It is important to understand the yield loss associated with different weed species if we use weed thresholds to optimize the economics of weed management. Corn yield loss is often variable among weed species. At a density of 2 weeds per foot of row, corn yield was reduced 10% by giant foxtail, 11 % by common lambsquarters, 18% by velvetleaf, and 22% by common cocklebur (Beckett et al. 1988, Lindquist et al. 1996). Yield loss is also variable among locations and years even at the same weed density (Cowan 1998, Jasieniuk et al. 1999). For example, a study in Illinois reported maximum corn yield loss from common lambsquarters was 12% in 1985, but no yield loss was observed in 1986 or 1987 (Beckett et al. 1988). Langston and Harvey (1994) reported 9 giant foxtail plants per foot of row did not reduce corn yield in 1993 but reduced yield by 18% in 1994. Lindquist et al. (1996) suggested caution should be taken when estimating crop yield loss solely on weed density in bioeconomic weed management models.