Johnsongrass

Johnsongrass Distribution and Biology

• Likely introduced to the United States before the 1800s and named for Col. William Johnson; johnsongrass was initially used as a forage crop.
• Johnsongrass (Sorghum halepense) has naturalized throughout the southern United States and West Coast but is now reported to be present throughout much of the continental United States, plus Hawaii, where it is considered invasive.
• Predominately grows as a weed in cultivated and no-till cropland, perennial field crops, hayfields, roadsides, and other wasteland areas.
• A rapidly growing warm-season perennial, often with erect multiple stems that grow from 2 to 8 feet in height. Mature plants are capable of reproducing by seed and thick, scaly rhizomes.
• Johnsongrass rhizomes initiate new shoot growth when soil temperatures reach 60 degrees. Seeds germinate when soil temperature reaches 70 degrees. Plants emerging from seed are capable of producing rhizomes within 3 to 4 weeks.
• Individual plants are able to produce nearly 30,000 seeds per plant. Even though the viability of johnsongrass seed in soil remains high for as long as five years, seed can remain viable in soil up to 10 years.
• When johnsongrass populations are high, it can be very competitive with soybeans. Johnsongrass densities reaching 30 stems per 100 square feet reduce season-long soybean yield between 59 and 88 percent. In another study, the amount of time required for johnsongrass to reduce soybean yield was eight weeks when soil moisture was abundant, but only three weeks under dry conditions.

Herbicide Resistance in Johnsongrass

• Since the 1990s, johnsongrass has evolved resistance to four different herbicide sites of action.
• The first report of glyphosate-resistant johnsongrass in soybeans came from Argentina in 2005. Documented reports of glyphosate resistance within the United States occurred in 2007 (Arkansas), 2008 (Mississippi) and 2010 (Louisiana).

Management of Johnsongrass in Soybeans

Best management practices to control johnsongrass in soybeans require efforts to control plants that emerge from seed and rhizomes. Johnsongrass can emerge before soybean planting, and new plants are likely to emerge after soybean emergence.

1. Control existing plants at planting. Johnsongrass present at planting should be managed with a burndown herbicide application or tillage. For populations susceptible to glyphosate, the most effective burndown treatment should include an application of glyphosate. Other herbicide options include foliar grass herbicides unless the johnsongrass population is resistant to Group 1 (ACCase inhibitor) herbicides. Foliar grass herbicide options include Assure® II (quizalofop-P-ethyl), Fusilade® DX (fluazifop-P-butyl), Fusion® (fluazifop-P-butyl + fenoxaprop-P-ethyl) or Select Max® (clethodim). For glyphosate-resistant populations, the use of a grass herbicide with ACCase site of activity or preplant tillage of plants with rhyizomes can sometimes encourage vegetative reproduction and proliferation of more johnsongrass plants.
2. A soil residual herbicide can help suppress plants emerging from seed. Although not always highly effective, a soil residual herbicide, such as Prowl® (pendimethalin) or Treflan^TM (trifluralin), can provide partial control of johnsongrass emerging from seed. Plants that have already germinated from seed or are emerging from rhizomes will not be controlled with a soil residual herbicide.
3. Apply an effective post-emergence herbicide. Most johnsongrass is treated after crop planting and the weed has emerged. If the johnsongrass is glyphosate-sensitive, one of the most effective treatments in glyphosate-tolerant soybean systems is glyphosate applied to actively growing plants at a minimum of 0.75 lb. acid equivalent per acre. Other effective choices include Group 1 (ACCase) type herbicides such as Assure® II, Fusilade® DX, Fusion® and Select Max®, which can be applied to conventional or glyphosate-tolerant soybean varieties. To reduce the potential for johnsongrass resistance in soybeans, it is important to rotate between glyphosate and ACCase inhibitor type herbicides. Also, rotating fields to other crops, such as corn, will allow use of other alternative herbicides that have Group 2 (ALS inhibitor) site of activity, such as nicosulfuron (e.g. Accent Q®, Steadfast® Q) or primisulfuron (e.g. Beacon®, Spirit®).
4. Scout fields 10 to 14 days later for effectiveness. If johnsongrass escapes initial control or new plants emerge, glyphosate may be applied a second time in glyphosate-tolerant soybeans or an ACCase type herbicide can be used on conventional or glyphosate-tolerant soybeans. Weeds not controlled with the second application should be tested for herbicide resistance.

Photo: Howard F. Schwartz, Colorado State University, Bugwood.org