I am deeply grateful to the people who assisted me in my long journey to a destination that often seemed unreachable. As the end of this journey approaches, it brings along the end of my program of study. I might have given up half way if it were not for the encouragement of my major professor, Dr. Robert Martin, who has been a role model to me and by his sincere dedication as a scholar and teacher. He has inspired me to become a competent researcher and, more important, a caring individual in my relationship with peers and, more importantly, those with whom I have conducted field research. My deep appreciation is also extended to my committee members Drs. Mark Gleason, David Acker, Larry Trede, and Karen Kessel. Their guidance, resourcefulness and support as a team shaped my academic and professional development. Special recognition and appreciation goes to Dr. George Jackson, Associate Provost and former Dean of the Graduate College, and to Dr. Gregory Geoffrey, the President of Iowa State University, for funding my assistantships. Their financial support has made this journey possible. I am most grateful to my family members (and their spouses who are too numerous to list) for their encouragement and prayers: my mom, Miriam Wambui, dad, Simon Kagima,
It is well established that pre-emergence herbicide activity is influenced by soil type, and recommended rates for most herbicides are adjusted to soil type. Current technology allows application rates to be adjusted on-the-go Variable Rate Application (VRA), adjusting the rate as soil type changes in a field. Most applicators offering this service base herbicide rates on soil organic matter content. However, sever~ other factors influence herbicide activity, most importantly weed density and the environment (weather). This research was done to determine which of these factors was most important in determining the optimum herbicide rate, the benefit ofVRA to the grower, and to determine the relative contribution of soil type, environment, and weed density to herbicide effectiveness. Experimental areas were selected in Ogden, Nashua, Hinds and Bruner Farms based on differing soil types and environmental factors. The experiment was established as a split plot design in which micro-plots were established within main plots. Each micro-plot received different seedbank levels of giant foxtail seeds. Herbicide treatments were the main plot, with foxtail seed inputs as the split plot. Foxtail populations and biomass in the augmented micro-plots were significantly higher than micro-plots with the native populations. Increasing foxtail seed banks cau_sed higher populations and biomass of weeds across all metolachlor treatments, while lower level seed banks maintained lower weed densities and biomass. Therefore, an increase in foxtail seed bank had a negative effect on herbicide efficacy. An increase in herbicide rate could not overcome the effect of higher foxtail populations.Reduced efficacy at high seedbank densities could be explained by increased genetic X diversity within the weed population or a dilution of the herbicide by the increase in weed population. Greater weed diversity would increase resistance or tolerance, extension of period of emergence and the likelihood of weeds being in areas where they might not be contacted by herbicides. Increasing weed population could result in a sufficiently higher weed density that plants might not absorb lethal amounts of herbicide. It is documented elsewhere that increasing weed density resulted in a decrease in herbicide efficacy and data from this study
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