In conservation agriculture systems, farmers gain many advantages from retaining crop residue on the soil surface, but crop residue retention in these systems may intervene with the activity of pre-emergence herbicides. A pot study was conducted to evaluate the effect of different rates of pre-emergence herbicides [imazethapyr (100 and 150 g a. i. ha-1), isoxaflutole (100 and 200 g a. i. ha-1), metolachlor (1.5 and 2.25 kg a. i. ha-1), pendimethalin (2.25 and 3.38 kg a. i. ha-1) and prosulfocarb + metolachlor (2.5 and 3.75 kg a. i. ha-1)] on seedling emergence and biomass of Echinochloa colona and Chloris virgata when applied in the presence of sorghum residue at rates equivalent to (0, 3 and 6 t ha-1). When seeds of E. colona and C. virgata were not covered with sorghum residue, the seedling emergence and biomass of both weeds was inhibited by 93–100% and 56–100%, respectively, with the application (both rates) of isoxaflutole, metolachlor, pendimethalin and prosulfocarb + metolachlor. Using sorghum residue resulted in lower herbicide efficacy on both weeds. At 3 t ha-1 sorghum residue, E. colona emergence and biomass reduced by 38–100% and 30–100%, respectively, with application of isoxaflutole, metolachlor and pendimethalin (both rates) in comparison with the no-herbicide treatment. Similarly, the emergence and biomass of C. virgata was also reduced by 92–100% and 25–100%, respectively. The results of this study suggest that crop residue may influence efficacy of commonly used pre-emergence herbicides and that the amount of crop residue on the soil surface should be adjusted according to the nature of the pre-emergence herbicides to achieve adequate weed control. [ABSTRACT FROM AUTHOR]