In this situation, remaining herbicide options for broadleaf weed control are Harmony Extra and similar products (Edition, Treaty Extra, Nimble, others), Harmony SG (Treaty, Harass, Volta), Buctril, Stinger and Starane. These can be applied until the flag leaf is visible (before Feeke's Stage 8). Buctril, Huskie, Stinger and Starane can be applied to wheat up to boot stage (before Feeke's Stage 9).
Wheat tolerance of 2,4-D or MCPA 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. Severe injury is highly probable when applied at Feeke's stages 9 and 10.
Minimize risk by applying the amine form of 2,4-D or reducing the 2,4-D ester rate. A much better alternative on wheat past Feeke's stage 6 is to use another broadleaf herbicide with a wider application window.
Rules for nitrogen carrier
Liquid urea-ammonium nitrate fertilizer (UAN) is a common herbicide carrier. The most common herbicide to be used in this manner is 2,4-D ester. The amine form is difficult to mix in UAN.
Liquid N can cause leaf burn, especially under hot, humid conditions. And, using a surfactant, required with some products, greatly increases leaf burn potential. To minimize that risk:
•Don't apply more than 20 pounds of N per acre in the form of UAN when using a surfactant with herbicide.
•Don't apply more than 40 pounds of N per acre in the form of UAN when no surfactant is used.
Avoid high-temperature, high-humidity days. Late afternoon applications carry less risk of leaf burn.
A note about Glory
Now that Glory (metribuzin) herbicide is labeled for winter wheat and barley to control ALS-resistant chickweed, there are questions about variety tolerance. For best crop safety, apply Glory in the spring, after tillering. Once the small grain has a root system of at least an inch, don't use more than 4 ounces per acre. And, don't include it in liquid fertilizers with surfactants.
In general, barley is more tolerant than wheat, but tolerance within each species greatly varies. Many varieties listed on the Glory label are no longer on the market.