Bromus secalinus
(rye brome)

Bromus secalinus is a serious grassy weed infesting winter rye (Secale cereale), winter wheat (Triticum aestivum) and other cereal crops. As its life cycle fully coincides with that of the cereal crop it infests, at harvesting it can become a major contaminant of cereal seed for sowing. Originating in Eurasia, it has been spread most likely through contaminated grain shipments to North America, Australia and elsewhere. It has proved invasive of disturbed land, dry prairies, limestone glades, meadows, vacant lots, abandoned fields, crop land, field margins, roadsides, railroad tracks and waste places, displacing native vegetation.

The extent of the native distribution of B. secalinus in Europe is confused, no doubt by the fact that its seed would have been widely distributed geographically in prehistoric times by early farmers and traders as a contaminant of cereal grain. As a consequence, some authorities cite it as a mainly European species growing all over western Europe (except the far south), as well as the Caucasus, Mediterranean region and Asia Minor, extending its eastward limit into parts of European Russia, especially the north-west of Central Russia but not as far north as the Arctic (Afonin et al., 2016). USDA-ARS (2016), on the other hand, gives its native range as eastern, central, southeastern and southwestern Europe, Algeria, Republic of Georgia and Turkey, while the Royal Botanic Gardens Kew (2016) limits the native distribution to southwestern, east-central and southern Europe, the Transcaucasus, the Middle East, Siberia and northern China, with northern Europe, Algeria and most of European Russia defined as the introduced Eurasian range.

According to Afonin et al. (2016), beyond Central Russia B. secalinus is adventive, from southern Siberia to Russia’s Far East. The Flora of China (Flora of China Editorial Committee, 2016) is unclear about its native status but includes it as found in Xinjiang, Gansu, Tibet, Taiwan and Japan.

B. secalinus is an introduced species across much of North America. Present in all contiguous states of the USA except North Dakota, as well as Alaska and Hawaii, and most provinces in western and eastern Canada (USDA-NRCS, 2016), B. secalinus is listed as invasive in Arkansas, Illinois, Tennessee, Georgia and Florida (Invasive.org, 2016). In Hawaii it has been recorded naturalized at Kipukapuaulu in the Hawaii Volcanoes National Park on the island of Hawaii (Wagner et al., 2005). In Canada, the species has been reported from the Yukon, British Columbia, Alberta, Ontario, Quebec, New Brunswick, Nova Scotia, Newfoundland and Prince Edward Island (Canadian Food Inspection Agency, 2015).

Having been accidentally introduced into Australia, B. secalinus is now naturalized in New South Wales, Victoria and Tasmania (Atlas of Living Australia, 2016). Populations of the species have also been observed in Mexico (eMonocot, 2016), Argentina and Chile (Royal Botanic Gardens Kew, 2016).

As a cereal seed contaminant difficult to identify and eliminate, the risk of introduction to other regions is high.

As an aggressive weed, rye brome invades open, disturbed areas below 1500 m elevation, dry prairies, limestone glades, disturbed grassy meadows, vacant lots, abandoned fields, crop land, field margins, roadsides, railroad tracks and waste places. It is a weed of winter wheat, rye and lucerne fields. This grass species is able to adapt to many soil types, from mesic to dry soils in full sun. It prefers heavy, clay, over-wet and cold soils (Baldwin et al., 2012; Darbyshire, 2003; Saarela and Peterson, 2016; Afonin et al., 2016; Invasive.org, 2016; Tennessee Exotic Pest Plant Council, 2016).

Rye brome is a noxious weed of cereal crops, especially wheat and rye but also barley, and of grass crops.

Genetics

B. secalinus is an allotetraploid with a chromosome number of 2n = 28 (Spalton, 2002; Saarela and Peterson, 2016)

Reproductive Biology

B. secalinus is an annual cross-pollinated grass reproducing exclusively by seed (Tennessee Exotic Pest Plant Council, 2016). According to Afonin et al. (2016), one plant can produce up to 5000-6000 caryopsides, although Nádasyné Ihárosi and Nagy (2010) give a figure of 800-1600 seeds per plant.

