Tall Fescue Roadside Mowing Reduction from Imazapic

Tall fescue [Lolium arundinaceum (Schreb.) S.J. Darbyshire] is commonly established along roadside rights-of-way in adapted zones due to its tolerance of drought, heat, and wear; however, its upright growth habit coupled with seedhead production can impair motorist vision. Field research was conducted in 2013 and 2014 to quantify tall fescue mowing requirements following imazapic {(±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid}, an herbicide commonly used for plant growth regulation, application (53 g a.i. ha^–1) alone, as well as tank-mixed with clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) + triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy]acetic acid} (158 + 473 g a.i. ha^–1). Plots were mown at a 23- or 30-cm foliage intervention height following treatment. Nontreated-mown and nontreated-nonmown controls were included for comparisons. Evaluated herbicide treatments caused 0 to 20% tall fescue injury, which recovered following mowing. Additionally, imazapic provided 100% tall fescue seedhead suppression through 56 days after treatment (DAT). Imazapic reduced tall fescue mowing requirements at both intervention heights through 70 DAT. Two mowing cycles were reduced across intervention heights in 2013, while three and two cycles were reduced at 23 and 30 cm heights, respectively, in 2014. Lastly, imazapic + clopyralid + triclopyr application increased tall fescue cover 84 DAT. This information will allow vegetation managers to more efficiently allocate resources to maintain clear motorist sightlines on tall fescue roadside rights-of-way.