Overwatch™ Herbicide

Frequently Asked Questions: Overwatch® Herbicide

Product Name: Overwatch® Herbicide

Active Constituent: Bixlozone (isoxazolidinone), powered by Isoflex™ active

Formulation Type: Suspension Concentrate (SC)

Proposed Use Rate: 1.25 L/Ha

Technical Highlights:

  • Long-lasting residual control of Annual ryegrass including herbicide resistant biotypes and regionally specific broadleaf weeds
  • New herbicide molecule - Group Q mode of action
  • Pre-emergent herbicide for application as an IBS (Incorporated By Sowing) treatment for use in Wheat, Barley and Canola
DISCLAIMER: This is not the Overwatch® Herbicide label. To view or print the label for this product, visit 20131-FMC-Overwatch-Herbicide-web-label_03022022.pdf (overwatchherbicide.com) Always follow label instructions.

We also encourage customers to reach out to their own agronomists with any questions they may have.
Recently Added FAQ's
1. Volatility
2. Rain and volatility
3. Lupins and bleaching
4. Lupin pot trials- Field "Research"
Existing FAQ's
1. FMC's Actions
2. Application
3. Drift & off-target transient bleaching
4. Mixing
5. Safety
6. Soil moisture
7. Transient bleaching
8. Volatility
9. Yield


Some agronomists theorize that Overwatch® Herbicide is highly volatile and suggest volatilization is causing bleaching in some off-target crops. 

The APVMA has classifications for how volatile a chemical is. Extensive studies found that Overwatch® Herbicide is not considered to be a highly volatile chemical.

This finding was concluded by considering a number of different factors including vapor pressure, water solubility, soil binding coefficient and formulation type.

Review of studies by the APVMA, concluded that volatilization and atmospheric transport following application are not likely to be significant for Bixlozone*.

  • FMC conducted wind-tunnel testing of this product. The amount of volatilization from an application rate of 300 g/ha was low and decreased with increasing distance from application site. The maximum amount detected (0.42% of total deposited) was at 1 metre from the application site; at 20 metres only 0.03% of the total deposited was detected. Sensitive chickweed plants positioned 1 metre from the application site exhibited low levels of transient bleaching (13% of leaf area bleached) and plants located 20m from the application site exhibited no bleaching symptoms 4 days after application.
  • The amount of volatilization detected in the wind tunnel testing was very low, and too low to cause anything more than a transient symptom on sensitive vegetation.

Overwatch® is not considered to be a relevant residue in air.

*Public release summary on evaluation of new AI Bixlozone in the product Overwatch® Herbicide.  APVMA product number 86427.  Dec. 2019.

2. Rain and volatility

Some agronomists have suggested that rain events a long time after application cause Overwatch® Herbicide to rise through soil, volatilize into vapor, and cause signs of bleaching in off-target crops.

This suggestion focusses on only one dimension of a multi-dimensional issue. The issue could involve impacts from spray drift, delayed symptom expression after drift, or low-level localized volatility, alone or in combination.

Any rainfall that occurs a long time after application will likely increase downward mobility of Overwatch® Herbicide (rather than promoting upward movement and subsequent volatility). Bixlozone is known to exhibit moderate mobility in soil.

FMC has considered possible explanations as to why we see such a long time between potential drift at the time of application, and expression of symptoms in off-target plants. Three possible explanations are delayed expression of drift symptoms, localized volatility at low levels, and plants not being in a suitable vegetative state to express drift symptoms when they occur.

  • Alternatively, it can occur when the amount of drift deposited on sensitive plant surfaces and the soil is so small that it will not express signs of bleaching until activated by rain.
  • Just like Overwatch® Herbicide requires adequate soil moisture for effective weed control, some amount of moisture will be required to activate the extremely low doses of active ingredient deposited on off-target foliage from spray drift or inversions at an earlier stage. To express, Overwatch® Herbicide requires susceptible vegetation to be (i) at a susceptible growth stage and (ii) actively metabolizing.

