Pre-emergent Herbicide Use Fact Sheet

Published: 7 Oct 2014

Understanding pre-emergent herbicides and how they interact with the environment

With the continued evolution of herbicide resistance, growers are being forced to introduce a range of different weed control tactics. A tactic that has rapidly increased in recent seasons is the use of pre-emergent herbicides, especially in the summer crop and fallow. To predict field performance of these herbicides, an understanding is needed of their chemical properties and how they interact with the environment.

The value of pre-emergent herbicides

When devising a weed control strategy, consider the use of pre-emergent herbicides as an additional tactic available to help drive weed numbers down. Used alone they usually will not achieve the objective of driving down weed seedbank numbers, but when used amongst a suite of tactics, they can be particularly effective.

Key Points ––

  • Knowing which weeds are in the paddock and where the weed seeds are located (shallow or deep) is important in selecting a herbicide to be applied. ––
  • ––Be aware of whether a herbicide is subject to volatilisation or photodegradation in order to determine an incorporation strategy that minimises loss to the environment. ––
  • ––Solubility influences how much rain is required for herbicide incorporation, how easily a herbicide will be taken up by a germinating weed and crop, and if a herbicide will be subject to moving down the profile, potentially causing crop injury or loss to leaching. –– ––
  • ––Sandy or low organic matter soils will have less binding and allow for greater herbicide availability for crop and weed uptake.
  • ––Herbicides that bind tightly to soil and organic matter generally require higher application rates, stay close to where they are applied (unless the soil moves), and persist for longer. ––
  • ––––Soil pH affects how long some herbicides persist for and how available they are for plant uptake and soil binding. ––
  • ––The persistence of a herbicide and the way in which it breaks down will dictate the length of residual control and plantback constraints to sensitive crops. ––
  • ––Rainfall after application is important for incorporation and availability to the weeds and crop. Rainfall and temperature also affect degradation. ––
  • ––Choice of application rate will affect length of residual, and possibly crop selectivity.

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