Journal of Agricultural Science; Vol. 10, No. 10; 2018 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 32 Tolpyralate Applied Alone and With Atrazine for Weed Control in Corn O. Adewale Osipitan 1 , Jon E. Scott 1 & Stevan Z. Knezevic 1 1 Northeast Research and Extension Center, Haskell Agricultural Laboratory, University of Nebraska-Lincoln, Concord, NE, USA Correspondence: Stevan Z. Knezevic, Haskell Agricultural Laboratory, University of Nebraska-Lincoln, Concord, NE, USA. Tel: 1-402-584-3810. Email: sknezevic2@unl.edu Received: June 27, 2018 Accepted: July 29, 2018 Online Published: September 15, 2018 doi:10.5539/jas.v10n10p32 URL: https://doi.org/10.5539/jas.v10n10p32 Abstract Tolpyralate, an HPPD (4-hydroxyphenyl-pyruvate dioxygenase) inhibitor, is a relatively new herbicide for weed control in corn. Field studies were conducted in 2015 and 2016 to evaluate the effective dose of tolpyralate applied alone or mixed with atrazine for weed control in corn. The treatments included seven rates (0, 5, 20, 29, 40, 50 and 100 g ai ha -1 ) of tolpyralate applied alone or mixed with a constant rate (560 g ai ha -1 ) of atrazine. The evaluated weed species were common waterhemp (Amaranthus rudis Sauer), common lambsquarters (Chenopodium album L.), velvetleaf (Abutilon theophrasti Medik), henbit (Lamium amplexicaule L.) and green foxtail (Setaria viridis L.). Overall, POST-application of tolpyralate resulted in 42-100% visual weed control, depending on the weed species and tolpyralate doses. Calculated dose of 19-31 g ai ha -1 (ED 90 ) of tolpyralate applied alone provided 90% visual control of common waterhemp, common lambsquarters, henbit, and velvetleaf. However, addition of atrazine significantly reduced the required dose of tolpyralate to 11-17 g ai ha -1 for the same level of control of these weed species; suggesting a synergy between the two herbicides. Keywords: corn, effective dose, HPPD, herbicide, tolpyralate, weeds 1. Introduction There is an increase in minimum and no-till systems in United States, which in reality depends heavily on herbicides as the main tool for weed control in corn (Heap & Duke, 2018). Due to widespread and repeated use of herbicides, weed species have developed resistance to most commonly used herbicides. A more recent example is the increase weed resistance to glyphosate. Glyphosate alone accounts for at least 35% of 86 million liters of herbicides used for pre- and post-emergence weed control in corn due to commercialization of glyphosate-tolerant (GT) corn in United States (Livingston et al., 2016). As of 2017, 17 weed species have been confirmed resistant to glyphosate across United States, of which at least 6 are present in Nebraska alone (Heap, 2017). Diversifying the use of herbicides by incorporating alternative modes of action for weed control in general and for managing herbicide resistant weeds in particular have been widely recommended (Owen, 2016; Lamichhane et al., 2017; Osipitan & Dille, 2017). Tolpyralate, an HPPD (4-hydroxyphenyl-pyruvate dioxygenase) inhibitor is a relatively new post-emergence herbicide for weed control in corn (Kikugawa et al., 2015; Morris et al., 2018). This new active ingredient blocks biosynthesis of carotenoids in plants through inhibition of HPPD enzyme resulting in the disruption of photosynthesis followed by death of sensitive plants (Kikugawa et al., 2015). Tolpyralate can be used as part of a diverse weed control program with herbicides of other modes of action. For example, a tank mix of tolpyralate with commonly used herbicides such as chloro-acetamides, dicamba, glyphosate and glufosinate provided excellent weed control (Tonks et al., 2015). In comparison to other HPPD-inhibitors, POST-application of tolpyralate provided weed control equal to or better than mesotrione, topramazone and tembotrione (Tonks, 2016). Currently, information is lacking on the effectiveness of tolpyralate applied alone or in mixture with atrazine for weed control in corn. Atrazine has been the cornerstone of chemical weed control in corn for over 40 years. Atrazine has been known to improve efficacy of several HPPD-inhibiting herbicides (Abendroth et al., 2006; Kohrt & Sprague, 2017). Therefore, the objective of this study was to determine the effective dose of one of the newest HPPD herbicide, tolpyralate, applied alone or mixed with atrazine for control of selected weed species in corn.