The Importance of Pre-emergent Herbicides in Australia

The Importance of Pre-emergent Herbicides in Australia

Anthony Stibbard

Feb 10, 2022

Pre-emergent herbicides play an integral role in both weed management and herbicide resistance management in Australian farming systems.

Pre-emergent herbicides take the pressure of post-emergent chemistry and allow growers to have a longer chemical rotation.

By rotating herbicide groups growers are reducing the selection pressure and the likelihood of developing herbicide resistance. Another major benefit of using pre-emergent herbicides is the reduction of competition from weeds as the crop is germinating.

If a successful pre-emergent spray has been applied, the crop can establish quicker and once it matures, it becomes far more competitive against weed infestations. 

Understanding your paddocks weed spectrum

The key to an effective pre-emergent program is understanding how the herbicides you are using interact with the environment and also the weed spectrum you are attempting to control.

Aligning your herbicide choice, and application strategy will result in an improvement in efficacy of pre-emergent herbicides.

Assessing the stubble load and ground cover

Once you have understood the weed spectrum in the paddock, the next step is to assess the stubble load and ground cover.

Both standing and laying stubble will intercept the pre emergent herbicide during spraying resulting in a patchy or uneven application.

Knowing how the herbicide will bind to organic matter will help in these types of decisions. For example, some herbicides bind tightly to organic matter, and even with subsequent rainfall will either very slowly wash into the soil, or in some cases with herbicides such as trifluralin, will not wash off at all.

Early assessment of groundcover will help make decisions to alter the herbicide strategy, use tools such as tillage or stubble management to result in a successful pre-emergent application. 

Herbicide loss through volatilisation

The next herbicide property which needs to be understood is the volatility of the herbicide. A volatile herbicide will transition to a gaseous state if left on the soil surface.

The longer it is left on the surface, the more it will break down. Herbicides such as Triallate and Trifluralin are highly volatile. When using volatile pre-emergent herbicides, incorporation needs to occur soon after spraying to minimise herbicide loss.

Other herbicides such as Atrazine or Pyroxasulfone have much lower volatility, in most instances mechanical incorporation is still recommended however some herbicides can be applied to the soil surface and incorporated via subsequent rainfall with minimal losses. 

Understanding the herbicide solubility is also important in successful pre-emergent strategy. A highly soluble herbicide such as clopyralid requires very little rainfall for incorporation.

However, this high solubility also means the herbicide will move readily with soil moisture. And can be prone to leaching into the root zone of the crop and in some instances crop damage can occur.

Conversely, a herbicide with a low solubility will require larger amounts of moisture to be incorporated and will not as readily move through the soil profile. 

Certain crops are also susceptible to crop damage from registered pre emergent herbicides. When poor seed placement, incorrect herbicide incorporation or large amounts of rainfall damage to the emerging crop can result.

Taking extra care at sowing time to physically displace the herbicide away from the seed without throwing the soil into the adjacent furrow is important.

Placing seed at depth also provides physical distance between the seed and the herbicide and reduces the likelihood of crop damage occurring. 

Pre-emergent herbicides are an important tool to minimise the impact of weeds in our cropping systems. Getting the most out of your applications is important for maximising returns and to keep the weed seed bank to a minimum.

There are some technicalities that go with pre emergent herbicides and no 2 herbicides work and interact with the environment in the exact same way.

Therefore, it is always important to follow the label recommendations and get some advice from your agronomist before deciding the best product to use. 

Learn more 

To find out more about our pre-em herbicide range of products available to help your farming operation, please visit fbn.com/en-au/direct.


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Anthony Stibbard

Feb 10, 2022