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February 7, 2024
by Dwight Lingenfelter
We are getting more calls each year about how to control roughstalk bluegrass in field crops. Unfortunately, it is becoming more of a problem in small grains and forage.
Roughstalk bluegrass is a cool-season perennial with short stolons, but it mostly spreads by seed. As it matures, it typically turns reddish-tan and sets seed before summer. It then goes dormant until fall and remains green during the winter months.
Unlike annual bluegrass, which does not grow more than a foot tall, roughstalk bluegrass can get up to 3 feet tall or higher, and it has been very evident in crop fields in Pennsylvania. Once it starts setting seed, no management tactics — not even an herbicide — will provide adequate control.
Fall and early spring are the best times to apply effective herbicides. Studies from Michigan State University show that Osprey, Axial XL/Bold and PowerFlex HL provide control of this weed.
However, Osprey applied at 1-2 inches tall in fall or early spring provides the best control. Fall treatment of these products provides effective early control — more than 90% — but spring emergence of roughstalk bluegrass can lead to escapes by harvest.
Late-spring applications are usually not as effective since the weed gets too tall. However, according to the Osprey label, no more than one application can be made during the growing season to a fall-sown wheat field. Therefore, if Osprey were already sprayed in fall, suppression of roughstalk bluegrass the following spring could only be done by applying metribuzin, PowerFlex HL or Axial XL/Bold.
The spring application must be sprayed early, when the weed is less than 3 inches tall. If Osprey is applied in fall, then an early-spring application should provide good control.
If you intend to double-crop soybeans after wheat, be aware of herbicide rotational restrictions.
In a pure stand of alfalfa, Select, or another clethodim formulation, can provide good control of roughstalk bluegrass after first cutting.
In university studies, Raptor has been shown to provide about 85% to 90% control, too. Products such as Gramoxone (paraquat) after first cutting and Velpar during alfalfa dormancy can provide 75% to 85% control.
Unfortunately, herbicide choice is rather limited in alfalfa or grass mixes. Pursuit tends to provide only about 50% to 60% control when applied after first cutting.
When applying any of these herbicides, it is important to include the correct adjuvants to optimize performance. Furthermore, herbicides such as Osprey and PowerFlex HL can be applied in a UAN-liquid nitrogen carrier, but certain guidelines must be followed, and temporary crop injury may occur.
For Osprey, the 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% volume per volume (1 quart per 100 gallons of spray solution).
For PowerFlex HL, the carrier should not be comprised of more than 50% liquid nitrogen fertilizer and should not exceed 30 pounds of actual nitrogen per acre. Use NIS at a maximum 0.25% volume per volume, instead of crop oil concentrate.
Also, keep in mind that herbicides applied in early spring can be slow under the typically cool conditions of March and early April. Remember that cool — less than 50 degrees F — and cloudy days can reduce herbicide activity and speed of control.
Lingenfelter is an Extension associate of weed science with Penn State Cooperative Extension.
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