Masters Theses

Date of Award

6-1976

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Larry S. Jeffery

Committee Members

Laurence N. Skold, Elmer L. Ashburn

Abstract

The residues of some preemergence herbicides used for weed control in cotton (Gossypium hirsutum L.) can present problems for alternate crops if cotton stands are not satisfactory and recommended planting dates for cotton have passed. The selection of an alternate crop to cotton may be dependent on the herbicides initially applied for preemergence weed control. Fluometuron [1,l-dimethyl-3-(a,a,a-trifluorom- tolyl)urea] is a widely used cotton herbicide which has been shown to cause injury to alternative crops. Tetrafluoron (3-tetrafluoroethyoxyphenyl-N,N-dimethyl urea) is an experimental substituted urea herbicide which compares favorably with fluometuron as a preemergence cotton herbicide. The objectives of this 1975 study were (1) to determine the tolerance of soybeans [Glycine Max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench.] to tetrafluoron residues when grown as alternate crops to cotton, (2) to compare the effect of tetrafluoron on these alternative crops with that of fluometuron. The soil types selected for this study were a Statler loan at Knoxville, Tennessee, and a Grenada silt loam at Milan, Tennessee. The five main treatments consisted of tetrafluoron applied at the rate of 3.4 kg/ha broadcast, 1.7 kg/ha broadcast, 1.7 kg/ha on a treated area basis restricted to a 36 cm band, fluometuron applied at the rate of 1.7 kg/ha broadcast and a nontreated check. At Knoxville, a sprayer calibration error resulted in a 40 percent increase in the amount of tetrafluoron applied to the banded area, therefore the band treatment was applied at 2.4 kg/ha. Each main treatment was divided into three subplots consisting of three, six, and nine week intervals from date of application to planting. The 1.7 kg/ha tetrafluoron and fluometuron broadcast treatments did not cause significant injury or yield reduction to grain sorghum at any planting date at Knoxville, but a six week waiting period was required before soybeans could be grown successfully following either treatment. The 3.4 kg/ha broadcast and 2.4 kg/ha band tetrafluoron treat-ments required a waiting period of at least six and nine weeks before grain sorghum and soybeans, respectively, could be grown without injury or yield reduction. At Milan, the 1.7 kg/ha tetrafluoron band application did not cause a significant reduction in either soybean or grain sorghum growth. The area treated with 1.7 kg/ha fluometuron broadcast required a six week waiting period before soybeans could be grown without injury, but only a three week period before grain sorghum could be grown successfully. After nine weeks the 1.7 kg/ha and 3.4 kg/ha tetrafluoron broadcast treated areas significantly reduced soybean response, while a six week waiting period was required before a grain sorghum could be grown successfully on these treated areas. Soybeans grew more successfully on an area broadcast with fluometuron than on an area broadcast with tetrafluoron at Milan. At both locations grain sorghum was more tolerant to both tetrafluoron and fluometuron than were soybeans. Results from oat bioassays indicated that both tetrafluoron and fluometuron residues remained primarily in the top 20 cm of the soil profile, but areas treated with 3.4 kg/ha tetrafluoron had detectable residue at all depths. Herbicide concentrations throughout the soil profile dissipated with time. The organic matter content of the soil and the water solubility of the herbicides are probably responsible for the differences seen between the two locations.

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