Marshmallow is a widely distributed, difficult to manage weed that has thrived in grain cropping regions due to the adoption of no-till, the reduction of livestock in the farming system, and the biological characteristics of the plant. Growers in southern Western Australia have indicated that late summer and early autumn germinations of marshmallow cause the most issues for management. Poor management of these early germinations during the fallow period results in very large plants present in crops at sowing, which mostly are beyond controllable and through the growing season that are extremely difficult to kill due to their size. These plants impact crop yield through competition and potentially causing grain contamination at harvest, can be host plants for some insects, and marshmallow's large canopy has the potential to intercept pre-emergent herbicides from reaching the soil surface and impacts the control of subsequent germinations

There is a gap in the understanding by growers on the biology and ecology of marshmallow and how this information determines the success of management decisions, on the best-practice techniques to manage late summer/early autumn germinations of marshmallow, and how to identify and manage escapees prior to sowing.

Seasonal Marshmallow Behaviour

Over the past two seasons there have been limited summer rainfall events. In 2023 there was not sufficient rain for the germination of marshmallow in the summer and as a result trials were delayed until a germination occurred in early Autumn. The plants in these early knockdown trials at Corrigin were small, similarly the plants in the in-crop and pasture trials also germinated later so they were not as big as they could’ve potentially been if they had germinated and survived over the summer period.

Late rain in 2024 created similar issues, however, a chemical fallow at Varley with large survivors offered an opportunity to look at those which had survived a summer spray. The other two knockdown trials occurred under similar conditions to 2023, however, the Corrigin trial targeted slightly drought stressed plants whereas the Kulin trial targeted them after a rejuvenating rain had occurred. These shifting dynamics influenced the ways in which herbicides were able to control the marshmallow.

2023 Key Findings

Knockdown

• New group 14 herbicides such as Terrad’or and Sharpen achieved faster brownout compared to tank mixes with older group 14s such as Hammer and Goal.
• The addition of a group 14 spike to glyphosate significantly improved early control- at 22DAA 1.2L Glyphosate 570 provided 49% control.

1.2L Glyphosate 570 +

• 20g Sharpen (100% control)
• 20g Terrad’or (93% control)
• 25ml Hammer (82% control)
• 80ml Goal (84% control)

In-Crop

• 1L/ha Quadrant provided the highest level of control. This herbicide contains picolinafen and diflufenican, which is having an additive effect on control.
• Treatments containing carfentrazone had very fast brown out but soon recovered, in the absence of crop competition.
• Group 4 chemistry (Condor and Paradigm) did not work well.

Pasture

Legume pastures are commonly the phase of the rotation where marshmallow grows freely and significantly increases the marshmallow weed seed bank that then needs management in subsequent crops.

• Raptor was the most effective treatment with 90% final control, however not significantly different to the Broadstrike treatments, which ranged from 75-84% in various mixtures.
• Buttress and Bromoxynil 200 provided inadequate control.
• Broadstrike caused the least biomass reduction of medic (12.5%), followed by Raptor (15%). Buttress and Bromoxynil 200 were the most damaging, causing ~30% biomass reduction.

General

• Crop competition improves the efficacy of herbicide mixes. This was evident in the improved control observed in the 2024 trial compared to 2023, where there was no crop.

2024 Key Findings

In-Crop

• Carfentrazone-based products (Affinity Force and Aptitude) mixed with MCPA, provided the most robust marshmallow control, with very little recovery over time. They did cause more crop bleaching, so this risk must be weighed up with the need for strong control.
• Quadrant provided robust control (85%), demonstrating the additive effect of picolinafen and diflufenican.
• Tigrex (MCPA + diflufenican) and Triathlon (MCPA + bromoxynil + diflufenican) provided similar levels of control, indicating that bromoxynil does not improve control.
• Intervix + MCPA provided suppression, particularly as the marshmallow became more stressed.

Knockdown

• Spraying when plants were moisture stressed resulted in poorer control than waiting for a good rain before spraying the knockdown (spraying early before stress develops is ideal).
• Only high rate of glyphosate (3L Crucial) could provide acceptable suppression of moisture stressed marshmallow.
• The addition of an oil to Goal 240 + Glyphosate significantly improved control, and numerically improved control of Hammer + Glyphosate.
• When comparing new and old group 14 spikes, the trend was towards better control from the new products (Terrad’or and Sharpen).
• The addition of 2,4-D ester to glyphosate improved control, providing no evidence of antagonism.
• Paraquat and paraquat + diquat (Reglone) was weak on marshmallow, providing <30% control.
• The addition of Sharpen and Terrad’or to paraquat significantly improved final control. Terrad’or + paraquat resulted in 79% final brownout.

