Arkansas Rice: Herbicide Application Considerations

Applying Herbicides to Dry Soil

As rains are supposed to subside in the coming week and more rice nears flood, residual herbicides will be starting to break. If you are preparing to make postemergence herbicide applications and conditions are becoming dry, it’s important to remember that control can be less than optimal so maximizing coverage is critical.

Herbicides such as Ricestar HT, Facet, Newpath, and Clincher generally require good to excellent soil moisture to have actively growing weeds that will rapidly take up the herbicides. If conditions begin to get too dry, we need to strongly consider being prepared to immediately flush (if younger rice) or get the crop to flood.

While not an ideal answer, at times it may be more appropriate to target postemergence applications for grasses after a shallow flood has been established. The downside here is that weed size will have increased and coverage potential decreased depending on the depth of the water.

There is no perfect answer, but environmental conditions and field management capability will dictate the best approach.

Many questions have arisen about spraying grass that is bleached from Command applications. In some cases, Command has the potential to “reachback” and control grasses that were emerging during application or already up at the time of application, especially early in the season.

With current temperatures forecasted, these grasses may be stunted today, but will most likely recover and will need to be re-treated timely. And yes, the herbicide will still work even though the grass has a white color.

Herbicides on Cut Soil

Herbicide options are slim on soils that have been recently leveled. There have been a few mistakes this year as we have moved rapidly to get fields planted and residual herbicides out.

Command should not be applied to cut ground, especially where deep cuts have occurred. There is the potential for reduced plant vigor and even stand loss if Command is used in these fields.

Facet can also cause issues on cut soils and should not be used pre-emergence or early postemergence. If rice is growing well and healthy, then it could be considered for use as a late postemergence application.

Regiment and Grasp should not be used on cut soils, particularly with deeper cuts.

Use of Clearfield, FullPage, or Provisia cultivars are generally advised due to tolerance to their respective herbicides. Newpath/Preface plus Beyond/Postscript or Provisia herbicides can be used successfully on cut soils and will help manage weeds successfully in the absence of Command and Facet.

Prowl and Bolero are good options for residual herbicides in cut soil situations. But remember these can only be applied delayed preemergence (after rice has germinated) or later.

Overall, we will need to rely on Prowl and Bolero as preemergence herbicides, and propanil, Ricestar, and Clincher as postemergence herbicides. As previously stated, Newpath/Preface, Beyond/Postscript, and Provisia may be used in their respective production systems and these technologies are strongly recommended for use on a recently cut field.

Spray Applications and Water Volume

There have been some recent comments made about needing aerial application volumes greater than 10 gallons per acre (GPA). This isn’t particularly true whether in regard to herbicide or fungicide applications.

In recent work focusing on fungicide applications using a ground rig, fungicides applied using 10 GPA were better at managing sheath blight than fungicides applied using 3 GPA. Applications made at 20 GPA were not better than those at 10 GPA. Keep in mind this is from a ground rig.

Applications made via ground versus aerial differ especially in respect to droplet sizes used. When discussing spray coverage, there is a highly significant interaction between droplet size and spray volume, and discussing one without the other does not paint the whole picture.

Droplet size impacts spray coverage to a larger extent than spray volume due to the exponential relationship between droplet diameter and the actual volume within that droplet. If we cut our droplet size in half with a fixed volume, we actually have 8x the number of droplets available for coverage.

In contrast, if we double our spray volume with a fixed droplet size, we increase the number of droplets available for coverage 2x. Coverage card data show that deposition from a plane at 5 GPA provides similar coverage to deposition by ground at 10 GPA when considering commonly used droplet sizes from each application method.

Recent aerial applicator surveys indicate that systemic herbicide applications average 5 GPA and contact herbicide applications average 7 GPA. Yes, some applicators use less, some use more.

Increases in application rate will certainly help with coverage and subsequently with weed or disease control. Going from 3 GPA to 5 GPA is a good thing and going from 5 GPA to 7 GPA is a good thing because we have more droplets with the potential to remain on a leaf surface.

But increasing much beyond that is not likely to cause substantial improvement and can be very difficult to achieve via aerial application considering payload capacity of current agricultural aircraft.