Unlike many factors that affect the development of weed resistance to herbicides (Stoltenberg 2004), herbicide selection intensity and can be directly affected by the grower. Herbicide selection intensity is determined by herbicide efficacy, persistence, and frequency of application (Gressel and Segel 1990). The greater the number of susceptible weeds that are exposed to a herbicide and killed, the greater the selection intensity upon that weed population. Reduced herbicide selection intensity will reduce the probability of resistance development and prolong the usefulness of a herbicide mode of action. However, it is essential to balance the benefits of responsible herbicide stewardship with the need to maintain satisfactory levels of weed management. One rationale for adopting an integrated approach to weed management is to reduce herbicide selection intensity on our weed populations.
Although the integration of weed management practices is recommended to reduce the potential for weed resistance to glyphosate (Boerboom and Owen 2006), research to quantify the effectiveness of such integration under field conditions is limited. To address this information need, field data from a long-term experiment (Stoltenberg and Jeschke 2007) at the University of Wisconsin Arlington Agricultural Research Station was analyzed to determine the probability (or likelihood) of occurrence of giant foxtail (Setaria faberi), redroot pigweed (Amaranthus retroflexus), common lambsquarters (Chenopodium album), velvetleaf (Abutilon theophrasti), and giant ragweed (Ambrosia trifida) resistance to glyphosate as affected by crop sequence, tillage system, and intensity of glyphosate use. The goal was to provide a quantitative assessment of the effectiveness of integrated weed management practices to reduce the risk of selection for resistance to glyphosate among some of our most common weed species.