Five rigid ryegrass populations suspected of being resistant to both glyphosate and oxyfluorfen were collected in southern Spain and tested under laboratory-controlled conditions. Four populations (Depuradora, Condado, AlamoRasilla, and Portichuelo) were treated with glyphosate for at least 15 consecutive years, and treatments during the last 5 yr were mixed with oxyfluorfen. The fifth population (4alamos) followed the same glyphosate treatment, although oxyfluorfen was never used to control it. Dose—response assays confirmed glyphosate resistance in all populations, with resistance indexes ranging from 11.7 to 37.5 (GR₉₀). Shikimate accumulation assays consistently supported these data, as the most glyphosate-resistant populations (Depuradora and Condado) displayed the lowest shikimate levels. Surprisingly, four populations (Depuradora, Condado, AlamoRasilla, and Portichuelo) displayed 7.93- to 70.18-fold more resistance (GR₉₀) to oxyfluorfen, despite limited selection pressure, showing a similar resistance pattern as that for glyphosate. The 4alamos population displayed oxyfluorfen GR₉₀ values that were similar to those observed in susceptible plants; however, this population was significantly more resistant in terms of plant survival (LD₉₀). Protoporphyrin IX accumulation assays supported the results of dose—response assays, in that the most oxyfluorfen-resistant populations accumulated less protoporphyrin IX. Although more studies are needed, it seems that these five glyphosate-resistant weed populations display a natural tendency to easily develop resistance to oxyfluorfen, with the populations that have higher resistance to glyphosate also having higher resistance to oxyfluorfen.

Nomenclature: Glyphosate; oxyfluorfen; rigid ryegrass, Lolium rigidum Gaudin LOLRI.