Key Recommendations

The results from this trial series revealed some key recommendations and considerations when targeting the control of marshmallow.  

Spraying Conditions

There was a large variation in the performance of herbicides based upon spraying conditions. In this series mostly this was associated with the health of the Marshmallow. In conditions when it was particularly dry there was a lack of efficacy from most herbicides and observable regrowth once conditions had improved. Maximising the ability of herbicides to be taken up should be a key consideration when managing the plant. This should involve looking at the health of the marshmallow as well as timing applications to maximise the uptake of the chemistry, with summer spraying this means avoiding high delta T and sunlight intensity for group 14 products.

Grey Banded Weevil

It was observed toward the backend of the trial series that Grey Banded weevils caused significant defoliation of marshmallow plants (figure 1). In the trial in Wickepin in 2023 their presence resulted in control that made it difficult to compare treatments. Whilst this isn’t a tool you can depend on, the presence of the weevil appears to be widespread and may contribute to final control if plants are not completely controlled by herbicide strategies. Their presence was not observed in the summer period, however there was very little marshmallow around during the time of this trial series. This reflects current knowledge that they typically emerge in autumn and winter, though their full lifecycle is poorly understood.

Choosing Herbicide Strategies

Glyphosate

The performance of glyphosate varied across the trial series. Control was vastly superior when it was applied in good growing conditions and the additive benefits of extra herbicides such as ester/DropZone and group 14’s gave acceptable levels of control. There were additional benefits from using the newer generation group 14’s such as Terrad’or and Sharpen with interchange between the best performers.

Broadly the best combination for knockdown control was a brew that contained:

1-1.5L Glyphosate 570 + 0.5L DropZone + 20g of Terrad’or or Sharpen + 1% Hasten

Paraquat

Paraquat based knockdown brews were not effective on larger marshmallow plants. As a standalone product it gave inadequate control and when paired with new generation group 14’s there was still regrowth.  Its best role will be in a double knock scenario to prevent recovery from the previous knockdown.

Group 14’s

The newer generation group 14’s gave higher levels of control than the older generations. Generally, Terrad’or gave the highest level of control followed closely by Sharpen. Carfentrazone was superior to oxyfluorfen on most occasions, although was able to achieve comparable control when an adjuvant was included.

Pastures

There are limited options to control marshmallow in medic pastures where it often proliferates. Group 2’s provided the highest level of control particularly with the use of:

40g Raptor + 1% Hasten + 1% Ammonium Sulphate

This was observed to cause and acceptable biomass reduction in the medic of 10-15%.

In Crop Management

For marshmallow control in a wheat or barley crop, there are a couple of options based on the situation.

1. When the marshmallow is <10cm diameter, a softer herbicide will provide control. A good option is:

800ml Tigrex

1. For larger plants, a stronger punch is required and would recommend:

A) 1L Quadrant OR B) 100ml Affinity Force + 330ml MCPA Amine 750

For option B, there is a risk of regrowth in the absence of good crop competition and more crop phytotoxicity.

Information Gaps

• The physiological response of Marshmallow to drying conditions need improved understanding. The notable decline in herbicide efficacy points toward a restriction of translocation or uptake or potentially a combination of the two.
• A better understanding of the role of adjuvants in improving uptake in adverse conditions. The 2024 trial looking at the adjuvants were applied to healthier marshmallow. Teasing out the advantages of the adjuvants would require higher stress spray conditions. It’s likely dusty conditions from higher speed spraying could also be a cause of reduced efficacy.
• The biology of the grey banded weevil could improve understanding of how its role could be better implemented in final control of marshmallow. Also, a review of whether current IPM strategies are damaging populations in crop or pasture.
• The use of spot spraying technology is likely to improve control in more adverse conditions if higher rates of herbicide or spray volume can be used.
• Alternative herbicides for control of Marshmallow in pastures. There was very good efficacy from group 2 herbicides, however, from a resistance management perspective additional tools are needed.