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The severity of drought is increasing owing to global climate change. Knowledge about the influence of drought on weeds and herbicide performance is necessary for optimising herbicide applications. Therefore, dose-response trials were conducted to investigate the influence of drought stress on the efficacy of four herbicides.Drought-stressed and non-stressed Hordeum spontaneum seedlings were exposed to nine doses of sulfosulfuron and sulfosulfuron + metsulfuron-methyl. Clodinafoppropargyl and mesosulfuron-methyl + iodosulfuron-methyl sodium were applied to drought-stressed and non-stressed seedlings of Avena sterilis subsp. ludoviciana. Leaf greenness index (SPAD), total chlorophyll content, stomatal conductance and photosynthesis rate of both species were measured. The efficacy of herbicides on H. spontaneum was not influenced by drought stress. In contrast, the performance of herbicides on A. sterilis subsp. ludoviciana was reduced under drought stress, where the ED 50 increased by 1.5-fold. The low doses (<2.2 g a.i. ha −1 ) of mesosulfuronmethyl + iodosulfuron-methyl-sodium caused hormesis on A. sterilis subsp. ludoviciana. Total chlorophyll content and SPAD index of both species increased due to drought stress. The stomatal conductance and photosynthesis rate of A. sterilis subsp. ludoviciana decreased under drought stress, while H. spontaneum was not influenced. The lack of detrimental effect of drought stress on sulfosulfuron and sulfosulfuron + metsulfuron-methyl performance might be attributed to similar stomatal conductance and photosynthesis rate of the drought-stressed and non-stressed H. spontaneum. The performance reduction in herbicides on drought-stressed A. sterilis subsp. ludoviciana might be owing to the reduction in stomatal conductance and photosynthesis rate. Using adjuvants, diverse formulations and early application of herbicides were suggested for future research to hamper the negative effect of drought on herbicides.

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