It, and other invasive plants, have become the scourge of the West, infesting millions of acres of pasture and rangeland and crowding out more desirable native plants. It’s cheatgrass, and it has so far been nearly impossible to control.
But change may be on the horizon.
“Cheatgrass, medusahead and jointed goatgrass are exotic annual grass species that increase fire frequency on rangelands,” says Ann Kennedy, a soil scientist/soil microbiologist with the Agricultural Research Service in Pullman, Wash.
Many ranchers and rangeland managers are trying to find ways to halt prolific spread of cheatgrass and medusahead, and reduce these invasive weeds in areas that are already dominated by them. A new weapon in this battle is naturally-occurring soil bacteria that suppress and inhibit root growth of these unwanted invaders. Kennedy was instrumental in discovering these helpful bacteria with research that began 30 years ago.
“My work involves soil quality, to help farmers produce more food and forage by looking at soil health and the microbes involved. Healthy soil increases crop production but also keeps more soil on the land, with less risk for erosion,” says Kennedy.
“Thirty years ago, when I was a post-doc student here in Pullman, we were looking at the poor growth of winter wheat in early spring—yellow strips in fields of young, green wheat. The soil we took from those areas was full of bacteria that inhibited wheat. Barley was not affected, so we wondered what was going on,” she says.
“We found these bacteria were present in early spring but not as much the rest of the year. The plants grew out of the yellowing but had fewer tillers than unaffected plants. They also had only one stalk and one wheat head. Then I wondered if grass weeds might show this same phenomenon of inhibited growth.”
Most of these bacteria did not inhibit crops. This was exciting to realize there might be bacteria available to thwart weed growth but not hurt crops—a potential biological weapon against certain weeds.
The search begins
Her team set out to search for the desired bacteria. They discovered that cheatgrass, medusahead, jointed goatgrass and a few other exotic noxious grass weeds gain their competitive advantage by germinating in the fall, growing their roots longer into the winter than the dormant perennials, and starting to grow again earlier in the spring than native plants or crops.
“The bacteria that inhibit weed grasses thrive in cold weather but are not around as much when the normal population of soil microbes are thriving. We set out to find the specific ones that inhibit weed grasses, looking at 5,000 isolates from the soil,” she says.
“The ones we found that inhibited both cheatgrass and wheat/native grasses, we discarded. We kept honing it down to the ones that would inhibit cheatgrass but not the bunchgrasses or any other desirable plants. We ended up with 14 strains of bacteria that were selective for cheatgrass, and a few that inhibited just cheatgrass. Some of them vigorously inhibited cheatgrass, medusahead and jointed goatgrass,” she says.
“We worked with a company that registered the cheatgrass-inhibiting bacteria as a bio-herbicide through EPA in 2014. We were working with another strain of bacteria that provided even better reduction of cheatgrass, medusahead and jointed goatgrass and put that second organism in for registration. Now we have the first-ever bio-herbicide registered by EPA and will soon have the second,” she says. An Idaho company will produce it.
Robbing the seedfbank
“On many rangelands, there is cheatgrass in the seedbank. Chemical herbicides only kill growing plants and won’t affect a seed. We can spray the bacteria onto an area and reduce the number of seeds in the seedbank over time,” explains Kennedy.
“If it survives long enough to get into the soil, it stays there two or three years or longer, and continues to inhibit cheatgrass. If we spray during cool temperatures in the fall, the bacteria multiply. They produce lots of grandchildren, and it’s the grandchildren that inhibit cheatgrass seed. The bacteria go dormant in summer and it’s actually the second or third year that inhibition of cheatgrass is visibly obvious.”
Collaborators have done field studies in many locations, establishing test plots up to 10 acres in size. These studies have consistently shown a 50% reduction in annual grass weeds within three years of a single bacterial application. In long-term field trials in the western U.S., application of bacteria resulted in almost complete suppression of these annual grass weeds in five to six years after one application. None of the annual grass weeds remained in the seed bank five to seven years after that single application.
Filling the space after cheatgrass
When cheatgrass is gone and leaves a void, another weed comes in unless a desired perennial is available. “Cheatgrass is very competitive against native rangeland species but then the area can be taken over by medusahead if the cheatgrass is gone. We can apply a certain bacterium to areas that have both cheatgrass and medusahead and inhibit both at the same time, but then we need to seed a desirable plant to fill that space.”
The nice thing about using bacteria to inhibit weeds is that these are naturally-occurring organisms that reside in most soils. “We just increase the number and tip the balance to more effectively inhibit cheatgrass,” says Kennedy.
“We can also coat seeds of a desirable plant with these bacteria. Then when you reseed an area, the bacteria are delivered along with the seed. We’ve found that it works very well when reseeding after a fire, to put the bacteria onto seeds you want to plant.” Then what grows back after the fire will be desirable plants rather than just cheatgrass.
“We need to stop that vicious cycle, and go back to bunchgrasses. I am hoping we can use this organism to help ranchers and federal land managers on BLM [Bureau of Land Management] and Forest Service rangelands have better forage and stop wildfires,” Kennedy says.
Many ranchers can’t afford to buy seed and drill it, which is the best way to establish new plants. “I am not a big proponent of the government coming in to do things, but when a fire goes through an area, there are funds for restoration. A lot of restoration efforts have not been well thought out. I’m hoping that if ranchers get hold of some of those funds—to buy the seed and rent a drill—putting bacteria on the seed will give them a leg up for better restoration and less cheatgrass,” she says.
This may be an excellent tool in the future, since chemical herbicides are not the whole answer. Bacteria in soil work at the seedbank level, whereas chemical herbicides merely kill growing plants. There are always seeds in the soil from earlier years, and this is why it’s so difficult to eliminate a noxious weed by using herbicides. We need to attack the seeds, and one way is to utilize weed-inhibiting bacteria.
After the bacteria are cultured, they can be sprayed from a ground rig or by air over rangelands or used as a coating on seeds, if applied at the right time. These bacteria have already been integrated into weed-management plans for cropland. “We are developing rangeland restoration plans that include the bacteria,” says Kennedy.
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