If access to MCPA (2-methyl-4-chlorophenoxyacetic acid) for post-emergent broadleaf weed control is an issue for growers, switching to 2,4-D will require an adjustment in timing of application.

Independent Consultants Australia Network (ICAN) weed specialist Mark Congreve says MCPA LVE can generally be applied from the three-leaf growth stage, depending upon application rate, whereas an application of 2,4-D must be delayed somewhat.

“For 2,4-D, this means waiting until the first node can be felt at the base, which normally means that tillering will have commenced and crops are typically at the five-leaf growth stage, although this can change with seasonal conditions,” says Mr Congreve, whose work in herbicide behaviour has been supported by the Grains Research and Development Corporation (GRDC).

“If 2,4-D is applied too early, the risk of crop injury is significant. Ensure the 2,4-D label is fully read and understood, especially if you are not experienced with early applications of 2,4-D.

“Crop varieties have different levels of tolerance. Additionally, 2,4-D ester is often more damaging than amine, and the addition of some tank mix products, or spraying oils, can further increase risk of damage.

“There may be some growers, from the younger generation especially, who have never had to seek an alternative to MCPA for post-emergent broadleaf control,” Mr Congreve says.

“They may therefore be unaware of the risk to establishing crops if they use 2,4-D too early.”

Mr Congreve encourages growers to seek expert advice if they are planning to spray with 2,4-D as other factors also need to be considered when determining the most appropriate application timing.

“Several broadleaf herbicides only carry label recommendations for tank mixing with MCPA, and not 2,4-D, so it would be wise to speak with the manufacturer before switching to 2,4-D as there may be crop safety issues or possibly legal constraints, depending upon state regulations.”

More information on post-emergent weed control is available in the GRDC’s Understanding post-emergent herbicide weed control in Australian farming systems manual, available at https://grdc.com.au/understanding-post-emergent-herbicide-weed-control.