Weed Control in Wheat and Carrier Selection

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 (and similar generic products) 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. Dicamba, 2,4-D, or MCPA 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, Starane/Pixxaro, or Quelex. Consider using 2,4-D and dicamba, 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 Penn State Agronomy Guide (Table 2.5-6) (PDF) 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, Osprey, PowerFlex HL, Axial, and Prowl H2O are most effective when applied in the fall. Products like Shieldex and Tolvera are weak on these winter annual grasses. 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.

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.

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 lb 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, since cereal crop injury may occur if it is applied in combination with fluid fertilizer, especially with the addition of surfactant.

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.