity and profitability of the cotton industry and the farming system. Over time, the industry is increasingly moving back to relying on residual herbicides to manage herbicide resistant weeds in cotton, in rotation crops and in fallows, with glyphosate now ineffective for controlling some grass and broadleaf weeds, and resistance to the grass herbicides (Group 1) and paraquat also becoming increasingly common.

With the increasing use of residual herbicides, comes the problem of crop safety to the following crop, an even more difficult issue in dryland cropping, where rotations are often determined by planting opportunities, but selection of a residual herbicide applied in the fallow may restrict these options. The issues are further complicated by the increasing range of herbicides available for fallow use, with multiple herbicides potentially applied and multiple applications within a fallow period common. On top of this, camera sprayers are becoming standard in the industry and it is common to apply higher than normal rates through these rigs. There is a big need to explore the potential for these herbicides and herbicide combinations and rates to damage following cotton and other rotation crops.

The primary research focus of this project was on the impact of residual herbicides applied to rotation crops and in fallows on the following cotton crops. Research was carried out over a series of seasons (replicated in time), on replicated, randomised field experiments, some with split-plot designs, in fields grown under typical commercial conditions. Components were tested at Warwick (Southern Qld), Narrabri (Northern NSW) and Leeton (Southern NSW). The experiments were on solid plant, irrigated cotton, picked with modified commercial pickers, and ginned to determine lint yield. Excessively wet conditions delaying herbicide applications and flooding in spring 2022 were challenging for some experiments.

Our research has highlighted: that a) residual herbicides used in rotation crops, fallows, and through camera sprayers have the potential to seriously impact following cotton crops, reducing seedling vigour and lint yield, and b) combinations of herbicides can be more damaging than expected, such that label crop-safety information may underestimate the potential damage from combinations.

However, the limitation of this work was that it could not test every potential herbicide, herbicide combination and application scenario. The largest of the experiments (at Narrabri) examined eight in-crop herbicides in two rotation crops (wheat and chickpea) and 9 fallow herbicide combinations, a total of 80 treatments, with four replications. While the findings from the results from this work are extremely valuable to industry, a common scenario in the cotton system would be two in-crop herbicides and two or more fallow herbicides, increasing the potential herbicide combinations four-fold or more (as herbicides other than the ones we used could have been included).

Future research should focus on the impacts of some of the more common multiple-herbicide strategies. The current work, for example, showed that the standard cotton herbicides are not causing issues for the following rotation crops. However, only single herbicides were considered, applied at cotton planting. The potential impact of multiple herbicides, some applied pre-crop and others in-crop is yet to be examined, but is likely to be damaging to rotation crops.

Further work is also needed to correlate herbicide soil and plant concentrations to crop damage as we note that with more residual damage occurring in the industry and increasing concern around damage from residual herbicides, there is currently no information that can relate herbicide concentrations in soils or plants to damage. Where a grower sees damaged plants, it is becoming increasingly common to test for herbicide residues. When a herbicide is detected from a laboratory sample, it is assumed that the detected herbicide has caused the damage. However, this may not be the case, and conversely, when no herbicide is detected, this does not necessarily indicate that a given herbicide is not causing the damage. We simply have no data to relate laboratory test results to plant damage and this needs to be addressed.

Herbicide screening showed that samples of awnless barnyard grass, feathertop Rhodes and windmill grass had some level of resistance to glyphosate, the Group 1 grass herbicides, paraquat and glufosinate. Theses levels of resistance have big implications for managing weeds. The resistance to glufosinate is surprising and unexpected and will diminish the value of this relatively new mode of action herbicide to the cotton system, especially if resistance occurs amongst broadleaf weeds.

The studies on emerging weeds highlighted the challenges of managing some of these weeds, especially as they develop herbicide resistance. Cotton growers and consultants need to be aware of these issues with these weeds and develop management strategies for fields where they become problematic.

This research has generated a bulk of experimental results that will be published in information sheets, articles and scientific papers, and delivered to industry through the CottonInfo network.

Already from this project there have been a series of scientific publications, articles, conference presentations, many presentations at grower meetings and conversations with growers and consultants. More presentations and articles will follow.