AN ABSTRACT OF THE THESIS OF Richard Lyle Chase for the degree of Doctor of Philosophy in Crop Science presented on November 6, 1978 Title: Control of purple nutsedge (Cyperus rotundus L.) with glyphosate. Abstract approved:Redacted for pnvacy --"1" Arnold P. A:0514y /f Research conducted in El Salvador, Central America, demonstrated that an interval of 3 days between application of glyphosate and tillage was sufficient to cause a 90% reduction in purple nutsedge (Cyperus rotundus L.) plants, while delays of 11-23 days generally gave slightly less control. A second application to the same plots 35 days following tillage resulted in greater than 90% reduction at all intervals. Approximately 3 months after the initial treatment the tuber population had been reduced to one half the original population. Germination of the remaining tubers was reduced more than 50%. Glyphosate applied during the dry season caused an average of 79% reduction in plant numbers as compared to 88% in the rainy season. However, in the dry season, the remaining plants had no competition from other weeds and after 5 months there was only a 40% reduction in nutsedge population. During the rainy season 1, 2 and 3 kg/ha were ecually effective, but 1 kg/ha was not sufficient in the dry season. In growth chamber studies, glyphosate at 2 kg/ha was more effective in reducing regrowth of purple nutsedge (Cyperus rotundus L.) scapes at 90% than at 50% relative humidity (r.h.), and more effective at -2 bars than at -11 bars of plant water potential.Regrowth of treated plants under water stress of -1 to -8 bars was reduced 54-60% while at -11 bars growth inhibition was only 34%. A time interval of as little as 8 h between application and excision was sufficient to result in a 47% reduction in regrowth at 90% r.h. None of the treated plants, except those clipped immediately after application, produced new shoots from the basal bulb, while all of the non- treated control plants produced one or more new shoots. Experiments using14C-glyphosatesubstantiate the results observed in the regrowth 14 studies. Three times more C-label was translocated into the under- ground parts of nutsedge at 90% than at 50% r.h. Twice as much translocated at -2 bars than at -11 bars of water stress. Control of purple nutsedge (Cyperus rotundus L.) with glyphosate by Richard Lyle Chase A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed November 1978 Commencement June 1979 APPROVED: Redacted for privacy Professor of Crop Science in charge of major Redacted for privacy Head of Department of Crop Science Redacted for privacy Dean of Graduate School Date thesis is presented November 6, 1978 Typed by Jeanie Chase for Richard Lyle Chase ACKNOWLEDGMENTS I would like to express my sincere appreciation to Dr. Arnold P. Appleby for his assistance and guidance throughout my doctoral program. Thanks are also extended to S. C. Fang, R. L. Stebbins, W. W. Chilcote, and D. 0. Chilcote as members of my graduate committee for their help and encouragement. Special appreciation is conveyed to Dr. Stanley F. Miller, director of the International Plant Protection Center, for financial assistance, counsel and advice. To the staff of that organization I am indebted and grateful for their comradship and help during the past four years. Larry Burrill was especially helpful with ideas, suggestions, and encouragement. I am also appreciative of the friendship and help of my fellow graduate students and all members of the Crop Science Department. Finally, I extend my gratitude and love to my wife, Jeanie, and to my children Kristie, David, Kathie, and Daniel for their patience and encouragement during a difficult two years. This thesis is composed principally of two articles written for publication in Weed Research: I. Effect of intervals between application and tillage on glyphosate control of Cyperus rotundus L. II. Effects of humidity and moisture stress on glyphosate control of Cyperus rotundus L. TABLE OF CONTENTS I. Effect of intervals between application and tillage on glyphosate control of Cyperus rotundus L. A. Summary 1 B. Introduction 2 C. Materials and methods 3 1. Rainy season 3 2. Dry season 4 D. Results 5 1. Rainy season 5 2. Dry season ........... E. Discussion ............ 10 F. Acknowledgments 13 G. Bibliography 14 II. Effects of humidity and moisture stress on glyphosate control of Cyyerus rotundus L. A. Summary ............... 15 B. Introduction 16 C. Materials and methods 18 1. Environmental effects on glyphosate toxicity . 19 2. Translocation of 14C-glyphosate 19 3. Degree of moisture stress on glyphosate toxicity 20 4. Time required for translocation . .....21 D. Results 22 1. Environmental effects on glyphosate toxicity . 22 2. Translocation of 14C-glyphosate ..... 24 3. Degree of moisture stress on glyphosate toxicity ........... ... .. 