Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Forestry and Environmental Conservation

Committee Member

Christina Wells, Committee Chair

Committee Member

Michael W Marshall

Committee Member

Matthew Cutulle

Committee Member

Patrick Gerard


Palmer amaranth (Amaranthus palmeri S. Watson) has quickly become one of the most economically important weeds in South Carolina soybean production. Its ability to rapidly accumulate root and shoot biomass allows it to effectively compete with crops for light, nutrients and water. To better understand the below-ground competition dynamics between soybean and Palmer amaranth, greenhouse and field experiments were conducted. In 2016 and 2017, field studies were initiated with a split-plot 3x2x2 factorial treatment design. Treatments factors consisted of: neighbor (Palmer amaranth, soybean, none), divider (with and without) and irrigation (irrigated and non-irrigated). The response variables measured were soil volumetric water content, leaf stomatal conductance and soybean yield (seed dry weight m-1 row). Differences between trial year and irrigation were observed, likely due to the rainfall differences between years. In general, soil water content was lower in the absence of a soil divider when water availability was limited (non-irrigated). In the non-irrigated field in 2016, Palmer amaranth competition depleted soil moisture more than soybean competition. In irrigated plots, Palmer amaranth reduced soybean stomatal conductance by 12% and 14% on the last 2 sampling dates respectively, compared to intraspecific soybean competition. A 56.8% increase in soybean yield was observed in non-irrigated treatments with Palmer amaranth as a neighbor when a divider was present compared to when a divider was absent. In both fields (non-irrigated and irrigated) and both trial years, Palmer amaranth as a neighbor caused a greater reduction in soybean yield than soybean as a neighbor. In general, the below-ground competition from Palmer amaranth reduced soybean yield more than above-ground competition.

A greenhouse study was performed to evaluate the effect of Palmer amaranth soil incorporated residues on the growth of soybean. The study was arranged in a completely randomized experimental design with 5 treatments and 5 replications. Treatments consisted of soybean grown in varying levels of Palmer amaranth residues or pitted morningglory (Ipomoea lacunosa L.) residues of varying concentrations incorporated into equal amounts of soil. Palmer amaranth residues of 160000 ppm and 80000 ppm significantly reduced soybean leaf area by 97% and 94% respectively. Overall, an increase in Palmer amaranth residue in the soil reduced soybean growth and development. This study demonstrated the allelopathic potential of Palmer amaranth residues and the sensitivity of soybean to those residues.



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