Physiology and Phenology

Anthesis in B. secalinus takes place from around May to July (Saarela and Peterson, 2016), with fruits produced in July-August. Where it is a weed of cereals, its life cycle coincides with that of the crop (Afonin et al., 2016). In Oklahoma in the south-central USA, rye brome germinates in the autumn (from late August to mid-November), increases its reproductive tillers as it grows vegetatively through the autumn and winter months, initiates its reproductive growth in mid-March, flowers and sets seed in May, and finally matures in early June (Armstrong and Peeper, undated).

Longevity

B. secalinus is an annual plant but has been reported to form a short-term persistent soil seed bank. Seeds are able to survive in soil for over 1 year but less than 5 years (Thompson et al., 1997).

Associations

Predominantly found as a very common and dominant weed of cereal crops (wheat, rye, barley), B. secalinus is also found in association with other grass species. For example, in Taiwan, it has been observed growing together with Dactylis glomerata, Festuca arundinacea, Miscanthus sinensis, Poa annua and Vulpia myuros (Jung et al., 2006).

Environmental Requirements

B. secalinus prefers full sun and mesic to dry conditions. Seeds are able germinate in both natural light and in darkness. Germination ability significantly depends on depth of burial in the soil, with the best germination achieved when diaspores are on the soil surface (Kapeluszny and Haliniarz, 2007).

In the USA, rye brome is common in the eastern portion of the hard red winter wheat and more humid soft red winter wheat regions where the soils are acid sandy or loamy (Peeper, 1984), while in northwestern Russia it has a preference for heavy, clay, over-wet and cold soils, being rare on loam and sandy loam soils (Afonin et al., 2016). However, the grass will adapt to practically any kind of soil, and in areas of exposed open ground it can reseed itself aggressively (Hilty, 2015). In northeastern Poland, where B. secalinus appears to be becoming more frequent in cereal crops, it was observed on all soil types and in all soil-agricultural complexes, but was most frequent in winter cereals on compacted soils with pH values from 5 to 7.5 (Rzymowska et al., 2010).

B. secalinus is a host of wheat High plains virus which has been reported in the USA since 1993 and can cause severe symptoms (stunting, chlorosis and mortality) in both maize and wheat (EPPO, 2016).

The following insect species have been recorded feeding on B. secalinus and other Bromus species:

• Stink bugs (Pentatomidae) Aelia americana, Cosmopepla lintneriana, Hymenarcys aequalis, Hymenarcys nervosa and Mormidea lugens;
• Leaf beetles (Chrysomelidae) Chalepus walshii and Oulema melanopus;
• Grasshoppers (Acrididae) Ageneotettix deorum, Arphia simplex, Eritettix simplex, Hippiscus ocelote, Melanoplus bivittatus, Melanoplus confusus, Melanoplus femurrubrum, Melanoplus keeleri luridus, Melanoplus packardii, Melanoplus sanguinipes, Orphulella speciosa, Pardalophora haldemanii, Phoetaliotes nebrascensis and Syrbula admirabilis.

Additionally, small rodents such as Microtus ochrogaster feed on seeds and foliage of B. secalinus, while immature foliage is eaten by white-tailed deer (Odocoileus virginianus), cattle and other mammalian herbivores. The seeds are also eaten by upland gamebirds and sparrows (Hilty, 2015).

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Wildlife habitat

Human food and beverage

• Emergency (famine) food

Ornamental

• Cut flower

B. secalinus is readily identified when it is in fruit. However before fruiting it is more difficult to distinguish this species from B. racemosus and B. commutatus (Bomble and Scholz, 1999; Spalton, 2002).

According to Parnell (1845), B. secalinus can be distinguished from other Bromus species in the apex of the large glume being situated half-way between the base of the glume and the summit of the second floret on the same side; and in the outer palea being rounded on the upper margin, with the breadth considerably greater than half its length.

In the UK, B. secalinus is often confused with B. hordeaceus and particularly B. commutatus. Diagnostic features of the three species are outlined by Moss (2015). In B. secalinus lower leaf sheaths can be hairless or hairy, but are always hairy in the other two species. Panicles are usually compact in B. hordeaceus but loose in the other two species. Spikelets are hairy or hairless in B. secalinus, but usually softly hairy in B. hordeaceus and hairless in B. commutatus. Mature spikelets of B. secalinus when viewed from the side have visible gaps, but usually no gaps are visible in the other two species. The most reliable diagnostic feature is the cross section of mature seeds, which is a deep V or U shape in B. secalinus but saucer-shaped in B. hordeaceus and B. commutatus.

