There are two types of herbicide resistance, cross resistance and multiple resistance. Cross resistance occurs when a weed biotype has gained resistance to more than 1 herbicide with the same mode of action. This can occur with herbicides in the same or in different families. For example, imidazolinone (Cadre, Pursuit, Scepter) and sulfonylurea herbicides (Accent, Classic, Envoke) have the same mode of action but are in different families. Multiple resistance occurs when a weed biotype has developed resistance to more than one herbicide brought about by different selection pressures. In other words, weeds that have developed resistance to herbicides with different modes of action. For example, some barnyardgrass (Echinochloa crus-galli) populations in California rice fields have been reported to be resistant to both the ACC-ase inhibitors (Clincher, Whip) and thiocarbamates (Ordram, Bolero). Consequently, multiple resistance can be a more difficult problem to address.