Field trials were conducted in 2004 and 2005 to identify sulfonylurea (SU) herbicides that would provide improved weed control, minimal soil residual, and crop safety to SU-resistant chicory. SU-resistant chicory had previously been selected in vitro for resistance to chlorsulfuron. Our research evaluated three commercial, nonresistant and three breeding lines of SU-resistant chicory. Each of the cultivars was treated POST at the two true-leaf growth stage with either foramsulfuron, rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, thifensulfuron, thifensulfuron plus tribenuron, triflusulfuron, flumetsulam, or imazamox at normal use rates. Established plant densities and root yields of SU-resistant chicory breeding lines were greater than or equal to the densities and root yields of commercial cultivars. The plant density of commercial chicory cultivars was reduced by rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, and thifensulfuron plus tribenuron, but SU-herbicides did not reduce the density of SU-resistant breeding lines. SU-resistant chicory differed in cross-resistance to SU-herbicides, with tribenuron causing the most crop injury and thifensulfuron, the least. Weed control varied between the SU-herbicides. The greatest reduction in weed biomass occurred with tribenuron, thifensulfuron plus tribenuron, and rimsulfuron plus thifensulfuron; the least reduction occurred with triflusulfuron, foramsulfuron, and rimsulfuron. Chicory root yields comparable to the hand-weeded treatment were achieved with rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron treatments. The SU herbicides that met the initial project objectives of crop tolerance and improved weed control were combinations of rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron.

Nomenclature: Imazamox; flumetsulam; foramsulfuron; rimsulfuron; thifensulfuron; tribenuron; triflusulfuron; chicory, Cichorium intybus L. ‘Beryl’, ‘Orchies’, ‘Turquoise’, ‘FD 28’, ‘FD 29’, ‘FD 30’.