Ultraviolet-B radiation is an environmental stress for plants and this situation could become aggravated in the next decades. In this study we used Italian ryegrass (Lolium multiflorum Lam.) as a model system to test whether an environmental stress derived from global change, such as UVB, can influence the efficacy of control procedures and evolution toward herbicide resistance. We grew three generations of Italian ryegrass plants with and without UVB light and subjected them to a series of diclofop-methyl [(+/-)-2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid, methyl ester] doses. The effect of selection history was tested with herbicide dose response. The effect of herbicide application on plant survival and biomass varied significantly among herbicide doses and with absence or presence of UVB light. In the absence of herbicide, the decrease in individual fecundity with increasing plant density was similar under both no-UVB and UVB light treatments. Only plants growing without UVB light increased production of reproductive structures in response to the decrease in density caused by herbicide application. Our study shows that UVB light was a weak stress factor for the ryegrass plants. However, when herbicide selection pressure was high, UVB light reduced the evolution toward herbicide tolerance. When selection pressure on the parental plants was lower, the two stress factors had a synergistic effect, causing changes in herbicide efficacy that in turn had demographic and evolutionary consequences. In the field, these interactions between stress factors might be of significance for annual weeds in which seed output is a major determinant in fitness.