ABSTRACT.- Cover crops constitute an interesting option for cultural weed control. Expansion of herbicide resistant weeds in Uruguay, has been associated with an increase use of pre-emergent herbicides for their control. Crops residues could decrease their effectiveness. The objective of these work was to evaluate the effect of Avena strigosa straw and moment and amount of rain, in Amaranthus spp. control effectiveness by flumioxazin and sulfentrazone. Two experiments were carried out for each herbicide in pots, in a greenhouse. The design was in randomized complete blocks, with five repetitions and factorial arrangement of treatments. In experiments I, factors were:I) amount of A. strigosa straw (0 and 6 t ha-1); and II) amount of rainfall (20 and 80 mm), for each herbicide. In experiments II, factors were:I) amount of A. strigosa straw (0 and 6 t ha-1); II) amount of rainfall (20, 60 mm); and III) Days between application and rainfall occurrence (1, 7 and 14). Weed emergencies were quantified at days 8, 22 and 39 after rainfall (DAR). Results were independently analyzed using a model with gaussian distribution. Sulfentrazone effectiveness in Amaranthus spp. control was not affected by studied factors. In the case of flumioxazin, straw effect was consistently detected. Amounts of 6 t ha-1 of oat straw reduced Amaranthus spp. control effectiveness of the herbicide. © 2022 Informa UK Limited, trading as Taylor & Francis Group