Glyphosate is regularly used to control annual bluegrass populations in dormant bermudagrass turf. A population of annual bluegrass not controlled by glyphosate at 840 g ha⁻¹ (glyphosate resistant, GR) was identified on a golf course in Humboldt, TN in 2010. Mature tillers of GR plants were established in a greenhouse and treated with glyphosate at 0, 210, 420, 840, 1,680, 3,360, and 6,720 g ha⁻¹. Mature tillers of a biotype known to be susceptible to glyphosate (SS) were also established in the greenhouse and subjected to the same treatments. At 14 d after treatment (DAT), glyphosate controlled the SS biotype > 95% at rates > 420 g ha⁻¹. Comparatively, the GR biotype was only controlled 76% with glyphosate at 6,720 g ha⁻¹. The rates required to provide 50% control (I₅₀ values) for SS and GR biotypes were 236 and 2,812 g ha⁻¹ respectively, resulting in a resistance factor of 12. Photochemical efficiency (F_v/F_m) values on SS plants treated with glyphosate at > 210 g ha⁻¹ measured 0.000 at 14 DAT, whereas F_v/F_m values on GR plants were not significantly different from the untreated control with glyphosate rates ≤ 840 g ha⁻¹ on the same date. In laboratory experiments, the SS biotype accumulated greater shikimate concentrations than the GR biotype 3 to 6 DAT. Future research should evaluate strategies for managing GR and SS annual bluegrass with alternative modes of action.

Nomenclature: Annual bluegrass, Poa annua L.; bermudagrass, Cynodon dactylon L. Pers.; glyphosate; photochemical efficiency.