Previous research has shown that pyridine growth regulator herbicides can affect seed production in annual grasses including downy brome, Japanese brome, wheat, and other cereal grain crops. Aminocyclopyrachlor is a pyridine carboxylic acid growth regulator herbicide that has recently been registered for broadleaf weed and brush control in nonagricultural areas, which may help facilitate release of native perennial grasses in native plant restoration sites. The influence of aminocyclopyrachlor on downy brome seed production was evaluated at multiple application rates and timings under controlled field conditions. The effect of aminocyclopyrachlor on seed production was compared with aminopyralid, another pyridine growth regulator herbicide. When applied to downy brome plants in the early vegetative stage (EPOST) at approximately 580 growing degree days (GDD), aminocyclopyrachlor at 320 g ae ha⁻¹ reduced seed germination by 50 to 88% in the first and second study years, respectively. Aminopyralid reduced seed germination by 94% in the first study year, but only 20% in the second year. When applied to downy brome plants in the early heading stage at approximately 1,235 GDD (LPOST), aminocyclopyrachlor at 320 g ae ha⁻¹ reduced seed germination by 100% both years. Aminopyralid reduced seed germination by 95% in the first year, and 81% in the second year. Other than the observed reduction in seed germination, herbicides did not produce any visible changes in downy brome aboveground plant growth or development. Because downy brome seeds are relatively short-lived in soil, aminocyclopyrachlor and aminopyralid applications to downy brome–infested rangelands and other natural areas could result in reductions in downy brome population densities over time. No published data exist on the effect of aminocyclopyrachlor on seed production of desirable perennial grasses in natural ecosystems, thereby suggesting the need for further research.

Nomenclature: Aminocyclopyrachlor, formerly DPX MAT28, 6-amino-5-chloro-2-cyclopropylpyrimidine-4-carboxylic acid, aminopyralid, 4-amino-3,6-dichloropyridine-2-carboxylic acid, downy brome, Bromus tectorum L. BROTE, Japanese brome, Bromus japonicus Thunb. ex Murr. BROJA, wheat, Triticum aestivum L.

Management Implications: Aminocyclopyrachlor (formerly DPX MAT28) is a growth regulator herbicide that has been registered for broadleaf weed and brush control in nonagricultural areas, which can facilitate the release of native perennial grasses in native or improved plant community sites. In addition to controlling many invasive broadleaf weeds in rangeland and other natural areas, this herbicide may significantly reduce seed production in downy brome. In this controlled study, application of aminocyclopyrachlor to downy brome plants in the vegetative stage reduced seed germination by 50 to 88%. When applied to downy brome plants in the early heading stage, aminocyclopyrachlor reduced seed germination by 100%, even though aboveground biomass of downy brome was not significantly reduced. Because aminocyclopyrachlor has soil residual properties and because of the relatively short viability of downy brome seeds in the seed bank, it may be possible to use this herbicide to reduce downy brome in infested rangeland areas. A reduction in the downy brome seed bank caused by aminocyclopyrachlor may facilitate the restoration of native or improved plant communities. How aminocyclopyrachlor will affect plant community composition may be difficult to predict, because this herbicide also affects native forbs and shrubs, and may reduce seed production of desired, perennial grasses. Further field testing is needed to assess the effects of aminocyclopyrachlor on long-term changes in plant community composition.