Mesotrione efficacy for annual bluegrass (Poa annua) control can be inconsistent. Amicarbazone is a photosystem II-inhibiting herbicide with activity against annual bluegrass. Field and greenhouse experiments were initiated in 2011 at the University of Tennessee evaluating the efficacy of mesotrione and amicarbazone for annual bluegrass control.

Annual bluegrass was collected from the East Tennessee Research and Education Center (Knoxville, TN). Single tillers were transplanted to cone-tainers filled with Sequatchie silt-loam soil. Plants were allowed to acclimate for 4 weeks and contained 5 to 7 tillers when treatments were applied. Nitrogen was soil-applied at 49 kg ha^-1 (46N:0P:0K) prior to treatment application. Treatments were arranged in a 2-by-2 factorial, completely randomized, design with ten replications. Treatments consisted of mesotrione (280 g ha^-1) or topramezone (14.5 g ha^-1) applied with amicarbazone (0 and 79 g ha^-1). An untreated-control was included for comparison. Herbicide treatments were applied singly with a nonionic surfactant (NIS) at 0.25% v/v and 340 L ha^-1 water using a spray chamber. Two experimental runs were conducted in 2011. Annual bluegrass control was evaluated visually on a 0 (no control) to 100% (complete control) scale and using chlorophyll fluorescence yield (F_v/F_m) at 3, 5, 7, 14 and 21 days after treatment (DAT). Aboveground dry biomass was measured 21 DAT.

Field experiments were conducted on a dormant bermudagrass (Cynodon dactylon) fairway overseeded with perennial ryegrass (Lolium perenne) at 440 kg ha^-1. Treatments were arranged in 2-by-4 factorial, randomized complete block, design with three replications. Treatments consisted of mesotrione (280 g ha^-1) or topramezone (14.5 g ha^-1) applied with amicarbazone (0, 79, 160 or 240 g ha^-1). An untreated-control, bispyribac-sodium (78 g ha^-1) and methiozolin (1.5 kg ha^-1) were included for comparison. All herbicide treatments were applied singly with NIS at 0.25% v/v and 280 L ha^-1 of water using small-plot spray equipment. Treatment responses were evaluated visually from 7 to 56 DAT. Grid counts were conducted 56 DAT as well.

In greenhouse experiments, amicarbazone alone provided < 5% annual bluegrass control on all rating dates. By 21 DAT, mesotrione only controlled annual bluegrass 44%. The addition of amicarbazone (79 g ha^-1) to mesotrione increased control to 74% by 21 DAT. Topramezone alone or in combination with amicarbazone provided < 10% annual bluegrass control on all rating dates. F_v/F_m and biomass data supported visual observations.

In field experiments, mesotrione provided 78% annual bluegrass control 56 DAT. Amicarbazone alone provided 58, 78 and 96% control at the 79, 160 and 240 g ha^-1 rates, respectively, 56 DAT. Mesotrione + amicarbazone provided > 96% control 56 DAT at all rates. These data indicate annual bluegrass control provided by mesotrione can be improved by the addition of amicarbazone.