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Suppression of Smooth Brome by Atrazine, Mowing, and Fire

Materials and Methods

Study site

The study was conducted on a 32 ha oldfield in management unit 4 within Pipestone National Monument, a national park unit in the Prairie Couteau region of southwestern Minnesota (44 1' N, 96 19' W; 512 m above sea level). The site was purchased by the National Park Service in 1956. Prior to that time, the site was used for row crop production. The site has been burned in early April to mid-May on a 3- to 4-year cycle since 1972. In 1989, the oldfield was dominated by seeded smooth brome and Kentucky bluegrass (Poa pratensis L.), 60% and 23% cover, respectively, but native prairie forbs had invaded from adjacent native prairie (Becker 1986). Included among these forbs were smooth blue aster (Aster laevis L.), purple prairieclover (Dalea purpurea Vent.), prairie wild rose (Rosa arkansana Porter), and whorled milkweed (Asclepias verticillata L.). Except for tall dropseed [Sporobolus asper (Michx.) Kunth], native grasses were absent. The soils of the site are classified as fine-silty, mixed Udic Haploborolls developed on loess over glacial till (Hokanson et al. 1976). Precipitation during spring and summer was highly variable but near the long-term average in 1990 and below average in 1991 (Table 1) (NOAA 1990, 1991).

Table 1. Total monthly precipitation (cm) and departures from normal monthly means for April - September 1990-91 at Pipestone, Minnesota.

April 3.95 -1.60 5.33 -0.23
May 12.08 3.45 7.95 -0.68
June 10.10 -0.23 8.20 -2.13
July 12.68 5.25 3.70 -3.73
August 7.93 -0.55 5.63 -2.85
September 1.03 -6.10 11.48 4.35


In September 1989, 16, 6 x 12 m plots were established at the oldfield site in a randomized complete block design which allowed 4 replications of 4 treatments. September smooth brome tiller density was determined for each plot by counting all current-year tillers within 10, 0.1m2 randomly placed microplots and was the blocking criterion. Plots were separated by a 12-m lane to eliminate herbicide drift to adjacent plots. In September 1990, 12 new, 16 x 12 m plots were established at the same site but in a completely randomized design. In 1990, replicates were reduced to 3 because of limited space.

Treatments were herbicide (atrazine), prescribed burn, mow-and-rake, and no treatment (control). Treatments were applied 19-20 May, 1990, and 22-23 May, 1991, when approximately 50% of the smooth brome tillers showed internode elongation. A single application of atrazine at 2.2 kg/ha was broadcast over the herbicide plots with a hand sprayer. Burn plots were treated using a back fire (Wright and Bailey 1982) under the following prescription: temperature between 10 and 27°C, wind from any direction between 16 and 32 km/hr, and relative humidity between 30 and 70%. Mow-and-rake plots were cut to 4-cm stubble height and raked to remove cuttings. The day following treatment applications, a mix of big bluestem seed previously collected from the park and purchased from a commercial source in eastern South Dakota, was sod-seeded (see Samson and Moser 1982) in all plots at 25 PLS/0.1 m2 using a Truax native grass drill in 1990 and a John Deere Powr-Till seeder in 1991.


During September 1990 and 1991, sod suppression was determined by counting smooth brome tillers in three randomly located (0.1 m2) microplots per treatment plot. Also in September 1990 and 1991, big bluestem seeding success was evaluated along five randomly located 1-m long transects per treatment plot that had been permanently marked along drill rows at the time of seeding. Each meter-long transect was divided into 20, 5-cm row segments. If 10% or more of the segments were occupied by one or more big bluestem seedlings, it was considered an acceptable stand. This acceptance level is approximately equal to at least 10 seedlings/m2 which is considered to be a good stand by the Great Plains Agricultural Council (Launchbaugh 1966).

All data were subjected to either an analysis of covariance (1989 plots) or an analysis of variance (1990 plots). An F-protected LSD test was used to separate the treatment means in both experiments (alpha level of 0.05). The analyses were performed using SOLO statistical software (BMDP Statistical Software Inc. 1991).

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