Degree Type


Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Micheal D. K. Owen


Iowa farmers rely on glyphosate (N-(phosphonomethyl) glycine) resistant crops for weed management in row crops. Inconsistent glyphosate control was reported in common waterhemp (Amaranthus tuberculatus (Mq. ex DC) and horseweed (Conyza canadensis (L.) Cronq.) populations. Therefore, an investigation was undertaken to ascertain the potential for selection of glyphosate resistance in common waterhemp and assess the possibility for transfer of glyphosate resistance in two Conyza species.;Rate responses verified that 0.62 kg acid equivalents (ae) of glyphosate ha-1 were required to reduce biomass by 50% ( GR50) in common waterhemp from Everly, Iowa, compared to 0.24 kg ae ha-1 of a pristine population from Paint Creek, Ohio. Recurrent selection was performed to isolate resistant and susceptible plants within the Everly population. Rate responses suggested that the frequency of resistant individuals increased in the first (S1) and second (S2) recurrent generations and that selection reduced the overall population variability to glyphosate. Interestingly, both the S1 and S2 populations selected for glyphosate resistance demonstrated sex ratios skewed towards maleness. Since variability for glyphosate resistance remained in the S2, a three-level selection strategy was used to isolate asexually propagated plants with a homogenous response to glyphosate. This research suggested that resistance can evolve in common waterhemp with a variable response to glyphosate.;Hybridization between the glyphosate resistant horseweed and dwarf fleabane (Conyza ramosissima Cronq.) ranged from 0% to 9% in assisted crosses and >95% in artificial crosses. The interspecific hybrid ( FH1 ) was phenotypically intermediate to both parents, but shared more homology to the dwarf fleabane parent. Stability, heterosis, and absence of reproductive barriers confirmed that the FH1 was fertile. Inheritance of glyphosate resistance in the FH1 followed the hybrid resistance model and the nuclear encoded, incompletely-dominant single gene (R-allele) model in the hybrid progeny ( FH2 ). We argue that adequate fitness and niche differentiation are prerequisites for successful hybrid adaptation in the environment and that hybridization in Conyza may complicate the containment of glyphosate resistance in current agroecosystems.



Digital Repository @ Iowa State University,

Copyright Owner

Ian A. Zelaya



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174 pages