Bromus tectorum (downy brome)

B. tectorum has tremendous phenotypic plasticity, with self fertilization permitting continuous replication of successful genotypes, and occasional out-crossing inducing genotypic variability and adaptation to new environments. Seeds have the advantage of simultaneous germination and through dormancy acquired in the seedbed, continuous germination and the development of large, persistent seedbanks. B. tectorum has an excellent long-distance seed dispersal mechanism through contamination of seed lots of crop species and through wool, hair, clothing, and vehicle contamination. B. tectorum is a highly invasive exotic weed that truncates succession to remain a dominant species for prolonged periods of time. B. tectorum is one of the few invasive annual exotic species that is a major weed of rangelands and agronomic fields in North America.

In Russia, Anisantha tectorum (L.) Nevski is used by some authors as the taxon for this species, but Bromus tectorum L. is universally used in publications in English (Kostivkovsky and Young, 2000). If near universal agreement is found on the scientific name, chaos prevails on common names. The Weed Science Society of America has assigned the common name of downy brome for B. tectorum. In the western USA it is widely known as cheatgrass. Other common names in the USA include bronco grass, six weeks grass and military grass. In western Canada it is known as nodding brome (Taylor and MacBryde, 1977). The Latin root for the specific name tectorum refers to the roof in apparent reference to B. tectorum growing on thatched roofs (Kostivkovsky and Young, 2000).

B. tectorum plants have a soft hispid pubescence throughout the leaves and stems. The leaf blades are flat and rather narrow, 2-3.5 mm broad. The stature of the plant and the number of tillers is highly variable. Plant height ranges from 2.5 to 50 cm. In dense stands, 8500 to 11,000 plants per m², B. tectorum may produce a single stem and a single diminutive panicle. In very sparse stands (10 per m²), as would occur the season after a wildfire, B. tectorum plants may reach 0.5 m in height and have a multitude of robust panicles of florets (Young et al., 1987). The panicle ranges from 6 to 20 cm in length and usually, but not always, nodding (Wilken and Painter, 2003). The spikelets are sub-cylindrical to slightly compressed. Glumes generally glabrous, but for all the inflorescence components, the presence and amount is highly variable. The upper glume is 7-12 mm long with three veins. The lower glume is 5-6 mm long with one vein. The florets are 3-7 mm long, the lemma body 9-13 mm long with 5-7 veins and the tip has two teeth 2-3 mm long. The caryopsis is about 2 cm long with a recurved, sharp-tipped awn. The colour of the caryopsis ranges from light straw to reddish brown, and is covered with silicon barbs that aid in attachment to wool, hair and clothing. The awn moves with wetting and drying forcing the sharp point of the caryopses deeper into wool or hair and even into the flesh, eyes, ears or toes of animals. B. tectorum rosettes often have bright red leaves during mid-winter, which apparently is due to nitrogen stress.

B. tectorum is widely distributed in central Asia (Kostivkovsky and Young, 2000), with the western edge of its native range generally given as the Balkans, Europe. The present distribution of B. tectorum in Europe extends to south-western Spain, but west of the Balkans, B. tectorum is considered to be an adventive species. B. tectorum is common in the Middle East and occurs across North Africa in areas with a Mediterranean-type climate but may be considered adventive west of Egypt (Meusel et al., 1965). To the east, the native range extends into China, probably restricted to Xinjiang province. Presence in India (USDA-ARS, 2003) is probably restricted to the north. This species has been so closely associated with winter cereal grain production and range livestock grazing for such an extended period of time, it is difficult to separate native habitat from where it has been introduced in pre-history.

