Wheat Herbicide Options and UAN Carrier Issues

By Dwight Lingenfelter

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

Here are some "best bets" for common problem weeds in the spring. But first, be sure the crop is not beyond the maximum growth stage listed on the herbicide label.  And keep in mind that generic alternatives are available for some of these products listed:

• Common chickweed: Harmony Extra is still effective on it in many parts of the state, however pockets of resistant populations starting to thrive. Tank-mix with Quelex or Starane Ultra or include Metricor (metribuzin) in your spray program if ALS-resistant chickweed is prevalent.
• Marestail: Products such as 2,4-D, dicamba, Huskie, Quelex and Stinger applied POST have activity on this weed.
• Speedwell species: There are several speedwell (Veronica) species, including corn, common, ivyleaf, and others in our area. Depending on the species, their life cycle is either annual or perennial. Many of the commonly used small grain herbicides provide little or no control of speedwell. Harmony Extra + Quelex has provided control of ivyleaf speedwell (better than either herbicide applied alone). The same with Harmony Extra + Starane Ultra. PowerFlex HL has provided control/suppression of certain speedwell species in some university trials. Finesse has a few annual and perennial speedwells listed on its label and usually provides about 80% control. Be cautious of crop rotation intervals if using Finesse. If speedwell is small, Aim provides some control/suppression. Metricor also has activity on speedwell when it's small.
• Downy brome: Herbicide options include Finesse, Osprey, and PowerFlex HL.
• Annual ryegrass: Axial, Finesse, Osprey, and PowerFlex HL have activity on this weed.
• Annual bluegrass: Active herbicides include Metricor, Finesse, and Osprey.
• Roughstalk bluegrass (RSBG): This tall (>3 feet), cool-season perennial is becoming more of a problem in small grains and forages across the state. As it matures it typically turns reddish-tan and sets seed prior to summer, goes dormant until the fall, and remains green during the winter months. Michigan State University recently conducted various field studies on this weed and found that Osprey, Axial XL/Bold, and PowerFlex HL provide control of RSBG. Osprey applied to RSBG (1-2" tall) in the fall or early spring provided the most consistent control (include NIS + AMS in the spray mixture). Fall treatments provide effective initial control (>90%) but spring emergence of RSBG leads to escapes by harvest. Late spring applications are usually not as effective since RSBG is too tall.

Keep in mind, herbicides applied in early spring can be slow under the typically cool conditions of 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.

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. University research at 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:

Other herbicides such as Osprey, PowerFlex HL, 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.