Seeds of a putative 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide–resistant tall waterhemp biotype from Henry County, IA, were collected from a seed corn field in fall 2009 after plants were not controlled following a POST application of mesotrione plus atrazine. The response of this biotype to various herbicide modes of action was evaluated in greenhouse and field tests. Under greenhouse conditions, the suspect biotype showed an eightfold decrease in sensitivity to mesotrione with a 50% control rate of 21 g ha⁻¹ compared with 2.7 g ha⁻¹ for the susceptible biotype. The biotype also had a 10-fold decrease in sensitivity to atrazine and a 28-fold decrease in sensitivity to thifensulfuron. Under field conditions, tall waterhemp was not controlled POST at the label rate of 1,100 g ha⁻¹ of atrazine. Tall waterhemp control was less than 60% at the label rates of three commonly used POST HPPD-inhibiting herbicides in seed corn: 105 g ha⁻¹ of mesotrione, 92 g ha⁻¹ of tembotrione, or 18 g ha⁻¹ of topramezone. Thus, this new tall waterhemp biotype is resistant to three herbicide modes of action: HPPD inhibitors, photosystem-II inhibitors, and acetolactate synthase (ALS) inhibitors.

Nomenclature: Atrazine; mesotrione; tembotrione; thifensulfuron, topramezone, tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer AMATU; corn, Zea mays L. ZEAMX