Based on herbarium specimens in the USA, B. tectorum was first collected in Pennsylvania in 1861, Washington in 1893, Utah 1894, Colorado in 1895, and Wyoming in 1900 (Yensen, 1981). In North America in 1950, B. tectorum occurred in Alberta, Canada, Mexico, and all the mainland USA except for the far south-eastern portion of the country (Hitchcock, 1950). Today, it is found in all states including Alaska and all of the Canadian Prairie Provinces (Cronquist et al., 1977). It has also spread into northern and western Europe and North Africa. B. tectorum was first noted as an exotic species in New Zealand in 1870 (Owen, 1996), and it is also present in parts of Australia and South Africa though dates of introduction are not known. USDA-ARS (2003) notes presence in South America but without specific locations and no other reports could be located. The history of introduction in North America is a comprehensive one. The completion of the trans-continental railroad in 1868 and the subsequent development of regional railroad networks greatly enhanced the rate of spread of exotic weed species in western North America (Young and Longland, 1996). The widespread adoption of steam powered grain threshing equipment which was moved from farm to farm was disastrous for spreading weeds. They were not cleaned between farms and the farmers saved their own grain, often contaminated with their neighbours weeds, for seed grain (Morrow and Stahlman, 1984). This process has been documented for the spread of the exotic annual Russian thistle (Salsola targus) (Young, 1988). The search for winter hardy strains of alfalfa in the late 1800s led to widespread importation of seed from central Asia under the general name of 'Turkestan' seed. These unregulated, uninspected importations were probably significant contributors to the exotic weed flora of western North America.Yensen (1981) recreated the historical developmental of B. tectorum for southern Idaho from it being first noted as a weed in cereal grain and alfalfa fields in the early 1900s. Rural roads were little more than dirt tracks throughout the big sagebrush steppe and B. tectorum spread along these roads as a ruderal species. Periodic grading of the road surface with soil brought up from boarding burrow pits assured sufficient disturbance to provide habitats for exotic invasive weeds. After a considerable lag period as a strict ruderal species, B. tectorum was suddenly noticed to be invading degraded stands of big sagebrush where the native perennial grasses had been killed by excessive, improperly timed, and continuous grazing. When the native perennial grasses had been killed, the density of big sagebrush, which is not preferred as a browse source by domestic livestock, increased, effectively closing the sites to establishment of perennial grass seedlings even if grazing was excluded (Robertson and Pearse, 1947). B. tectorum invaded these brush stands that had virtually no herbaceous understorey and provided the fine-textured fuel with sufficient continuity of cover to ignite and allow wildfires to spread from shrub to shrub. B. tectorum populations exploded in the burned areas and effectively truncated plant succession to continued dominance by the exotic annual weeds. The agricultural economic depression that occurred after World War I brought the abandonment of many sub-marginal farms in the intermountain area of the USA. These abandoned farms were rapidly colonized by invasive exotic annual weeds which led to B. tectorum dominance. The only break in this process during the first half of the 1900s in the western USA was that the ranges were so excessively grazed that herbivory by domestic animals served to biologically suppress annual grasses (Emmerich et al., 1993).B. tectorum on rangelands was largely confined to the big sagebrush zone from its introduction until the 1980s. This zone is characterized by annual precipitation of 200-350 mm and loam-textured surface soils that are not affected by accumulations of soluble salts. During the 1980s, B. tectorum suddenly spread to the salt deserts of the intermountain area (Young and Tipton, 1990). As the name implies, these areas are characterized by salt-affected soils and annual precipitation of 100-150 mm. The spread of B. tectorum into these areas made possible wildfires as a stand renewal process for the first time. At about the same time, B. tectorum also spread into higher elevation, higher precipitation coniferous woodlands. Within the sagebrush zone, B. tectorum became a much more dominant species as grazing management systems were implemented that included rotational deferment until after seed ripening or a complete year-long rest from grazing. This abundance of B. tectorum fuel led to the occurrence of regional firestorms that burned huge areas of rangelands in a very short period of time.

Planting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. In most of the USA, B. tectorum is not a regulated noxious weed meaning that it can occur in seed lots as long as it does not exceed the 'other weed species' limit, applying even to certified seed lots. As such, it is likely to spread to other areas where it is not yet present as a seed contaminant. For example, the revegetation of the right-of-way for a natural gas pipe line in Alberta, Canada by seeding certified crested wheatgrass seed grown in Idaho, USA, resulted in a strip of B. tectorum across the south-eastern Alberta prairie. The transactions involved in purchasing the seed were all perfectly legal because B. tectorum fell under the 'other weed species' category in the USA. Even if you purchase certified seed, it is worthwhile to have the seed sampled to identify the non-legally noxious weed seeds that may contaminate the seed lot.

