Proceedings of the Sixty-first Annual Meeting of the Northeastern ...
Proceedings of the Sixty-first Annual Meeting of the Northeastern ... Proceedings of the Sixty-first Annual Meeting of the Northeastern ...
132 RECENT FINDINGS ON THE FIELD BEHAVIOR, GENETICS, AND MECHANISM OF GLYPHOSATE-RESISTANCE IN HORSEWEED. D.I. Gustafson, M.T. Faletti, G.R. Heck, R.D. Sammons, and M.B. Spaur, Monsanto Company, St. Louis, MO. ABSTRACT We have undertaken a series of investigations to help understand the field behavior, genetics, and mechanism of glyphosate-resistance in horseweed (Conyza canadensis). Our on-going field studies are focused on the long-term impact of various agronomic practices on horseweed populations, such as crop rotation, use of coherbicides, and tillage. We have also studied the relative fitness of resistant and sensitive biotypes under field conditions. Our genetics studies show that glyphosate-resistance in horseweed is inherited through a single nuclear gene, one that is either wholly or partially dominant. We have further investigated whether target site mutation can be implicated as a mechanism for glyphosate resistance in horseweed. Three EPSPS (5-enol-pyruvylshikimate-3- phosphate synthase) genes were found in the species and identical amino acid sequences were determined for the corresponding genes in both sensitive and resistant biotypes. Expression of these genes was comparable in response to glyphosate and in gross organ distribution across the biotypes, leading to the conclusion that glyphosate resistance in horseweed is not due to EPSPS target site mutation, overexpression or gene amplification. Instead, our mechanism research shows that translocation of glyphosate in the resistant biotype is reduced, relative to the sensitive biotype. The biomolecular process responsible for this phenomenon is the subject of current research, both at Monsanto and at other institutions. A number of potential hypotheses have been investigated and will be discussed. Taken together, our findings have many practical implications for growers attempting to manage this important weed. In particular, our results suggest that timely applications of glyphosate and auxin-type herbicides at sufficient rates should be extremely effective in reducing populations of the resistant biotype. 114
133 HORSEWEED EMERGENCE, SURVIVAL, AND SEEDBANK DYNAMICS IN SOUTHEASTERN INDIANA AGROECOSYSTEMS. W.G. Johnson, V.M. Davis, and K.D. Gibson, Purdue Univ., West Lafayette, IN. ABSTRACT Horseweed (Conyza canadensis) is an increasingly common and problematic weed in no-till soybean production in the eastern cornbelt due to the frequent occurrence of biotypes resistant to glyphosate. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual non-glyphosate herbicides, and burndown application timing on the population dynamics of glyphosate resistant (GR) horseweed and crop yield. A field study was conducted from 2003 to 2005 in a no-till field located at a site that contained a moderate infestation of GR horseweed (approximately 1 plant m -2 ). The experiment was a splitplot design with crop rotation (soybean-corn or soybean-soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying in-field horseweed plant density, seedbank density, and crop yield. Horseweed densities were collected at one month after spring applied burndown herbicides (MAB), one month after postemergence applications (MAP), and at the time of crop harvest or 4 MAP. Viable seedbank densities were also evaluated from soil samples collected in the spring prior to germination, in the summer prior to seed rain, and in the fall following seed rain. Crop rotation did not influence in-field horseweed or seedbank densities at any data census timing. Burndown herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season long in-field horseweed densities and protecting crop yields since horseweed in this region behaves primarily as a spring emerging summer annual weed. Horseweed seedbank densities declined rapidly in the soil by an average of 76% for all systems over the first ten months prior to new seed rain. Despite rapid decline in total seedbank density, seed for GR biotypes remained in the seedbank for at least two years. Therefore, to reduce the presence of GR horseweed biotypes in a local no-till weed flora, integrated weed management (IWM) systems should be developed to reduce total horseweed populations based on the knowledge that seed for GR biotypes are as persistent in the seed bank as GS biotypes. 115
