Agricultural production systems that reduce the use of in-crop herbicidescould greatly reduce risks of environmental damage and the development ofherbicide-resistant weeds. Few studies have investigated the long-termeffects of in-crop herbicide omissions on weed seedbank community size andstructure. A crop-rotation study was sampled 10 yr after a strictly annualrotation and an annual/perennial rotation were exposed to different in-cropherbicide omission treatments. In-crop herbicides were applied either in allannual crops (control), omitted from oats only, or omitted from both flaxand oats. Seedbank densities were greatest when in-crop herbicides wereomitted from flax and oats, and this treatment also reduced crop yield.Shannon-Wiener diversity differed among crops in the annual crop rotationand among herbicide omission treatments in the perennial rotation. Herbicideomissions changed the weed-community structure in flax and in wheat andcanola crops in the annual rotation enough to warrant alternate controlmethods in some treatments. The magnitude of the effects on the seedbankparameters depended largely on the competitive ability of the crop in whichherbicides were omitted. No yield response to omitting herbicides in oatsindicated that standard weed management practices have reduced weedpopulations below yield-loss thresholds.