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.

Annual
Grass / sedge
Herbaceous
Seed propagated

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.

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.

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.

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.

Reproductive Biology

The 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 Phenology

B. 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 Requirements

B. 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.

Associations

On 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).

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.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control

On rangelands, the establishment of perennial grasses effectively suppresses B. tectorum so that it remains persistent, but only as a minor component of vegetation communities. The problem with this approach is in the establishment of perennial grass seedlings in the face of competition from B. tectorum, and which was found to be almost impossible in western North America. During the 1940s it was determined that the exotic perennial crested wheatgrass (Agropyron desertorum) could be established on sagebrush rangelands (Young and McKenzie, 1982) and although weed control is still required for its establishment, seedlings are much more competitive compared to those of the native perennial grasses. In areas of winter wheat production, if cultural, environmental, and economic restraints permit, periodic switching to spring-planted cereal grains permits suppression by herbicide application or tillage before planting of the grain crop. In addition, immobilization of nitrogen followed by planting seeds of a non-leguminous native shrub, antelope bitterbrush (Purshia tridentata) that symbiotically fixes nitrogen, has been highly successful on B. tectorum infested rangelands (Young and Clements, 2002).

Mechanical Control

On rangelands and much of the winter wheat production areas in the USA, deep mouldboard ploughing to bury the B. tectorum seedbank is not physically or economically feasible. Post-emergence mechanical tillage as a seedbed preparation is feasible, but on arid rangelands this forces a delay in planting until sufficient B. tectorum has emerged for the tillage to have sufficient efficacy before the onset of the summer drought.

Chemical Control

On rangelands in the western USA, there are two basic approaches for the use of herbicides for control of B. tectorum as a means of establishing seedlings of perennial grasses. The first system is to wait until a significant portion of the germinable seeds of B. tectorum have emerged in the early spring and then apply a contact herbicide such as paraquat (Evans et al., 1967). Seeding of perennial grass could be carried out simultaneously with the sprayer mounted on the front of the drill. This practice never became popular because of paraquat toxicity to humans and the failure to apply it aerially, and glyphosate was not a good substitute for paraquat probably because of the delay in B. tectorum mortality during the cold spring period after application of this herbicide. The second system involves the creation of a fallow with herbicides, with atrazine applied in October or November and B. tectorum controlled during the following growing season with the site being seeded to perennial grasses the following October, one year after the application of the herbicide (Eckert and Evans, 1967). If the summer rains are good in the fallow year, lower successional levels of exotic invasive broadleaved weeds will flourish on the fallow and certainly during the seedling year of the perennial grasses.

In winter cereal production, B. tectorum germinates very close to the same time as the winter wheat or barley (Ogg, 1994). Selective control of B. tectorum during the first 3 weeks following emergence is critical in reducing competition. In conventionally tilled wheat, diclofop has been the most effective soil-applied herbicide for control of B. tectorum, killing B. tectorum as it germinates and reducing B. tectorum populations by up to 95% allowing wheat yields to increase by 30-40% (Stahlman, 1984).

Metribuzin is the only herbicide available that when applied post-emergence will control B. tectorum selectively in winter wheat (Swan and Whitesides, 1988), but to be effective it must be applied before the weed begins to tiller extensively. The margin of crop safety with metribuzin is very narrow and is based on the ability of wheat to metabolize metribuzin rapidly and its ability to root deeply early in its life cycle and thus avoid uptake of the herbicide (Devlin et al., 1987). The use of metribuzin in wheat is restricted to fine textured soils with more than 1% organic matter (Ogg, 1994). A special formulation of atrazine was used in the Pacific Northwest, USA, to reduce B. tectorum in winter wheat, and applied before the wheat was seeded with the seeding done with a deep furrow drill. The openers on the drill moved the herbicide on the soil surface to the side and B. tectorum was controlled between, but not within the rows (Ogg, 1994).

Much of the winter wheat production in the Pacific Northwest of the USA takes place on fine textured, aeolian soils, often on steep slopes and erosion is a serious problem. Conservation tillage practices that leave litter to protect the soil surface may have promoted an increase in B. tectorum, considering the seedbed ecology of the species (Evans and Young, 1970). Winter wheat production without tillage (no-till) has significant popularity in the Pacific Northwest. In no-till winter wheat a granular formulation of triallate plus trifluralin applied to the soil surface before planting has controlled B. tectorum (Ogg, 1994). The granulars are not absorbed by litter on the soil surface so they are more effective than liquid applications and a deep furrow drill is necessary to move the herbicide away from the wheat seeds.

Biological Control

Classical biological control of grasses such as B. tectorum poses problems regarding specificity, noting that many crops are also grasses. There has been considerable interest in using naturally occurring soil micro-organisms for weed control (Grey et al., 1995), with Kennedy (1994) having screened thousands of isolates of soil bacteria for the inhibition of B. tectorum germination or seedling growth. Several isolates were found that slow the root growth of B. tectorum giving wheat a competitive advantage (Kennedy et al., 1989, 1991), but the practical problem with application of mycoherbicides is in obtaining infection in the field. B. tectorum can be infested with smut (Ustilago spp.), and Meyer et al. (2001) proposed the use of U. bullata as a biological control agent for B. tectorum.

Integrated Control

In the Pacific Northwest, USA, some winter wheat farmers appear to tolerate the presence of B. tectorum whereas others are destroyed by the weed. Those that successfully suppress B. tectorum maintain an active programme that identifies and maps infestations. Rotation to spring-planted cereal crops on infested fields is used to reduce seedbanks. Contact herbicides are used to control spot infestations and herbicidal control practices used correctly in winter wheat stands help to reduce competition from B. tectorum. On rangelands, ranchers can graze B. tectorum infestations or watch them eventually burn in uncontrollable wildfires. Grazing management that involves rest from grazing or deferment of grazing until after seed ripening of B. tectorum have proven to be disasters. Weed control to allow establishment of perennial grasses followed by quality grazing management relegates B. tectorum to a minor species in most rangeland communities.