As with the introduction of any new product we continue to monitor observations in the paddock and review those observations as they arise.

3. Lupins and bleaching

Some reports and social media posts show a field of lupins that the post or report says exhibit signs of bleaching.

Approximately 99% of the 1.2 million ha of paddocks to which Overwatch® Herbicide was applied this year has not reported off-target crop bleaching. 

However, some off-target bleaching of lupins was observed in nearby paddocks. That observation showed that bleaching occurring from 300 meters to up to 1 kilometre away from the treated paddocks.

Ongoing monitoring by FMC is observing strong recovery from transient bleaching in the overwhelming majority of cases.

FMC has received numerous reports of Lupin crops immediately adjacent to Overwatch® Herbicide treated paddocks that did not experience any transient bleaching at all.  Further investigation of these cases revealed that application of Overwatch® in the neighbouring paddocks was consistent with label instructions and good spray practices.

4. Lupin pot trials

Some individuals have conducted their own tests by placing potted lupins into paddocks treated with Overwatch® Herbicide.  Some of these potted lupins have demonstrated symptoms of bleaching.

FMC is aware that different people have been undertaking their own tests. We have not been involved with the methods they have been using. We are aware that some potted lupins have demonstrated transient bleaching, and some potted lupins have shown no transient bleaching.

We have been involved with scientific trials that were reviewed by the APVMA. Our results demonstrated that low-level volatility from treated soil is possible and is confined to a relatively short distance away from the application site.

It is therefore plausible that a sensitive plant such as lupins placed within an Overwatch® treated paddock may show symptoms of transient bleaching.

However, the validated data indicates the amount of volatilization causing such bleaching is very low, and too low to cause anything more than a transient symptom on sensitive vegetation such as lupins. By itself, this amount of volatilization cannot move very far, and decreases in concentration the further it moves from the source.  

1.1 What actions are FMC taking?

FMC representatives are discussing the situation with the grower, obtaining all the relevant background information and inspecting the crop/s in question. At the same time all the relevant facts are documented, along with pictures, samples and other relevant items required to resolve the situation or provide an explanation. Should the FMC representative not be able to offer a solution or explanation, an FMC technical expert will visit the grower for a more detailed investigation.

1.2 Where are the specific geographical areas that reports are coming in from? Name the closest towns.

Most of the reports of off-target effects are from the central part of the WA wheat belt.

1.3 What do you do when you visit a farm based on receiving a report?

Once a report is received, an FMC representative will contact the farmer to discuss the observation, and if necessary, visit the farm to inspect the crop/s in person and follow up if required.

1.4 Who gets involved?

The local FMC representative, and/or technical expert and local agronomist.

1.5 If I have any questions or concerns, where can I get more information?

FMC Australasia works closely with industry to support the release of all its new products.

Our team remains committed to answering any questions our customers may have and provide technical support via our qualified agronomy team. If you are unsure who your local FMC team member is, please call FMC on 1800 901 939.

2.1 Some growers and agronomists are proposing to use lesser rates of Overwatch® Herbicide next season to reduce the amount of in-crop bleaching.

The use of any herbicide at reduced rates has the potential to compromise resistance management for the entire cropping system. The use rate on the label for of Overwatch® is 1.25L /ha and it is not recommended that a lesser rate be used. The registered rate was demonstrated to be the rate of optimal herbicidal efficacy and crop selectivity as confirmed by the APVMA efficacy and crop safety review during the registration process.

2.2 Should Overwatch® Herbicide have been used with disc seeders?

That was not recommended. The Overwatch® label specifically provides that:

“Overwatch® should be applied prior to sowing and incorporated by sowing using knife point and press wheel equipment”, and that “Crop safety from the use of disc seeders is variable”. Growers should follow the Overwatch® label.

