Tolpyralate Applied Alone and With Atrazine for Weed Control in Corn

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) of tolpyralate applied alone or mixed with a constant rate (560 g ai ha) 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 (ED90) 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 for the same level of control of these weed species; suggesting a synergy between the two herbicides.


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).

Site Description
The experiments were conducted in 2015 and 2016 at the Haskell Agricultural Laboratory of the University of Nebraska-Lincoln in Concord, NE (42.37 o N, 96.68 o W).The soil type of the experimental sites was Kennebec series silty clay loam (fine-silty, mixed, mesic Cumulic Hapludolls) with 0 to 2% slopes, 2.8% and 4.5% organic matter in 2015 and 2016, respectively.The soil pH was 6.3 and 5.8, respectively in 2015 and 2016.The GT corn, Pioneer 35F40 was seeded within the first week of June at moderate rate of 61,700 seeds ha -1 with row spacing of 76 cm apart in both years.The field tillage practice was no-till in 2015 and conventional-till in 2016.Soybean was previously cultivated on the experimental fields, with weed control mainly glyphosate-based.Total rainfall from April 1 to October 30 was 67.3 cm in 2015 and 61.0 cm in 2016.Average daily temperature was 23 and 25 o C in 2015 and 2016, respectively.

Experimental Design
The experiments were established as a randomized complete block design with 14 treatments (Table 1), in 4 replicates.The treatments include 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.A commercial formulation of tolpyralate, ShieldEx (tolpyralate 400SC, ISK Biosciences, Concord, OH, USA) has an estimated label rate of 34 g ai ha -1 .
The adjuvants used for all treatments were: High surfactant oil concentrate, HSOC (0.05% v/v, Destiny HC, WinField Solutions, Shoreview, MN, USA), and ammonium sulfate, AMS (20 g L -1 , DSM Chemicals North America Inc., Augusta, GA, USA).Each of the experimental plots was 2 m width by 8 m length with five weed species seeded perpendicular to GT corn rows.The seeded weed species included common waterhemp (Amaranthus rudis L.), common lambsquarters (Chenopodium album), green foxtail (Setaria viridis), velvetleaf (Abutilon theophrasti), and henbit (Lamium amplexicaule) (Azlin Seed Service, Leland, MS, USA).The weed species were seeded with push planters 76 cm apart 4 days before planting the GT corn.Treatments were applied post-emergence of corn at V3 stage (~3 weeks after planting), while weeds were 9 to 13 cm tall.Herbicide applications were made using a CO 2 -pressurized backpack boom sprayer calibrated to deliver 140 L ha -1 at 276 kPa through four 110015-VP flat spray nozzle tips (Turbo TeeJet, Spraying systems Co., P.O.Box 7900, Wheaton, IL 60187) with a boom length of 2 m.

Data Collection
Visually rated weed control on the scale of 0 (no injury) to 100% (dead plant) were collected at 7, 14, 21, 30 and 60 days after treatment (DAT).The visual rating was based on symptoms such as bleaching, chlorosis, and necrosis compared to untreated control.Weed biomass was also collected within 0.25 m 2 quadrant at 60 DAT.Corn was harvested from two middle rows of each plot in October each year, utilizing a combine (Almaco SP40, Nevada, IA, USA) with yield reported at 15% moisture.

Data Analysis
Analysis of variance was conducted to test for interaction between treatment and year of study using PROC GLM procedure in SAS 9.4 software (SAS Institute Inc, 100 SAS Campus Dr, Cary, NC 27513).A four-parameter log-logistic regression model was used to analyze the relationship between herbicide rates, and visual weed control, weed biomass or corn yield (Knezevic et al., 2007): where, Y was the visual weed control, weed biomass or corn yield, C was the lower limit, D was the upper limit, X was the rate of tolpyralate, E was the effective dose (ED50) of tolpyralate that provides a 50% visual control or weed reduction, and B is the slope around E.
The ED 90 values (dose that provided 90% weed control or biomass reduction) were calculated for both tolpyralate alone and tank-mixed with atrazine and corresponding weed species (Knezevic et al., 2018).The ED 90 values between the two curves (tolpyralate alone versus mixed with atrazine) were compared for statistical differences utilizing standard errors (SE).The regression analyses were conducted using R statistical software, version 3.4.1 (R Core Team, 2017).

Results and Discussion
There was no significant interaction between years and treatments on weeds and corn yield responses, thus, data from both years were combined and regression curves fit to 30 and 60 DAT observation dates for each weed

Figure
Figure 4. D