Weed populations in arable elds have often been found to be distributed heterogeneously in time and space (Marshall, 1988; Thornton et al., 1990; Wiles et al., 1992; Johnson et al., 1996; Gerhards et al., 1997; Christensen and Heisel, 1998; Nordmeyer et al., 2003; Gerhards and Christensen, 2003). They often occur in aggregated patches of varying size or in stripes along the direction of cultivation. The spatial distribution of weeds, however, has mostly been disregarded in weed-management decisions, and herbicides or mechanical weed-control methods are applied uniformly across the whole eld. The use of eld-scale mean density estimates in spatially heterogeneous weed populations results in under-prediction of yield loss at locations where weed density is high and over-prediction in parts of the eld where weed densities are low or weeds are absent (Lindquist et al., 1998; Brain and Cousens, 1990). Spatial variation in weed density must, therefore, be considered in weed-management strategies. When there is large within-eld variation in weed density, variable-rate patch spraying based on the need for weed control may reduce the area treated with herbicides and hence the economic and environmental costs.