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Suppression of Smooth Brome by Atrazine, Mowing, and Fire

Introduction


Several herbicides, including atrazine [2-chloro-4-(ethylamino)-6-(isopropyl amino)-s- triazine], have been used to control introduced smooth brome (Bromus inermis Leyss.) in order to enhance the vigor of remnant warm-season grasses or to suppress existing sod before seeding. For example, in southeastern Nebraska in overgrazed native pastures dominated by smooth brome, Waller and Schmidt (1983) found atrazine, applied in late April at 2.2 kg/ha, shifted dominance from cool- to warm-season grasses. First year results showed atrazine reduced the relative species composition of smooth brome by 91% compared to the control. In south central Nabraska, Dill et al. (1986) found that atrazine applied once in the spring at 3.3 kg/ha reduced smooth brome dominance in seeded warm-season pastures when remnants such as big bluestem (Andropogon gerardii Vitman) were present. Also in south central Nebraska, Bush et al. (1987) found application of atrazine at the time of seeding big bluestem and other warm-season grasses was an effective sod suppression treatment.

Atrazine suppresses smooth brome but may also impact native prairie species. For example, at the Samuel H. Ordway, Jr. Memorial Prairie, South Dakota, Plumb (1988) found atrazine applied at two rates (2.2 and 3.3 kg/ha) in the fall negatively affected both C3 and C4native plants in a seeded area heavily infested with smooth brome. Similarly, atrazine applied at 1.1 kg/ha reduced standing crop of prairie forbs by 74% on a loam soil in west Texas (Petersen et al. 1983). In a restoration study in eastern Nebraska, Bragg and Sutherland (1989) found no seeded forbs in any plot treated with either atrazine or 2,4-D [(2,4-dichlorophenoxy)acetic acid] due either to the effect of the herbicide or to strong competition from seeded grasses, such as switchgrass (Panicum virgatum L.). Rosburg and Glenn-Lewin (1992) found atrazine decreased several annual forbs but the effect was significant only in the year of treatment. Considering these various effects, Gillen et al. (1987) concluded that atrazine application should be avoided if enhancement of forb diversity of a prairie is a primary objective of a restoration.

As an alternative to atrazine, mowing or prescribed fire has been suggested as a means to suppress smooth brome when applied at a critical growth stage. For example, Teel (1956), Reynolds and Smith (1962), and Eastin et al. (1964) demonstrated that smooth brome was most easily damaged by intensive defoliation after tiller internode elongation began in the spring. Furthermore, in eastern Nebraska, Willson (1991) showed a 50% reduction in smooth brome tiller density after a prescribed burn at the time of tiller elongation (mid-May). Kirsch and Kruse (1972) also found cover of smooth brome decreased by 50% or more after a 26 May burn in North Dakota, presumably at tiller elongation or after.

The advantage of mowing or burning is that these treatments may avoid the potential negative effects of herbicides on prairie forbs while producing sufficient smooth brome sod suppression to allow warm-season grass establishment following seeding. In order to assess any such advantages, our study was designed to compare the effects of atrazine, fire, and mowing on the suppression of smooth brome and evaluate the impacts of sod suppressing treatments on seedling establishment of big bluestem.


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