<p>Barnyardgrass (<i>Echinochloa</i> spp.) is an invasive weed that poses significant challenges to diverse agricultural systems, mainly in flooded rice and others rainfed crops, due to its herbicide resistance and competitive traits. Understanding the differences in growth and development between resistant and susceptible biotypes under varying water regimes is crucial for effective management strategies. The aim of this study was to evaluate differences in growth and development of resistant and susceptible barnyardgrass biotypes to acetolactate synthase (ALS)-inhibiting herbicides in two water regimes. The experiment was conducted in a greenhouse at Federal University of Santa Maria between 2018 and 2019. The experimental design adopted was a completely randomized design with five repetitions per treatment. The treatments consisted of two barnyardgrass biotypes, resistant and susceptible, and flood and dry water regimes: flooded and non-flooded. Plant cycle was evaluated, using phenology and Haun Scale, plant height and seed dormancy. Regardless phenology and Haun Scale showed greater leaf emission when biotypes were submitted to irrigation, giving advantage to the resistant biotype, due to its initial development. Initial development allowed the resistant biotype to occupy the ecological niche in advance. The biotypes in flooded regime showed better initial growth, continued to grow for a longer period and were higher than the plants non-flooded. The resistant biotype in flooded exhibits greater initial growth, growth rate, and extended growth period as observed in phenology and by the Haun scale. Additionally, flooding affects barnyardgrass seed dormancy, promoting a longer period for dormancy in both resistant and susceptible biotypes than in non-flooded conditions. Timely herbicide use, water management, and integrated strategies are essential to prevent resistant barnyardgrass from reaching critical growth and reproductive stages.</p>