Broomsedge populations have increased substantially over the last decade on roadsides in Georgia. The invasiveness of this species might have resulted from imazapic use for bermudagrass growth regulation and the limited use of MSMA on roadsides. The objectives of this research were to evaluate (1) differential growth inhibition of bermudagrass and broomsedge to imazapic, (2) susceptibility of isolated acetolactate synthase (ALS) enzymes of bermudagrass and broomsedge to imazapic, (3) broomsedge control with tank mixtures of imazapic with MSMA, and (4) the influence of imazapic on absorption and translocation of ¹⁴C-MSMA. In greenhouse experiments, imazapic reduced bermudagrass shoot biomass ~ 2 times more from the nontreated than broomsedge. Isolated ALS enzymes of bermudagrass were ~ 100 times more susceptible to inhibition by imazapic than broomsedge. In field experiments, imazapic provided no control of broomsedge, but MSMA alone controlled broomsedge 81% at 12 mo after initial treatments (MAIT). Broomsedge control was reduced to 45% when MSMA was tank mixed with imazapic at 12 MAIT. In laboratory experiments, imazapic tank mixtures did not reduce broomsedge absorption or translocation of ¹⁴C-MSMA. Overall, bermudagrass is more susceptible to imazapic due to greater target-site inhibition than broomsedge. Results emphasize the importance of MSMA use for broomsedge control, but agronomists should avoid tank mixtures with imazapic to reduce potential antagonism.

Nomenclature: Bermudagrass, Cynodon dactylon (L.) Pers × C. transvaalensis Burtt-Davy ‘Princess-77’, broomsedge, Andropogon virginicus L.