Take home messages

• Both bixlozone (Overwatch®) and fomesafen (Reflex®) are relatively long persistence herbicides. Ensure labelled plantback conditions are fully met before planting rotational crops.
• Length of soil persistence (carry over) will be a factor of
  • starting concentration in the soil (application rate less any losses prior to incorporation)
  • soil type
  • climatic conditions (particularly summer rainfall) post-application.

Background

The behaviour of residual herbicides when applied to the soil is a function of the starting dose rate in the soil, individual chemical properties of the herbicide, and the environment where they are placed. Understanding these factors and how they interrelate gives users a moderate to high level of predictability of performance. Detailed discussion on how chemical properties interact with the environment they are placed in can be found in Soil Behaviour for Pre-emergent Herbicides.

Reflex® (fomesafen) and Overwatch® (bixlozone) are two relatively new herbicides to the Australian grains industry. Reports from some users in the southern grains region have indicated that Rresidue carry over of these herbicides has continued to be reported this season by some users in the Southen grains region. This paper will focus on these herbicides in particular and how they are predicted to behave under Australian environmental conditions.

Getting to the soil

For any residual herbicide to perform as expected it needs to enter the soil, ideally as an even deposition and close to the applied rate. If herbicide is prevented from reaching the soil in the full dose, then length of residual activity may be reduced.

Factors affecting soil entry include:

• application rate
• presence of green plant material. Any herbicide deposited on green plant material is likely to enter the green leaf material. For some herbicides (including Reflex® and Overwatch®), this may add to post-emergent control of existing weeds, however this volume of herbicide will therefore not be available in the soil for ongoing residual activity.

• stubble interception - Herbicides with a very high Koc value (for example, trifluralin Koc = 15 800, see Pesticide Properties Database) have a high affinity to bind with organic matter and will be almost impossible to wash off stubble once the spray deposit has dried.

Overwatch® (Kfoc[1] = approximately 400, see Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide) does not bind particularly strongly to stubble and Reflex® (Koc = 50, see Pesticide Properties Database) has very low affinity for stubble binding. Both would generally be expected to be washed off the stubble, provided there is reasonable rainfall following application.

• incorporation time - For some herbicides that have potential to be lost to volatilisation or ultraviolet light degradation, the time to soil incorporation is important.

Data suggests that neither Reflex® nor Overwatch® are particularly prone to UV loss if not incorporated. However, there may be slight loss of Reflex® should the herbicide be not be physically incorporated and there is no rainfall for several weeks, particularly if this is an early autumn application under high light intensity (for example, dry sowing in March).

Volatility loss is dependent on the ambient temperature, surface type (for example, soil, plant, stubble), moisture level, wind blowing across the surface, time to incorporation and vapour pressure of the particular herbicide. While vapour pressure is a laboratory calculation under controlled and contained situations and is only one factor involved in atmospheric losses, it does provide some indication of the relativity of potential losses between herbicides.

Typically losses from herbicides with a vapour pressure less than 1mPa (@ 20oC) are negligible under most situations. Reflex® has a published vapour pressure of 4 x 10-3mPa (see Pesticide Properties Database) so loss to volatilisation is considered negligible. Herbicides with vapour pressure above 5-10mPa may experience some losses to volatilisation, especially under warmer temperatures, with labels of these herbicides tending to recommend physical incorporation soon after application to reduce the potential for loss.

Overwatch® has a published vapour pressure of 2.3mPa (@ 25oC) (see Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide). This suggests that there may be a low potential for some minor loss prior to incorporation, especially if conditions are warmer following application. (For comparison, other common herbicides with similar published vapour pressure include clopyralid, pendimethalin and s-metolachlor which have vapour pressures of 1.4, 3.4 and 3.7mPa respectively (see Pesticide Properties Database)). Once incorporated into the soil, any losses are likely to be negligible.

• seeding systems - In broadacre grains production, herbicide incorporation is typically a combination of physical soil movement of the planter (if sowing after the herbicide is applied) and rainfall soon after application. A well set up knife point and press wheel sowing system should provide ‘reasonable’ soil incorporation, but there can be large differences between operators in the level of incorporation achieved. Where incorporation is sub-standard, there may be some ongoing losses from volatilisation or UV degradation for the herbicide where this is important.

