By Dwight Lingenfelter
As wheat approaches the stem elongation and jointing growth stage, be cautious of certain herbicide applications. Once wheat has passed Feeke’s Stage 6 (i.e., when the first node of stem is visible), the risk of herbicide injury from 2,4-D, MCPA, Clarity/dicamba, or Curtail increases and application of these herbicides is not recommended. In this situation, the remaining herbicide options for broadleaf weed control are Harmony Extra (similar products: Edition, Treaty Extra, Nimble, others), Harmony SG (similar products: Treaty, Harass, Volta), and Finesse can be applied to wheat until the flag leaf is visible (before Feeke’s Stage 8). While Maestro, Huskie, Stinger and Starane can be applied to wheat up to boot stage (before Feeke’s Stage 9). Refer to wheat growth stage graphic ( Figure 2.5-1 ) from the Penn State Agronomy Guide for more details.
Each spring there are questions about the risks associated with 2,4-D or MCPA application to wheat past Feeke’s Stage 6. Wheat tolerance of 2,4-D is highest between Feeke's stages 3 and 6 and is lowest in Feeke’s Stages 9 and 10. Between stages 6 and 9, sensitivity to 2,4-D gradually increases as wheat growth stage advances. Thus, the risk of injury increases as wheat growth stage advances between stages 6 and 9. Severe injury is highly probable when 2,4-D is applied at Feeke's stages 9 and 10. In most cases, herbicide injury can set back growth of the crop and potentially cause reduced yields since the crop can’t recover in a timely manner.
It is recommended that application of 2,4-D to wheat be made after wheat has reached Feeke's stage 3 but prior to Feeke's stage 6. If growers choose to apply 2,4-D at later stages, they need to understand the associated risk. This risk can be minimized by applying the amine form of 2,4-D or reducing the rate of a 2,4-D ester. A much better alternative on wheat past Feeke's stage 6 is to use another broadleaf herbicide with a wider application window that is effective on the weeds present in the field.
Source : psu.edu