Downy brome

Bromus tectorum L.

Images above: Left: Downy brome young plants (Randall Prostak, University of Massachusetts). Right: Downy brome plant (Joseph DiTomaso, University of California, Davis).

Identification

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” or 7.6 cm) and narrow (0.13” = 0.3 cm wide) 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 4-36” (10-91 cm) tall, are coated throughout with short, downy hairs.  Ligules are translucent, 0.1” (0.25 cm) long, and slightly jagged or frayed.  Auricles are absent.  Blades are 1.3-8.3” (3.2-21 cm) long by 0.1-0.3” (0.25-0.76 cm) wide, flat, and sharply pointed at tips.  Sheaths are open at the top and pink-veined.  Roots are fibrous; unlike other grasses, roots do not form at the nodes of tillers.  Inflorescences are 1.5-8.0” (3.8-20 cm) 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 (0.5-2.0” = 1.3-5.1 cm long) and contain 3 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” (1.0-1.8 cm) long awn.  Awns become sharp and stiff with seed maturity.  Seeds are 0.2-0.3” (0.5-0.8 cm) long if the outer chaff is removed or 0.4” (1.0 cm) if it is still tightly attached.  Seeds are linear or lance-shaped, 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” (0.5 cm).  Common velvetgrass (Holcus lanatus L.) has sheaths that overlap one another and hairs on the back of the ligule.

Management

Reduced tillage practices that retain surface residue creates a favorable environment for downy brome in fall-seeded crops (Morrow and Stahlman 1984).  Crop rotation with spring-planted crops helps control downy brome in winter wheat cropping systems.  Because downy brome is a winter annual, spring 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 seed bed preparation (Morrow and Stahlman 1984) and density is reduced in the subsequent crop (Anderson 1994).  Dryland crop rotations that include summer fallow should be cultivated after wheat harvest (Young et al. 2014) 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 to be positioned near the soil surface for emergence.  As a consequence, plowing the current year's seed crop under before planting is an effective management tool (Wicks 1997).  Few seeds are dormant when shed from the parent plant and many will germinate too deeply to emerge following fall plowing.  Moreover, most of the dormant seeds will die before later tillage events return them to the surface.  Downy brome seeds are susceptible to solarization with clear plastic, an approach that may be suitable for small-scale ecological restoration (Orr et al. 2019).

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” (60 cm), since uniform stands prevent establishment of downy brome.  Fire and overgrazing are highly associated with invasion of this species (Chambers et al. 2007, Knapp 1996).

Ecology

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 the extreme southeastern corner of the U.S.A.  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:  3.0 mg (Upadhyaya et al. 1986); 2.5-3.7 mg including chaff (Hulbert 1955); 1.9 mg for a subalpine population, 2.2 mg for a rangeland population, and 3.0 mg for a cropland population (Ehlert et al. 2019).

Dormancy and germination:  A substantial but variable proportion of downy brome seeds can germinate when shed from the parent plant.  Dormancy of the remaining seeds is broken by 1 to 3 months of after-ripening (Allen and Meyer 2002).  The rate of after-ripening increases with increasing soil moisture (Bair et al. 2006).  In the field, germination is typically delayed by dry soil conditions until the onset of fall rains, and by then all seeds can germinate.  Optimal germination temperature increases with seed age, generally ranging from 59-68 °F (15-20 °C) during the normal fall germination period (Hulbert 1955), however, germination can occur at temperatures as low as 32 °F (0 °C) (Mack and Pyke 1984).  Another experiment found greater germination at 55-63 °F (13-17 °C) than at 70-77 °F (21-25 °C) (Ehlert et al. 2019).  Temperature fluctuation does not promote germination.  Some seeds will germinate in as little as 2 days, but complete germination of apparently non-dormant seeds may require 5 weeks or longer.  Unlike most weeds, light has either no influence or can inhibit germination of downy brome (Hulbert 1955, Steinbauer and Grigsby 1957).  Nitrate stimulates germination.

Seed longevity:  Maximum seed longevity in undisturbed soil has been variously reported at 2 to 5 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 (Burnside et al. 1996, Mack and Pyke 1983, Wicks 1997).  Seeds buried 1" (2.5 cm) in soil have a very low (<1%) survival rate (Wicks 1997).  Fire can reduce the seedbank to less than 3% of that in unburned areas, but subsequent seeding from surviving seed can reestablish the seedbank after only one season (Humphrey and Schupp 2001).