2.3 Why are disc seeders not recommended for use with Overwatch® Herbicide?

Disc seeders vary in their ability to move herbicide treated soil out of the furrow, thereby minimizing the risk of germinating seeds coming into contact with the herbicide. When using a disc seeder the reduced soil disturbance means that there is greater potential for treated soil or the herbicide solution to come into contact with the germinating seed due to lack of positional selectivity. If there is heavy rainfall after sowing, the herbicide solution or herbicide treated soil may be washed into the seeding slot where it can come into contact with the germinating seed.

Disc seeders can also actively move herbicide treated soil into the furrow slot during the sowing operation. Another potential risk of disc seeders relates to the ability of the closing wheel to successfully close the furrow in wet conditions. If the seeding slot is not fully closed over, the potential for increased crop damage may be higher.

2.4 When applying Overwatch® Herbicide, what is the recommended set up of the spray rig?

It is recommended that Overwatch® be applied in at least 80 L/ha as a coarse quality spray.

The most preferred application regime for Overwatch® also includes:

  • A maximum travel speed of 20 km/hr, preferably 16 km/h
  • The minimum boom height that still ensures double overlap
  • TeeJet TTI nozzles operated at between 3 and 5 bar and angled backwards to direction of travel to reduce horizontal movement
  • The addition of a drift mitigating adjuvant in the tank mix at label rates will further minimise droplet production and enhance soil deposition

2.5 What can growers do prior to, or during sowing that can help Overwatch® Herbicide do its job effectively?

As with all products, following the label directions and precautions will ensure optimum product performance.

3.1 Is Overwatch® Herbicide prone to drift?

Like every other herbicide, drift from Overwatch® can occur when Best Management Spray Practices* are not followed. It is droplets that drift, and whatever the droplets contain will be carried with them.

The three types of drift that may occur are droplet drift, vapour drift and particulate drift. Particulate drift can also include the movement of product carried by dust from strong winds or seeding machinery. These are aggravated by surface temperature inversion, and high relative humidity conditions that prolongs the survival of airborne droplets.

*GRDC. Sprayright – to avoid drift. Spray Best: Best Practice Guide for Ground Spray of Pesticides.

3.2 What are the key factors to minimising drift?

Good spray application management employs a number of techniques to minimise off-target movement.

  • Spray under desirable weather conditions including appropriate temperatures and no inversions
  • Select the correct droplet size
  • Spray when there is a consistent cross wind. Avoid spraying under still or too windy
  • Do not spray with the wind direction toward sensitive areas
  • Do not spray when there is excessive wind Do not spray when there is no wind
  • Do not operate with a boom height that is too high
  • Avoid spraying with volatile products

3.3 We hear that the chemical is being drifted onto neighbouring crops – is this normal and should farmers be concerned?

No, this is not normal, because drift, regardless of which herbicide is being used, should not occur when good spray practices are followed.

FMC will continue to monitor the situation and work with industry partners and will provide an update once more information becomes available.

3.4 How many farmers have reported they have experienced issues with spray drift?

Of the approximately 1.2 Million ha’s treated in Australia, we are aware that less than 1% are showing signs of off-target bleaching. This involves 50-60 farmers.

3.5 When applying Overwatch® Herbicide, what special precautions are required to minimise the risk of spray drift?

For all pesticide applications, follow Best Management Spray Practices that employ a number of techniques to minimise drift (see point 2).

Best Management Spray Practices that are recommended for Overwatch® are detailed under the Spray Drift Restraints heading on the product label and include:

  • Spray quality should be no smaller than Coarse.
  • DO NOT apply if there are hazardous surface temperature inversion conditions present at the application site during
  • Adhere to mandatory downwind buffer zones specified on the product label

3.6 What are typical conditions under which surface temperature inversions are likely to appear?

Surface temperature inversions usually occurs on clear, calm mornings and nights. Windy or turbulent conditions may prevent inversion formation*

Inversions can transport pesticides for long distances, and not necessarily into adjacent paddocks only.

