Bromus tectorum L.
Other common names: downy brome grass, slender chess, early chess, downy chess, cheatgrass, drooping brome, wall brome, cheatgrass brome, slender brome, drooping brome grass, thatch grass
Family: Grass family, Poaceae
Habit: Winter annual grass, sometimes a summer annual
Description: A very long (3 inches) and narrow (0.125 inch) seed leaf emerges perpendicular to the soil. Young, true leaves of the seedling are rolled in the bud, unrolling as they emerge from the stem in an upward twisting or spiral pattern. Blades have soft, fuzzy hairs at the tip and on their undersides. Sheaths are white with reddish or maroon bases and short, fuzzy hairs. Mature plants, reaching 4–24 inches tall, are coated throughout with short, downy hairs. Ligules are translucent, 0.1 inch long and slightly jagged or frayed. Auricles are absent. Blades are 1.25–8.25 inches long by 0.1–0.3 inch wide, flat and sharply pointed at the tips. Sheaths are open at the top and pink-veined. Roots are fibrous; unlike many other grasses, roots do not form at the nodes of tillers. Inflorescences are 1.5–8 inches long, branched, nodding, oat-like panicles. Panicles have numerous soft, white hairs and are frequently purple-hued and shiny, especially when the seed is ripe. Individual spikelets are large (0.5–2 inches long) and contain three to 10 flowers. The fuzz-coated, papery outer chaff of each spikelet has one ridge; the outer chaff of each flower within the spikelet is wooly and has a 0.4–0.7 inch-long awn. Awns become sharp and stiff with seed maturity. Seeds are 0.2–0.3 inch long if the outer chaff is removed or 0.4 inch if it is still tightly attached. Seeds are linear or lanceolate, yellow to red-brown and grooved.
Similar species: Field brome (Bromus arvensis L.), which is also called Japanese brome, is best distinguished from downy brome by the outer chaff of the spikelets, which has three ridges rather than one. Unlike downy brome, cheat (chess) (Bromus secalinus L.) is largely hairless and has awns no longer than 0.2 inch. Common velvetgrass (Holcus lanatus L.) has sheaths that overlap one another and hairs on the back of the ligule.
Reduced tillage practices that retain surface residue create a favorable environment for downy brome in fall-seeded crops. Crop rotation with spring-planted crops helps control downy brome in winter wheat cropping systems. Because downy brome is a winter annual, seedbed preparation for a spring-planted crop will destroy the previous year’s seedlings, and relatively few new seedlings will germinate in the spring crop. Similarly, delaying planting of fall-sown crops allows more time for downy brome to germinate, so that more seedlings are destroyed during seedbed preparation and density is reduced in the subsequent crop. Dryland crop rotations that include summer fallow should be cultivated after wheat harvest or should begin early enough in the spring of the fallow year to prevent seed set.
Downy brome seeds are relatively short lived in the soil and need near-surface conditions for emergence. As a consequence, plowing the current year's seed crop under before planting is an effective management tool. Few seeds are dormant when shed from the parent plant, and many will germinate too deeply to emerge following fall plowing. Seeds that are dormant will die before later tillage events return them to the surface.
The dense mat of fine roots produced by downy brome can reduce soil moisture levels below the tolerance point of perennial forage species like bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve ssp. spicata]. Consequently, reducing downy brome populations before seeding new pastures is advisable. In existing pastures, avoid grazing practices that produce gaps larger than 24 inches, since uniform stands prevent establishment of downy brome. Fire and overgrazing are highly associated with invasion of this species.
Origin and distribution: Downy brome is a native of the Mediterranean region of Europe, North Africa and the Middle East. It has been introduced widely in other parts of Europe and Asia, and in Australia, New Zealand and South Africa. In North America, it occurs from Alaska and southern Canada throughout the United States and into Mexico, except in the extreme southeastern corner of the United States. As an agricultural weed, it is primarily a problem in the western half of the continent where rainfall occurs primarily in fall and winter followed by dry summers.
Seed weight: Mean population seed weight is 2.5–3.7 mg, including the chaff.
Dormancy and germination: A substantial but variable proportion of downy brome seeds are able to germinate when shed from the parent plant. Dormancy of the remaining seeds is broken by one to three months of after-ripening. The rate of after-ripening increases with increasing soil moisture. In the field, germination is typically delayed by dry soil conditions until the onset of fall rains, and by then all seeds are able to germinate. Optimal germination temperature increases with seed age, generally ranging from 59–68°F during the normal fall germination period. However, germination can occur at temperatures as low as 32°F. Temperature fluctuation does not promote germination. Some seeds will germinate in as little as two days, but complete germination of apparently non-dormant seeds may require five weeks or longer. Unlike most weeds, light has either no influence on or can actually inhibit germination of downy brome. Nitrate, however, stimulates germination.
