Downy brome (Bromus tectorum L. # BROTE) has developed into a severe weed in several agricultural production systems throughout North America, particularly on rangeland and in winter wheat (Triticum aestivum L.). Several million hectares of winter wheat, pastureland, alfalfa (Medicago sativa L.), grass seed fields, and overgrazed rangeland, as well as other crops, have been invaded by this annual grass since its introduction into this hemisphere. Downy brome is most abundant in the Great Basin and Columbia Basin areas of the western United States, but is found throughout the continental United States and parts of Canada and Mexico. In some cases, the vegetation on overgrazed rangeland consists totally of downy brome, while winter wheat growers in the western United States proclaim it as their worst weed problem. Changes in tillage practices that are currently being implemented for the control of soil erosion coupled with the lack of selective herbicides for the control of downy brome have aided its increase and spread.

Downy brome (Bromus tectorum L. # BROTE), also known as cheatgrass, downy chess, broncograss, Mormon oats, and junegrass, was introduced into the United States from Europe, apparently during the middle of the nineteenth century (11, 21). According to Mack (23), downy brome entered British Columbia, Washington, and Utah around 1890; and by 1928 it had reached its present range, occupying much of the perennial grassland in Washington, Idaho, Oregon, Nevada, Utah, and British Columbia. Today, downy brome is a widespread weed throughout most of Canada, Mexico, and the United States, except for the southeastern United States (5, 17). Some consider downy brome to be an important forage because it provides most of the early spring grazing for livestock in western United States rangeland (21). However, it is also considered a troublesome weed in rangeland (31), winter wheat (Triticum aestivum L.) (27), several other crops (29), and noncropland (32).

The sagebrush (Artemisia)-grass ecosystem is an extensive and important rangeland type in the western United States. Estimates of total area of this ecosystem vary from 38 million ha (21, 22) to 109 million ha (3). Of the sagebrush-grass rangelands, 88% are degraded to the point that they are producing 50% or less of their forage potential (22). Low forage production on these rangelands has been caused by overgrazing and other past land abuses (24) resulting in severe depletion of native perennial grasses, dominance of brush, and, on millions of hectares, downy brome (Bromus tectorum L. # BROTE) dominance of the understory.

As an agrestal weed, downy brome (Bromus tectorum L. # BROTE) is most troublesome in winter wheat (Triticum aestivum L. # TRIAE) and alfalfa (Medicago sativa L. # MEDSA) because of the ability of downy brome to reproduce prior to crop harvesting (70, 76). Geographically, downy brome is most common in the western half of the United States where annual rainfall totals 15 to 55 cm and autumn rainfall ranges from 5 to 12 cm (10, 11, 86, 98). In contrast, cheat (Bromus secalinus L. # BROSE) is more common in the eastern portion of the hard red winter wheat producing regions and in the more humid soft red winter wheat region. Behrendt and Hanf (12) state that cheat has been more common on acid sandy or loamy soils while downy brome commonly occurs on dry, mostly alkaline, sandy to loamy soils. Since soils are frequently alkaline in the relatively dry climate of the western regions (61), it is not clear exactly what role pH plays in affecting geographical distribution of the various Bromus spp. Recent serious problems with downy brome in the New England nursery industry (69) and orchards (57) indicate that downy brome could thrive under a relatively wide range of edaphic and climatic conditions. However, as a weed problem in wheat, the common prevalent conditions of low annual precipitation and alkaline soils should not go unnoticed in the development of chemical weed control practices.

Downy brome (Bromus tectorum L. # BROTE) may be a troublesome weed in winter small grains, perennial legumes, perennial grasses grown for seed, and orchards. In Nebraska, winter wheat (Triticum aestivum L.) yields have been depressed 30% by downy brome populations of 11 to 22 plants/m2 (8). In Oregon, downy brome densities of 108 to 160 and 538 plants/m2 reduced yields by 40 and 92%, respectively (22, 23). In Idaho, wheat yields were depressed 20 to 40% with 55 to 110 plants/m2 (18). Downy brome has also been shown to lower yields and quality of hay from infested fields of alfalfa (Medicago sativa L.) (29). In perennial grass seed fields, downy brome may cause a serious seed quality problem (16). Some grass seed lots have contained as much as 50 to 75% downy brome seed.