*NSW Department of Primary Industries. Reducing herbicide spray drift (nsw.gov.au)

3.7 What happens to non-target crops impacted by Overwatch® Herbicide    spray drift?

Available data indicates that the bleaching effects on non- target crops are transient. Initial reports show that any bleaching effects are not lasting and that crops recover from bleaching.

3.8 There are reports of bleaching symptoms similar to that caused by Overwatch® Herbicide on roadsides and other natural vegetation. How can this be explained?

We are aware of some reported bleaching of weeds like Bifora and Sow thistle on the side of roads. Given its high level of sensitivity to Overwatch®, these weeds are often the first to exhibit bleaching symptoms. The incidence of such symptoms is indicative of off-target drift. Use of best practices for spray drift management will enable growers to prevent this from happening.

The effect on the weeds that has been reported does not mean that Overwatch® is the only chemical present. While the symptoms caused by Overwatch® on these weeds is most obvious, the effects of other herbicides can also be seen in many cases.  If there are Fine particles in the spray mixture they will drift – including every other herbicide in the spray tank.

Growers concerned about off-target drift should seek agronomic advice and where appropriate report spray drift to the APVMA by submitting an on-line Adverse Exposure Report via the link:


3.9 How many cases of off-target bleaching are you aware of?

To our knowledge FMC has received a number of reports of off-target bleaching, mainly in WA. Of the approximately 500 000 ha’s treated in WA, we are aware that only about 1% are showing signs of off-target bleachin58g. The company is looking into each report received to better understand the situation. 

3.10 Are there any crops that are more sensitive to drift from an Overwatch®      Herbicide application?

After one season of full-scale commercial use, Lupins, Serradella, Tillage radish, Chicory and Biserulla appear to be  more sensitive to Overwatch® spray drift.

3.11 Is the use of a drift-mitigating agent recommended for Overwatch® Herbicide application?

A drift mitigating adjuvant may be added to further minimize fine droplet production, and to enhance soil deposition in  standing stubble.

A greater proportion of coarse droplets in the spray mixture will further reduce the potential for particulate drift and vapour drift to occur.

4.1 Does the mixing of Overwatch® with other chemicals change its volatility profile?

  • Mixing of Overwatch® with another product is unlikely to have any significant impact on the volatility profile of either product in the mix.
  • Volatility of any herbicide is inherently determined by its vapor pressure, water solubility and sorption to soil.
  • Volatility may be affected by physical factors including temperature, humidity, soil type and others.
  • Changes in volatility occurs because of physical, rather than chemical, factors.
  • Mixing Overwatch® with other chemicals does not create a physical effect that would lead to a change in the volatility profile, like the factors referred to above.

Certain Emulsifiable Concentrate (EC) formulations and spray adjuvants are known to increase the percentage of fine droplets during application.   Fine droplets will not fall to ground under their own weight and will be taken wherever the wind carries them.

4.2 Some people have been saying that they are seeing only symptoms of Overwatch®, and not other mix partners, in drift situations. What is FMC’s response?

When Overwatch® is used in combination with other herbicides, all products in the tank have the potential to cause symptoms in different ways, some of which are more difficult to detect.

5.1 Are the labels on the drums of Overwatch cautious enough? Do they need to be tightened?

The Overwatch® label is based on extensive research over 10 years. The label provides very detailed information on how to use the product and precautions to be observed. The label has been extensively reviewed and approved by the APVMA and technical experts. As per industry practice, all labels are periodically reviewed and updated as required.

5.2 Are you comfortable with the product as it is described on the label? Given some commentary on social media, do you think something will need to change next season?

FMC is comfortable that the information provided, and Directions for Use as it currently sits, are sufficient to ensure the responsible use of Overwatch® when these label directions are adhered to.

Future changes may include updates to stewardship, stakeholder awareness, marketing, and knowledge transfer materials to further educate users around the transient nature of bleaching, as well as best spray practices.

