Weed Technology Management of glyphosate-resistant Palmer – www.cambridge.org/wet amaranth (Amaranthus palmeri) in 2,4-D , glufosinate-, and glyphosate-resistant soybean 1 2 3 4 Research Article Chandrima Shyam , Parminder S. Chahal , Amit J. Jhala and Mithila Jugulam 1 2 Cite this article: Shyam C, Chahal PS, Jhala AJ, Graduate Student, Department of Agronomy, Kansas State University, Manhattan, KS, USA; Postdoctoral Research 3 Jugulam M (2021) Management of glyphosate- Associate, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, NE, USA; Associate resistant Palmer amaranth (Amaranthus Professor, Department of Agronomy and Horticulture, University of Nebraska-Lincoln, Lincoln, NE, USA and palmeri) in 2,4-D–, glufosinate-, and 4Professor, Department of Agronomy, Kansas State University, Manhattan, KS, USA glyphosate-resistant soybean. Weed Technol. 35:136–143. doi: 10.1017/wet.2020.91 Abstract Received: 7 May 2020 Glyphosate-resistant (GR) Palmer amaranth is a problematic, annual broadleaf weed in Revised: 7 July 2020 soybean production fields in Nebraska and many other states in the United States. Soybean Accepted: 29 July 2020 TM First published online: 18 August 2020 resistant to 2,4-D, glyphosate, and glufosinate (Enlist E3 ) has been developed and was first grown commercially in 2019. The objectives of this research were to evaluate the effect of herbicide Associate Editor: programs applied PRE, PRE followed by (fb) late-POST (LPOST), and early-POST (EPOST) fb Kevin Bradley, University of Missouri LPOST on GR Palmer amaranth control, density, and biomass reduction, soybean injury, and ’ Nomenclature: yield. Field experiments were conducted near Carleton, NE, in 2018, and 2019 in a grower sfield 2,4-D chlorimuron-ethyl; cloransulam-methyl; infested with GR Palmer amaranth in 2,4-D–, glyphosate-, and glufosinate-resistant soybean. flumioxazin; glufosinate; glyphosate; Sulfentrazone þ cloransulam-methyl, imazethapyr þ saflufenacil þ pyroxasulfone, and chlor- imazethapyr; metribuzin; pyroxasulfone; imuron ethyl þ flumioxazin þ metribuzin applied PRE provided 84% to 97% control of GR saflufenacil; sulfentrazone; Palmer amaranth; Amaranthus palmeri S. Watson; soybean; Palmer amaranth compared with the nontreated control 14 d after PRE. Averaged across Glycine max (L.) Merr. herbicide programs, PRE fb 2,4-D and/or glufosinate, and sequential application of 2,4-D or glufosinate applied EPOST fb LPOST resulted in 92% and 88% control of GR Palmer amaranth, Keywords: respectively, compared with 62% control with PRE-only programs 14 d after LPOST. Biomass reduction; density reduction; early- Reductions in Palmer amaranth biomass followed the same trend; however, Palmer amaranth POST followed by late-POST; PRE followed by late-POST; soybean yield density was reduced 98% in EPOST fb LPOST programs compared with 91% reduction in PRE fb LPOST and 76% reduction in PRE-only programs. PRE fb LPOST and EPOST fb LPOST Author for correspondence: programs resulted in an average soybean yield of 4,478 and 4,706 kg ha−1, respectively, com- Mithila Jugulam, Professor, Department of pared with 3,043 kg ha−1 in PRE-only programs. Herbicide programs evaluated in this study Agronomy, Kansas State University, Manhattan, KS 66502. Email: [email protected] resulted in no soybean injury. The results of this research illustrate that herbicide programs are or Amit J. Jhala, Associate Professor, available for the management of GR Palmer amaranth in 2,4-D–, glyphosate-, and glufosinate- Department of Agronomy and Horticulture, resistant soybean. University of Nebraska-Lincoln, Lincoln, NE, 68583-0915. Email: [email protected] Introduction Commercialization of herbicide-resistant (HR) crop technology in the late 1990s led to a turning point in the history of weed management (Reddy and Nandula 2012). Glyphosate-resistant (GR) soybean and corn (Zea mays L.) were rapidly adopted by growers in the United States because of its ease of use, cost-effectiveness, and broad-spectrum weed control (Green and Owen 2011). Currently, HR soybean constitutes 94% of the total soybean planted in the United States (USDA 2019). HR soybean including a single trait such as glyphosate resistance or glufosinate resistance, or multiple HR traits such as glyphosate and dicamba resistance are grown in the United States. The cultivation and widespread adoption of GR corn and soybean after their com- mercialization reduced the use of residual herbicides because of flexibility in application timing, excellent weed control, and wide margin of crop safety with glyphosate (Green 2012); however, repeated application of glyphosate resulted in the evolution of GR weeds (Heap and Duke 2018). Currently, 48 weed species have been reported to have evolved resistance to glyphosate globally, including 17 species in the United States (Heap 2020). Native to the southwestern United States, Palmer amaranth has been ranked as the most troublesome weed in agronomic cropping systems in the United States in a survey conducted © The Author(s), 2020. Published by Cambridge