In North America, from a livestock rearing perspective, control (reducing competitiveness) of less desirable cheatgrass Bromus tectorum (and also Japanese brome B. japonicus) may increase desirable perennial grass species yields. From a nature conservation viewpoint, control may be desirable to restore or enhance native plant communities. In this study, undertaken at Fort Keogh Livestock and Range Research Laboratory (46º22’N, 105º5’W),Montana (northwest USA), the efficacy of three soil-active herbicides in controlling these two annual Bromus species was trialed.
The trials were carried out on stands of crested wheatgrass Agropyron cristatum, pubescent wheatgrass pubescent wheatgrass Thinopyrum intermedium, Russian wildrye Psathrostachyis juncea, and a mixed stand of native western wheatgrass Parsopyrum smithii and blue grama Bouteloua gracilis. The four sites were infested with cheatgrass and also had some Japanese brome.
Atrazine, propham and pronamide (each mixed with water) were applied in a single autumn application (October 1982) i.e. Bromus post-emergence (brome seedlings about 25-50 mm tall) at two rates: atrazine - 0.6 and 1.1 kg/ha, propham - 3.4 and 6.7 kg/ha; and pronamide - 0.6 and 0.8 kg/ha.
Herbicides were applied on 9.1 x 9.1 m plots (by 3.7 m hand-propelled boom sprayer), with two replicates at each site (6 treated plots: 3 herbicides, 2 rates and untreated controls).
Grass yields were estimated by clipping three, 0.l8 m² subplots/main plot in early July (i.e. peak standing crop) in 1983 and 1984. Crude protein, phosphorus and total nonstructural carbohydrate content of perennial grasses were also measured over the two years subsequent to treatment.
Standing crop of the two annual Bromus in herbicide plots averaged 91% less than the controls in the first, and 47% less in second year post-treatment.
Pronamide exhibited substantially better Bromus control in the second year post-treatment (1984: range 4-295 kg/ha) than atrazine (range 4-679 kg/ha) and propham 4-452 kg/ha).
Perennial grass yields in most of the herbicide treatment plots were significantly increased the first year post-treatment.
Perennial grass crude protein was increased in atrazine plots (e.g. in 1983 controls ranged 6.3-8.6, treated plots ranged 8.1-13.1).
Additionally, atrazine (0.6 kg/ha) proved the most cost-effective herbicide for reducing the two annual brome species and increasing perennial grass yields.