Natural Dispersal (Non-Biotic)B. tectorum seeds are too heavy for wind to be a major factor in dispersal. Vector Transmission (Biotic)On rangelands, rodents collect and scatter hoarded seeds of B. tectorum (LaTourrette et al., 1971). Rodents recover seeds from some of these caches for consumption, while others germinate and produce viable seeds. The barbs on the lemma, palea, and awns of B. tectorum caryopses are very effective in aiding seed dispersal. The seeds stick in animal fur and also human clothing. Agricultural PracticesPlanting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. In most of the USA, B. tectorum is not a regulated noxious weed meaning that it can occur in seed lots as long as it does not exceed the 'other weed species' limit, applying even to certified seed lots. Accidental IntroductionPlanting contaminated seed and feeding contaminated hay or grain to livestock are common means of dispersal of B. tectorum. The use of B. tectorum-infested cereal straw in erosion control during construction projects is a common means of dispersal for this species. Also, accidental dispersal by farmers or walkers is another means, because if you walk through a B. tectorum stand at seed maturity, you rapidly find your socks full of seeds unless you are wearing tall boots.Intentional IntroductionThere are folk stories that B. tectorum was deliberately spread by stockmen in the Intermountain Area, USA, after the native perennial grasses were severely depleted by excessive grazing although these stories have never been documented or verified. The invasion rate of B. tectorum is sufficiently fast enough that intentional enhancement by humans was probably not necessary.

It is a very serious agronomic weed in winter wheat (Triticum aestivum) fields of the Pacific Northwest and the central and southern Great Plains of the USA (Young et al., 1984). Although the crop and season of planting are the same, the Pacific Northwest and the Great Plains of the USA have grossly different climates. The weed causes tremendous yield losses in winter wheat and barley (Hordeum vulgare) production and is a spring weed problem in alfalfa (Medicago sativa) hay production. It is a very troublesome species in bluegrass (Poa pratensis) seed production fields. B. tectorum is also a serious problem on rangelands, and often encountered as a weed of orchards and vineyards.

There are a host of annual Bromus spp. with more or less similar ecological requirements that can be associated with B. tectorum in specific environments. In the southern intermountain area of the USA, red brome (Bromus madritensis ssp. rubens) replaces B. tectorum and introgressive hybrids may exist. Occasionally, Japanese brome (B. japonicus), ripgut (B. diandrus), or rattlesnake brome (B. briziformis) may occur in the same communities as B. tectorum, but once in flower it is easy to distinguish the different species of Bromus spp. The only native annual grass that can be confused with B. tectorum in the seedling stage is six weeks fescue (Vulpia octoflora). The juvenile foliage of six weeks fescue is much finer in texture and the plants more diminutive than B. tectorum. Annual wheatgrass (Eremopyrun triticeum) is becoming increasingly abundant in B. tectorum stands in the intermountain area. At maturity the compact spike of this grass is so different from the nodding panicle of B. tectorum there is no problem in identification. Vegetatively, annual wheatgrass has so few leaves it does not resemble B. tectorum. Over vast areas of rangelands, B. tectorum has been replaced by medusahead (Taeniatherum caput-medusae). The seed, foliage, seed heads, and phenology of medusahead make it easy to distinguish from B. tectorum, and matures 2-4 weeks later than B. tectorum making infestations easy to distinguish from a considerable distance. In winter wheat fields in the Pacific Northwest, USA, jointed goatgrass (Aegilops cylindrica) has become a serious winter annual weed in addition to B. tectorum.

It is an opportunistic species with few absolutely restrictive requirements. B. tectorum is such a widespread species you should not be surprised where one may encounter the species, even in your socks (WSSA, 2003). B. tectorum is an exotic invasive annual grass in North America (Klemmedson and Smith, 1964). In the semi-arid to arid environments of western North America it is found in environments similar to central Asia where it originally evolved. B. tectorum is often described as a classic ruderal species found along roads, fence lines, and irrigation structures. B. tectorum currently dominates millions of hectares of degraded rangelands in the intermountain area between the Sierra-Cascade and Rocky Mountains in western North America. The bulk of this area was formerly big sagebrush (Artemisia tridentata)/native perennial bunch (caespitose) grass plant communities (Hull and Pechanec, 1947). B. tectorum dominance ranges from portions of the salt desert shrub steppe through the big sagebrush/bunchgrass zone to coniferous woodlands and even to true native perennial grasslands. Generally, the habitat of B. tectorum is described as disturbed plant communities, Daubenmire (1940) conclusively demonstrated that this exotic annual grass could invade high ecological condition bluebunch wheatgrass (Pseudoroegneria spicatum) communities that had always been protected by grazing by large herbivores.

