What Is Brome Grass? Identification and Significance

Brome grass is any member of the genus Bromus, a large group of grasses containing roughly 150 to 200 species found on every continent except Antarctica. Some bromes are prized forage crops that feed livestock across temperate regions; others rank among the most damaging invasive weeds in North America and Australia. The genus spans annuals, biennials, and perennials, and understanding which brome you are looking at makes a real difference in whether you should encourage it, tolerate it, or fight it.

How to Recognize a Brome Grass

Brome grasses share a handful of features that help separate them from look-alikes. The seed heads typically form open, drooping panicles rather than tight spikes. Individual spikelets are flattened and contain multiple florets, each usually tipped with a straight or slightly bent awn. The leaf sheaths are closed or nearly closed for most of their length, which is a useful field clue since many other common grasses have open sheaths. When you roll a brome stem between your fingers, the cross-section feels round rather than flattened or triangular.

Beyond those shared traits, species vary a lot. Smooth brome (Bromus inermis) has broad, flat leaves that often show a faint W-shaped crease partway across, and its awns are short or absent. Cheatgrass (Bromus tectorum) is much finer, with hairy leaf sheaths and long, conspicuous awns that twist as the seed dries. Ripgut brome (Bromus diandrus) earns its name with stiff, sharp awns that can injure grazing animals. Red brome (Bromus rubens) is compact and turns a distinctive reddish-purple at maturity. Knowing the species matters because the genus includes both some of the best cool-season pasture grasses and some of the worst rangeland weeds on the planet.

Smooth Brome as Forage

Smooth brome is the species most people encounter on purpose. It was introduced to North America from Eurasia in the late 1800s as a hay and pasture grass, and it remains one of the most widely planted cool-season perennial grasses across the northern Great Plains and western Canada. The appeal is straightforward: it establishes quickly, tolerates cold winters, and produces palatable forage with decent protein content.

A grazing trial comparing smooth brome, meadow brome, and hybrid brome found that all three species delivered similar nutritive value and similar steer performance over a 60-day grazing season, with steers gaining about a kilogram per day regardless of which brome they ate.¹ That consistency is part of what makes bromes attractive to ranchers. They are not flashy performers, but they are reliable ones.

Pasture management can push that reliability further. When smooth brome pastures are interseeded with legumes like alfalfa or clover rather than simply fertilized with synthetic nitrogen, cattle grazing those mixed pastures end up with less fiber in their diet and produce dung with higher nitrogen concentrations. That nitrogen-rich dung decomposes faster and moves more nutrients back into the soil, creating a self-reinforcing cycle that reduces the need for purchased fertilizer.² Russian researchers have also explored boosting smooth brome productivity with bio-stimulant treatments, reporting green-mass yields above 11 tonnes per hectare from treated stands.³

When Smooth Brome Escapes the Pasture

The same traits that make smooth brome a dependable forage plant make it a formidable invader once it moves into native prairie. It spreads aggressively through underground rhizomes, forms dense monocultures, and has frequently eliminated native plant species from sites it colonizes throughout the Great Plains.⁴ The species is now considered one of the most widespread exotic grasses in North American prairies, having successfully invaded both cool-season and warm-season native grasslands.

What makes smooth brome particularly hard to dislodge is a feedback loop it creates with the soil. Its leaf litter decomposes faster than native grass litter, which raises soil nitrogen levels. Smooth brome thrives in nitrogen-enriched soil, so as it breaks down its own leaf tissue, it makes the ground more hospitable for itself and less hospitable for nitrogen-adapted native species. On top of that, established smooth brome plants can shuttle nutrients through their rhizome network to younger shoots pushing into new, nutrient-poor territory.⁴ It is, in effect, building its own supply chain as it expands.

Cheatgrass and the Western Fire Cycle

If smooth brome is the brome that overstays its welcome in prairies, cheatgrass (Bromus tectorum) is the brome that has reshaped entire landscapes in the arid and semi-arid West. An annual grass introduced from Eurasia, cheatgrass germinated in disturbed soils across western North America during European settlement and spread along routes closely tied to patterns of human immigration.⁵ Today it dominates millions of hectares of rangeland, particularly in the sagebrush steppe that stretches across Nevada, Idaho, Utah, and neighboring states.

