Spring Weed Control in Wheat and UAN Carrier Issues

By Dwight Lingenfelter

As populations of winter annual weeds become more prevalent in March and April, they compete with wheat by slowing the rate of crop development thus potentially reducing yield. Harmony Extra (or other products that contain thifensulfuron and tribenuron) is still a commonly used herbicide for broadleaf control. Clarity/dicamba, 2,4-D, MCPA, or Finesse can improve the control of some winter annuals and perennials while Stinger is the most effective small grain herbicide for thistles.

A weedy small grain field in spring that was recently sprayed.

Here are some "best bets" for common problem weeds in the spring (but first make 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 and Quelex are still effective on it in many parts of the region, however, there are pockets of resistant populations starting to thrive. Tank-mix with Metricor (metribuzin) or Starane Ultra 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 and many of the commonly used small grain herbicides are not effective. Harmony Extra + Quelex or Starane Ultra has provided control of ivyleaf speedwell. PowerFlex HL, Metricor, or Aim provides some activity of certain speedwell species when small. Finesse has certain speedwells listed on its label and usually provides about 80% control of them. Be cautious of crop rotation intervals if using Finesse.
• Downy brome: herbicide options include Finesse, Osprey, and PowerFlex HL.
• Annual ryegrass: Osprey and PowerFlex HL tend to be the most consistent, while Finesse has activity on this weed and Axial is effective if the population is not Group 1 resistant.
• Annual bluegrass: active herbicides include Metricor, Finesse, and Osprey.
• Roughstalk bluegrass (RSBG): Studies at Michigan State University indicate that Osprey, Axial XL/Bold, and PowerFlex HL provide control of this weed. However, Osprey applied to RSBG (1-2" tall) in the fall or early spring provided the most consistent control (include nonionic surfactant (NIS) + ammonium sulfate (AMS) in the spray mixture). Fall treatments provide effective initial control (>90%) from these products 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 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.

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). 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:

Herbicides such as Osprey, PowerFlex HL, and Quelex can be applied in a UAN carrier, but certain guidelines must be followed, and temporary crop injury may occur.

Osprey: Fertilizer spray solution should not exceed 15% liquid nitrogen (1.5 gallons of liquid nitrogen in 10 gallons of spray solution per acre) and it must include NIS of 0.25% v/v (1 quart per 100 gallons of spray solution).

PowerFlex HL: Carrier should not be composed of more than 50% liquid nitrogen fertilizer and should not exceed 30 pounds of actual nitrogen per acre. Use NIS at a maximum of 0.25% v/v instead of crop oil concentrate.

Quelex: Spray solutions containing liquid nitrogen fertilizer can be used. If NIS is included, use a maximum rate of 0.25% v/v. Do not use crop oil concentrate or methylated seed oil.  Additional adjuvants are not needed if tank mixing with 2,4-D ester or MCPA ester and liquid nitrogen fertilizer.

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

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.