The importance of pre-emergent herbicides in Australia

By taking the pressure of post-emergent chemistry and allowing growers to have a longer chemical rotation, pre-emergent herbicides play an integral role in both weed management and herbicide resistance management in farming systems.

Through the rotation of herbicide groups, growers can reduce 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. Applying a successful pre-emergent spray can help the crop establish more quickly; once it matures, it also becomes far more competitive against future weed infestations.

Understanding Your Operation's Weed Spectrum

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

Aligning your herbicide choice with your application strategy will improve the efficacy of pre-emergent herbicides.

Assessing the Stubble Load and Ground Cover

Once you've developed an understanding of the weed spectrum in the paddock, the next step in developing a pre-emergent herbicide strategy 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 can help you make a more informed decision. 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.

Using tools such as tillage or stubble management can help you achieve a successful pre-emergent application.

Herbicide Loss Through Volatilisation

The next assessment to make when developing your pre-emergent herbicide strategy 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 the herbicide will break down. Herbicides such as Triallate and are highly volatile. When using volatile pre-emergent herbicides, incorporation needs to occur soon after spraying in order 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 developing a 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. This means it can be prone to leaching into the root zone of the crop and, in some instances, crop damage can occur. Conversely, an herbicide with a low solubility will require larger amounts of moisture to be incorporated and will not as readily move through the soil profile.

Avoiding Crop Damage from Herbicides

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

To prevent this, take extra care at sowing time to physically displace the herbicide away from the seed without throwing the soil into the adjacent furrow. Placing seed at depth also provides physical distance between the seed and the herbicide, reducing the likelihood of crop damage.

Follow Herbicide Label Guidance

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

It is important to note that there are some technicalities that go with pre-emergent herbicides; no two 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.