Field studies were conducted to compare the response of one inbred (‘CL161’) and two hybrid (‘CLXL729’ and ‘CLXL745’) Clearfield (CL) rice cultivars to imazamox. Imazamox was applied at 44 and 88 g ai ha⁻¹ to rice in the panicle initiation (PI) and PI plus 14 d (PI 14) growth stages and at 44 g ha⁻¹ to rice in the midboot growth stage. Maturity of hybrid CL cultivars was delayed following imazamox at 44 g ha⁻¹ applied at PI 14 and midboot. Furthermore, imazamox at 44 g ha⁻¹, applied at midboot, delayed maturity of CLXL745 more than CLXL729. Expressed as a percentage of the weed-free control plots, rough rice yields for CLXL729 were 91% following imazamox at 44 g ha⁻¹ applied at PI 14, 78% following imazamox at 44 g ha⁻¹ applied at midboot, and 77% for imazamox at 88 g ha⁻¹ applied at PI 14. Rough rice yield for CLXL745 was 77 to 92% of the control following all imazamox treatments. All imazamox treatments reduced CLXL745 rough rice yield compared with CL161. Rough rice yield, pooled across CL cultivar, varied with imazamox treatment between years, and these differences may have been a consequence of lower temperatures and solar radiation in the first year. Hybrid CL cultivars CLXL729 and CLXL745 were less tolerant than was CL161 when imazamox was applied at nonlabeled rates (88 g ha⁻¹) and/or timings (PI 14 or midboot). Because of variability in rice growth stages and irregularities in imazamox application in commercial fields, inbred CL cultivars should be planted where an imazamox application will likely be required.

Nomenclature: Imazamox; rice, Oryza sativa L