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By Chris Carlson
Missoula Parks and Recreation, Research and Monitoring Program 

New Biological Herbicide for Cheatgrass


Cheatgrass (downy brome, Bromus tectorum) is known throughout the Western U.S. as one of our most widespread and difficult to control weeds. Grain farmers, ranchers, dog walkers and conservationists all curse its early growth habit and ability to spread and persist into disturbed and undisturbed settings. Currently, few strategies exist which provide adequate control of cheatgrass. But there may be hope yet!

Ann Kennedy, a researcher with the Agricultural Research Service at Washington State University, has isolated a strain of naturally occurring soil bacteria that shows great promise in providing biological control of cheatgrass, medusahead and jointed goatgrass, exotic annual grasses which pose a threat to agricultural and natural ecosystems.

The bacterium (Pseudomonas flourescens strain ACK55; P.f. ACK55) works by colonizing the roots of hosts when they germinate in the fall. The bacteria is considered a pre-emergent herbicide that controls a variety of annual grass weeds, including medusahead and jointed goatgrass in addition to cheatgrass.

After the bacteria become established, they attack their host by emitting a secondary metabolite that acts to limit radicle formation, root growth and tiller initiation of the target species. Infected grasses show reduced vigor, over-winter less successfully and have a less viable seed bank.

The Kennedy lab tested this strain widely to ensure that it inhibits only the target species, not desirable crops or native grasses. It is considered more selective than another strain of the same bacteria (D7), which was shown to impact some native grasses.

Although the bacteria takes three to five years to establish and show results, initial research suggests that this biological herbicide holds great promise. In field trials in wheat systems, the use of P.f. ACK55 has reduced the cover of cheatgrass by more than 90 percent after three years and led to increased crop yields. In rangeland systems in the Great Basin, field studies consistently show about a 50 percent reduction in cheatgrass within three years.

Because the bacteria is a biological agent, it ideally provides long-lasting control with low toxicity as compared to traditional herbicides. It needs moist soil conditions to establish and is recommended for application in the late fall, when temperatures are below 55 degrees Fahrenheit and precipitation is expected. Effectiveness of the bacteria will decline if applied to dry soil, heavy litter residue or before the summer drought begins. The agent may be applied as a liquid spray or as a seed coat for native or crop species.

The P.f. ACK55 biological herbicide is currently being reviewed by Federal agencies and is expected to be available to the public within the next 18 months. The herbicide is currently undergoing field trials in Montana. They started in the fall of 2014, including two sites in Missoula County.

As with other weed management efforts, it is recommended that this herbicide be applied as part of an integrated weed management strategy. If used without accompanying re-vegetation efforts, cheatgrass could be replaced by other weeds rather than desirable vegetation. Please contact your local weed district for more information.


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