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The war on cheatgrass takes aim

Heather Smith Thomas for Progressive Forage Published on 30 August 2017

“Cheatgrass (Bromus tectorum) is an annual grass that invades pastures and increases fire frequency,” says Dr. Ann Kennedy, a recently retired soil scientist/soil microbiologist from ARS (Agricultural Research Service) USDA at Pullman, Washington.

Many ranchers and range managers are trying to reduce its prolific spread.

A new weapon in this battle is a naturally occurring soil bacterium that inhibits root growth. Kennedy discovered these helpful bacteria in research that began more than 30 years ago when she was a postdoctoral student at Washington State University in Pullman, a wheat-growing area.

“We were looking at poor growth of winter wheat in early spring. The soil we took from yellow strips in those fields was full of bacteria that inhibited wheat,” she says.

These bacteria were very selective, however, and did not affect barley. This led to a search for bacteria that could selectively thwart weedy grasses. Her team discovered that cheatgrass, medusahead, jointed goatgrass and a few other exotic noxious grass weeds gain a competitive advantage by germinating in the fall, growing roots for a longer time than the dormant perennials and starting to grow again earlier in the spring.

The bacteria that inhibit weed grasses thrive in cold weather. Kennedy’s team looked at more than 5,000 types of bacteria, searching for one that would inhibit cheatgrass but not bunchgrasses or any other desirable plants.

On many rangelands, cheatgrass seeds exist in the seedbank. “Herbicides only kill growing plants and won’t affect a seed – unless it germinates and becomes a plant. We can spray these bacteria onto an area, however, and reduce the number of cheatgrass seeds in the seedbank over time and eventually eradicate it,” Kennedy says.

If the bacteria sprayed over the area survive long enough to get into the soil, it stays there for two or three years or longer and continues to inhibit cheatgrass. “If we spray in the fall when it’s cool, before winter, the bacteria love those temperatures and multiply. They go dormant in summer but multiply again in cool weather.” By the second or third year the reduction in cheatgrass is visibly obvious.

Her studies consistently showed 50 percent reduction in cheatgrass within three years of a single bacterial application.

In long-term field trials in the West, one application of bacteria resulted in almost complete suppression in five or six years, when desirable plants such as winter wheat, perennial bunchgrasses and other native plants were present. No cheatgrass remained in the seedbank five to seven years after that single application of bacteria.

The problem is that when the cheatgrass is gone and leaves a void, another weed comes in unless a desired perennial is available. “You may need herbicides to make sure no broadleaf weeds come into that bare ground,” Kennedy says.

The nice thing about using bacteria to inhibit weeds is that these are naturally occurring organisms in most soils. “We just increase the number. Under natural conditions, these bacteria increase a little during fall and winter but then their numbers drop. We increase the number and tip the balance to more effectively inhibit cheatgrass.”

It’s not a magic fix. “These bacteria can’t do it by themselves. We need a management plan to make sure desirable plants fill in those gaps. We’ve found that the bacteria work really well to reduce cheatgrass when reseeding after a fire, when we put the bacteria onto seed we’re planting.”

These bacteria are very safe. A toxicology/pathology study by an independent lab showed they are not a danger to any mammals, birds or insects. The bacteria thrive in cool fall weather when cheatgrass and medusahead are accelerating root growth and then go dormant during warmer months. After four or five years, the bacteria die out – after they’ve done their job.

Large numbers, after being cultured, can be sprayed (from a ground rig or by air) over rangelands or used as a seed coating if applied at the right time for them to survive. These bacteria have already been integrated into weed-management plans for cropland, and there are now rangeland restoration plans that include the bacteria.

Currently, ARS is working with EPA to register the bacteria as a weed suppressant. “It will take about 1.5 years before it is a marketable product. The decision regarding which company will produce and sell the bacterial product will be made by USDA-ARS, but it looks definite that a company or companies will be selected to market this,” says Kennedy.

Late fall grazing reduces cheatgrass

Robert Alverts (Science and Management Consulting, Tigard, Oregon) helped with a cooperative cheatgrass control project between University of Nevada, Burns District BLM, Oregon Cattleman’s Association, Oregon Beef Council, Harney County Court and Drewsey Valley Ranch in Oregon on a 14,000-acre grazing allotment – reducing cheatgrass with fall grazing.

Due to the limited grazing period and small number of cows (less than 400), it is taking several years to change the plant community and effectively reduce cheatgrass dominance, but there is visible progress every year.

This is only working because the BLM authorizes late-season grazing on this allotment. Most BLM allotments don’t have provisions for fall grazing in their allotment management plan; ranchers cannot graze in late fall.

“Late-season grazing doesn’t hurt the land or grass. Forage is dormant and this allows us to hammer annual plants like cheatgrass. At that time of year, cows prefer annuals over rank, mature bunchgrasses,” Alverts says. Mature annuals like cheatgrass are softer (not as stiff, and therefore more palatable).

By that time of year the sharp seeds have dropped off. After fall rain, cheatgrass softens up. “When we distribute protein supplement around a pasture to attract cattle to certain areas, it works well. The supplement enables them to digest fiber, and they can eat a lot more of it.

We keep moving the supplement around the pasture, to influence how cows graze cheatgrass,” Alverts says. Over time, reduction in cheatgrass enables perennial grasses to come back and dominate plant communities.

Removing the covering of cheatgrass reduces next year’s production. Cheatgrass has lots of seeds that remain viable for several years, but when the next crop grows in the spring, without that cover from former years’ thatch to protect it from full sunlight, it doesn’t grow as vigorously.

It needs litter or protection from old dead grass to shade new seedlings. If cattle keep grazing it off every year, this reduces cheatgrass’ competitive edge on perennials, creating a plant community of more robust perennials.  end mark

PHOTO: New tools – naturally occurring bacteria and late fall grazing – are winning the battle against spreading cheatgrass (also called downy brome). Photo by Sal Gomez.

Heather Smith Thomas is a freelance writer based in Salmon, Idaho. Email Heather Smith Thomas

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