Differential Responses to Preemergence and Postemergence Atrazine in Two Atrazine‐Resistant Waterhemp Populations

Waterhemp [Amaranthus tuberculatus (Moq) Sauer] is a difficult‐to‐control dicot weed in the United States. Atrazine [6‐chloro‐N‐ethyl‐N’‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] is commonly used for preemergence (PRE) and postemergence (POST) waterhemp control in maize (Zea mays L.). Previous research reported that atrazine metabolism via glutathione S‐transferase (GST) activity contributes to atrazine POST resistance in two waterhemp populations from Illinois, designated MCR (McLean County, Illinois, resistant) and ACR (Adams County, Illinois, resistant). Objectives were to quantify responses of these populations to atrazine PRE and determine if the combination of a GST inhibitor and atrazine PRE or POST increases their control. Dose‐response analyses indicated MCR was resistant to atrazine PRE relative to ACR or WCS (Wayne County sensitive; herbicide‐sensitive population), despite MCR and ACR exhibiting equivalent levels of atrazine resistance POST. The ACR response to atrazine PRE (LD50) was intermediate compared with MCR and WCS. Seedling survival of ACR was reduced by 4‐chloro‐7‐nitrobenzofurazan (NDB‐Cl; a GST inhibitor) and atrazine PRE more than atrazine PRE alone, but not in MCR. Atrazine following NBD‐Cl applied POST inhibited seedling growth in ACR, but not in MCR. Enhanced atrazine activity with NBD‐Cl further supports rapid metabolism via GSTs as the main atrazine‐resistance mechanism in ACR. GST(s) that metabolize atrazine in MCR may not have been completely inhibited by NBD‐Cl, indicating that similar yet distinct atrazine‐resistance mechanisms exist in MCR compared to ACR. In conclusion, atrazine PRE (with or without NBD‐Cl) still controls ACR when applied at typical field‐use rates in maize, but the length of residual activity may be shorter than in sensitive populations. Core Ideas Two waterhemp populations are resistant to atrazine POST due to increased metabolism by GST enzymes. Since atrazine can also be soil applied, quantifying the levels of resistance in these populations is important for weed management in maize. The MCR population was resistant to atrazine PRE and displayed a higher level of resistance to atrazine PRE than the ACR population. NBD‐Cl, a metabolic inhibitor of GST enzymes, enhanced atrazine activity in the ACR population but not MCR. The length of residual activity of atrazine may be shorter for controlling these populations in maize compared with sensitive populations.