Diagnostic Characteristics
The ascending or stiff branches of the open panicle are a key characteristic distinguishing Bromus inermis and Bromus erectus from a group of similar native bromegrasses. The awnless lemmas, from which the species derives its Latin name, distinguish Bromus inermis from both the introduced and similar Bromus erectus and from the native Bromus pumpellianus.
Habitat
In its native range smooth brome grows on roadsides, riverbanks, edges of fields and woods and pastures. Bromus inermis was first recognized as a potential forage grass in Europe because of its drought resistance (Kennedy 1899).
Within the United States a variety of agricultural strains have been developed from two natural strains, a "northern" and a "southern" strain. The southern strain is more tolerant of drought and heat than the northern strain (Newell and Keim 1943).
Greenhouse and field tests in Wisconsin suggest that smooth brome has better root growth in sandy than in silty soils (Lamba et al. 1949). Artificial aeration of silt loam produced better root growth than in unaerated soils (Lamba et al. 1949). Smooth brome has a high mortality rate on organic soils (Myhr et al. 1966). The possible relationship between poor soil aeration and mortality has not been investigated. Tolerance of smooth brome to spring flooding is estimated at 24 days (Seamands 1979). Salt tolerance is considered "moderate" (Seamands 1979). The sun-loving habit of smooth brome is substantiated by research showing that seed production, numbers of shoots and rhizomes, and dry weight of all plant parts are reduced by shade (Watkins 1946, Dibbern 1947).
Reproduction
Smooth brome is a cool season grass, beginning its growth early in the spring and growing late into the fall. New shoots emerge in the Chicago region as early as mid-March, when night temperatures are below freezing (Lamp 1952). Flowering primordia first become observable in early April in the Chicago region (Lamp 1952, Gall 1947) reaching 1 to 1.5 cm in length by early May near Ames, Iowa (Knobloch 1944).
Stem elongation varies from early May (Reynolds and Smith 1962) to late April in Wisconsin and Illinois (Lamp 1952). Boot stage is reached in mid-to-late May in Illinois and Wisconsin (Lamp 1952, Reynolds and Smith 1962, Knieval et al. 1971, Okajima and Smith 1964). In Illinois, plants are fully headed and blooming occurs during the first two weeks of June (Lamp 1952) whereas in Alberta, full head occurs in mid-June and pollination in July (Evans and Wilsie 1946). Seeds ripen in July in Illinois (Lamp 1952) and in August in Alberta (Evans and Wilsie 1946). Carbohydrate levels are lowest in the spring when the plant is at boot stage, but increase during internode elongation until heading (Teel 1956). In Wisconsin field studies, Reynolds and Smith (1962) report maximum total stored carbohydrates in mid-July when seed is mature, with smaller peaks in earliest spring in the pre-boot stage and in the fall.
Because of the importance of smooth brome as a forage crop, reliable seed crops are necessary and numerous studies have been conducted to determine the requirement for floral initiation and optimal conditions for seed production.
Smooth brome generally requires a period of vernalization under short day conditions followed by a long photoperiod for panicle production (Newell 1950, Kirshin et al. 1974, 1977) best fulfilled by short autumn days and longer spring days. The required time of exposure to low temperatures is short, 1 to 14 nights (Gardner and Loomis 1951) enabling some early- emerged spring shoots to grow under short days of early spring, be vernalized during a cold spell, and develop flowering panicles within the same year.
In greenhouse studies in the Chicago area, Lamp (1952) found a relationship between internode elongation and reproductivity. A minimum of 5 to 14 leaves must be developed during or before primordia formation if the plant is to flower in any given year. Flowering occurs at daylengths of 17 to 18 hours (Gall 1947, Evans and Wilsie 1946) when temperatures are warm. In Alberta studies Evans and Wilsie (1946) observed little flowering when air temperatures were below 60 degrees Fahrenheit.
Under experimental conditions the requirements for floral induction and flowering can differ from those reported above. Sprague (1948) observed some flowering in plants that had been maintained in a heated greenhouse and Allard and Evans (1941) report a critical period of 13 hours of daylength for flowering to occur.
Smooth brome is an open-pollinated self-incompatible species. Synchronous flowering is common (McKone 1985) with pollination occurring from plants up to 50 m away (Jones and Newell 1946). In Minnesota field studies seed set of open pollinated plants were 29% and in nurseries as high as 36% (Lowe and Murphy 1955) to 37% (Nielson 1951). The number of seeds produced has a very wide range. Lowe and Murphy (1955) report 47 to 160 seed heads per plant, with 156 to 10,080 viable seeds per plant. Maturing seeds are subject to predation by Itonidid midges and Chalcid flies (Nielson and Burks 1958). Kramer (1975) suggests that seeds may be transported and sequestered by ants, resulting in creation of new brome patches on anthills.
Smooth brome is a rhizomatous, sod-forming species. The first adventitious roots develop within 5 days of germination (Knobloch 1944). Rhizome formation begins as early as 3 weeks after germination (Wagner 1952) to as late as 6 months (Knobloch 1944).
The drought resistance of smooth brome is probably accounted for in part by its deeply penetrating root system. Dibbern (1947) reports Bromus roots reaching a depth of 4.7 feet. Lamba et al. (1949) found 21% by weight of brome roots between depths of 16 and 40 inches, 10% between 8 and 16 inches and 64% in the top 8 inches. This heavy concentration of total root mass near the surface is the result of smooth brome's creeping rhizomatous habit. Individual rhizomes are reported to have a longevity of one year (Evans and Ely 1935). Old brome fields develop a "sod bound" condition in which shoot density is reduced and symptoms of nitrogen deficiency are exhibited (Meyers and Anderson 1942). Benedict (1941) attributes this condition to a carbon/nitrogen imbalance (perhaps because of the sheer mass of dead rhizomes) but Grant and Sallans (1964) suggest that the decomposing roots may actually produce an allelopathic substance inhibitory to further brome root development.