Date of Award

12-1969

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

B. J. Gassett

Second Advisor

U. S. Jones

Third Advisor

A. E. Schwertz

Abstract

The influence offol iar applications of chloroxuron, chloroxuron plus surfactant, and 2, 4-DB on the response of soybeans and the control of cocklebur was studied underfield conditions in 1965 and 1 966 at two locations in South Carolina . Concurrent applications of each herbicide were made to soybeans with a natural cocklebur infestation to determine effective weed control and to determine response of soybeans when free of weeds. Early season control of cocklebur, within the limits of crop injury tolerance, was achieved using 4. 0 lb. / A chloroxuron or chloroxur on plus surfactant at 0. 5 to 1. 0 lb. / A applied when weeds were 2 to 3 inches tall. When weeds were growing under conditions of low soil moisture, more chloroxur on was required to achieve effective weed control. Chloroxur on and chloroxuron plus surfactant did not reduce soybean yields when applied to soybeans growing free of weeds. A 0. 2 lb . I A r ate of 2 , 4-DB applie d at mid-bloom stage of soybean development effectively controlled cocklebur; however , weed competition had by then prevailed throughout most of the growing season. Where the higher rates of 2 , 4-DB were applied on soybeans free of weeds , there was a trend toward lower yields, but the reduction in yield was not always significant. Prolonged weed competition and the risk of crop injury renders 2, 4-DB useful primarily as a salvage treatment to control cocklebur if early season control methods are ineffective . The effects of temperature and sunlight on the dissipation of soil surface-applied and soil-incorporated diuron, fluometuron, chloroxuron, and prometryne were studied in a series of field-growth chamber experiments conducted in 1966 and 1967. Bioassay responses relative to a standard bioassay were utilized to evaluate the amount of herbicidal dissipation. The loss of herbicidal effectiveness from the soil surface applied and sunlight-exposed treatments can be attributed in part to direct photodecomposition and to vaporization induced by soil surface temperatures of 120 to 130 F. Such losses would be of particular significance with chemicals applied during periods of prolonged hot, dry weather. Shallow incorporation generally proved effective in reducing herbicide dissipation, thus lengthening the period of effectiveness. To more fully evaluate the significance of high temperatures on dissipation of the four herbicides, a study was conducted exposing soil surface-applied and soil-incorporated treatments to temperatures of 40F, lOOF, 130F, and l60 F. Bioassay responses showed no significant differences be tween soil-incorporated and surface-applied chemicals as to level of dissipation . A progressive loss in herbicidal effectiveness with increasing temperature was shown for diuron, fluomturon , and prometryne, with prometryne being the most susceptible to vaporization. Diuron required a temperature of 160 F before showing significant losses, while fluometuron failed to show a significant reduction in effectiveness with increasing temperature. Conversely, chloroxuron showed a significant increase in effectiveness with an increase in temperature. Dissipation of the four herbicides over time from a glass surface after exposure to sunlight and temperatures of 40 F and 130 F was determined using the ultraviolet range of a Beckman DU spectrophotometer. Prometryne again proved to be the most unstable. In fact, prometryne losses from the 130 F treatment were greater than the progressive loss of chemical observed for the sunlight-exposed treatments. Conversely , the sunlight-exposed treatments for fluometuron, chloroxuron, and diuron showed a progressive loss of chemical in excess of that demonstrated by their respective treatments exposed to 130 F. The greater losses observed for these three herbicides suggest the importance of photodecomposition whenever they are applied to the soil surface.

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