Agronomy and Horticulture Department


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A THESIS Presented to the faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Agronomy, Under the Supervision of Professor Mark L. Bernards. Lincoln, Nebraska: July, 2011

Copyright 2011 Roberto Javier Crespo


Dicamba-resistant soybeans are being developed to provide an additional herbicide mechanism of action that can be used in soybean, and to provide a tool to help manage or mitigate the evolution of other herbicide-resistant weed populations. The objectives of this thesis were to assess the risk of common Nebraska weeds developing resistance to dicamba, quantify baseline dose-response to dicamba of high-risk weed species, and survey the variability in dicamba dose-response among populations of those species. Twenty-five weed scientists were asked to estimate the risk likelihood of ten weed species evolving resistance to dicamba following the commercialization of dicamba-resistant soybean. Palmer amaranth (Amaranthus palmeri), common waterhemp (Amaranthus rudis), kochia (Kochia scoparia) and horseweed (Conyza canadensis) were rated the highest risk species. Ten populations of horseweed, 73 populations of kochia, and 41 populations of common waterhemp were collected across Nebraska. Greenhouse dose-response studies using a range of dicamba doses (0 g ae ha-1 up to 35,840 g ha-1, depending on the species) were conducted on 10 horseweed populations, 10 kochia populations, and 4 common waterhemp populations that represented a range of susceptibility to dicamba in preliminary experiments. Visual injury and dry weight data were measured 28 days after treatment (DAT), data was fit to a four-parameter log-logistic equation, and the dicamba doses necessary to achieve 90% visual injury (I90) or reduction in dry weight (GR90) were calculated for each population. There was a three-fold difference in dicamba dose necessary to achieve I90 between the least and most susceptible horseweed populations, a 18.4 fold difference among kochia populations, and a 1.5 fold difference among common waterhemp populations. Similar variation in susceptibility for each species was calculated for GR90 values. Two or three replications of plants were allowed to grow for 84-228 DAT. The maximum dicamba dose (g ha-1) at which a population was able to reproduce was 280 for horseweed, 8,960 for kochia, and 560 for common waterhemp. One population of kochia was classified as “dicamba-resistant.” Individuals who adopt dicamba-resistant soybean should use multiple methods to control high-risk species to reduce the risk of dicamba-resistant weeds becoming widespread.

Adviser: Mark L. Bernards