PNW 474 • October 1994 Downy Brome Bromus tectorum L.

P.E. Nesse and D.A. Ball

owny brome (Bromus tectorum L.), also known Das cheatgrass, was intro- duced into the North American continent from the Mediterra- nean area of Europe. It was identified in the eastern United States in 1861, and by 1914 this aggressive weed had spread throughout the continent. Downy Brome is of particular importance in the northern Great Plains, intermountain west, and the inland Pacific Northwest. It is present to a lesser extent in most of the con- tinental United States except for the extreme southeastern tier of states. In much of the Pacific North- west and intermountain west, Figure 1.—Downy brome in the vegetative growth stage. downy brome provides an im- portant source of spring forage on arid grazing lands. How- Downy brome is a major ever, the forage quality of weed problem in winter wheat, downy brome decreases as perennial grass seed, and alfalfa plants mature. Protein content in much of the Pacific North- drops to about 3 percent as west and Great Plains. Downy plants mature, and long slender brome infestations of 10 and 50 awns on the seed head can limit plants per square foot can re- feed intake by irritating and duce winter wheat yields by 40 puncturing the soft tissues in- and 92 percent, respectively. side the mouth of grazing ani- Downy brome is especially mals. troublesome in drier production areas where crop rotations are mostly limited to winter wheat followed by a year of summer fallow (winter wheat-summer Philip E. Nesse, Extension agent, Gilliam/Morrow counties; Daniel A. fallow rotation). Ball, assistant professor of weed science, Columbia Basin Agricultural Figure 2.—Magnified view of the collar Research Center, Pendleton; Oregon region of a downy brome culm showing State University. characteristic pubescence.

