This was a banner year for johnsongrass in many regions. Most everyone has an opinion about johnsongrass, and those opinions span the spectrum of deep hatred to “wish I had more of it.”
“Cattle love johnsongrass,” asserts Dirk Philipp, an associate professor in animal science at the University of Arkansas. “Once they enter a paddock that contains this rhizome-producing, warm-season perennial grass, they will go straight for it no matter its growth stage. It is a nutritious, palatable forage that can contribute to spring and summer grazing.”
Unfortunately, Philipp notes that johnsongrass also possesses several negative characteristics. It produces prussic acid under stress, which can result in cattle fatalities if grazed at the wrong time or in high enough quantities. Philipp says to be mindful of the prussic acid dangers, as the precursors can accumulate during drought and after a killing frost event. He cautions to avoid grazing fresh regrowth of johnsongrass after a rain event.
Johnsongrass is also labeled a noxious weed in about 20 states. The species has proven to be a troublesome pest in row-crop systems, causing significant economic damage.
Even though cattle readily eat johnsongrass in pastures, its tall growth habit can shade out desirable forage species and/or impede their growth through allelopathic chemicals. Philipp says its prolific growth habit will also allow it to outcompete native grasses such as big bluestem and switchgrass.
Multiple control options
If you’re in the invasive weed camp and want to control johnsongrass proliferation, Philipp suggests several possible measures.
“Johnsongrass is sensitive to repeated and close grazing that will deplete reserves located in the rhizomes,” he explains. “There are anecdotal reports from producers who even wish for more johnsongrass simply because it is a good forage, but their high grazing pressure over time is detrimental to johnsongrass survival. Johnsongrass won’t be a problem in regularly grazed fescue or bermudagrass pastures as its palatability is high compared to these two species.”
Controlling johnsongrass with herbicides can get complicated, according to Philipp. Chemical control success is often dependent on location, level of infestation, current and future land use, and access to the land with machinery and equipment.
The three main herbicide types that are useful for johnsongrass control are glyphosate, imazapic (Plateau), and sulfosulfuron (Outrider). Glyphosate is a nonselective herbicide that can be used effectively by spot spraying or applying with a weed wiper on the generally taller johnsongrass plants.
Imazapic is a selective herbicide that is labeled for grassy and broadleaf weeds. A large number of desirable native grasses and forbs are sufficiently tolerant to this active ingredient, so it is a good option in native grass stands. Sulfosulfuron works well in bermudagrass.
“Chemical control has to be timely,” Philipp asserts. “In my experience, once plants are taller than 20 inches, they will tolerate the labeled herbicide rate and survive.”
A final control strategy is to cut off the johnsongrass seedheads before they become viable. The animal scientist says that seed dispersal contributes disproportionally to the spread of johnsongrass.