Reproductive BiologyThe breeding system of B. tectorum is apparently an example of that theorized by Allard (1965) for largely self-pollinated species of annual grasses that are environmentally conditioned to occasional out-crossing. Under this breeding system, if an individual B. tectorum plant is introduced to a site where genotypically it is a good fit to the environmental potential of the site, it populates the site with offspring with stable duplicates of the desirable genotype through self fertilization. A healthy amplitude for phenotypic plasticity helps this one-fits-all genotype to be very successful. Occasionally, environmental conditions would be adequate to allow these self-pollinated species to cross-pollinate and produce hybrid offspring (Allard, 1965). In applying this concept to B. tectorum, Young and Evans (1976) took an additional step and suggested that for the first generation after hybridization, the offspring would express heterosis because essentially two inbred lines selected for survival in the environment of residency were being crossed. The population density of B. tectorum on a range site that has not burned for several years is 5000-10,000 plants per m² (Young et al., 1969), but which may shrink to 10 plants per m² following a wildfire. The reduction in herbaceous plant density coupled with the loss of perennial shrubs after fire, combine to enable each B. tectorum plant to have a much greater potential to spread and the resulting B. tectorum plants are huge in comparison to those that existed in dense stands before the wildfire. The post-fire B. tectorum plants produce multiple tillers that extend flowering over a prolonged period and the decreased plant density allows a greater amount of soil moisture per B. tectorum plant. Improved moisture relations, which normally limits B. tectorum growth, increases the chances that the floret will be sufficiently open to allow the anthers to be exerted.Physiology and PhenologyB. tectorum can either be a true annual with germination in early spring and reaching maturity in the early summer of the same year, or a winter annual with germination in the autumn, over-wintering as a flat rosette of leaves on the soil surface and sending up flowering tillers the next spring (Harris, 1967). In the Pacific Northwest, USA, autumn germination occurs every year and B. tectorum is a true winter annual which is why it is so competitive with winter wheat. In the Great Basin, USA, B. tectorum germinates in the autumn about once every 5 years. With either autumn or early spring germination, B. tectorum tillers begin rapid elongation from mid-April to mid-May. Most accessions of B. tectorum plants require vernalization before they will flower (Hulbert, 1955; Finnerty and Klingman, 1962), achieved by germinating seeds at 5°C. B. tectorum plants in the field will always flower if soil moisture is available; and most accessions collected from salt desert environments will flower in the greenhouse without vernalization. Phenology of B. tectorum plants is extremely variable depending on field environmental conditions. Flowering can initiate as early as late April or as late as early July on the same site in different years.B. tectorum seeds are initially not dormant at maturity, or may have short-term after-ripening requirements (Young et al., 1969; Milby and Johnson, 1987). Only a small portion of the annual seed production is required to provide plants that completely occupy a given area the next season. B. tectorum seeds that do not find safe sites for germination acquire a dormancy that permits the building of seedbanks and it is the development of these seedbanks that makes the control of B. tectorum such a prolonged and difficult problem. The acquired seed dormancy breaks down gradually over 3-5 years. Germination of seeds with the acquired dormancy can be enhanced by enrichment of the germination substrate with nitrate or gibberellin. B. tectorum seeds have specific requirements for safe sites for germination. The seeds have very low germination rates on the surface of seedbeds in semi-arid or arid environments. Litter coverage or micro-topography in the seedbed surface is required for successful germination under field conditions (Evans and Young, 1970, 1972). B. tectorum seeds can germinate at very cold seedbed temperatures (Evans et al., 1970) and germination will occur at a constant 0°C or alternating temperatures to as low as 0°C (Young and Evans, 1982).Very small amounts of nitrogen have large influences on the dynamics of B. tectorum populations. Fertilization with ammonium sulphate over a stand of established perennial grasses, with B. tectorum as a component of the community, can result in the death of the perennials as B. tectorum out-competes the perennials for soil moisture (Kay and Evans, 1965). Fertilization of B. tectorum stands without perennial grasses can result in extreme increases in herbage production (Kay, 1966). Immobilization of nitrogen with a carbon source or the inhibition of nitrification severely decreases the establishment and growth of B. tectorum populations (Young et al., 1998a). This aspect of B. tectorum physiology has not been developed into a commercially successful control measure, but the reciprocal (fertilization with nitrogen) certainly enters into the management of this species. In the reclamation of mining spoils in the USA, it appears to be desirable to add nitrogen fertilizers to raw rock dump spoils but such fertilization will always create a B. tectorum problem (Young et al., 1998b). Environmental RequirementsB. tectorum has successfully invaded so many contrasting environments in North America it is obviously a generalist in terms of environmental requirements. This is accomplished through great phenotypic plasticity and secondly through the apparent potential to rapidly evolve new genotypes. Generally, B. tectorum thrives as a winter annual, but germination in the autumn is not essential for the annual grass to persist. It is not a dominant species in truly warm deserts, even those with some winter precipitation. B. tectorum occurs, but is not a dominant annual grass in the mild Mediterranean climates of cis-montane California and southwestern Oregon, USA. B. tectorum has invaded the salt deserts of the Intermountain Area of western North America, but the true level of tolerance to salt-affected soil is not known with precision. Precipitation, both the amount received and the periodicity of moisture events are critical factors in the population density, herbage and seed production. On rangelands, B. tectorum populations can disappear over vast areas for as long as 3 years during droughts, but they will return when the drought ends.AssociationsOn rangelands, B. tectorum is closely associated with the seral continuum it often culminates. This continuum is largely composted of exotic annual herbaceous broadleaved and grass species. B. tectorum can truncate succession in this continuum for extended periods of time (at least 75 years). However, the B. tectorum-dominated sites are open to the invasion by other exotic annuals, biannual, or perennial weed species. B. tectorum can be grazed to the point it is replaced by lower seral stages dominated by broadleaf herbaceous exotic weeds, but if the grazing pressure is relaxed, B. tectorum returns as the dominant species (Young et al., 1969).

