Control of Amaranthus tuberculatus var. rudis (common waterhemp) with pre and post-emergence herbicides in Zea mays L. (maize)

Amaranthus tuberculatus var. rudis (common waterhemp) was first identified in Ontario, Canada in 2002. Eight trials were conducted over a 2-year period (2003 and 2004) at two locations to determine the efficacy of pre-emergence and post-emergence herbicides for the control of A. tuberculatus in maize (Zea mays L.). A. tuberculatus was resistant to the acetolactate synthase inhibiting herbicides at both locations and was resistant to the photosystem II inhibiting herbicides at one location; therefore, results varied by location. A. tuberculatus interference in maize resulted in yield loss of up to 38%. Isoxaflutole plus atrazine, s-metolachlor/atrazine, mesotrione, or S-metolachlor/atrazine plus mesotrione when applied pre-emergence all resulted in greater than 96% control of A. tuberculatus at both locations at 70 days after emergence. Dicamba, dicamba/atrazine, and mesotrione plus atrazine when applied post-emergence all provided greater than 90% control of A. tuberculatus at both locations at 70 days after application. Herbicide treatments which effectively controlled A. tuberculatus resulted in maize yields which were equivalent to the weed-free check. Herbicide selection will have to be site-specific depending on the resistance pattern of A. tuberculatus at each site.