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Species Abstracts of Highly Disruptive Exotic Plants

at Scotts Bluff National Monument

Bromus tectorum

Park: Scotts Bluff National Monument (SCBL)
Species: Bromus tectorum L.
Common Name: Downy brome, downy chess, early chess, drooping brome, cheatgrass brome, wild oats, military grass
Urgency Ranking: Medium
Overall Ranking: 4
Significance of Impact: 56
A. Current impact: 29
B. Ability to be a pest: 27
Feasibility of Control: 29

Taxonomic Description:
Downy brome is a winter annual grass, forming small tufts (20-60 cm tall), from a fine fibrous root system. Stems are erect and slender. Leaf blades are flat (1-19 cm long and 1-5 mm wide) and pubescent. Ligules are membranous, truncate, and lacerate (0.9-3.5 mm long). Lower sheaths are pubescent, and upper sheaths are glabrous. The inflorescence is a dense, drooping panicle (4-21 cm long) and is pale green to purplish in color. Branches are thin, flexuous, and pubescent. Spikelets are four- to seven-flowered (11-24 mm long and 3-6 mm wide) and pubescent or villous. Glumes are unequal. The first glume is subulate, one-nerved (4.5-7.5 mm long and 1 mm wide), and the second is three-nerved (7-11 mm long and wider than the first). Lemmas are bifid, lanceolate (9-12 mm long), pubescent, and awned. Awns are slender (7-17 mm long) and straight. Paleas are shorter than the lemma. Lemma pubescence, phenology, winter hardiness, height and openness of the panicle are highly variable. Several ecotypes have been identified.

Downy brome is native to Eurasia and the Mediterranean region. It has spread to Europe, southern Russia, west central Asia, North America, Japan, South Africa, Australia, New Zealand, Iceland, and Greenland. It was first identified in the United States in 1861 in New York and Pennsylvania. By 1928, downy brome had reached its present range. It occurs throughout the United States (including Hawaii and Alaska), except for portions of Alabama, Georgia, South Carolina, and Florida. It grows on rangelands, pastures, prairies, fields, waste areas, eroded sites, and roadsides. It is most abundant in the Great Basin and Columbia Basin of the western United States.

Downy brome grows in many climatic areas. It is found primarily in the 150-560 mm precipitation zone. Downy brome will grow in almost any type of soil. Research has shown that it is most often found on coarse textured soils and does not grow well on heavy, dry, and/or saline soils. Downy brome has been found growing on B and C horizons of eroded areas and areas low in nitrogen. It grows in a relatively narrow range of soil temperatures. Growth starts at 2.0-3.5°C and stops when temperatures exceed 15°C. Litter promotes germination and establishment of seedlings.

Downy brome is a winter annual. It germinates in the fall, over winters as a seedling, then flowers in the spring. If fall precipitation is limiting and spring moisture is adequate, the seeds will germinate in the spring, and the plants will flower that summer. Downy brome is cleistogamous and self-fertile, with no evident out-crossing. The plants typically flower from mid-April through June. Florets are dispersed within a week of maturity by wind, attachment to animal fur, or small rodents. Seeds move as a contaminant in hay, grain, straw, and machinery. An abundant seed producer, downy brome has been reported as having a production potential in excess of 300 seeds per plant. Under optimal conditions, it may produce 450 kg of seed per hectare with about 330,000 seeds/kg. Seed production per plant is dependent on plant density. Plants as small as 2.5 cm in height may produce seed.

Seeds will maintain a high viability in dry storage, lasting in excess of 11 years. In the field, under buried conditions, seeds will loose their viability in 2 to 5 years. The primary limit to germination is adequate fall, winter, and/or spring moisture. Seeds can withstand extremely high soil temperatures. Seeds germinate the best in the dark or in diffuse light. They do not need to be in contact with bare soil to germinate, and a litter cover generally will improve germination. However, seeds will germinate more quickly when covered with soil. Seedlings rapidly emerge from the top 2.5 cm of soil, and a few plants emerge from depths of 8 cm. No emergence occurs from seeds buried 10 cm below the surface.

Downy brome possesses an extensive root system that grows through the spring. It will produce roots to depths of 18-20 cm prior to sending out far-reaching lateral roots. These lateral roots are one of the keys to the survival of this plant. One study showed that downy brome had the capability to reduce soil moisture to the permanent wilting point to a depth of 70 cm, reducing competition from other species. Downy brome grows rapidly and may produce dry matter at a rate of 2.9 g/mm2/day.

Several widespread and dense populations of downy brome occur at Scotts Bluff National Monument (SCBL) combining to cover an area of 11 to 50 hectares. These plants are found in disturbed sites as well as in mid-successional sites disturbed 11-50 years ago. Downy brome occurs throughout the park with dense populations concentrated in the old field areas. Downy brome has the potential for long term modification or retardation of succession and poses a threat to the areas' primary resources. Also, it has a significant negative visual impact on the vegetation of SCBL.

A number of control options exist for downy brome. An important consideration in controlling this species is that the seeds have the potential to remain viable in the seed bank for 2 to 5 years. Also, many sources of new propagules surround SCBL. In addition, if desirable vegetation is scarce or absent control will be of little value. Most control methods harm other plants and may result in a disturbance that will favor reinvasion by downy brome or other exotic species.

Fire, mowing, grazing, tillage, and interseeding of competitive native plants have all been shown to reduce populations of downy brome. The results of wildfires on this species have been considered to be a catastrophic stand renewal process. This is because wildfires often occur at the worst time for perennial plants. One result is that open ground is created for readily colonizing species such as downy brome. Based on the fact that this is a cool-season annual, it could be assumed that prescribed fire in late spring could be a valuable tool in controlling this species, especially on an area where the preferred vegetation is primarily warm-season grasses. A prescribed fire should kill seedlings and further reduce the surface seed bank. A study conducted on spring burning of the closely related Japanese brome (Bromus japonicus Thunb. ex Murr.), showed that consecutive annual burns reduced brome density and standing crop.

Mowing has been shown to reduce seed production when the stand is mowed within 1 week following flowering. This reduces seed production, but does not eliminate it because later developing plants will escape mowing and will produce seed. Late fall and early spring grazing has been shown to significantly reduce plant numbers. However, heavy grazing will promote downy brome invasion. Spring or fall tillage will significantly reduce plant numbers. Although a strongly competitive crop, it will not tolerate shading.

The majority of the work on the chemical control of downy brome has focused on infestations in agricultural crops. Chemical control research in prairies has been primarily limited to AAtrex (atrazine). Herbicides active on downy brome in various crops include Hoelon (diclofop), Kerb (pronamide), Nortron (ethofumesate), AAtrex (atrazine), Princep (simazine), Amizol (amitrole), Arsenal (imazapyr), Hyvar (bromacil), Oust (sulfometuron methyl), Cyclone (paraquat), and Roundup (glyphosate). Many herbicides are not specific to downy brome or may not be specifically licensed for this use. It is important to read and follow all herbicide label directions.

Research into the biological control of downy brome is limited. It is known that rabbits and mice will feed extensively on this species as do migratory grasshoppers (Melanoplus sanquinipes). Downy brome is often infected with a head smut (Ustilago bulleta Berk.) that, when severe, may reduce seed yield. Some research has been conducted on pink snow mold (Fusarium nivale) as a biological control agent, but information has yet to be released. In addition to these molds and smuts, over 20 diseases of downy brome have been reported.


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