B. tectorum plants become infested with smut (Ustilago spp.) in certain years, but there is no evidence that natural infections have any lasting influence of the persistence or dominance of B. tectorum populations (Stewart and Hull, 1949). The seed pathogen Podosporiella verticillate is probably very significant in reducing viable B. tectorum seeds in seedbanks (Kreitlow and Bleak, 1964; Young et al., 1969).

It is estimated that as few as 100 B. tectorum plants/m² reduced winter wheat production by 27-36% (Young et al., 1984) and B. tectorum is a major problem in winter cereal production in North America. B. tectorum infests an estimated 5.7 million ha of cropland in the western USA and control measures cost US$350 million dollars annually (Mitch and Kyser, 1987; Ogg, 1994). Wildfires fuelled by B. tectorum are also a significant economic cost. For example, in Nevada, USA during a 10-day period in 1999, 0.64 million ha of rangeland burned in wildfires largely fuelled by B. tectorum. It cost US$38 million to suppress these fires and US$42 million in emergency restoration efforts on the burned area (Young and Sparks, 2002). Similar wildfire events are repeated in various portions of the Intermountain Area, USA annually.

B. tectorum increases the chances of ignition, rate of spread and expanding the season of wildfires, reducing the interval between re-occurring fires. The effect is to eliminate native woody species and truncate succession among herbaceous species to leave B. tectorum as the dominant species. At the same time, B. tectorum-dominated communities remain open to invasion by other exotic, invasive weeds. Accumulations of B. tectorum herbage mature in late spring and increase the chance of ignition and the rate of spread of wildfires. The natural wildfire season started in mid August when the herbage of native perennial grasses matured. The early maturity of B. tectorum extends the wildfire season through the warmest periods of the summer. B. tectorum provides the continuity fuel that allows the spread of wildfires from shrub to shrub. Such wildfires are a threat to human life and property, destroy the native sagebrush plants that do not re-sprout after the aerial portion of the plant is burned, destroy valuable forage resources, and the resulting burns are subject to accelerated wind and water erosion.

B. tectorum probably has the highest name recognition among exotic, invasive weeds in the Intermountain Area, USA and even suburban and urban residents fear wildfires fuelled by B. tectorum. The seeds often cause injury to the ears, eyes and mouths of pets such as dogs and horses as well as to humans. Livestock such as cattle and sheep can also be injured.

B. tectorum is the most abundant forage species on many intermountain area rangelands of the USA (Murray and Klemmedson, 1968) and this was true by at least the mid 1900s (Fleming et al., 1942). There are many disadvantages to basing a range livestock industry on B. tectorum versus perennial grasses, but grazing B. tectorum is a reality on millions of hectares of rangelands in North America.