Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Sharon Clay


bradyrhizobia, enlist soybean, herbicide, nodulation, weeds, xtend soybean


Foliar-applied Bradyrhizobia to V4 soybean has been reported to increase yield up to 5%. However, a stand-alone product application may not be practical. Applying with other treatments such as post-emergence herbicide application may be economical but herbicide and/or additives may be deleterious to rhizobial growth. A laboratory study investigated the impact of herbicides (glyphosate and dicamba), additives (an oil to improve absorption and spreading; and AMS used to overcome hard water impacts on glyphosate), and herbicide + additives on bacterial growth. Optical density (OD) measurements at the wavelength of 650 nm assessed solution turbidity, a surrogate measure of bacterial growth. Glyphosate, dicamba, and AMS, as stand-alone treatments, reduced OD values by 98, 64 and 100%, respectively, compared to control (deionized water + inoculant) after 72-hr. Herbicide + additives, however, had OD values 25 % greater than the control. Therefore, applying bradyrhizobia with post-emergence herbicide applications at labeled rates with typical mixtures of surfactants/additives should not be harmful to the bacteria. Field experiments were conducted at three South Dakota locations for two years where Enlist E3 or Xtend soybean varieties were planted early, mid, or late season. Treatments included preemergence (pre), pre + post emergence auxin herbicides (2,4-D or dicamba), or herbicide solutions mixed with bradyrhizobia to examine weed control, soybean nodulation and activity, yield, and seed protein. Pre-only herbicides resulted in poor weed control and reduced yields. Pre + post emergence treatments improved weed control and yield, with early and mid-planting having greater yields than late planting. Uncontrolled weeds in the pre and pre + auxin-based treatments were mostly grasses including barnyardgrass (Echinochloa crus-galli), volunteer wheat (Triticum aestivum), large crabgrass (Digitaria sanguinalis), green foxtail (Setaria viridis) and volunteer corn (Zea mays). Rhizobia application did not impact soybean nodulation, yield, or seed protein in 27 out of 30 treatments. The exception was dicamba + glyphosate + rhizobia that enhanced nodulation numbers (+30%) and activity (+54%) in one location in one year for all three planting dates compared to dicamba + glyphosate, although yield and seed protein content were similar among these treatments.

Library of Congress Subject Headings

Soybean -- Weed control.
Soybean -- Yields.

Number of Pages



South Dakota State University



Rights Statement

In Copyright