Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

The value of pre-emergent herbicides

When devising a weed control strategy, pre-emergent herbicides can be a valuable additional tactic to help drive weed numbers down. Used alone they often do not achieve the objective of driving down weed seed bank numbers, but when used amongst a suite of tactics, they can be particularly effective.

Benefits of pre-emergent herbicides include:

• Offers an alternate mode of action to many post-emergent options
• Reduced selection pressure on subsequent post-emergent herbicide applications
• Removal of early season weed competitive pressure on a crop often protects crop yield better than later applied post-emergent applications, especially in weedy paddocks
• Cost savings, especially in the fallow where multiple knockdown applications may be required
• Reduced time pressure on other spraying operations, both in crop and in fallow
• Major role in patch eradication where a weed blowout can be GPS logged and a pre-emergent herbicide applied to manage the patch
• After a cultivation event, there will always be some weed seed in a position in the soil profile that is ideal for germination. Applying a pre-emergent herbicide after the last cultivation can manage these weeds that would otherwise emerge and ultimately return additional seed to the soil, and
• Some crops have few post-emergent options (e.g. grass weed control in sorghum, or broadleaf weed control in pulses) and hence often rely on pre-emergent herbicides for in-crop weed control.

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