Around 1900, downy brome Bromus tectorum (native to Eurasia) was introduced to the arid and semiarid western USA, where it is now considered invasive on many grazing lands. Experimental herbicide control trials of downy brome and evaluations of the effect on perennial forage grass production were conducted at three locations on Nebraska rangeland.
Soils at the study sites ranged from a silty clay loam to loamy sand. Primary forage grasses were native western wheatgrass Agropyron smithii and blue grama Bouteloua gracilis.
The experiment was a randomized complete block, with four replications of each treatment applied (by a tractor-mounted sprayer) to 3.05 x 10.67 m (10 x 35 ft) plots.
Herbicide treatments were: atrazine, cyanazine, metribuzin, and simazine at 0.5 and 1.0 lb/acre; glyphosate and terbacil at 0.25 and 0.5 lb/acre; secbumeton at 1.0 and 2.0 lb/acre; and paraquat at 0.25 to lb/acre applied in the spring (early March). These treatments (except glyphosate and paraquat) were also applied in autumn (November), as was metribuzin at 0.33 lb/acre.
Percentage downy brome cover was visually estimated in the year of application (excluding autumn treatments) and 1 and 2 years following application. Forage yields were determined by clipping samples, drying and weighing.
Two months after spring application on the silty clay loam soil, excellent downy brome control was achieved; all treatments except cyanazine at 0.5 lb/acre (72%) and paraquat at 0.25 lb/acre (85%) resulted in 95-100% control. Five months after application, glyphosate and paraquat had become totally ineffective, the effect of cyanazine at 0.5 lb/acre had decreased to 40%, and simazine at 0.5 lb/acre from 98% to 78%. All other treatments maintained effective brome control. Autumn application was generally not aseffective.
Downy brome control was broadly similar on the coarse-textured soils (silt loam, loamy sand) but generally less effective.
Non-target injury to native perennial cool-season forage grasses was greater following spring than autumn, applications. Forage grass production was significantly increased only by simazine a 1 lb/acre (686 lb/acre or 126% increase compared to untreated plots). Atrazine, cyanazine, metribuzin and terbacil at 0.5 lb/acre, and metribuzin at 0.33 lb/acre also increased yields by more than 300 lb/acre when applied in spring on the silt loam soil in the year after application.
In autumn applied herbicide plots, although forage production was not significantly increased, there was a trend toward increased production.
Overall, atrazine, metribuzin and simazine effectively controlled downy brome, with control and forage grass production greater when applied in spring.