The influence of glufosinate in auxinic-based herbicide systems by Grace Flusche Ogden, B.S. A Thesis In Plant and Soil Science Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Peter A. Dotray, Ph.D. Chair of Committee Katie L. Lewis, Ph.D. J. Wayne Keeling, Ph.D. Mark Sheridan Dean of the Graduate School December, 2020 Copyright 2020, Grace Ogden DEDICATION To Jack. Texas Tech University, Grace Flusche Ogden, December 2020 ACKNOWLEDGMENTS I would like to give my sincere appreciation and gratitude to Dr. Peter Dotray for serving as a mentor and providing the opportunity to study weed science. I would like to thank Drs. Katie Lewis and Wayne Keeling for their guidance, feedback, and support of this project. I admire and respect all who served on my committee and am honored to have had the opportunity to work with them. My fellow graduate students Ubaldo Torres, Delaney Foster, and Kyle Russell have been an invaluable help throughout this process. Bobby Rodrigues was always there for technical support and to lend a helping hand. Andrew Dotray provided a boost of morale on hot summer days. Thank you to each of them for their time and effort to help this project reach completion. This project would not have come to fruition without the financial support of Legacy Monsanto Company and the field presence of John Everitt. Thank you to John for his field support and inquisitive nature. Texas Tech University and Texas A&M Agricultural Experiment Station provided the facilities for this project and countless opportunities to share this research with the community. Dr. Jaroy Moore and Beau Henderson kindly let the Weed Science team grow our rouge weeds and understood the need for our unsightly research blocks. Kathy, Robyn, Gail, Joann, and Diann encouraged me and always met my questions or requests with kindness. Thank you to both institutions and their wonderful staff. My family served as my biggest source of encouragement throughout this journey and I am immensely thankful for their support. My husband, Jace, spent many evenings and weekends helping with these projects and encouraging me to keep the faith. My parents gave the best pep talks and were always eager to hear about what I was learning. Thank you to all my family for fostering my weed science interests and playing the “What Weed is This?” game on Saturday afternoons. ii Texas Tech University, Grace Flusche Ogden, December 2020 TABLE OF CONTENTS DEDICATION.................................................................................................................... i ACKNOWLEDGMENTS ................................................................................................ ii ABSTRACT ...................................................................................................................... iv LIST OF TABLES ........................................................................................................... vi CHAPTER I: LITERATURE REVIEW .........................................................................1 Literature Cited ....................................................................................................... 11 CHAPTER II: THE INFLUENCE OF GLUFOSINATE IN DICAMBA BASED HERBICIDE SYSTEMS .................................................................................................17 Abstract ................................................................................................................... 17 Introduction ............................................................................................................. 19 Materials and Methods ............................................................................................ 21 Sequential Applications of Glufosinate and Dicamba with and without Acetochlor ........................................................................................................ 21 Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals ............................................................................................................ 23 Results and discussion ............................................................................................. 25 Sequential Applications of Glufosinate and Dicamba with and without Acetochlor ........................................................................................................ 25 Sequential Applications of Glufosinate or Dicamba at 3, 7, and 10 Day Intervals ............................................................................................................ 29 Literature Cited ....................................................................................................... 32 CHAPTER III: THE INFLUENCE OF GLUFOSINATE IN 2,4-D BASED HERBICIDE SYSTEMS .................................................................................................41 Abstract ................................................................................................................... 41 Introduction ............................................................................................................. 43 Materials and Methods ............................................................................................ 45 Sequential Applications of Glufosinate and 2,4-D Choline ............................. 46 2,4-D Choline Tank-Mix Applications ............................................................ 47 Results and Discussion ............................................................................................ 49 Sequential Applications of Glufosinate and 2,4-D Choline ............................. 49 2,4-D Choline Tank-Mix Applications ............................................................ 50 Literature Cited ....................................................................................................... 53 SUMMARY AND CONCLUSIONS ..............................................................................62 iii Texas Tech University, Grace Flusche Ogden, December 2020 ABSTRACT Palmer amaranth (Amaranthus palmeri S. Watson) is native to the southwestern United States and for decades has been one of the most common weeds in West Texas. In recent years, this weed has been considered one of the most troublesome across the southern United States. The management of Palmer amaranth has changed since the discovery of glyphosate resistant populations in 2005. Dicamba and 2,4-D tolerant cotton (Gossypium hirsutum L.) systems were introduced in 2017 and provide a new opportunity to manage glyphosate resistant populations of Palmer amaranth. The use of glufosinate (Liberty® 280 SL) in auxinic-based herbicide systems may not only improve the management of glyphosate-resistant Palmer amaranth, but also be effective against new developments of herbicide resistance to group 4 modes of action. Four field studies were conducted in a non-crop environment in Lubbock, Texas in 2018 and 2019 to determine the influence of sequential spray order and role of glufosinate when used in a system with dicamba or 2,4-D to control Palmer amaranth at different growth stages. Palmer amaranth control decreased as Palmer amaranth size at initial application increased for both dicamba and 2,4-D studies. A difference in efficacy based on herbicide order was observed for < 10 cm Palmer amaranth in the sequential applications of glufosinate and dicamba with and without acetochlor study. Glufosinate followed by dicamba was less effective than dicamba followed by glufosinate at multiple rating dates in both years. Dicamba + acetochlor followed by glufosinate provided greater Palmer amaranth control than dicamba followed by dicamba at one or more rating dates across all weed sizes. The addition of acetochlor reduced new weed flushes when applied to > 30 cm Palmer amaranth in 6 out of 8 treatments. No treatment controlled > 30 cm Palmer amaranth more than 55%. No consistent difference in efficacy based on herbicide order was observed for > 10 cm Palmer amaranth. Sequential applications of glufosinate or dicamba at 3, 7, and 10 day intervals evaluated the influence of sequential spray timing of dicamba and glufosinate. Sufficient evidence was not obtained in these studies to alter sequential application timing interval recommendation from current label restrictions. Sequential applications of glufosinate and 2,4-D choline evaluated the influence of sequential spray order when applying glufosinate and 2,4-D choline with or without pyrithiobac when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. When applied to 7 to 15 cm iv Texas Tech University, Grace Flusche Ogden, December 2020 Palmer amaranth, 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 100% 10 days after sequential application in 2018 and 97% 21 days after sequential application in 2019. When applied to 25 to 30 cm, Palmer amaranth 2,4-D choline + glyphosate followed by glufosinate controlled Palmer amaranth 98% 11 and 21 days after sequential application in 2018. 2,4-D choline provided greater control when used in the initial application and followed by glufosinate than glufosinate followed by 2,4-D choline in both weed sizes. 2,4-D choline tank-mix applications evaluated the efficacy of tank-mixing 2,4-D choline, 2,4-D choline + glyphosate, and glufosinate when controlling 7 to 15 cm and 25 to 30 cm Palmer amaranth. Seven days after initial application, 2,4-D choline + glufosinate controlled 7