If the herbicide is applied post-sowing pre-emergent (PSPE) or early post-emergent in-crop, then incorporation is totally reliant on subsequent rainfall, and environmental losses may continue to occur until adequate incorporating rainfall is achieved.

• spray drift - Every spray application results in the production of some small droplets. Typically, droplets with a volume mean diameter (VMD) of less than 150µm do not have enough weight for gravity to pull these towards the spray surface and run the risk of remaining suspended in the atmosphere for long periods of time and potentially moving off the paddock (Table 1). This may result in problems with spray drift, however also results in less herbicide reaching the soil.

Table 1: Typical percentage fines of droplets with a VMD of <152µm produced by various nozzle spray qualities.

*See Overwatch® Herbicide – volatility versus spray drift

Soil availability

Once in the soil, some of the herbicide will initially bind to soil and organic matter, with the remainder unbound and ‘available’ in either the soil water or air spaces. For a pre-emergent herbicide to be able to be taken up by the plant, some herbicide needs to be unbound and freely available. Herbicides with extremely high soil binding (for example, glyphosate, paraquat) are not active via soil uptake.

The percentage split between bound and unbound herbicide differs between herbicides and is determined by the herbicide binding coefficient (Koc) and soil type. Soil binding coefficients are generally presented as an average and range across different soil types. For the same herbicide, the binding coefficient will typically be lower when applied to light/sandy soils or those with low organic matter (indicating more herbicide will be ‘available’ at any point in time), while the binding coefficient is generally higher for heavier or high organic matter soils (more herbicide is bound and less available). As there is generally more herbicide ‘available’ to the plant in light sandy or low organic matter soils at any point in time, it is typical to see more adverse crop effects in these soils, all other factors being equal.

The binding coefficient for Reflex® is relatively low, with Overwatch® having slightly increased binding. However, for both herbicides, it would be expected that there is likely to be substantial ‘available’ herbicide present in the soil water phase. As the ‘available’ herbicide is used up (taken up by plants or lost to microbial degradation), some previously bound herbicide is released back into the soil water to maintain an equilibrium.

To understand soil mobility, the solubility of the herbicide also needs to be considered. Solubility of Overwatch® and Reflex® are both relatively low at 40mg/L (see Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide) and 50mg/L (see Pesticide Properties Database) (at 20oC), respectively. In summary, this means that Reflex® is largely unbound to soil and organic matter, but the lower solubility will mean that a higher volume of rainfall will be needed to move the herbicide down the soil profile. The additional binding of Overwatch® will mean that more herbicide is likely to remain closer to the soil surface, however that does not exclude the possibility of some herbicide moving deeper in the profile, particularly under heavy rainfall and lighter soil types with larger air spaces.

As Reflex® is somewhat mobile, it is likely that the germinating crop will come into contact with some herbicide and crop tolerance is based on the crop being able to rapidly metabolise the herbicide. Crops with high tolerance can sustain a PSPE application in most situations (that is, herbicide placed directly above the crop row), although damage may still be evident under conditions where metabolism is reduced. For less tolerant crops, label directions may require reduced rates and incorporation by sowing (IBS) only, which removes some treated soil from the planting furrow. While IBS reduces the total amount of herbicide exposure, a moderately mobile herbicide, such as Reflex®, may move back into the planting line with following rainfall, so IBS alone does not completely remove the risk of crop injury.

The increase in soil binding of Overwatch® allows users to better utilise positional safety for crop tolerance. The lower soil mobility means there will be higher herbicide concentration in the 0-2cm zone than further down the profile, so planting at ≥3cm depth typically reduces crop exposure. Additionally, where crops are planted IBS with knife points and press wheels, some herbicide treated soil will be removed from the planting furrow and into the interrow, further increasing crop safety.

For all herbicides (including Reflex® and Overwatch®), the risk of crop injury is greatest when there is a combination of additional stress on the emerging seedling (disease, other herbicide residues, slow emergence, poor conditions for metabolism, for example waterlogging).

