Although some herbicides are available for control of broadleaf weeds on rangeland, currently no herbicides are registered for selective control of weedy annual grasses in perennial forage grasses. Field experiments were conducted to determine the tolerance of several perennial forage grass species to preemergence (PRE) and postemergence (POST) applications of imazapic, a herbicide that controls certain weedy annual grasses. In PRE studies, perennial forage grasses were seeded 1 d after spraying with several rates of imazapic. The grass species by herbicide rate by location and the grass species by herbicide rate interactions were not significant for plant height and biomass 395 d after treatment (DAT). Expressed as a percentage of the untreated control, imazapic applied at 18 to 140 g/ha reduced height of all grass species 10 to 18%, whereas 280 g/ha of imazapic reduced height 39%. Imazapic applied at 18 to 70 g/ha reduced biomass 12 to 26%. Biomass was reduced 51 and 63% when imazapic was applied at 140 and 280 g/ha, respectively. Thus, rates of imazapic required to control downy brome likely will excessively injure perennial forage grass seeded 1 DAT. In POST studies, imazapic was applied to 1-yr-old stands of perennial forage grass. A dose– response model provided a good fit for grass species biomass and height data. In year 1, biomass and height of orchardgrass, smooth brome, and meadow brome were reduced 14 to 29% more than those of bluebunch, crested, intermediate, and western wheatgrass as imazapic rate increased, which implies that the wheatgrasses were more tolerant to imazapic. However, in year 2, slope of regression lines did not differ among grass species, implying that all forage grass species responded the same to increasing rates of imazapic. Plant height of all grass species decreased 25 to 56% when compared with the untreated control, and biomass decreased 28 to 59% as imazapic rate increased from 18 to 280 g/ha. As discussed previously, rates of spring-applied imazapic required for downy brome control severely injured perennial forage grasses whether applied PRE or POST. The level of tolerance of perennial forage grasses to imazapic depended on herbicide dose and perhaps environmental differences between years.
Nomenclature: Imazapic; bluebunch wheatgrass, Agropyron spicatum (Pursh) Scribn. & Smith. #3 AGRSP; crested wheatgrass, Agropyron cristatum (L.) Gaertn. # AGRCR; downy brome, Bromus tectorum L. # BROTE; intermediate wheatgrass, Agropyron intermedium (Host) Beauv. # AGRIN; meadow brome, Bromus erectus Huds. # BROER; orchardgrass, Dactylis glomerata L. # DACGL; smooth brome, Bromus inermis Leyss. # BROIN; western wheatgrass, Pascopyrum smithii Rydb. # PASSM.
Additional index words: Biomass, picloram, plant density, tolerance.
Abbreviations: DAT, days after treatment; OM, organic matter; POST, postemergence; PP, preplant; PRE, preemergence.