- Page 82 and 83: 82 A UNIFYING FRAMEWORK FOR SPECIES
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- Page 108 and 109: 108 PRE AND POST EMERGENT ANNUAL BL
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- Page 112 and 113: 112 Figure 1. Grass cover following
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- Page 122 and 123: 122 THE IR-4 PROJECT: UPDATE ON HER
- Page 124 and 125: 124 PEDIGREE OF A PESTICIDE. D.R. S
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- Page 128 and 129: 128 APPLICATIONS FOR SULFENTRAZONE
- Page 130 and 131: 130 HORSEWEED: FROM OBSCURITY TO TH
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- Page 136 and 137: 136 THE MASSACHUSETTS EXAMPLE: ONE
- Page 138 and 139: 138 Supplemental NEWSS Abstracts (p
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- Page 142 and 143: 142 EVALUATION OF AN HERBICIDE APPL
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- Page 148 and 149: 148 THE 2002 FARM BILL AND ITS EFFE
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- Page 152 and 153: 152 A NEW STANDARD FOR DEFINING AQU
- Page 154 and 155: 154 PORTRAIT OF THE HEALTH STATUS O
- Page 156 and 157: 156 TEN YEARS OF VARIABLE WATER MIL
- Page 158 and 159: 158 Northeastern Weed Science Socie
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- Page 170 and 171: 170 2005, Hilary Sandler and Brent
- Page 172 and 173: 172 (Ornamentals), Rakesh Chandran
- Page 174 and 175: 174 Total Expenses $38,227.24 Total
- Page 176 and 177: 176 PUBLIC RELATIONS Brent Lackey A
- Page 178 and 179: 178 • 2 nd place team: Guelph tea
- Page 180 and 181: 180 WSSA REPRESENTATIVE Jeffrey Der
132<br />
RECENT FINDINGS ON THE FIELD BEHAVIOR, GENETICS, AND MECHANISM OF<br />
GLYPHOSATE-RESISTANCE IN HORSEWEED. D.I. Gustafson, M.T. Faletti, G.R.<br />
Heck, R.D. Sammons, and M.B. Spaur, Monsanto Company, St. Louis, MO.<br />
ABSTRACT<br />
We have undertaken a series <strong>of</strong> investigations to help understand <strong>the</strong> field<br />
behavior, genetics, and mechanism <strong>of</strong> glyphosate-resistance in horseweed (Conyza<br />
canadensis). Our on-going field studies are focused on <strong>the</strong> long-term impact <strong>of</strong> various<br />
agronomic practices on horseweed populations, such as crop rotation, use <strong>of</strong> coherbicides,<br />
and tillage. We have also studied <strong>the</strong> relative fitness <strong>of</strong> resistant and<br />
sensitive biotypes under field conditions.<br />
Our genetics studies show that glyphosate-resistance in horseweed is inherited<br />
through a single nuclear gene, one that is ei<strong>the</strong>r wholly or partially dominant. We have<br />
fur<strong>the</strong>r investigated whe<strong>the</strong>r target site mutation can be implicated as a mechanism for<br />
glyphosate resistance in horseweed. Three EPSPS (5-enol-pyruvylshikimate-3-<br />
phosphate synthase) genes were found in <strong>the</strong> species and identical amino acid<br />
sequences were determined for <strong>the</strong> corresponding genes in both sensitive and resistant<br />
biotypes. Expression <strong>of</strong> <strong>the</strong>se genes was comparable in response to glyphosate and in<br />
gross organ distribution across <strong>the</strong> biotypes, leading to <strong>the</strong> conclusion that glyphosate<br />
resistance in horseweed is not due to EPSPS target site mutation, overexpression or<br />
gene amplification.<br />
Instead, our mechanism research shows that translocation <strong>of</strong> glyphosate in <strong>the</strong><br />
resistant biotype is reduced, relative to <strong>the</strong> sensitive biotype. The biomolecular process<br />
responsible for this phenomenon is <strong>the</strong> subject <strong>of</strong> current research, both at Monsanto<br />
and at o<strong>the</strong>r institutions. A number <strong>of</strong> potential hypo<strong>the</strong>ses have been investigated and<br />
will be discussed.<br />
Taken toge<strong>the</strong>r, our findings have many practical implications for growers<br />
attempting to manage this important weed. In particular, our results suggest that timely<br />
applications <strong>of</strong> glyphosate and auxin-type herbicides at sufficient rates should be<br />
extremely effective in reducing populations <strong>of</strong> <strong>the</strong> resistant biotype.<br />
114
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