Pre-emergent chemicals aren’t cheap. So when spraying without a knockdown, you’ll want to do a few things to ensure you get the most out of them.

First off, when using pre-ems, you should aim to get as much chemical as possible onto the soil and not on the stubble.
There are a couple of ways you can achieve this:

• using a larger droplet
• higher pressure
• high water rates
• narrow fan angle to decrease the amount of double overlap
• And if possible, have rear-facing nozzles to offset the forward momentum of the boom if you are driving over 20km/h. 

Secondly, mechanical incorporation is essential to move the chemical into the soil if there hasn’t been any rain to wash in the chemical. If this doesn’t happen, you risk losing chemical activity to photodegradation.

Why does that happen? Well, if a herbicide sits on top of the soil without being incorporated by rainfall or mechanics, it reacts with sunlight, breaks down, and is lost to the weed control system. Volatilisation happens, and the chemical will transition to a gaseous phase. Therefore, these products must be incorporated soon after application to avoid significant losses to the atmosphere and a reduced level of weed control. (GRDC, Pre-Emergent Herbicides, 2015)

Below is a table from WeedSmart of the environmental losses of some pre-em chemicals:

Table is taken from “WeedSmart webinar – considerations for pre-emergent herbicides with dry sowing” 2021

A couple things to note:

– In dry conditions, Luximax will need some mechanical incorporation if not washed in with rain, as it can be lost to volatility. ENSURE THAT YOU ARE STILL GETTING GOOD SOIL THROW OUT OF THE FURROW.

– If trifluralin is sprayed onto the stubble and isn’t incorporated within 4 hours of application or washed off before it dries, it will bind to the stubble and will not come off until the stubble has broken down.

So then, if it does rain, what happens to our pre-em chemicals? This will depend on a few factors, such as soil type, the amount and intensity of the rainfall, and the herbicide properties. 

Highly mobile herbicides will move with the wetting front down into the soil profile, and so in dry soils could potentially move a long way and into the seed zone. This is why some of the new products that can potentially see crop damage recommend sowing at 3cm depth (i.e. Overwatch and Luximax).

See table below:

Table taken from “WeedSmart webinar – considerations for pre-emergent herbicides with dry sowing” 2021

Once the chemical is in the soil, there are two ways that the herbicide will enter the weed seedling. The first way is to be taken up by roots dissolved in water. The second way is the direct movement across the epidermis into either roots or coleoptilar node. Both entry pathways still require ‘some’ soil moisture in the herbicide zone, and all herbicides will fail in completely dry soils. 

Taken from “WeedSmart webinar – considerations for pre-emergent herbicides with dry sowing” 2021

Some herbicides will be able to perform better than others in dry conditions. 

See table:

If you have any questions about your herbicide strategy or sowing plan, please call one of our agronomists.