Tennessee Exotic Pest Plant Council (2016) lists the following species as landscape alternatives that may be confused with B. secalinus: Andropogon ternarius, Calamagrostis canadensis and Chasmanthium latifolium.

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.

Prevention and Control

Control techniques for B. secalinus are limited, with the main focus being on its control in cereal crops, especially wheat. There is little focus on its removal from natural areas. Effective control measures for the weed in wheat are described in detail by Armstrong and Peeper (undated).

Prevention

The control of rye brome plants growing along borders, fence rows and roadsides before they produce seed can prevent spread into cereal fields. All farm equipment to be used in wheat fields should be cleaned thoroughly to prevent transfer of seed and seedlings (Armstrong and Peeper, undated). Cereal seed needs to be properly cleaned to eliminate B. secalinus and other weed seeds. Hauhouot et al. (1998) showed that rye brome seeds could be devitalized after secondary cleaning using mechanical devices such as hammer and roller mills mounted on the combine harvester. These dehulled the seeds and fractured the seed embryos, thus significantly reducing weed seed germination.

Where B. secalinus has invaded natural areas, cutting and mowing of populations before flowering will reduce the seed bank (Tennessee Exotic Pest Plant Council, 2016).

Cultural Control and Sanitary Measures

Cultural practices that encourage a vigorous winter wheat crop will help it to gain a competitive advantage over rye brome. Tillage can control weed seedlings if done before the wheat crop is sown or established. The wheat seed sown should be brome-free. Increased sowing rates and narrower row spacing help to increase the competitive ability of the wheat. Delaying wheat sowing till after the brome germinates allows seedlings to be destroyed with tillage or herbicides. Deep ploughing can effectively bury brome seed (Armstrong and Peeper, undated).

Crop rotation accompanied by herbicide application has shown increases in total net production of wheat. Stone et al. (2006) found that rotating out of winter wheat for one growing season increased grain yield in the succeeding wheat crop by up to 42%, while dockage due to B. secalinus was reduced by up to 87%.

Although wheat harvesting equipment can be adjusted to discharge a larger proportion of the rye brome seeds back into the field rather than into the grain bin, the concentration of weed seeds left behind can increase the difficulty of controlling infestations (Stone et al., 2001).

Movement Control

B. secalinus is recognized as a noxious weed seed under the US Federal Seed Act. This designation does not allow seed lots with over 300 rye brome seeds per pound of certified seed to be imported into the following states: Alabama, Arkansas, Delaware, Florida, Georgia, Kansas, Louisiana, Mississippi, New Jersey, Oklahoma, South Carolina, Tennessee and Texas. India and Taiwan totally prohibit the importation of B. secalinus (Ontario Ministry of Agriculture, Food and Rural Affairs, 2016).

Chemical Control

As a result of recent advancements in herbicide chemistry, effective herbicides are available for rye brome control in wheat. Several herbicides selectively kill rye brome when applied post-emergence, with minimal if any injury to the wheat. These include propoxycarbazone-sodium, propoxycarbazone-sodium + mesosulfuron-methyl, pyroxsulam and chlorsulfuron + metsulfuron-methyl. Sulfosulfuron can be applied both pre- and post-emergence. Imidazolinone herbicides can be used for rye brome control only with imidazolinone-resistant wheat (Armstrong and Peeper, undated). Using herbicides with different modes of action is important in control regimes to prevent the development of herbicide resistance in weed species. Resistance of B. secalinus biotypes in wheat crops to several Group 2 herbicides (imazamox, propoxycarbazone-sodium, pyroxsulam and sulfosulfuron) has already been reported in Kansas and Oklahoma (Heap, 2016).

As virtually all of these herbicides can control other grassy weed species, their use would be inappropriate in conditions where B. secalinus is growing with mixed native grasses.

Synowiec and Kalemba (2015) reported that essential oil distilled from Heracleum sosnowskyi seeds has potential to be used as a natural herbicide against weeds in maize; even a relatively small dose of 0.2 g/litre caused a significant reduction in germination and seedling growth of B. secalinus.

Most of the literature regarding B. secalinus concentrates on the species as a weed of cereal crops. There appears to be little research on it as an invasive impacting on natural plant communities and ecosystems, such as the North American prairies, or on its control in these situations.

11/04/2016   Original text by:

Lukasz Tymo, CABI, UK