5.3 How does FMC impart stewardship information about Overwatch® Herbicide and other products, and educate users on their appropriate use?

To qualify as a selling Agent for Overwatch®, all distributors were required to complete on-line stewardship training on the safe and responsible way to use the product. Agents are expected to impart this knowledge when recommending Overwatch® to growers.

In addition, direct marketing materials, commercial field demonstrations and training modules are being used to educate growers and advisors about Overwatch® as well as the appropriate way to use it.
6.1 Some agronomists assert that rain events long after the application & incorporation of Overwatch® Herbicide causes it to start rising through the soil, volatilise into a vapour phase, and cause bleaching symptoms in off-target crops. What are FMC’s comments on this assertion?

Rainfall or moisture generally tends to move downwards by processes of diffusion and advection, and usually serves to move any herbicide deeper into the soil profile. This phenomenon has been evident in many cases where excessive rainfall occurred after the application and incorporation of Overwatch® – resulting in more pronounced bleaching from the germinating plants coming into contact with the herbicide. The Overwatch® label identifies this risk.

The active ingredient Bixlozone is known to exhibit moderate mobility in soil, and significant rainfall events long after application will only serve to increase this downward mobility rather than promoting upward movement and subsequent volatility. This downward movement is likely to be further enhanced by microporous flow though channels in the soil which will have been exacerbated by warm dry weather causing cracks in the soil to extend downward.

A more likely explanation for what is perceived as “delayed drift” is that the off-target bleaching effect is delayed because the amount of drift that occurred around the time of application did not express any symptoms until reactivated by moisture on the sensitive plant or soil surface where it landed, and the plant starts to grow actively. Just like Overwatch® requires adequate soil moisture for effective weed control, some amount of moisture will be required to activate the active ingredient deposited on off-target foliage or soil from spray drift or inversions at an earlier stage.
7.1 FMC says it is working with industry partners to address the reports of transient bleaching. What are these reports – are they from farmers or someone else (health department, agronomists, resellers, others)?

The reports we have heard are cases of transient bleaching that farmers are either unfamiliar with, or deem to be excessive from the use of a herbicide, and generally where there were adverse weather conditions These reports are mainly from farmers, resellers, or agronomists, or from commentary on social media.

7.2 What is happening to crops that exhibited symptoms of elevated bleaching earlier in the season?

Ongoing monitoring is showing promising recovery of crops from transient bleaching effects. Some crops have already fully recovered from the transient bleaching effect.

There are still a few areas where FMC are yet to observe that crops have recovered from the transient bleaching effect. Those areas continue to be monitored but, importantly, it is still some weeks before FMC would expect that the transient bleaching effects would have entirely resolved in all crops.

7.3 For crops that exhibited elevated bleaching, and which have not yet recovered, or do not recover - why is that so?

There are specific variables applicable to each situation. These include insufficient separation between seed and herbicide, insufficient soil throw, overlapping, heavy rains on sandy soils soon after application, every condition or combination of conditions that increases plant stress, combined effects of other herbicides in the mix, etc. Refer to the product label for the comprehensive list of conditions that may cause elevated bleaching. Some of the issues referred to may, on their own or combined, cause irreparable damage to the crop.

7.4 What is happening to Lupin crops affected by off-target drift earlier in the season?

Just as FMC is seeing for other crops, as referred to above, ongoing monitoring is showing recovery of Lupin crops from transient bleaching effects caused by off-target drift.

Left: a field of Lupins showing symptoms of bleaching.
Left: a field of Lupins showing symptoms of bleaching.
ight: the same field, four weeks later
Right: the same field, four weeks later.
FMC has received reports that Lupin crops immediately adjacent to Overwatch®-treated paddocks are not experiencing any transient bleaching effects, and upon investigation of those reports FMC has noted that the application of Overwatch® in the neighbouring paddocks was consistent with its the label instructions and good spray practices.