Seed longevity: Maximum seed longevity in undisturbed soil has been variously reported at two to five years. The great majority of seeds germinate the first fall after being shed. Only a few of the remainder survive until the following fall, and survival of any remaining seeds is negligible. Seeds buried 1 inch in the soil have a very low (less than 1%) survival rate. Fire can reduce the seed bank to less than 3% of unburned areas, but subsequent seeding from surviving seed can reestablish the seed bank after only one season.
Season of emergence: Most seedlings emerge in the fall following significant rains. If conditions remain mild, emergence may continue into the winter. Some seedlings emerge in the spring, but seed production by spring emerging plants is highly variable. Flexibility in reproductive success over a wide range of emergence periods adapts this species to environments with highly variable conditions.
Emergence depth: Most seeds produced seedlings from depths of less than 2 inches, but a few seedlings emerged from more than 3 inches.
Photosynthetic pathway: C3
Sensitivity to frost: Downy brome is a highly frost-tolerant winter annual that survives temperatures of 10°F with little damage. Although aboveground growth ceases during cold weather, root growth continues in soil as cold as 37°F. Overwinter survival rates are generally high, with over 50% of plants surviving from seedling to maturity, but death from frost-heaving can occur when temperatures alternate above and below freezing.
Drought tolerance: Downy brome does not develop a deep root system in the fall and thus is sensitive to fall drought. By flowering time, however, roots penetrate from 20–50 inches or more. Under highly variable soil moisture conditions, downy brome usually has an advantage over competing species, thus accounting for its high invasive potential.
Mycorrhiza: Downy brome can be mycorrhizal but does not require a fungal associate. This characteristic allows establishment in disturbed sites with minimal mycorrhizal fungi.
Response to fertility: Downy brome tolerates extremely infertile soil conditions. Nevertheless, the species is highly responsive to N and P. Growth continues to increase with additional P, even at high P rates. The growth rate in soil already invaded by downy brome is higher than in uninvaded soil, a phenomena partially explained by increased availability of N, P, Mn and S in downy brome infested soil.
Soil physical requirements: Downy brome thrives on a wide range of soil types, but not on saline soils or very dry soils. It will grow on badly eroded land.
Response to shade: Downy brome is moderately shade tolerant. Moderate shade can promote growth.
Sensitivity to disturbance: Seed production by downy brome can be greatly reduced by mowing the plants between flowering and the dough stage of seed development. Mowing cannot completely eliminate the weed at any stage in plant development, however, because some plants can resprout if mowed before the dough stage, and seeds will mature on the clippings if the plant is cut at the dough stage or later. Mowing or grazing downy brome at any stage in the life cycle reduces ultimate plant size and seed production.
Time from emergence to reproduction: Plants typically emerge in the fall and flower in mid- to late spring, and seeds mature about four weeks later. Flowering is earlier in populations from hot, arid regions compared to those from cool, moist forested regions.
Pollination: Downy brome flowers self-pollinate.
Reproduction: Plants typically produce all their seeds in a single burst of reproduction before dying. Occasionally plants will rejuvenate and produce a second burst of reproduction following rain. Plants growing in favorable conditions can produce more than 500 seeds each, but most plants in more typical situations produce 10–80 seeds each.
Dispersal: Downy brome spread through the western United States primarily in bedding and manure of transported cattle, but also in straw used for packing dry goods and in contaminated crop seed. The awns stick in clothing and in the skin and fur of animals. Seeds are also dispersed by rodents. Seeds disperse in irrigation water.
Common natural enemies: Head smut (Ustilago bullata) is very common in some areas and can greatly reduce seed production. The fungus Pyrenophora semeniperda kills a large proportion of dormant seeds, particularly in dry sites and years, and infects a greater proportion of seeds as they age.
Palatability: Downy brome is not eaten by people. Immature plants are excellent forage for livestock and make up a substantial part of spring forage for cattle in much of the west. The stiff awns of mature downy brome irritate the mouths and digestive tracts of livestock.