Season of emergence:  Most seedlings emerge in the fall following significant rains (Mack and Pyke 1984).  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 (Hulbert 1955, Mack and Pyke 1983).  Flexibility in reproductive success over a wide range of emergence periods adapts this species to environments with highly variable conditions (Chambers et al. 2007, Mack and Pyke 1983).

Emergence depth:  Most seeds produce seedlings from depths of less than 2" (5 cm) but few seedlings emerge from more than 3" (7.5 cm) (Hulbert 1955, Morrow and Stahlman 1984).

Photosynthetic pathway:  C3.

Sensitivity to frost:  Downy brome is a highly frost-tolerant winter annual that survives temperatures of 10 °F (-12 °C) with little damage (Cici and Van Acker 2011).  Above ground growth ceases during cold weather, but root growth continues in soil as cold as 37 °F (3 °C).  Over-winter survival rates are generally high, with over 50% of plants surviving from seedling to maturity (Mack and Pyke 1983), but death from frost-heaving can occur when temperatures alternate above and below freezing (Mack and Pyke 1984).

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 20-51” (50-130 cm) or more (Hulbert 1955).  Under highly variable soil moisture conditions, downy brome usually has an advantage over competing species, thus accounting for its high invasive potential (Chambers et al. 2007).

Mycorrhiza:  Downy brome can be mycorrhizal but does not require a fungal associate.  This characteristic allows establishment on disturbed sites with minimal mycorrhizal fungi (Knapp 1996).

Response to fertility:  Downy brome tolerates extremely infertile soil conditions.  Nevertheless, the species is highly responsive to N and P (Blackshaw et al. 2003, Blackshaw et al. 2004, Morrow and Stahlman 1984).  Growth continues to increase with additional P even at high P rates.  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 (Blank and Morgan 2013).

Soil physical requirements:  Downy brome thrives on a wide range of soil types, but not on saline soils or very dry soils (Morrow and Stahlman 1984).  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 dough stage of seed development.  Mowing cannot completely eliminate the weed at any stage in plant development, however, because some plants can re-sprout if mowed before the dough stage, and seeds can mature on the clippings if the plant is cut at the dough stage or later (Hulburt 1955).  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, flower in mid- to late spring and seeds mature about 4 weeks later (Hulbert 1955, Mack and Pyke 1983).  Flowering is earlier in populations from hot, arid steppe regions compared to those from cool, moist forested regions (Rice and Mack 1991).

Pollination:  Downy brome flowers self-pollinate.

Reproduction:  Plants typically produce all the 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 (San Martin et al. 2021), but most plants in more typical situations produce 10 to 80 seeds each (Mack and Pyke 1983, Rice and Mack 1991).  In eleven site-years from central Oregon and Washington, seed retention at the beginning of winter wheat harvest averaged 43%.  Considerable variability in seed retention was observed, with highest retention during a cool, wet season that delayed plant development and lower retention under warmer and drier conditions (San Martin et al. 2021).  Seed retention at winter wheat harvest in the central Great Plains averaged 75% with considerable variation (20 to 90%), however, shattered seeds were determined by recovery from the soil surface which could have underestimated shattering and overestimated retention rates (Soni et al. 2020).  

Dispersal:  Downy brome spread through the western U.S.A. primarily in bedding and manure of transported cattle, but also in straw used for packing dry goods, and in contaminated crop seed (Mack 1981).  The awns stick in clothing and in the skin and fur of animals.  Seeds are also dispersed by rodents.  Seeds disperse in irrigation water (Wilson 1980).

Common natural enemies:  Head smut (Ustilago bullata) is very common in some areas and can greatly reduce seed production (Mack and Pyke 1984).  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 (Meyer et al. 2007).  This fungus more readily infects smaller seeds at temperatures > 68° F (20° C) (Ehlert et al. 2019).

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 (Morrow and Stahlman 1984).  The stiff awns of mature downy brome irritate the mouths and digestive tracts of livestock.

