FIVE is the critical number for farmers looking to make the switch from the use of MCPA phenoxy based broadleaf herbicides to 2,4-D based products.
Leading weed specialist Mark Congreve, Independent Consultants Australia Network (ICAN) said cereal plants needed to be more mature to avoid crop damage when using 2,4-D than MCPA.
With MCPA croppers can normally apply the product safely from the three-leaf stage onwards, however with 2,4-D Mr Congreve said farmers needed to wait until the first node could be felt at the base of the plant, generally around the five leaf stage.
Farmers are switching from MCPA to 2,4-D in some cases due to difficulties getting access to MCPA.
"If 2,4-D is applied too early, the risk of crop injury is significant," Mr Congreve said.
"Ensure the 2,4-D label is fully read and understood, especially if you are not experienced with early applications of 2,4-D."
He added that farmers should study up on the difference in risk from variety to variety and in the different forms of 2,4-D.
"Crop varieties have different levels of tolerance," he said.
"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."
Mr Congreve encouraged 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."