Vigorous stands of perennial grasses can effectively provide long-term control of many invasive plants on rangelands. However, in degraded conditions, successful reestablishment of perennial grasses can be compromised by invasive annual grasses, such as downy brome. Propoxycarbazone-sodium is a selective herbicide currently labeled for downy brome control in small grains, but its potential use on rangelands is unknown. Studies were conducted from 2004 through 2008 at three rangeland sites in Colorado and Nebraska to evaluate downy brome control and perennial grass injury with propoxycarbazone-sodium and imazapic. Propoxycarbazone-sodium provided satisfactory downy brome control with grass injury equal to or less than imazapic when rainfall followed the fall application. A second set of studies was conducted from 2007 to 2008 at Lingle, WY, and Scottsbluff, NE, to determine the plant-back interval and postemergence application response of seven perennial grass species to propoxycarbazone-sodium and imazapic. Grass tolerance to both herbicides was good when applied 90 and 120 d before planting (DBP). However, grass injury increased as plant-back interval decreased. The greatest impact on plant biomass was observed from herbicide applied at planting or after planting. Crested and intermediate wheatgrass (Agropyron cristatum and Thinopyrum intermedium) biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass (Pascopyrum smithii) and Russian wildrye (Psathyrostachys juncea) showed tolerance to imazapic applied before planting. Smooth brome (Bromus inermis), sheep fescue (Festuca ovina), and orchardgrass (Dactylis glomerata) showed the least amount of tolerance to propoxycarbazone-sodium and imazapic.

Nomenclature: Propoxycarbazone-sodium; imazapic; downy brome, Bromus tectorum L.; crested wheatgrass, Agropyron cristatum (L.) Gaertn.; intermediate wheatgrass, Thinopyrum intermedium (Host) Barkworth & D.R. Dewey; orchardgrass, Dactylis glomerata L.; Russian wildrye, Psathyrostachys juncea (Fisch.) Nevski; sheep fescue, Festuca ovina L.; smooth brome, Bromus inermis Leyss.; western wheatgrass, Pascopyrum smithii (Rydb.) Á. Löve

Interpretative Summary: The establishment of perennial grasses that can provide long-term control of invasive plants can be jeopardized by infestations of downy brome. Propoxycarbazone-sodium is a selective herbicide for downy brome control in small grains, but its performance on established perennial grass stands in rangelands was unknown. A study was conducted in Colorado and Nebraska and indicated satisfactory downy brome control with propoxycarbazone-sodium at 45 g ha⁻¹ (0.04 lb ac⁻¹) when applied in fall. Downy brome control was consistent when fall applications were followed by seasonal rainfall. An additional finding of the study was that grass injury was equal to or less than that caused by imazapic. In a second study, the response of seven perennial grasses to propoxycarbazone-sodium at 60 g ha⁻¹ and imazapic at 105 g ha⁻¹ applied 120, 90, and 30 d before planting (DBP), at planting, and 30 d after planting (DAP) was examined. Grass tolerance to both herbicides was good for applications 90 and 120 DBP. Grass injury increased as herbicide applications were made closer to planting. Crested and intermediate wheatgrass biomass production was not affected when herbicides were applied 90 or 120 DBP. Western wheatgrass and Russian wildrye showed tolerance to imazapic applied before planting. When planting orchardgrass, smooth brome, or sheep fes