Degradation and dissipation

As mentioned previously, the length of soil persistence is a factor of the initial rate of herbicide reaching the soil; the speed of breakdown of the herbicide; and the environmental conditions following application.

Reflex® and Overwatch® have relatively long persistence compared to many other herbicides. The published DT50 value (days of time for 50% of the herbicide to dissipate) for Reflex® is an average of 86 days, with a range across different soils tested of 59 to 112 days (see Pesticide Properties Database). For Overwatch®, the published values are 93 days (range 46-267) when not incorporated, or 179 days (range 37-446) if incorporated soon after application (see Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide).

These relatively long DT50 values (or half-lives) will mean that extended plantbacks are required for sensitive crops. Closely follow label advice on plantback periods, or updated advice as provided by the manufacturer. Both FMC (Overwatch®) and Syngenta (Reflex®) provide additional information onf their respective web pages.

For both herbicides, microbial degradation is the primary route for breakdown. Microbes attack herbicide residues to utilise the herbicide as an alternative food source (organic carbon). To ensure adequate levels of functioning soil microbial activity, there is a requirement for an ongoing food source (soil organic matter), available soil moisture and adequate soil temperature.

Highest levels of soil microbes are usually near the soil surface, as this is generally where the organic matter is concentrated. Soils with very low organic matter (that is, less than 1.5%) often have extended persistence of most residual herbicides.

Inversion tillage, that buries the soil surface containing the organic matter and microbes and replaces this with soil from depth that is low in organic matter and associated microbes, can be particularly problematic. Persistence of any residual herbicide is likely to be greatly extended in this situation, at least until organic matter levels can be rebuilt at the soil surface.

If the soil surface dries out, then no herbicide degradation will be occurring in this zone until the soil rewets. In southern farming systems, often with substantial dry periods over summer, this may result in an extension of time for herbicide degradation. For optimum herbicide degradation, consider the number of weeks of moist topsoil over the summer months, in addition to total amount of rainfall. Non-wetting sands can also result in uneven degradation within the soil profile, due to patches of wet and dry soil and resulting microbial activity.

Speed of microbial activity is also influenced by temperature. As soil temperature drops in winter, microbial activity slows. Therefore, soil moisture over the spring/summer months is typically more important for herbicide degradation than soil moisture during winter. In addition, soil microbes prefer a relatively neutral soil pH and no major soil constraints. Conditions outside of this may reduce the efficiency of microbial operation.

Due to the medium to long persistence of Reflex® and Overwatch®, it will be critical to ensure that label plantback advice is followed very closely. Where label plantback conditions have not been achieved, the most advisable strategy will be to switch planting intention to a more tolerant crop (that is, one with a shorter plantback period).

If labelled plantback conditions have only just been achieved, there may be benefit in conducting a soil bioassay in the field to be planted. A bioassay is where some seeds of the intended following crop are planted into soil in advance of the expected planting date and any herbicide symptoms observed on emerging seedlings. The Syngenta Reflex® web page provides information for appropriate bioassay protocols.

Bioassays are best done in the field to be planted, as any herbicide remains in situ. However, this often means that test strips require regular watering if there is no rainfall to ensure the planted seeds have adequate conditions for emergence. Taking soil from the field to ‘pot up’ at home may be convenient to ensure pots can be easily watered, however runs the risk of mixing and diluting herbicide residues and may lead to false results.

Additionally, where possible, look for evidence of emerging weeds in autumn. If ‘sensitive’ weeds emerge without damage following the autumn break (for example, brassicas or sowthistle for Reflex®, lupins or sowthistle for Overwatch®), then users could expect an increased level of confidence that the majority of residues may have dissipated.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support.

References

Pesticide Properties Database

Public Release Summary on the evaluation of the new active bixlozone in the product Overwatch® Herbicide

Overwatch® Herbicide – volatility versus spray drift

Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

Overwatch® Herbicide Plant-back Guide

Reflex® plantback information and bioassay procedure

Contact details

Mark Congreve
ICAN
PO Box 718, Hornsby NSW 1630
0427 209 234
mark@icanrural.com.au
@congreve_mark