Downy brome (Bromus tectorum L.) is found throughout Nebraska and is one of the most serious weeds in western Nebraska. Lack of control can be costly to both crop and livestock producers. It is especially troublesome in alfalfa, winter wheat-fallow rotations, continuous winter wheat, rangeland, waste areas, roadsides, shelterbelts, fence rows, and railroad rights-of-way. It invades overgrazed pastures and rangelands and is spread when the long awns on seeds attach to cattle. Seeds also are spread by hay, combines, grain trucks, and in contaminated winter wheat seed.
Downy brome is known by a variety of names including cheatgrass, cheatgrass brome, downy bromegrass, military grass, wild oats, downy chess, and cheat. Downy brome may be called cheat, but cheat (Bromus secalinus L.) is actually a different weed. Cheat always heads out above the wheat. Two other annual bromes often are confused with downy brome: Japanese brome (Bromus japonicus Thunb. ex. Murr), which is more common in western Nebraska, and hairy chess (Bromus commutatus Schrad), which is more common in eastern Nebraska. Both are more prevalent in pastures and waste areas but can be found in winter wheat. This discussion of downy brome also pertains to Japanese brome and hairy chess.
Downy brome is a winter annual and thrives in all soils. This weed has an extensive shallow root system and roots with many hairs that enable the plant to extract much of the soil water. A downy brome density of 50 plants per square foot can remove soil water to the permanent wilting point to a depth of about 2.5 feet. Downy brome is thus very competitive with winter wheat for soil water and nutrients. When under stress, plants only 1 to 2 inches tall can produce seed. Plants under stress from tillage or harsh environments divert more of their photosynthetic energy to seed production than undisturbed plants or plants growing in more productive environments.
At Alliance and North Platte, moderate (one to two plants per square foot) to heavy infestations of downy brome have reduced wheat yields 30-80 percent. Downy brome also can dramatically reduce first harvest alfalfa yields by competing for early season moisture. It also can severely reduce forage quality. Overgrazed rangelands are more easily invaded by downy brome, which reduces economic returns of the grassland.
Downy brome is a palatable grass before the seed heads emerge but becomes unpalatable with maturity. Mature downy brome can injure livestock by causing infection in the eyes or mouth. Mature plants also are a serious fire hazard.
Downy brome usually begins growing in the fall or early spring. Reproduction is by seed and seedling plants must be vernalized to produce seed. In medium-textured soils, the optimum seed depth for downy brome emergence is less than 1 inch; however, seedlings have emerged from a 4-inch depth in western Nebraska. The plant tillers profusely, depending on time of germination. In early spring the plant continues to tiller, joints, and sets seed (see Figure 1). The plant grows from 6 to 24 inches tall. Height depends on available soil water, fertility, and plant competition. At emergence, leaves are about 1/32 of an inch wide and brownish-green. As the plant and seed reach maturity, leaves turn purplish-tan.
- Leaves — Leaf blades are flat and 2 to 6 inches long. Blades and sheath are hairy.
- Ligule — Prominent membrane with frayed margin to almost 0.10 inch long.
- Auricle — None
- Roots — Fibrous and relatively shallow.
- Stems — Smooth, slender, and erect, protruding from a much-branched base.
- Panicle and seed — Panicles are 2 to 6 inches long, slender, and dropping to one side. Spikelets are numerous, five to eight flowers with slender straight awns, 1/2 to 3/4 inch long, each attached to a hairy, buff-brown, narrow seed about 1/2 inch long. The seeds are light and fluffy — about 200,000 per pound. Initial seed germination rates usually are high. A heavy infestation can produce as much as 400 lbs or 80 million seeds per acre. Some seeds will remain viable more than two years when placed near the soil surface because of natural dormancy or unfavorable conditions for germination.
Downy brome is best managed by integrating cultural and chemical control measures to eliminate seed sources, contain its spread, and kill existing weeds before they can compete with crops and pastures for water resources. Crop rotation is one of the most effective control measures; however, this is not a viable option in pastures or in some cropping systems in western Nebraska. The following sections describe two control options — eliminating seed sources and rotating crops — as well as strategies best suited to specific cropping systems.
Eliminating Seed Sources Outside the Fields
- Till and crop roadside ditches when possible or seed to a perennial grass.
- Seed perennial, cool-season grasses such as crested wheatgrass or smooth brome in waste areas and field borders. Vigorous stands of grasses or grass-legume combinations are highly competitive with downy brome and other annual weeds. See section on Reestablishing Desirable Grasses in Waste Areas for additional information.
- Sometimes mowing can be effective in reducing seed production, but it will not eliminate downy brome. Mowing must be timely and close to the ground. More than one mowing may be necessary to prevent tillers from producing seed. Mowing is useful for small infestations in pastures, roadsides, and waste areas where cultivation or herbicides are not feasible. Mowing also may reduce competition so that desirable perennial grass may reestablish.
- On cultivated fields, destroy weeds before they produce seed.
- Plant clean seed. Downy brome seeds often are found in small grain and grass seed.
- Use herbicides that do not kill established perennial grasses around field borders.
- Control small patches or area infestations before they spread.