The ecological damage from cheatgrass centers on fire. Native sagebrush steppe typically burns on cycles measured in decades, sometimes a century or more. Cheatgrass dries out early in summer, blanketing the landscape with fine, continuous fuel. Fires ignite more easily, spread faster, and return more frequently. Native perennial grasses and shrubs like sagebrush often cannot recover fast enough between fires, so the burned areas fill in with more cheatgrass, which dries out, burns again, and so on. The U.S. federal government has committed to managing this invasive-grass-fire cycle across six western states, proposing roughly 17,700 kilometers of fuel breaks along with millions of hectares of fuel-reduction treatments. Yet analysis of past federal seedings found that over a quarter of those revegetated sites had burned again, representing close to 17 percent of the total seeded area, with some locations requiring repeated and costly retreatments.⁶

How Brome Grasses Reshape the Soil Beneath Them

Both smooth brome and cheatgrass change the soil they grow in, though the mechanisms differ. The smooth brome feedback loop described above runs through faster litter decomposition and nitrogen enrichment. Cheatgrass takes a similar path but in dryland systems where the consequences ripple differently.

A long-running replicated field study found that soils beneath cheatgrass had greater concentrations of nitrate, organic carbon, and organic nitrogen than soils under native sagebrush and perennial grass communities. Rates of nitrogen cycling, including both gross and net nitrogen mineralization and nitrification, were all faster beneath cheatgrass, measured through the full soil profile down to 60 centimeters. Researchers concluded that cheatgrass provides soil microbes with lower-carbon-to-nitrogen substrates than native plants, which accelerates the breakdown of organic matter and the release of available nitrogen.⁷

This is not good news for restoration. When you try to re-establish native species on soil that has been chemically reshaped by decades of brome grass dominance, the natives are returning to a playing field that no longer suits them. Many native sagebrush-steppe species are adapted to low-nitrogen, slow-cycling soils. Pumping nitrogen into those soils favors fast-growing weedy species, including cheatgrass itself, at the expense of the slow, conservative growers that once dominated.

Red Brome and the Seed Bank Question

Red brome (Bromus rubens) is another invasive annual brome that has spread through arid ecosystems, particularly the Mojave Desert. It shares some of cheatgrass’s fire-promoting tendencies but differs in an important way: its seed bank is relatively short-lived. Research in the Mojave found that only a small proportion of red brome seeds stayed viable for two years. The species compensates by producing enormous numbers of seeds each growing season, meaning that even if just a small fraction survive in the soil for more than a year, the seed bank can still contribute meaningfully to future recruitment.⁸

This matters for control strategies. A weed with a seed bank that persists for many years requires patience, since seeds germinating years after the parent plant was killed can restart an invasion. Red brome’s reliance on prolific annual seed production rather than a durable seed bank suggests that preventing seed set for even one or two consecutive years could substantially reduce its presence. The trade-off is that missing a single year of control in a high-rainfall season can allow red brome populations to bounce back rapidly.

Herbicide Resistance Is Already Emerging

Where brome grasses are crop weeds, farmers typically rely on selective herbicides to control them. Ripgut brome (Bromus diandrus) is a serious problem in Australian grain fields, and resistance to commonly used herbicides has begun to appear. In one Australian population with a history of repeated clethodim use, researchers found that a mutation conferring resistance to a class of herbicides called ACCase inhibitors had become fixed in the local brome population, meaning every tested plant carried it. The resistance went undetected by the farmer for some time, and researchers warned that resistance to a second herbicide, quizalofop, was likely to follow.⁹

The pattern is familiar from other weedy grasses: heavy reliance on a single herbicide mode of action selects for resistant individuals, which then dominate the population. Rotating herbicide chemistries, integrating non-chemical control methods like crop rotation and strategic tillage, and testing for resistance before it becomes obvious are all standard recommendations. The brome grass situation in Australia is a warning for grain-growing regions worldwide where annual bromes are creeping into fields.