7.5 I am not clear about what bleaching means – please explain this?

Bleaching is a generic term used to describe a crop symptom where the leaf colour changes from green (normal) to a lighter shade. Bleaching can be complete (whole leaf) or partial (e.g. leaf edges). It can occur on all leaves, or just some. Bleaching is not unique to Overwatch® and is caused by various herbicide modes of action. Bleaching can be irreversible, or transient in nature. In the case of Overwatch®, bleaching is transient in early crop stages under typical growing conditions.

Below is a picture that shows bleaching occurring in a plant. This picture is indicative only.
Bleaching occurring in a plant
7.6 How many cases of bleaching are you aware of?

Crops treated with Overwatch® may exhibit some degree of transient bleaching, from which they will recover.

7.7 Should farmers expect to see bleaching after using the Overwatch® Herbicide?

Yes, due to its mode of action, transient bleaching is commonly associated with Overwatch®, the degree of which is influenced by crop type, variety, application technique, use rate and a range of environmental factors (selectivity by positioning).

7.8 If a farmer’s crop is bleached, does this mean lower yields for this crop?

No. Bleaching is transient in the early stages of the crop under in typical growing conditions and does not impact yield.

7.9 What is the detail around the trials that have been done – what did you test and what are the outcomes?

Overwatch® has been extensively tested over 7 years in a range of weather conditions and across all geographies by FMC and is approved for use throughout Australia by the APVMA. FMC conducted three pot studies and 95 small plot replicated field trials on farmer’s properties around Australia, to generate the data package to support the efficacy and crop safety of the package.

8.1 Is Overwatch® Herbicide (Bixlozone) considered to be a volatile herbicide?  Some reports suggest that the product moves around…

Overwatch® Herbicide (Bixlozone) is not considered to be a             volatile herbicide, because:

  • According to the APVMA, studies indicate that volatilisation and atmospheric transport following application are not likely to be significant for bixlozone*.
  • Investigations about potential volatility of Bixlozone under controlled conditions in a wind tunnel showed the following:

For an application rate of 300 g Bixlozone/ha the amount of volatilisation from the application was low (maximum 0.42% at 1m from the application site 72 hours after application) and was seen to decrease with increasing distance from the application site, with <0.05% of the application rate  detected at 20 m from the application site.

The degree of bleaching of the sensitive indicator plant Stellaria media (Chickweed) associated with the volatilisation of Bixlozone was also seen to be low, with a maximum of 13% total leaf area bleached in plants located 1m from the application site. In plants located 20m from the application site no bleaching was observed. Any bleaching was seen to occur 7 days after application, but not before.

These observations correlate with the degree of volatilisation observed in the APVMA report.

Overwatch® is not considered to be a relevant residue in air.

* Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide. APVMA product number 86427. December 2019

8.2 Was Overwatch® Herbicide fit for commercial launch in Australia?

Yes. Overwatch® was fit for commercial launch in Australia. The complete regulatory package was evaluated by the APVMA, which is a world class regulator committed to  regulatory and scientific excellence.

A complete registration package takes over 10 years to generate, and costs approximately $280 million* to compile. This includes an extensive range of studies covering toxicology, metabolism, residues, environment, efficacy, and       crop safety.

Bixlozone was first developed in the 1980’s, and studies have been undertaken since then. A substantial volume of  information about this product exists.

*Phillips McDougall

The Cost of New Agrochemical Product Discovery, Development and Registration in 1995, 2000, 2005-8 and 2010-2014. R&D expenditure in 2014 and expectations for 2019

9.1 In some cases, Overwatch® Herbicide is damaging crops and farmers are worried about yield penalties come harvest time. Will farmers experience yield penalties for Wheat, Canola, and Barley?

The result from 7 years of R&D trial data is that there is no yield penalty in wheat, barley or canola associated with the transient bleaching that is characteristic of Overwatch®. It passed the APVMA efficacy and crop safety review because there is no yield penalty when Overwatch® is used according to the registered product label under typical growing conditions.
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