Waterhemp seed was collected in fall 2010, and greenhouse bioassays were conducted during that winter and spring. An initial study was also conducted during the 2011 season to determine dose response for mesotrione at the field level.

Comparisons of dose response curves clearly showed mesotrione resistance level at a minimum of four times the normal rate (shown as 4X). For example, 90% control of a susceptible population was achieved with 3 oz./acre of Callisto applied POST (1X rate), while the resistant populations needed a minimum of 12 oz./acre (4X rate), and in some cases as much as 18 oz. (6X).

HPPD-inhibiting herbicides can still be useful components of the crop production system, but obviously their use pattern should be carefully managed. HPPD-inhibiting herbicides are pigment inhibitors (carotenoid biosyntesis inhibitors). Carotenoid is a red pigment that protects the green pigment (chlorophyll) from harmful UV lights. By stopping the production of carotenoids, the green pigment (chlorophyll) is destroyed by UV light, the plant tissue turns white, and eventually dies.

That’s why HPPD herbicides are commonly called “bleachers.” Besides Callisto (mesotrione), this mode of action includes: Laudis (tembotrione), Balance Flexx (isoxaflutole), Huskie (pyrasulfutole), and Impact (topramezone) and some premixes: Capreno (thiencarbazone-ethyl+tembotrione) and Corvus (thiencarbazone-ethyl + isoxaflutole).