by the Weed Science Society of America (Van Wychen 2019). It is also one of the most eco- University Press on behalf of Weed Science nomically important weeds in agronomic crops in the United States (Beckie 2011; Chahal Society of America. et al. 2018). Palmer amaranth is a prolific seed producer (Keeley et al. 1987) and can survive and produce a high amount of seeds even under moisture-stressed situations (Chahal et al. 2018). It has a high photosynthetic rate, resulting in a fast growth rate and considerable biomass accumulation compared with other Amaranthus species (Ehleringer 1983; Jha and Norsworthy 2009), along with continuous emergence throughout the growing season, leading to season-long crop interference (Jha and Norsworthy 2009). In addition to the aforementioned weedy Downloaded from https://www.cambridge.org/core. IP address: 97.98.140.109, on 29 Jan 2021 at 14:08:55, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/wet.2020.91 Weed Technology 137 characteristics, the evolution of herbicide resistance in Palmer Corteva™ Agriscience and was commercialized in 2019 in the amaranth has made control of this weed especially challenging United States. However, little scientific literature exists pertaining in cotton (Gossypium sp. L.) and soybean. GR Palmer amaranth to the most effective herbicide programs for managing GR Palmer was first reported in Georgia (Culpepper et al. 2006), and since amaranth in this new soybean production system. Therefore, the then, 27 other U.S. states have documented the presence of GR objectives of this research were to investigate and compare the Palmer amaranth (Heap 2020). effect of three preformulated herbicide mixtures applied PRE alone Glyphosate is a systemic, nonselective POST herbicide that tar- or fb a late-POST (LPOST) application of either 2,4-D or glufosi- gets 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the nate or both and sequential applications (early-POST [EPOST] fb chloroplast of sensitive plants, causing inhibition of aromatic LPOST) of 2,4-D or glufosinate on Palmer amaranth density, bio- amino acid production (Bentley 1990). Current mechanisms of mass, crop injury, and yield in a multiyear field study in Nebraska. target-site resistance to glyphosate in Palmer amaranth have been due to either mutation in the EPSPS gene, the molecular target of glyphosate (Dominguez-Valenzuela et al. 2017), or overamplifica- Materials and Methods tion and expression of the EPSPS gene (Chahal et al. 2017; Gaines Site Description et al. 2010; Koo et al. 2018). Reduced glyphosate absorption and ’ translocation have also been reported to impart nontarget-site resis- Field experiments were conducted in 2018 and 2019 in a grower s tance to glyphosate in Palmer amaranth (Dominguez-Valenzuela field infested with GR Palmer amaranth in Thayer County, et al. 2017; Nandula et al. 2012). Considering the extent of GR Carleton, NE (40.30°N, 97.67°E) (Chahal et al. 2017). The field Palmer amaranth populations, effective management programs was rain fed without supplementary irrigation. Palmer amaranth in soybean should focus on methods that reduce survival, seed pro- was the predominant weed species at the research site. The soil duction, and transfer of herbicide-resistance alleles. at the experimental site was silt loam with 63% silt, 19% sand, Palmer amaranth interference can cause substantial yield losses 18% clay, 2.63% organic matter, and 4.8 pH. The previous crop in agronomic crops. For example, a Palmer amaranth density of at the site was no-till soybean. The experiment was arranged in 3 plants m−2 caused 60% yield loss in soybean in Arkansas a randomized complete block design with four replications. Individual plots were 3 m wide (four soybean rows spaced (Klingaman and Oliver 1994), and Bensch et al. (2003) reported – 79% soybean yield loss at a density of 8 plants m−2 in Kansas. 0.76 m apart) and 9 m long. Glyphosate-, 2,4-D , and glufosinate- resistant soybean with 2.5 maturity group was no-till planted at a Early emergence (0-1 wk after crop emergence) and establishment −1 of Palmer amaranth reduce soybean yield compared with late rate of 322,000 seeds ha at a depth of 3 cm on May 10, 2018, and emergence (2-8 wk after crop emergence) (Korres et al. 2019). May 6, 2019. Van Acker et al. (1993) reported the critical period of weed control in soybean ranged from the second soybean-node growth stage Herbicide Treatments (V2) to the beginning pod growth stage (R3). However, in a recent Herbicide programs included PRE, PRE fb LPOST, and EPOST fb multilocation and multiyear study conducted in Nebraska, the LPOST applications with a total of 15 treatments, including a non- critical period of weed removal in soybean was delayed from treated control (Table 1). Herbicides were applied using a – – þ V1 V2 to V4 R5 using PRE herbicides such as saflufenacil CO -pressurized backpack sprayer calibrated to deliver 200 L ha−1 þ þ 2 imazethapyr pyroxasulfone and saflufenacil imazethapyr at 210 kPa equipped with a 2-m wide spray boom equipped with (Knezevic et al.