Previous research showed growth regulator herbicides, such as picloram and aminopyralid, have a sterilizing effect on Japanese brome (Bromus japonicus Thunb.) that can reduce this invasive annual grass's seed production nearly 100%. This suggests growth regulators might be used to control invasive annual grasses by depleting their short-lived seed banks. The goal of this study was to extend the previous Japanese brome research to downy brome (Bromus tectorum L.), the most damaging invasive annual grass of U.S. grasslands. In a greenhouse, we found picloram did not greatly influence downy brome seed production, while point estimates suggest aminopyralid reduced seed production 55 to 80%. If not for a highly abnormal retillering response that we somewhat doubt would occur in the field, point estimates suggest aminopyralid would have reduced downy brome seed production approximately 90% when applied at the heading stage and approximately 98% when applied at three earlier growth stages. Our greenhouse study should encourage field studies designed to further explore the potential for using growth regulators to control downy brome and other invasive annual grasses.
Nomenclature: Picloram; aminopyralid; downy brome, Bromus tectorum L.; Japanese brome, Bromus japonicus Thunb.
Management Implications: Growth regulator herbicides, such as aminopyralid, picloram and dicamba, are widely used to control broadleaf weeds in grasslands. In recent studies, we discovered these herbicides interfered with reproductive processes in the invasive annual grass Japanese brome, thereby reducing its seed production nearly 100% in the greenhouse and field. This suggests growth regulators might be used to control invasive annual grasses by depleting their short-lived seed banks. Compared to currently used invasive annual grass herbicides, such as glyphosate and imazapic, growth regulators have the advantage of being less damaging to desirable perennial grasses, though all herbicides currently used for invasive annual grass control can severely damage desirable forbs and shrubs.
The objective of the current study was to extend our Japanese brome research to downy brome, a much more widespread and damaging congener of Japanese brome. In a greenhouse, we found that picloram was not effective against downy brome while aminopyralid greatly reduced downy brome seed production. This encouraging result should promote field studies designed to more fully evaluate the potential for using aminopyralid to control downy brome.
Our findings will perhaps prove most applicable to areas co-dominated by invasive annual grasses and invasive forbs, such as yellow starthistle, spotted knapweed, etc. Downy brome often dominates sites after herbicides are used to control invasive forbs. It may sometimes be possible to overcome this problem by using one appropriately timed growth regulator herbicide application to simultaneously target both invasive forbs and invasive annual grasses.