NC State Extension

Perennial Grass Control

Perennial grassy weeds commonly found in soybeans include johnsongrass and bermudagrass. Bermudagrass, especially in narrow-row soybeans, often does not severely reduce yields. Heavy infestations of johnsongrass, however, can reduce yields up to 80 percent.

Regardless of the program chosen to control johnsongrass or bermudagrass, it is important to maintain control in the rotational crops and to control the weeds in field borders and adjoining areas to prevent reinfestation of treated fields. To suppress bermudagrass in corn, incorporate Eradicane or Sutan+. Use the higher rates suggested on the labels and cultivate to supplement chemical control. Johnsongrass can be controlled in corn with Accent or Beacon applied postemergence.

Johnsongrass and bermudagrass can be controlled along field borders and fence rows and around in-field obstructions with foliar applications of Roundup. See the Roundup label for details.

Although not critical for control, a tillage program before planting soybeans will aid in control of these perennial grasses. Using a moldboard plow or chisel plow followed by several diskings spaced at 4- to 6-week intervals during the fall and winter is most effective. This tillage cuts the rhizomes into small pieces, which stimulates sprouting the following spring, and also brings some of the rhizomes to the soil surface where winter weather will kill them. If winter tillage is not possible, tillage just before planting is still of some benefit.

Johnsongrass Control with Wiper Applicators

Light infestations of johnsongrass can be controlled in soybeans with Roundup applied with a wick bar or other type of wiper applicator. For heavy infestations, one of the control programs outlined below is preferred.

The johnsongrass should be a minimum of 6 inches above the soybean canopy and the wiper applicator should be carried at least 2 inches above the top of the soybeans. Better results are obtained when more of the weed is exposed to the herbicide solution.

For wick bars, mix 1 gallon of Roundup with 2 gallons of water. Do not add surfactant. For other types of wiper applicators, see the owner’s manual or the Roundup label. Drive a maximum of 5 mph and decrease speed in areas with dense johnsongrass. Better results will be obtained if the weeds are wiped from two directions. Do not apply when weeds are wet.

Johnsongrass Control with Postemergence Overtop Herbicides

Johnsongrass can be controlled in soybeans with postemergence applications of Assure II, Bugle, Fusilade DX, Poast, Poast Plus, and Select. There are no soil type limitations nor significant rotational restrictions with these herbicides and there is no crop injury. If good control is obtained in soybeans, the rhizome johnsongrass population in the following year should be reduced 85 percent or more. However, a control program for seedling johnsongrass will be necessary in the following crop. Seedling johnsongrass can be controlled in corn with preplant-incorporated Eradicane, Sutan+, or Sutazine or a postemergence application of Accent or Beacon.

Although not critical, it is better to begin the program in soybeans with a soil- applied herbicide that controls seedling johnsongrass. Then apply one of the postemergence herbicides (Assure II, Bugle, Fusilade DX, Poast, Poast Plus, or Select) when the johnsongrass is actively growing and at the proper size for treatment. Application of these herbicides to johnsongrass under drought stress may result in inadequate control. In most cases, one application will be sufficient. However, a second application may be made if necessary to control regrowth or newly emerged plants. Do not mix any broadleaf herbicides with the postemergence grass herbicides when treating johnsongrass.

As an alternative, a postemergence application of Pursuit could be considered when both johnsongrass and broadleaf weeds controlled by Pursuit are present. Pursuit usually gives good initial control of johnsongrass. In some cases, Pursuit alone will provide adequate johnsongrass control. In others, enough regrowth will occur to justify a later application of Assure II, Bugle, Fusilade DX, Poast, Poast Plus, or Select. The Pursuit should be applied when the broadleaf weeds are within the recommended growth stages and before the johnsongrass is more than 8 inches tall.

Johnsongrass Control with Fall Roundup Application

If properly applied, a fall application of Roundup will effectively control rhizome johnsongrass. Roundup is normally applied following corn harvest but may also be applied to set-aside land or to cotton during the defoliation treatment. Corn should be harvested 6 weeks before the first killing frost. After corn harvest, rotary mow the field to remove excess trash and to stimulate regrowth of the johnsongrass foliage. Do not till. Allow the johnsongrass to regrow until it has reached the boot to head stage. This will normally take about 3 to 5 weeks. Apply 1 quart of Roundup in 5 to 10 gallons of water per acre and add 2 quarts of nonionic surfactant per 100 gallons of spray solution. Use flat fan nozzles at a pressure of at least 30 pounds per square inch (psi). If the spray volume is greater than 10 gallons per acre, increase the Roundup rate to 2 quarts per acre.

For best results, the johnsongrass must be actively growing, not under drought stress. There should be at least 7 days between application and the first killing frost. Delay tillage for 10 days after treatment. After the 10-day waiting period, the field can be tilled and small grain or a cover crop planted if desired. If seedling johnsongrass emerges after Roundup application, tillage is suggested before the seedlings exceed 2 weeks of age to prevent new rhizome production.

An integral component of the fall Roundup program is control of seedling johnsongrass the following season. There should be no need to repeat the Roundup treatment for several years if seedling johnsongrass is controlled in all crops in the rotation. To control seedling johnsongrass in soybeans, use an effective soil-applied or postemergence herbicide and cultivate as needed.

Although fall application of Roundup is preferred, spring application is also an option. With spring application, delay land preparation and allow the johnsongrass to grow undisturbed until the boot stage is reached. Apply Roundup as described above, wait 7 to 10 days after application, and then incorporate Prowl, Sonalan, or Treflan. Spring Roundup application delays soybean planting until early to mid-June, which reduces yield potential. Hence, control with postemergence herbicide is preferred to spring application of Roundup.

Bermudagrass Control with Postemergence Overtop Herbicides

Bermudagrass can be controlled in soybeans with postemergence applications of Assure II, Fusilade DX, Poast, Poast Plus, or Select. One application of any of these herbicides in combination with cultivation or narrow rows is usually sufficient to avoid a soybean yield reduction. However, two applications may be made if needed. Bugle does not control bermudagrass.

Apply one of the postemergence herbicides when the bermudagrass is actively growing and at the proper size for treatment. Do not tank-mix the grass herbicides with a broadleaf herbicide. If a second application is needed, better spray coverage will be obtained if drop nozzles are used to semidirect the herbicide under the soybean foliage.

Cultivation will enhance control. However, do not cultivate for 7 days before or after herbicide application. Continue bermudagrass suppression in the following crop. In corn, incorporate Eradicane or Sutan+ and cultivate.

Bermudagrass Control with Fall Roundup Application

The program for bermudagrass is similar to that previously described for johnsongrass. Corn should be harvested at least 6 weeks before the first killing frost. Do not till. When the bermudagrass is 8 to 10 inches tall or when some seedheads can be seen, apply 3 quarts of Roundup per acre in a spray volume of 10 to 40 gallons per acre using flat fan nozzles and a pressure of 30 to 40 psi. (The label suggests 3 quarts for partial control, 5 quarts for full control. Tests in North Carolina have shown little difference between the 3- and 5-quart rates.) Roundup must be applied at least 14 days before the first killing frost. Delay tillage for 14 days after application. Small grains or a cover crop can then be sown if desired. The following spring, plant soybeans in narrow rows or use cultivation as needed to keep the row middles free of bermudagrass until the soybean canopy closes. Spot treatment with Assure II, Fusilade DX, Poast, Poast Plus, or Select is an option for escapes. Continue suppression in the following crop.

Broadcast application of Roundup at 3 quarts per acre is expensive. To reduce costs, spot spray the Roundup only on infested areas. A 2 percent solution can be used in hand sprayers for treatment of small spots.

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