Abstract
The biologically-effective-dose of tolpyralate, a new 4-hydroxyphenyl-pyruvate dioxygenase (HPPD)-inhibitor, applied alone or tank-mixed with atrazine, for the control of multiple-herbicide-resistant (MHR) waterhemp [Amaranthus tuberculatus (Moq.) J. D. Sauer] has not been studied in corn. Seven field experiments were conducted during a three-year period (2018, 2019, 2020) in Ontario, Canada with MHR waterhemp to determine: 1) the dose-response of MHR waterhemp to tolpyralate and tolpyralate plus atrazine, and 2) the relative efficacy of tolpyralate and tolpyralate plus atrazine to post-emergence corn herbicides, dicamba/atrazine (500/1000 g·ha−1) and mesotrione + atrazine (100 + 280 g·ha−1). Tolpyralate + atrazine (120 + 4000 g·ha−1) caused 13% corn injury at one site two weeks after application (WAA), which was observed as transient foliar chlorosis and bleaching of new leaves. At 12 WAA, the predicted dose of tolpyralate for 50% control of MHR waterhemp at Cottam and on Walpole Island was 8 and 2 g·ha−1, respectively; the predicted dose of tolpyralate + atrazine for 50% control of MHR waterhemp at Cottam and on Walpole Island was 5 + 160 and 1 + 21 g·ha−1, respectively. The difference in predicted dose at the two sites is likely due to differences in MHR density and resistance profile. Applied at the registered rate, tolpyralate (30 g·ha−1) and tolpyralate + atrazine (30 + 1000 g·ha−1) controlled MHR waterhemp similar to dicamba/atrazine and mesotrione + atrazine across sites. This study demonstrates that tolpyralate + atrazine, applied POST, provides season-long control of MHR waterhemp in corn.
Highlights
IntroductionWaterhemp has been documented in 19 states of the United States and three Canadian provinces including Ontario [1] [2]
Control was lower at Cottam due to greater waterhemp density and biomass and differences in the resistance profile compared to Walpole Island (Table 4 and Table 7)
All MHR waterhemp populations were resistant to acetolactate synthase (ALS), photosystem II (PS II), enolpyruvylshikimate-3-phosphate synthase (EPSPS), and protoporphyrinogen oxidase (PPO)-inhibitors; Cottam contained a higher proportion of ALS, PS II, and PPO-resistant individuals (Table 1)
Summary
Waterhemp has been documented in 19 states of the United States and three Canadian provinces including Ontario [1] [2]. The spread of waterhemp across Ontario and its ability to proliferate in corn production are facilitated by several biological characteristics, which include a prolonged emergence pattern, rapid growth rate, dioecious reproductive system, and prolific seed production [3] [4] [5]. Waterhemp exhibits an aggressive, indeterminate growth habit allowing it to compete with agricultural crops and reach over 3 m in height in competitive environments [3] [10]. Waterhemp typically produces 300,000 seeds per plant when competing with a growing crop; a plant can produce up to 4.8 million seeds, contributing vast quantities of seed to the soil seed bank [4]
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