A Pacific Northwest Extension Publication • Oregon • Idaho • Washington Identification before winter wheat. Mature plants also can be produced Downy brome is a winter- from seeds germinating in the annual grass producing erect or spring, although seed produc- spreading culms that are slen- tion is much more prolific from der and 12 to 24 inches high. It autumn germinating plants. produces numerous tillers and Newly produced downy brome is characterized by the presence seed requires a short after-rip- of fine hairs covering leaf ening period for best germina- sheaths and blades. The panicle tion. (seed head) is rather dense, Beyond this initial after-rip- slender, drooping, often purple, ening period,␣ little seed dor- with spikelets nodding, and 2 to mancy remains, and 6 inches long. Seeds are narrow, germination occurs if favorable 1 5 ⁄4 to ⁄8 inch long and with awns temperature and moisture con- 5 up to ⁄8 inch long. ditions exist. Seed germination often exceeds 95 percent for seeds buried within 2 inches of the soil surface. Seeds buried Ecology below 2 inches may germinate Downy brome is adapted to but will rarely emerge above climates with annual precipita- the soil surface. tion from 6 to 22 inches. It can Seedling emergence also is colonize a wide range of soil restricted by soil compaction. conditions on both disturbed Downy brome seeds are viable and undisturbed sites. in the soil for 2 to 3 years, In addition to its success as a which makes a 2-year crop rota- cropland weed, downy brome tion of winter wheat with fal- can inhabit salt desert shrub low or other crop ineffective, by communities, sagebrush steppe, itself, for control of downy Downy brome inflorescence (left) and stem and ponderosa pine and Dou- brome. (right). glas-fir forest. Once established, Under ideal conditions, a it can become the dominant dense infestation of downy herbaceous plant species. The brome can produce over 500 and can reach a maximum area dominated by downy pounds of seed per acre depth of 3 to 4 feet as downy brome in the intermountain (1 pound of seed contains brome plants mature. In a dense west has been estimated at over approximately 250,000 seeds). stand, the extensive, fibrous 100 million acres. Dry matter production of root system can extract all the Downy brome seed germina- downy brome on rangeland available moisture from the tion typically occurs in late sites can vary from less than 100 upper soil profile. summer or autumn shortly after lb/acre on the poorest sites to Downy brome responds dra- fall rains when the soil tempera- more than 2000 lb/acre on the matically to nitrogen fertiliza- ture is about 70°F. Seeds can better sites. tion of winter wheat. continue to germinate in the Rainfall, or lack of it, plays a Surface-applied nitrogen, either autumn at soil temperatures key role in determining dry as commercial fertilizer or as between 35 and 40°F if soil matter production of downy manure, has been shown to moisture is adequate. Estab- brome, as does competing veg- triple downy brome height and lished plants overwinter in the etation and growing season yield. Research has shown that vegetative stage, resume temperatures. Roots can con- actively growing downy brome growth again in the early tinue to grow at soil tempera- can negate the benefits to win- spring, and mature in May or tures of 37°F during the winter, ter wheat from spring-applied, June, typically 4 to 6 weeks topdress nitrogen. Control prevent new seed production destroyed by using a nonselec- and deplete the downy brome tive herbicide or by tillage. Control of downy brome in seed reserve in the soil. Yields The use of nonselective herbi- rangeland generally consists of from spring crops can be con- cides in early spring followed burning, using nonselective siderably less than yield from by late spring tillage also has herbicides, and stocking rate winter wheat. However, winter been effective in maximizing management. Vigorously grow- wheat yield losses from downy soil moisture storage in fal- ing desirable grass and forb brome need to be a part of com- lowed land. Herbicides with species compete with downy parisons of alternative crop soil residual activity also can be brome and help prevent the options and rotations. used to control winter annual establishment of infestations. grasses during the winter and Prevention of overgrazing and Mechanical and Chemical spring of the fallow year. This management for optimum Control practice can reduce the neces- range production are effective sity for early nonselective herbi- approaches for downy brome Another strategy for control cide treatment in the spring. control. is to perform shallow tillage Selective chemical control of after downy brome emergence downy brome can be obtained Rotation in the fall, but prior to planting in alfalfa, but is more difficult winter wheat. This control re- in cereal crops such as winter A number of strategies have quires timely rains for downy wheat because of the relatively been used with some success brome germination. Waiting for close botanical relationship be- for control of downy brome in adequate precipitation for tween the weed and winter winter wheat. The most effec- downy brome germination can wheat. Several selective herbi- tive control is to lengthen the delay wheat planting and may cides are available as preplant period between winter wheat result in reduced grain yield. incorporated treatments in cere- crops by including 2 or more Moldboard plowing of winter als or as postemergence treat- years of various combinations wheat stubble in the fall after ments for downy brome in of spring crops with or without harvest to deeply bury downy growing winter wheat. fallow. brome seed traditionally has For further, in-depth informa- In the lower precipitation been used as the primary con- tion on downy brome manage- areas, rotations could include: trol for downy brome in in- ment in winter wheat rotations, winter wheat-summer fallow- fested winter wheat crop refer to the PNW Conservation spring wheat-summer fallow; rotations. While this control Tillage Handbook Series publica- winter wheat-spring barley/ generally has been successful in tion, “Managing Downy Brome other crop-summer fallow; or limiting infestations, it can re- under Conservation Tillage winter wheat-spring barley/ sult in severe soil erosion over Systems in the Inland North- other crop-spring wheat-sum- the winter due to elimination of west Crop-Fallow Region” mer fallow. crop residue on the soil surface. (chapter 5, No. 15). In higher precipitation areas It also can result in reduced For current herbicide control with annual cropping, rotations overwinter soil moisture com- strategies for downy brome, including 2 years of spring pared to moisture retained by refer to the PNW Weed Control crops, such as spring barley, leaving wheat stubble on the Handbook, and contact local canola or spring peas, between surface. county Extension agents and winter wheat crops can effec- A fallow management prac- agricultural professionals. As tively reduce downy brome tice that helps deplete downy with all crop protection chemi- populations. These types of brome seed reserves in the soil cals, read and follow label di- rotations allow for fall and/or is to allow downy brome to rections and understand their spring destruction of germinat- germinate in wheat stubble proper use. ing downy brome over a period from autumn into early spring. of 2 or more years. This can Downy brome can then be Photographs provided by Larry Burrill (Figures 1 and 2), Extension weed control specialist emeritus, Oregon State Univer- sity; and author Daniel Ball (Figures 3 and 4). The black-and-white illustrations were reproduced, with permission, from La Rea Dennis, Gilkey’s Weeds of the Pacific North- west (Corvallis, OR, Oregon State Univer- sity, 1980); © La Rea J. Dennis. Pacific Northwest Extension publications contain material written and produced for public distribution. You may reprint writ- ten material, provided you do not use it to endorse a commercial product. Please reference by title and credit Pacific North- west Extension publications. To reproduce material used with permission on page 2 in this publication, please contact the original source. Pacific Northwest Extension publications are jointly produced by the three Pacific Northwest states—Oregon, Washington, and Idaho. Similar crops, climate, and topography create a natural geographic unit that crosses state lines. Since 1949 the Figure 3.—Mature downy brome plant showing characteristic growth habit. PNW program has published more than 450 titles. Joint writing, editing, and pro- duction have prevented duplication of effort, broadened the availability of faculty specialists, and substantially reduced the costs for participating states. Published and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, by the Oregon State University Exten- sion Service, Lyla Houglum, interim direc- tor; Washington State University Cooperative Extension, James J. Zuiches, director; the University of Idaho Coopera- tive Extension System, LeRoy D. Luft, director; and the U.S. Department of Agri- culture cooperating. The three participating Extension Services offer educational programs, activities, and materials—without regard to race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, and disabled veteran or Vietnam-era veteran sta- tus—as required by Title VI of the Civil Rights Act of 1964, Title IX of the Education Amendments of 1972, and Section 504 of the Rehabilitation Act of 1973. The Oregon State University Extension Service, Wash- ington State University Cooperative Exten- Figure 4..—Mature downy brome panicles in a field of ripening winter wheat. sion, and the University of Idaho Cooperative Extension System are Equal Opportunity Employers. Published Octo- ber 1994. 50¢/50¢/50¢