| Summary: | A failure of the EPSPS-inhibiting herbicide glyphosate to control several populations of <i>Hordeum murinum</i> subsp. <i>leporinum</i> (or <i>H. murinum</i>) occurred in southern Spain after more than fifteen applications in both crop (olive, orchards, and citrus) and non-crop (dry areas, roadsides and ditches) areas. Eight out of 18 populations studied were resistant (R) to glyphosate with R factors higher than four based on GR<sub>50</sub>. These populations also had the highest values of LD<sub>50</sub> and the lowest levels of shikimic acid accumulation. Two adjuvants tested increased glyphosate efficacy in both susceptible (S) and R populations thanks to better spray foliar retention. Moreover, PS I-, PS II-, and ACCase-inhibiting herbicides, in pre- or post-emergence, proved to be the best chemical alternatives with different sites of action (SoA) to control both S and glyphosate-R populations. This study represents the first report worldwide of glyphosate resistance in <i>H. murinum</i> found in very different crop and non-crop areas from southern Spain. To design chemical strategies to implement integrated weed management programs for glyphosate-R <i>H. murinum</i>, both adjuvants and herbicides with alternative SoA as well as application timings should be considered.
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