Each field has a unique weed community that is composed of several species, and each species is composed of numerous biotypes. Every management tactic, whether cultural, mechanical or chemical, is more effective on some weeds than others. Those weeds (species or biotypes) highly susceptible to a tactic will decline in prevalence, whereas those less affected will increase. Thus, weed communities are in constant flux in response to the weed management practices currently used. Failure to respond to these weed shifts frequently results in control failures and increased management costs.