Predicting Herbicide Dissipation in Container Nursery Crop Production - A Method for Improving Herbicide Performance and Reducing Hand Weeding

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Title: Predicting Herbicide Dissipation in Container Nursery Crop Production - A Method for Improving Herbicide Performance and Reducing Hand Weeding
Author: Judge, Caren Ann
Advisors: Joseph C. Neal, Chair
Jerome B. Weber, Co-Chair
Ross B. Leidy, Member
Abstract: In southeastern U.S. container nursery crop production, frequent applications of preemergence herbicides supplemented by hand weeding are relied upon for broad-spectrum weed control during the growing season. Experiments were conducted to determine the aqueous concentrations required for weed control, to relate this to surface-applied rates, and to determine trifluralin dissipation in container substrates. Petri dish experiments were conducted to determine the aqueous concentration required for control of common nursery weeds including ) were also included as potential bioassay species. Herbicides evaluated were isoxaben, oryzalin, and trifluralin. The relative response of weeds to aqueous concentrations was similar to that observed in efficacy trials. Concentrations of Gallery required for 80% inhibition (I80) were 1.3 and 0.4 for eclipta, 0.4 and 3.9 for hairy bittercress, 0.5 and 0.3 for spotted spurge, 1.5 and 0.3 for large crabgrass, and 1.6 and 0.1 ug ai/mL for lettuce shoot and root, respectively. I80 values for Surflan were 9.8 and 0.4 for eclipta, 5.9 and 1.6 for hairy bittercress, 1.2 and 1.4 for spotted spurge, 1.2 and 0.1 for large crabgrass, and 17.4 and 0.6 ug ai/mL for lettuce shoot and root, respectively. I80 values for Treflan were 73.8 and 3.4 for eclipta, 17.3 and 7.4 for hairy bittercress, 6.2 and 9.2 for spotted spurge, 1.1 and 0.5 for large crabgrass, 7.2 and 8.4 for lettuce, and 0.9 and 1.2 ug ai/mL for perennial ryegrass shoot and root, respectively. Oat I80 values were well beyond the concentration range tested and extrapolated values were not realistic for Gallery and Surflan. However, Treflan I80 values were 0.5 and 2.1 ug ai/mL for shoot and root inhibition, respectively. Treflan dose-response experiments were conducted in the greenhouse and outdoors to determine the surface-applied rates necessary for preemergence control of large crabgrass and perennial ryegrass. The rates of Treflan in the greenhouse were 0.07 to 2.24 kg ai/ha and outdoors were 0.14 to 4.48 kg ai/ha. Percent control was estimated 3 and 6 weeks after treatment (WAT); shoot fresh weights were measured 6 WAT. In the greenhouse, approximately 1.0 kg ai/ha was necessary to control both species. Outdoors, less than 2.0 kg ai/ha was needed for control of both species 3 WAT. By 6 WAT, 2.6 and 3.4 kg ai/ha were required to control perennial ryegrass and large crabgrass, respectively. Additionally, an experiment was conducted at two locations to determine the dissipation of Preen (trifluralin) in a pine bark plus sand potting substrate and to compare grass inhibition over time. Preen was surface applied at 4.5 kg ai/ha, then large crabgrass and perennial ryegrass were seeded 0, 1, 2, 4, 6 and 8 WAT. Shoot and root growth were measured two weeks after seeding. Substrate samples taken 0, 1 and 3 days after treatment and 1, 2, 4, 6 and 8 WAT from the top 2 cm of the potting substrate were extracted and trifluralin content quantified by gas chromatographic techniques. In the March to May test, 50% weed growth occurred approximately 7 WAT, at which time weeds emerged and flourished. However, in the May to July test, trifluralin dissipated more rapidly. Growth (50%) occurred within 2 to 7 weeks after application, depending on species. These data suggest that trifluralin residues in the surface of the potting substrate decrease rapidly after application and slowly thereafter reaching what may be considered critically low levels approximately 4 to 6 weeks after application. These results suggest that for trifluralin, the common reapplication interval of 8 to 10 weeks in the southeastern U.S. may need to be shortened to 4 to 6 weeks.
Date: 2002-01-14
Degree: MS
Discipline: Horticultural Science

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