Date of Award

January 2016

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Botany and Plant Pathology

First Advisor

William G Johnson

Committee Member 1

Stephen C Weller

Committee Member 2

Steve G Hallett

Committee Member 3

Fred Whitford


Water is the primary carrier for herbicide applications; therefore, carrier water quality factors such as pH, hardness, temperature, or turbidity could be critical for influencing herbicide performance. Currently, there is no information on the effect of carrier water temperature on herbicides applied for weed control in agronomic crops. In previous research, carrier water pH or hardness was found to influence herbicide efficacy; however, the results were variable depending upon herbicides and weed species. The antagonism of coapplied agrochemicals on glyphosate efficacy were reported by previous researchers. In contrary, the enhancement of glyphosate efficacy was illustrated by water conditioning adjuvants such as ammonium sulfate (AMS). At present, dicamba or 2,4-D resistant crops have been developed and the newer formulation of dicamba or 2,4-D and their premixed formulation with glyphosate will be applied on these crop systems. Likewise, hydroxyphenylpyruvate dioxygenase (HPPD); or mesotrione, glufosinate, isoxaflutole (MGI) resistant soybeans are being developed which will allow the use of mesotrione herbicide. Gaining knowledge on effect of carrier water quality will help for optimizing spray solution for herbicide application on herbicide resistant crops that will be available in the near future. Therefore, the objectives of this research were to evaluate the influence of water temperature, spray-solution holding duration, pH, hardness, coapplied foliar fertilizers, and water conditioning adjuvants on low volatility formulation of dicamba and its premix with glyphosate, 2,4-D choline and its premix with glyphosate, glufosinate, and mesotrione efficacy for problematic broadleaf weeds control. A series of field and greenhouse experiments were conducted to accomplish the research objectives. Herbicide spray solution storage time, ≤ 24 h after mixing herbicide, did not affect efficacy of premixture of dicamba plus glyphosate; 2,4-D choline; glufosinate; or mesotrione on giant ragweed (Ambrosia trifida L.), horseweed [(Conyza canadensis (L.) Cronq.], Palmer amaranth (Amaranthus palmeri S. Wats.), and pitted morningglory (Ipomoea lacunosa L.). When considering spray solution temperature, premixed dicamba plus glyphosate efficacy was reduced at 5 C compared with 22 or 39 C. Horseweed, Palmer amaranth, and pitted morningglory control with 2,4-D choline was greater with spray-solution at 22 C compared with 5 C. Spray-solution at 5 C or 56 C negatively influenced glufosinate efficacy for horseweed or pitted morningglory control. Likewise, Palmer amaranth control was reduced at least 13% with spray-solution at 5 or 56 C compared with 22 C for mesotrione application. There was no interaction of carrier water pH and hardness on 2,4-D choline or premixed dicamba plus glyphosate efficacy for weed control. The significant main effect of carrier water pH or hardness was observed on herbicide efficacy. Dicamba or 2,4-D, and their premixed with glyphosate; or glufosinate efficacy was reduced with carrier water at alkaline pH compared with acidic pH. While, mesotrione activity on horseweed was reduced with carrier water pH 4 or 9 compared with pH 6.5. Coapplied Zn foliar fertilizer reduced dicamba, premixed dicamba plus glyphosate, and mesotrione efficacy. Coapplied Mn foliar fertilizer reduced 2,4-D choline and premixed 24-D choline plus glyphosate efficacy. While coapplied Zn or Mn fertilizer did not influence glufosinate efficacy compared to no foliar fertilizer. Dicamba and premixed dicamba plus glyphosate efficacy was improved with the addition of potassium phosphate dibasic (PPD). Addition of AMS resulted greater efficacy of 2,4-D choline, premixed 2,4-D choline plus glyphosate, glufosinate, and mesotrione for giant ragweed, horseweed, and Palmer amaranth control. Carrier water hardness and water conditioning adjuvants did not have an interaction effect on herbicides efficacy, except for giant ragweed control with 2,4-D choline and its premixed formulation with glyphosate, and Palmer amaranth control with glufosinate. Increased carrier water hardness reduced herbicide efficacy in a linear trend for giant ragweed, horseweed, and Palmer amaranth control. Overall, the increased carrier water hardness from 0 to 1000 mg L-1 reduced herbicide efficacy at least 11%. Use of water conditioning adjuvants: DPP or AMS improved herbicide performances in the presence of hardness cations. The results of this research demonstrated that carrier water quality: temperature, pH, or hardness; coapplied foliar fertilizers; and water conditioning adjuvants influenced herbicide efficacy. Therefore, an optimum spray solution will be crucial for obtaining the greatest performance from herbicides applied on herbicide resistant crops in the near future. Moreover, spray solution should be adjusted to an optimum condition depending upon herbicide chemistry.