This blog covers split apps of pre-M herbicides and is an update on our recent turf blog on using pre-emergent herbicides. It discusses some newer research and split or sequential applications for better control. Using split apps of pre-M herbicides can potentially control certain weeds a lot better. A split application is where you apply a pre-emergent herbicide at lower rates in multiple treatments. Advantages of split applications are:
  • A reduction in the likelihood of turf injury and
  • They may extend weed control later into the season.
 
 
 
 

Advantages of using split apps of pre-M herbicides.

Comparative advantages of split applications over a single treatment are:
  • Reduction the potential for turf injury,
  • Worth considering if freely draining rootzones i.e. sand and
  • Extending and improving weed control later into the season.

 

 

Considerations when using split apps of Pre-M herbicides.

The main consideration is the timing of the second application of the herbicide. However, applying the second dose early in the season will provide the more consistent weed control. Delaying this too long is counter productive as applying the second dose 4-8 weeks after the first will provide the most consistent weed control.
 
By making the second application whilst the first application is still effectively working reduces the risk of weeds establishing due to a “control window” developing. So ideally the split treatment should be made while the first application is still effective.
 
As most pre-emergent herbicides are lipophilic, they are rapidly absorbed into the cuticle of plants. Once there they will not be washed off with rain, and so will not contribute to weed control.

 

Waterlogging.

With the huge amount of rain areas such as NSW and QLD have had in the last few months the performance of pre-emergent herbicides has to be discussed. So what impact does saturated soil have on how pre-emergent herbicides perform?

If you are using the split application approach a smart move is to consider increasing the rate of application. How much to increase the rate is an educated guess and depends on site-specific conditions but increasing the rate by 25% on the second preemergent application is certainly reasonable

 

Pre-emergent herbicide half life dependant upon soil moisture content

Active Ingredient

Trade Name

Approximate half-life in moist soil

Approximate half-life in saturated soil

Indaziflam

Specticle

120 days

120 days

Pennmag

39 days

24 days

Oryzalin

Prolan

45 days

5 days

Oxadiazon

Ronstar

60 days

30-60 days

Pendimethalin

Stomp

45 days

12 days

Prodiamine

Barricade

56 days

28 days

Trials using split applications of pre-emergent herbicides.

  • 2020. A trial looking at Freehand and Specticle. A split application (half rate followed by half rate three weeks later) controls weeds as effectively as the full rate, while providing extended control over the full rate.
  • 2011. Equivalent summergrass control occurs when using split applications of prodiamine, pendimethalin, and dithiopyr. This is regardless of which herbicide is used for the first and/or second application.
  • When applying the same amount of active ingredient, split applications are more effective. They also give more consistent control of summergrass than single applications.
  • 2012. Research conducted at CAFS evaluated the effects of mowing height on the efficacy of single and split applications of preemergence herbicides for summergrass control.
  • When mowed at 14mm, using split applications provides greater control than single applications.
  • At 50mm, no significant differences were detected between single and split application regimes.
  • Studies conducted at Penn State University using Dispatch Sprayable tank-mixed with pre-emergent herbicides give better summer grass control than pre-emergent herbicides alone.
  • 2011 work shows that watering-in pre-emergent herbicides immediately after application with 2.5 to 5mm of water results in up to 50% improved summer grass control in a study on fairway-height Kentucky bluegrass.
  • Contrary to what you might think, a 2014 study shows that applications of pre-emergent herbicides do not affect divot recovery.
Using a good quality surfactant will improve split applications of pre-emergent herbicides