References:

  • Allen, P. S., and S. E. Meyer.  2002.  Ecology and ecological genetics of seed dormancy in downy brome.  Weed Science 50:241-247.
  • Anderson, R. L.  1994.  Characterizing weed community seedling emergence for a semiarid site in Colorado.  Weed Technology 8:245-249.
  • Bair, N. B., S. E. Meyer, and P. S. Allen.  2006.  A hydrothermal after-ripening model for seed dormancy loss in Bromus tectorum L.  Seed Science Research 16:17-28.
  • Blackshaw, R. E., R. N. Brandt, H. H. Janzen, T. Entz, C. A. Grant, and D. A. Derksen.  2003.  Differential response of weed species to added nitrogen.  Weed Science 51:532-539.
  • Blackshaw, R. E., R. N. Brandt, H. H. Janzen, and T. Entz.  2004.  Weed species response to phosphorus fertilization.  Weed Science 52:406-412.
  • Blank, R. R., and T. Morgan.  2013.  Soil engineering facilitates downy brome (Bromus tectorum) growth - A case study.  Invasive Plant Science and Management 6:391-400.
  • Burnside, O. C., R. G. Wilson, S. Weisberg, and K. G. Hubbard.  1996.  Seed longevity of 41 weed species buried 17 years in eastern and western Nebraska.  Weed Science 44:74-86.
  • Chambers, J. C., B. A. Roundy, R. R. Blank, S. E. Meyer, and A. Whitaker.  2007.  What makes Great Basin sagebrush ecosystems invasible by Bromus tectorum?  Ecological Monographs 77:117-145.
  • Cici, S. Z. H., and R. C. Van Acker.  2011.  Relative freezing tolerance of facultative winter annual weeds.  Canadian Journal of Plant Science 91:759-763.
  • Ehlert, K. A., Z. Miller, J. M. Mangold, F. Menalled, and A. Thornton.  2019.  Temperature effects on three downy brome (Bromus tectorum) seed collections inoculated with the fungal pathogen Pyrenophora semeniperda.  Invasive Plant Science and Management 12:150-154.
  • Hulbert, L. C.  1955.  Ecological studies of Bromus tectorum and other annual bromegrasses.  Ecological Monographs 25:181-213.
  • Humphrey, L. D., and E. W. Schupp.  2001.  Seed banks of Bromus tectorum-dominated communities in the Great Basin.  Western North American Naturalist 61:85-92.
  • Knapp, P. A.  1996.  Cheatgrass (Bromus tectorum L) dominance in the Great Basin desert.  Global Environmental Change 6:37-52.
  • Mack, R. N.  1981.  Invasion of Bromus tectorum L. into western North America: an ecological chronicle.  Agro-ecosystems 7:145-165.
  • Mack, R. N., and D. A. Pyke.  1983.  The demography of Bromus tectorum: Variation in time and space.  Journal of Ecology 71:69-93.
  • Mack, R. N., and D. A. Pyke.  1984.  The demography of Bromus tectorum: The role of microclimate, grazing and disease.  Journal of Ecology 72:731-748.
  • Meyer, S. E., D. Quinney, D. L. Nelson and J. Weaver.  2007.  Impact of the pathogen Pyrenophora semeniperda on Bromus tectorum seedbank dynamics in North American cold deserts.  Weed Research 47:54-62.
  • Morrow, L. A., and P. W. Stahlman.  1984.  The history and distribution of downy brome (Bromus tectorum) in North America.  Weed Science 32, Supplement 1:2-6.
  • Orr, M. R., R. J. Reuter, S. J. Murphy.  2019.  Solarization to control downy brome (Bromus tectorum) for small-scale ecological restoration.  Invasive Plant Science and Management 12:112-119.
  • Rice, K. J., and R. N. Mack.  1991.  Ecological genetics of Bromus tectorum III. The demography of reciprocally sown populations.  Oecologia 88:91-101.
  • San Martín, C., M. E. Thorne, J. A. Gourlie, D. J. Lyon, and J. Barroso.  2021.  Seed retention of grass weeds at wheat harvest in the Pacific Northwest.  Weed Science 69: 238–246.
  • Soni, N., S. J. Nissen, P. Westra, J. K. Norsworthy, M. J. Walsh, and T. A. Gaines.  2020.  Seed retention of winter annual grass weeds at winter wheat harvest maturity shows potential for harvest weed seed control.  Weed Technology 34:266–271.
  • Steinbauer, G. P., and B. Grigsby.  1957.  Field and laboratory studies on the dormancy and germination of the seeds of chess (Bromus seculinus L.) and downy brome (Bromus tectorum L.).  Weeds 5:1-4.
  • 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.
  • Wicks, G. A.  1997.  Survival of downy brome (Bromus tectorum) seed in four environments.  Weed Science 45:225-228.
  • Wilson, R. G. Jr.  1980.  Dissemination of weed seeds by surface irrigation water in western Nebraska.  Weed Science 28: 87-92.
  • 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.