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Suppression of Smooth Brome by Atrazine, Mowing, and Fire

Results and Discussion

Sod Suppression

Atrazine was the only treatment to significantly decrease smooth brome tiller densities (77% in 1990 and 70% in 1991) from untreated controls (P=0.0004, P=0.02) (Fig. 1). The reduction in smooth brome tillers following atrazine applications in our study was consistent with the results from other herbicide studies (Waller and Schmidt 1983, Dill et al. 1986, Plumb 1988, Rosburg and Glenn-Lewin 1992).

Although burning reduced smooth brome densities by 16% in 1990 and 37% in 1991, post-treatment values were not significantly different from the untreated controls (P=0.29, P=0.17) (Fig. 1). The reduction in smooth brome density following fire was not as great as the reduction in a companion study at Mead, Nebraska, (Willson 1991), nor in other studies reported from the Midwest (Old 1969, Kirsch and Kruse 1972, Boehner 1986) even though the application of fire at Pipestone was timed to correspond with the period of smooth brome tiller internode elongation. At this stage, fire kills the tiller by removing the growing point and secondary tillering is reduced as a result of low carbohydrate reserves (Kuneluis et al. 1974). The difference between the responses at Pipestone and Mead was most likely due to differences in interspecific interactions. At the Pipestone study site, smooth brome was the dominant grass with a cover estimate of over 50% (Becker 1986). At Mead, smooth brome and big bluestem were co- dominates (T. Bragg, pers. comm.). Although the Pipestone burn treatment was timed to kill smooth brome tillers and minimize secondary tiller production, secondary tillers were not subjected to competitive stress due to the lack of a warm-season grass component at the study site.

As with burning, a single mowing in May had no significant effect on smooth brome tiller density although density decreased 16% in 1990 and increased 10% in 1991 (P=0.31, P=0.68) (Fig. 1). The absence of significant effects of mowing may have been due to mowing above a majority of the tiller growing points. Mowing height and the uneven ground surface in the plots could result in cutting that would not significantly affect smooth brome. Also, as in the burn plots, secondary tillers were not affected by the presence of warm-season grasses.

GIF - Figure 1
Figure 1. Mean smooth brome tiller density per 0.1 m2 following treatments (CONT = unburned control, BURN = prescribed burn, ATRA = herbicide (atrazine), and MOW = mow and rake) in 1990-91 at Pipestone National Monument, Minnesota. Means with the same letter within years are not significantly different (P>0.05, LSD).

Sod-Seeding Big Bluestem

Sod-seeding big bluestem was unsuccessful (zero seedlings in September) for all treatments in both 1990 and 1991. The reasons for this were not apparent since seeds were viable (germination tested at 76% in 1990 and 88% in 1991) and seed to soil contract had been maximized. The absence of any seedling establishment in 1990 prompted the change from a grass drill to a power seeder for better seed-to-soil contact and potentially greater germination.

Weather may have been a factor since precipitation following sod-seeding was below normal most of the growing season months in both years (Table 1). Such conditions have been shown to effect big bluestem establishment (Bush et al. 1989). Bush et al. (1989), however, also found that with favorable precipitation, sod-seeding resulted in a complete stand failure in 1981, but a successful stand in 1982. No explanation for the difference is apparent.

Another possible explanation for seeding failure may be related to the degree treatments suppress existing sod. Competition from resident plants must be removed in order to establish seeded grasses (Sprague et al 1960, Bush et al. 1989). For example, in upland pastures dominated by Kentucky bluegrass, Samson and Moser (1982) found sod-seeded switchgrass stands were successful only with 85% or greater suppression of residual herbage. In our study, maximum suppression of smooth brome tiller density was 16% following mowing and 37% following fire. Both treatments probably did not suppress smooth brome sod enough to allow big bluestem seedling survival. In the atrazine plots, reductions in smooth brome tiller densities were large (77% and 70%) in both years of our study, but also may have been insufficient to allow big bluestem seedlings to survive.

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