Giant ragweed is a challenging weed species to manage in many cropping systems due to its extended period of emergence, rapid growth rate, and high degree of competitiveness with crop species. Giant ragweed has become increasingly prevalent in glyphosate-resistant cropping systems, possibly due to later-emerging plants which avoid exposure to glyphosate. A study conducted in the late 1960’s found that nearly all giant ragweed emerged before May 1 in central Illinois (Stoller and Wax 1973). This early emergence suggested that giant ragweed populations could be effectively managed by preplant tillage or other preplant weed management practices. However, recent research has found relatively late emergence of giant ragweed, with emergence of some biotypes occurring throughout June (Hartzler et al., 2002), potentially later than the timing of post-emergence herbicide applications.