Masters Theses

Date of Award

5-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Tom Mueller

Committee Members

Robert Hayes, Donald Tyler, Elmer Ashburn

Abstract

The residual activity and rate of dissipation of soil-applied herbicides are important factors in modern agricultural crop production; impacting herbicide efficacy, rotational crops, production economics and the environment. Field and laboratory experiments were conducted in 1992 and 1993 at the University of Tennessee Plant Sciences Field Laboratory in Knoxville, to examine the comparative residual concentration and activity of several corn and soybean herbicides, and the effect that a crop has on their dissipation. The site was of the Sequatchie loam soil series (fine-loamy, siliceous, thermic, Humic Hapudult) pH. 6.1 and 1.6 % organic matter (21). The site had a naturally occurring infestation (> 100 plants per m2 of common cocklebur (Xanthium strumarium L.). which was used as the indicator species in the weed response measurements. The study utilized a split-plot design. Herbicide treatments were replicated four times, and randomized within crop blocks. Whole plots were crop (corn, soybean, no-crop) and sub-plots were herbicide treatment. All plots were sampled approximately 0, 1,2, 3, 4, 6, 8, and 12 wk after application at the 0 to 8 cm soil depth. Measurements of weed control (weed density, height and biomass) were taken at approximately 4, 6, and 8 wk after application. Weed response data indicated that imazaquin, atrazine, and cyanazine controlled common cocklebur better than did clomazone and metribuzin. Soil samples were extracted and quantified using high performance liquid chromatography (HPLC). Each crop + herbicide treatment was treated as an individual regression line, and herbicide dissipation was examined by comparing half-life (DT50) values. Atrazine, metribuzin, and clomazone half-lives were averaged over treatments and seasons and were approximately 27, 22, and 55 days respectively, when determined by empirically fitting the first order degradation model to the data, and 25, 23, and 44 days respectively when determined by the best fit of the linear quadratic formula. DT50 values determined for atrazine and metribuzin are in agreement with previous studies that place atrazine DT50 in the 18 to 25 day range, and metribuzin half-life in the 16 to 50 day range. Atrazine and metribuzin dissipation was more rapid in no-crop plots than in corn or soybean plots in 1993, indicating a relationship between herbicide dissipation and crop presence. Moisture data collected in 1993 revealed that no-crop plots had consistently higher moisture contents than either corn or soybean plots. Soybean and corn appeared to remove more water than the vegetation, primarily common cocklebur, in the no-crop plots. High temperatures and low soil moisture combined to possibly inhibit soil microbial activity, thus suppressing atrazine and metribuzin degradation in the 1993 growing season. Clomazone DT50 values determined in this study differ from earlier studies that place the DT50 of clomazone in the 15 to 45 day range. This might be due to differences in both the application method (PRE vs PPI) and environmental conditions or the analytical method (bioassay vs chemical extraction). The effect of a soybean crop on clomazone dissipation remains unclear. However, positive correlations were made between cocklebur growth (density, height, biomass) and crop presence, indicating that there is a direct relationship between herbicide dissipation and crop presence. Relative dissipation based on chemical analysis was atrazine = metribuzin > clomazone. Few significant correlations existed between herbicide DT50 values and herbicide concentration to cocklebur growth, although one would expect DT50 to be an index of activity.

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