The critical knowledge of crop-weed competition period is important for designing weed management strategies in cropping systems. Field experiments were conducted in 2012 and 2013 growing seasons at the Agricultural Research Institute, Kahramanmaras, Turkey to determine the effect of three nitrogen (N) rates (0, 150 and 300 kg N ha(-1)) on the critical period for weed control (CPWC) in cotton. A four parameter log-logistic model was fit to the two sets of relating relative crop yield to data obtained from increasing durations of weed interference and lengths of weed-free period. In both the years and regardless of the N application rate, the relative yield of cotton decreased with increasing duration of weed-interference and increased with increasing duration of weed-free period. The application of N fertilizer delayed the beginning of the CPWC in cotton. At the 0 kg N ha(-1), the CPWC ranged from 44 to 537 GDD (growing degree days) (7-51 days after crop emergence [DAE]) in 2012 and 87-605 GDD (14 -57 DAE) in 2013 based on the 5% acceptable yield loss. At the 150 kg N ha(-1), the CPWC ranged from 124 to 597 GDD (18-56 DAE) in 2012 and 183-551 GDD (22-49 DAE) in 2013. Depending on the N application rate, weed-free conditions need to be established as early as one week after crop emergence and maintained as late as eight weeks after crop emergence to avoid more than 5% loss in cotton yield. These findings could help cotton producers improve the cost effectiveness and efficacy of their weed management programme under different N application rates. (C) 2015 Elsevier Ltd. All rights reserved.