Control of Johnsongrass (Sorghum halepense) in tall fescue (Festuca arundinacea) stands

Johnsongrass is a nonnative invasive species introduced as a forage crop from the Mediterranean region (Miller and Miller 1999). This perennial species has become naturalized and is found in 47 of the 50 United States (Alaska, Minnesota, and Maine being the exceptions) (USDA 2004). Johnsongrass has become problematic along roadsides in Kentucky due to its aggressive and prolific nature and rapid growth habit. Johnsongrass reproduces by seed and by rhizomes which adds to its invasive nature. This plant can cause line of sight issues, maintenance concerns along guardrails, and unsightly rights-of-way. The past management regime for Johnsongrass for the Kentucky Transportation Cabinet has been chemical treating infestations with an ACCase type herbicide (e.g. Fusion®, a.i. fluazifop + fenoxaprop) along the guardrails and vehicle recovery areas and mowing operations for areas outside these areas. Unfortunately, there have been reported cases of Johnsongrass developing resistance to the ACCase type herbicides in agricultural settings in Kentucky (Obermeier et al 1998). Graminicide resistant Johnsongrass has also been reported in Mississippi, Tennessee, Virginia, Louisiana, and Texas (weedscience.org 2003). Although no cases have been officially documented on KTC property, the potential for herbicide resistance exists to make this invasive species more of a problem. Evaluation of herbicide chemistries with modes of action different to that of graminicides (ACCase inhibitors) needs to be evaluated for efficacy to provide the KTC a cost effective alternative to Fusion for an annual application rotation. Outrider® (a.i. sulfosulfuron) was shown to be an effective control option for Johnsongrass. Outrider has a different mode of action (ALS inhibitor) than that of graminicides and may prove to be a feasible control option and\or rotation partner with current control strategies. The goal of the trials presented here was to evaluate the efficacy of Envoy (a.i. clethodim), Fusion, and Outrider in controlling Johnsongrass and the effect that the compounds have on tall fescue stands.