Pennsylvania Wheat: Weed Control and UAN Carrier Issues

Populations of winter annual weeds will become more prevalent in late March/early April and can compete with wheat and barley and slow the rate of crop development potentially reducing yield. If winter annual weeds like common chickweed, henbit, purple deadnettle, marestail/horseweed, and others emerge with the small grain and are left unchecked, the potential impact on yield could be great.

Harmony Extra is still probably the most broad spectrum herbicide for broadleaf control. Harmony SG contains only one of the active ingredients (thifensulfuron) in Harmony Extra and thus does not have the same weed control spectrum. Clarity/dicamba, 2,4-D, MCPA, or Finesse can improve the control of some winter annuals and perennials and Stinger is the most effective small grain herbicide for thistles.

Harmony SG or Harmony Extra should be included where control of chickweed is desired, since these are the only herbicides that control this weed but if it is ALS-resistant than you will need to include MetriCor or Starane. Consider using 2,4-D and Clarity, Huskie, or Quelex if horseweed/marestail is a problem in small grains.

Prowl H2O is also labeled for use up to growth stage 7 but it must be applied before weeds germinate or tank mixed with effective post herbicides. Refer to the Agronomy Guide (Tables 2.5-6) for additional details on herbicide effectiveness.

Herbicides applied in early spring can be slow under the typically cool conditions in March and early April. Remember that cool (less than 50 F) cloudy days can reduce herbicide activity. Applications this early are not likely to effectively control dandelions or Canada thistle. These weeds would be more effectively controlled with a later spring application.

Weedy grasses, such as downy brome, cheat, annual bluegrass, annual ryegrass, and roughstalk bluegrass are also becoming more of a problem in Pennsylvania. The wheat herbicides available for control of grasses, Axial, Finesse, Osprey, PowerFlex, and Prowl H2O are most effective when applied in the fall. Some of these products can work in the spring, but to be effective, the weeds must be small. Or in the case of Prowl, it must be applied before weeds germinate.

Keep in mind, not all the mentioned weedy grasses are controlled by each of these herbicides; some are more effective on certain species.

Herbicide selection for broadleaf weeds in winter wheat past Feeke’s Stage 6:

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).

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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.

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.

Guidelines for liquid fertilizer carriers and herbicides:

Liquid urea-ammonium nitrate fertilizer (UAN) is a common spray solution carrier for herbicides in wheat in our region. We typically recommend no more than a 50:50 water/UAN ratio. The most common herbicide used in this manner is 2,4-D ester at 1 pint/A (2,4-D amine is difficult to mix in UAN).

Application of herbicide in liquid nitrogen can cause leaf burn from the nitrogen, especially under hot, humid conditions; and the addition of other herbicides or fungicides to these mixtures will likely increase the risk for crop injury. This risk increases with later wheat growth stages because more leaf area is exposed to the treatment and recovery time is shorter.

Applications of 2,4-D should be made in the spring to actively growing wheat following tillering (Feeke’s stage 3) but prior to jointing (Feeke’s stage 6). In addition, the use of surfactant (required with herbicides such as Harmony Extra) greatly increases leaf burn potential. Research at Michigan State Univ. has demonstrated that excessive leaf burn from high nitrogen rates combined with surfactant can reduce wheat yield. To minimize this risk:

  • Do not apply more than 20 lbs of nitrogen per acre in the form of UAN when using a surfactant with herbicide.
  • Do not apply more than 40 lbs of nitrogen 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.

Below are the specific adjuvant recommendations for Harmony SG and Harmony Extra SG:

Carrier Situation Rate of NIS/100 gallons
Water normal 1 quart
Nitrogen diluted with water (>50% N) normal 0.5 to 1 pint
Liquid nitrogen fertilizer garlic >8” 0.5 pint
Liquid nitrogen fertilizer garlic <8” None
Liquid nitrogen fertilizer with 8 fl oz of 2,4-D None
Water with 8 fl oz of 2,4-D 1 pint

Other herbicides such as Osprey, PowerFlex, and Quelex can be applied in a UAN carrier but certain guidelines must be followed. Aside from these products, other herbicides may allow the use of liquid nitrogen fertilizer as a carrier, but make sure to review the product label for details.

Osprey may be applied using a liquid nitrogen solution as the spray carrier, but the fertilizer spray solution should not exceed 15% liquid nitrogen (1.5 gallons of liquid nitrogen in 10 gallons of spray solution per acre). A non-ionic surfactant at a maximum concentration of 0.25% v/v (1 quart per 100 gallons of spray solution) is required in spray solutions containing liquid nitrogen carrier. Crop injury may occur.

PowerFlex HL may be applied in spray solutions containing liquid nitrogen fertilizer but should not be composed of more than 50% liquid nitrogen fertilizer and should not exceed 30 lb of actual nitrogen per acre. Use a non-ionic surfactant at a maximum of 0.25% v/v instead of crop oil concentrate. Temporary crop injury may result when liquid nitrogen fertilizer is used as the spray carrier.

Quelex may be applied in spray solutions containing liquid nitrogen fertilizer. Run a tank mix compatibility test before mixing Quelex in fertilizer solution. Mix and disperse Quelex granules in clean water as a pre-slurry before adding to liquid fertilizer solution. If using a non-ionic surfactant when Quelex is applied in spray solutions containing liquid nitrogen fertilizer, use non-ionic surfactant at a maximum rate of 0.25% v/v. Do not use crop oil concentrate or methylated seed oil. Additional adjuvants are not needed when using Quelex in tank mix with 2,4-D ester or MCPA ester and liquid nitrogen fertilizer solutions. Temporary crop injury may result when liquid nitrogen fertilizer is used as the spray carrier.

Metribuzin application via a liquid nitrogen fertilizer carrier is typically not recommended.

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