Populations of winter annual weeds spring to life in late March and early April. And Penn State Weed Specialists Bill Curran and Dwight Lingenfelter say now's the time to jump these small grain yield-sappers.
If common chickweed, henbit, purple deadnettle, horseweed and others emerge unchecked with the small grain, potential impact on yield could be great – especially with local July wheat bids now in the neighborhood of $6.40 a bushel.
Here are Curran's and Lingenfelter's weed control recommendations. First though, remember that early-spring herbicide response can be slow under the typically cool conditions in March and early April. Application on cool (less than 50 degrees Fahrenheit) cloudy days aren't likely to effectively control dandelions or Canada thistle. These weeds would be more effectively controlled with a later spring application.
Harmony Extra is still probably the most broad spectrum herbicide for broadleaf control. Harmony SG doesn't have the same weed control spectrum.
Clarity, Banvel, 2,4-D, or MCPA can improve the control of some winter annuals and perennials. Stinger is the most effective small grain herbicide for thistles.
Harmony SG or Harmony Extra should be included where control of chickweed is desired. They're the only herbicides that control this weed. But if it is ALS-resistant chickweed, you'll need to include Glory or Starane. Consider using 2,4-D and Clarity if horseweed/marestail is a problem in small grains.
Grasses growing more troublesome
Weedy grasses, such as downy brome, cheat, annual bluegrass, annual ryegrass, and roughstalk bluegrass are becoming more of a problem. Wheat herbicides available for control of grasses – Axial, Finesse Grass and Broadleaf, Maverick, Osprey and PowerFlex – are most effective when fall-applied. Some of them can work during spring, but the weeds must be small.
Broadleaf herbicide choices
Once wheat has passed Feeke's Stage 6 (when the first stem node is visible), risk of herbicide injury from 2,4-D, MCPA, Banvel/Clarity, or Curtail increases. Then, these herbicides aren't recommended.
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.