New weed resistance hits Nebraska
A population of common waterhemp has developed resistance to postemergent application of HPPD-inhibiting herbicides (e.g., Callisto, Laudis, and Impact). This resistance occurred in a seed corn production system where these HPPD-inhibiting herbicides were repeatedly used over the last five years.
The good news is that the resistance was identified at only one location. The bad news is that this can easily happen in other fields, as HPPD-inhibiting herbicides are commonly used in Nebraska and the Midwest.
Resistance is a phenomenon that usually results from the repeated use of the same chemical. This can occur with herbicides and many other pesticides. In addition to the weeds already found in Nebraska to be resistant to glyphosate — marestail, giant ragweed, kochia — a new type of weed resistance has developed in the state.
Repeated use of the same herbicide can easily result in the evolution of weed resistance, regardless of the type of herbicide used. We have forgotten the time of atrazine and ALS resistance (15-30 years ago) and with the recent developments of glyphosate resistance, Mother Nature is reminding us that weeds can develop resistance to any type of chemical we develop.
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/ac 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).