Restoring Land That Brome Has Taken Over

Getting native vegetation back on brome-dominated land is expensive and uncertain, but the economics can pencil out when you factor in the cost of repeated wildfires. Modeling of cheatgrass-invaded sagebrush steppe showed that increasing investment in post-fire revegetation reduces long-term fire-management costs by more than enough to offset the cost of the restoration work, in addition to delivering environmental benefits.¹⁰

One promising approach is what researchers call “assisted succession.” Rather than trying to establish native species directly into cheatgrass-dominated ground, you first introduce a competitive perennial like crested wheatgrass, which can suppress cheatgrass. Once the crested wheatgrass has stabilized the site, you drill native grasses and shrubs into the established stand. A field trial testing this method found that native species emerged in greater numbers when seeded into the crested wheatgrass matrix than when seeded directly into cheatgrass-dominated soil. The crested wheatgrass effectively served as a bridge between weedy monoculture and native diversity.¹¹ The approach worked especially well in years with normal or below-average precipitation, conditions under which direct seeding into cheatgrass often fails.

What Climate Change Means for Cheatgrass

If the cheatgrass problem feels intractable now, warming temperatures may make it worse. An experimental study in semi-arid grassland found that warming more than tripled cheatgrass biomass and seed production, driven by a combination of increased recruitment and increased individual plant growth. The boost held up with and without competition from native species and in both wet and dry years, even though warming simultaneously reduced soil moisture. Elevated carbon dioxide, by contrast, had little effect on cheatgrass invasion and actually reduced soil and plant nitrogen.¹²

The implication is that rising temperatures alone could substantially expand the range and density of cheatgrass, intensifying the fire cycle in places already struggling with it and potentially pushing cheatgrass into higher-elevation or more northerly habitats where it currently has a smaller footprint. Land managers planning restoration or fuel-break construction today are, in effect, planning for a moving target.

Fungal Partnerships and Forage Improvement

Not all brome research focuses on fighting invasions. Some agronomists are exploring ways to improve forage bromes by harnessing beneficial fungi. Endophytic fungi, organisms that live inside plant tissues without causing disease, can boost grass growth, improve drought tolerance, and even produce defensive chemicals that deter insect pests. In South American pasture systems, a Chilean brome species (Bromus valdivianus) was successfully inoculated with the fungal endophyte Epichloë uncinata as well as several native fungal strains. The most effective inoculants colonized over 80 percent of treated plants and triggered measurable increases in plant height, tillering, leaf development, and dry biomass. Chromatographic analysis detected production of auxins and loline-type alkaloids, compounds known to deter certain insect herbivores.

This line of work is still early-stage, but it hints at a future where forage brome varieties come pre-loaded with fungal partners that enhance productivity and pest resistance. If successful, it could reduce reliance on synthetic fertilizers and pesticides in brome-based pasture systems, a meaningful shift given how widely smooth brome and its relatives are planted across the temperate world.

Telling Beneficial Bromes from Harmful Ones

Because the genus contains both valued pasture grasses and destructive invaders, accurate identification matters more with brome than with most grass genera. A few practical guidelines help sort them out:

• Growth habit: Perennial bromes like smooth brome and meadow brome form persistent sod through rhizomes. Annual bromes like cheatgrass, ripgut brome, and red brome complete their life cycle in one season, dying off by midsummer and leaving dry, flammable residue.
• Awn length: Short or absent awns usually point toward smooth brome or meadow brome, the forage species. Long, prominent awns are typical of the weedy annuals.
• Hairiness: Cheatgrass and ripgut brome have noticeably hairy leaf sheaths and sometimes hairy blades. Smooth brome, true to its name, has relatively smooth, hairless foliage.
• Setting: If you find a brome in a planted pasture or hayfield, it is probably smooth or meadow brome. If you find it in rangeland, roadsides, grain stubble, or recently burned ground, the weedy annual species are far more likely.

When in doubt, your local extension office or a regional herbarium can confirm an identification. Getting the species right is the first step toward deciding whether you are looking at a forage asset, a fire hazard, or a crop weed with evolving herbicide resistance.