Timing of harvest aids continues to be a difficult decision for producers. Producers and crop advisors often are tempted to wait as long as possible on young immature boll in the top of the plant before making the decision to defoliate. These bolls are often insect damaged, small, and account for little additional gain but the perception of additional lint is difficult to overcome. Validation of the heat unit (HU) concept of timing defoliation beyond the last effective boll population as defined by COTMAN would allow producers to make this decision with greater confidence and allow for an earlier harvest. The objective of this study is to evaluate the heat unit based concept for defoliation timing to traditional methods.