As the early growing season unfolds in south-west Victoria, a familiar yet critical challenge emerges for growers; managing weeds to effectively safeguard yields and optimise crop establishment.

In a region where early dry sowing has been a necessity weeds germinating alongside crops significantly threaten establishment and yield.

Post-emergent herbicides are a strategy to manage weed populations. However, with prolonged dry periods and cool conditions upon us, there are some key considerations for effective post-emergent herbicide application.

Weather conditions at the time of application can have a significant impact on how it enters and is translocated within the plant.

• In dry conditions, plants move water out of the cuticle, which increases the density of the waxy layer and reduces the plant’s water loss. However, this makes penetration of herbicides more challenging.

• Dry conditions also lead to closure of stomata, which reduces the rate of photosynthesis and therefore the efficacy of herbicides that target the Photosystem I (PSI) and PSII pathways, such as Groups 5, 6 and 22.

• Reduced photosynthesis decreases the translocation of metabolites in the phloem, which affects herbicides such as Group 1 and glyphosate which rely on phloem mobility.

When to apply:

Avoid application immediately after stress events such as frost, excessive heat or waterlogging, where plant metabolism and translocation can be slowed or completely stopped. In particular, Group 1 herbicide efficacy can be significantly reduced in cold or frosty conditions. Selectivity of Group 2 herbicides is also reduced when a crop is under stress conditions. Plants can take a few days to recover from these stress events, therefore allowing this time before herbicide application goes a long way in improving post-emergent herbicide efficacy.

For further information on the actions and considerations of post-emergent herbicide application, refer to the GRDC manual – Understanding Post-Emergent Herbicide Weed Control In Australian Farming Systems (2023).

 By Cam Barr, Research & Extension Officer