Annual bromes (downy brome and Japanese brome) have been shown to decrease perennial grass forage production and alter ecosystem functions in northern Great Plains rangelands. Large-scale chemical control might be a method for increasing rangeland forage production. Although fall application has been shown to be the most effective and least likely to impact co-occurring native species, spring germination of downy brome may reduce the efficacy of fall-only herbicide application. We assessed the impact of a low glyphosate dose rate (210 g ha⁻¹) applied to rangelands in fall or in fall and spring on nontarget species and on annual brome abundance at two sites in eastern Montana over 2 yr. We tested the following hypotheses: (1) nontarget effects are greater with spring herbicide application, (2) fall and spring herbicide application are necessary for effective downy brome control, and (3) fall herbicide application is sufficient to control Japanese brome. Few nontarget effects occurred; two dicotyledonous species exhibited small increases in response to herbicide. We found that that a single fall application reduced downy brome cover and seed bank density, but after the second fall application in the following year, downy brome did not continue to show a response to herbicide. After 2 yr of fall herbicide application, Japanese brome had denser seed banks in plots where herbicide had been applied. Blanket glyphosate application on rangelands is an unreliable method for controlling annual brome invasions in the northern Great Plains.

Nomenclature: Glyphosate; downy brome, Bromus tectorum L. BROTE; Japanese brome, Bromus japonicus Thunb. ex Murr. BROJA.

Management Implications: Broad-scale low-rate glyphosate application over rangelands infested with annual bromes is an attractive option for increasing forage production of perennial grasses in the northern Great Plains. When we assessed the impact of glyphosate applied in fall or in fall and spring on annual bromes (downy brome and Japanese brome) and nontarget species abundance at two sites over 2 yr, we found that a fall application of 210 g ha⁻¹ reduced downy brome cover and increased Japanese brome seed bank. Negative nontarget effects were not observed. Because Japanese brome and downy brome tend to co-occur in northern Great Plains rangelands and their negative impacts to perennial grass production are similar, broad-scale low-rate glyphosate application is likely not a reliable option for rangeland improvement.