Weed Characteristics Summary Table
|Growth habit||Seed weight (mg)||Seed dormancy at shedding||Factors breaking dormancy||Optimum temperature for germination (F)||Seed mortality in untilled soil (%/year)||Seed mortality in tilled soil (%/year)||Typical emergence season||Optimum emergence depth (inches)|
|medium||2.5–3.7||Variable||ni||59–68||99||–||fall and spring||0–2|
|Photosynthesis type||Frost tolerance||Drought tolerance||Mycorrhiza||Response to nutrients||Emergence to flowering (weeks)||Flowering to viable seed (weeks)||Pollination||Typical & high seed production (seeds per plant)|
|C3||high||high||variable||high||4||4||self||50 & 500|
General: The designation “–” signifies that data is not available or the category is not applicable.
Growth habit: A two-word description; the first word indicates relative height (tall, medium, short, prostrate) and second word indicates degree of branching (erect, branching, vining).
Seed weight: Range of reported values in units of “mg per seed.”
Seed dormancy at shedding: “Yes” if most seeds are dormant when shed, “Variable” if dormancy is highly variable, “No” if most seeds are not dormant.
Factors breaking dormancy: The principle factors that are reported to break dormancy and facilitate germination. The order of listing does not imply order of importance. Abbreviations are:
scd = seed coat deterioration
cms = a period subjected to cold, moist soil conditions
wst = warm soil temperatures
li = light
at = alternating day-night temperatures
ni = nitrates
Optimum temperature range for germination: Temperature (Fahrenheit) range that provides for optimum germination of non-dormant seeds. Germination at lower percentages can occur outside of this range. The dash refers to temperature range, and the slash refers to alternating day/night temperature amplitudes.
Seed mortality in untilled soil: Range of mortality estimates (percentage of seed mortality in one year) for buried seeds in untilled soil. Values were chosen where possible for seeds placed at depths below the emergence depth for the species and left undisturbed until assessment. Mortality primarily represents seed deterioration in soil.
Seed mortality in tilled soil: Range of mortality estimates (percentage of seed mortality in one year) for seeds in tilled soil. Values were chosen for seeds placed within the tillage depth and subjected to at least annual tillage events. Seed losses are the result of dormancy-breaking cues induced by tillage, germination and deterioration of un-germinated seeds.
Typical emergence season: Time of year when most emergence occurs in the typical regions of occurrence for each weed. Some emergence may occur outside of this range.
Optimum emergence depth: Soil depths (in inches below the soil surface) from which most seedlings emerge. Lower rates of emergence usually will occur at depths just above or just below this range.
Photosynthesis type: Codes “C3” or “C4” refer to the metabolic pathway for fixing carbon dioxide during photosynthesis. Generally, C3 plants function better in cooler seasons or environments and C4 plants function better in warmer seasons or environments.
Frost tolerance: Relative tolerance of plants to freezing temperatures (high, moderate, low).
Drought tolerance: Relative tolerance of plants to drought (high, moderate, low).
Mycorrhiza: Presence of mycorrhizal fungi. “Yes” if present; “no” if documented not to be present, “unclear” if there are reports of both presence and absence; “variable” if the weed can function either with or without, depending on the soil environment.
Response to nutrients: Relative plant growth response to the nutrient content of soil, primarily N, P, K (high, moderate, low).
Emergence to flowering: Length of time (weeks) after emergence for plants to begin flowering given typical emergence in the region of occurrence. For species emerging in fall, “emergence to flowering” means time from resumption of growth in spring to first flowering.
Flowering to viable seed: Length of time (weeks) after flowering for seeds to become viable.
Pollination: “Self” refers to species that exclusively self-pollinate, “cross” refers to species that exclusively cross-pollinate, “self, can cross” refer to species that primarily self-pollinate, but also cross-pollinate at a low rate, and “both” refers to species that both self-pollinate and cross-pollinate at relatively similar rates.
Typical and high seed production potential: The first value is seed production (seeds per plant) under typical conditions with crop and weed competition. The second value, high seed production, refers to conditions of low density without crop competition. Numbers are rounded off to a magnitude that is representative of often highly variable reported values.
Hulbert, L.C. 1955. Ecological studies of Bromus tectorum and other annual bromegrasses. Ecological Monographs 25: 181–213.
Mack, R.N., and D.A. Pyke. 1983. The demography of Bromus tectorum: Variation in time and space. Journal of Ecology 71: 69–93.
Upadhyaya, M.K., R. Turkington and D. McIlyride. 1986. The biology of Canadian weeds. 75. Bromus tectorum L. Canadian Journal of Plant Science 66: 689–709.
Young, F.L., A.G. Ogg, Jr. and J.R. Alldredge. 2014. Postharvest tillage reduces downy brome (Bromus tectorum L.) infestations in winter wheat. Weed Technology 28: 418–425.