26 4. Time required for translocation 28 E. Discussion 30 F. Acknowledgments 33 G. Bibliography 34 III. Appendices A. Appendix A 36 B. Appendix B ............ 43 IV. Additional References 52 EFFECT OF INTERVALS BETWEEN APPLICATION AND TILLAGE ON GIITHOSATE CONTROL OF CYTERUS ROTUNDUS L. Research conducted in El Salvador, Central America, demonstrated that an interval of 3 days between application of glyphosate and tillage was sufficient to cause a 90% reduction in purple nutsedge (Cyperus rotundus L.) plants, while delays of 11-23 days generally gave slightly less control. A second application to the same plots 35 days following tillage resulted in greater than 90% reduction at all intervals. Approximately 3 months after the initial treatment the tuber population had been reduced to one half the original population. Germination of the remaining tubers was reduced more than 50%. Glyphosate applied during the dry season caused an average of 79% reduction in plant numbers as compared to 88% in the rainy season. However, in the dry season, the remaining plants had no competition from other weeds and after 5 months there was only a 40% reduction in nutsedge population. During the rainy season, 1, 2 and 3 kg/ha were equally effective, but 1 kg/ha was not sufficient in the dry season. 2 INTRODUCTION Purple nutsedge (Cuerus rotundus L.) is considered by Holm & Herberger (1969) to be the world's worst weed. Some of the principal reasons for this are its ability to proliferate rapidly,to produce tubers that can remain dormant and to compete vigorously. In El 2 Salvador, plant populations of 2,000 plants/m have been reported (Chase, Vides & Reyes, 1977). A 143% reduction in maize yield 2 resulted from a nutsedge population of 700 plants/mthat was not cultivated until 30 days after maize planting (Chase,unpublished). Other researchers have reported on the effectiveness of glyphosate in controlling this weed. Magambo & Terry (1973) in Tan- zania achieved 95-100% control of foliage with 2kg/ha. In Hawaii, glyphosate applied to nutsedge that had been undisturbed for 12 weeks killed the foliage and more than 75% of the tubers(Zandstra & Nishimoto, 1975). In Colombia, Gomez (1976) found that increased control of foliage, as well as a reduction in tuber number and germi- nation of the remaining tubers, resulted when tillage followed appli- cation of glyphosate. Because glyphosate is readily bound to soil(Sprankle, Meggitt & Penner, 1975), crops can be planted following application withlittle risk of injury. In this study, we were interested in determining how many days were necessary between application of glyphosateand tillage to obtain optimum control of purple nutsedge during the rainy and the dry seasons in El Salvador. 3 MATERIALS AND METHODS The experiments in this study were conducted at the San Andres Experiment Station, San Andres, El Salvador, from July 1975 to June 1976. The commercial formulation of the isopropylamine salt of glyphosate was used in all experiments. During the rainy season, which lasts approximately 5 months, rainfall averages approximately 13 mm per day. Very little rainfall occurs in the dry season. Rainy season Glyphosate was applied at 1, 2 and 3 kg/ha to a heavy infestation (approximately 1000 plants/m2) of flowering purple nutsedge plants. The sandy loam soil had been disked 35 days earlier. Plots were tilled twice with a rotary tiller to a depth of 10 cm at 3, 7, 11, 15, 19 and 23 days following application. A non-sprayed treatment was til- led 3 days following application. Thirty days following each tillage plants were counted in four 0.1m2 quadratsper plot and averaged. A second application was made 35 days following each tillage in an attempt to further reduce the population by controlling tubers that germinated following the tillage. Plots were again tilled and plants counted as previously described. Applications were made with a CO2 pressurized plot sprayer delivering 280 1./ha. Plot size was 2 x 4 m in a randomized block design with a factorial arrangement of treatments. Each treatment was replicated four times. Sixty days following the last tillage of each treatment, four soil samples 20 cm deep x 20 cm in diameter were taken from each plot 4 and the tubers were counted. One hundred tubers from each plot were covered with moist paper towels, placed at room temperature (about 25 °C) for 5 days and checked for germination. Approximately 8 months following the final application the trial area was disked. Thirty days later the plants were counted. Dry season Glyphosate at 1, 2 and 3 kg/ha was applied to nutsedge plants at the beginning of the dry season. Treatments were made to non- flowering plants 21 days following disking of the sandy loam soil and to flowering plants 35 days following disking, as in the rainy season study.