Movement, Dissipation, and Impacts of Isoxaben

(Snapshot TG) in Nursery Runoff Water

Chris Wilson, Raj Bandary, and Ted Whitwell	Melissa Riley
Department of Horticulture	Department of Plant Pathology
Clemson University	Clemson University
Ray Cooper
Dow Elanco, Charlotte, NC

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Introduction

Weed management strategies often require several herbicide applications in containerized plant nurseries each year due to continuing emergence of weeds and degradation/movement of applied herbicides. Granular preemergent herbicide formulations are preferred by the nursery industry due to their ease of application. These materials are applied over-the-top of containerized crops and then activated with irrigation. Depending on plant architecture and pot spacing, much of the applied herbicide may land on non-target surfaces surrounding the target pots, where it is available to move offsite in runoff water and into runoff collection ponds used for water recycling. Ronstar losses of 51 to 55% have been reported for containers spaced 9 in apart and 79 to 80% for containers spaced 12 in apart. Others detected the herbicides simazine and metolachlor in surface runoff water from a container nursery in North Carolina. We have also detected three other commonly used herbicides (Surflan (oryzalin), OH-2 (oxyfluorfen + pendimethalin) in runoff and runoff-collection pond water at nurseries in the piedmont and coastal regions of South Carolina.

Concern exists over the ultimate fate and effects of nontarget herbicide concentrations on non-target organisms in the environment. This concern is particularly important in nurseries that recycle their irrigation water because reapplication of water containing herbicide residues may injure ornamental crops. We earlier detected 0.005 ug/ml oxyfluorfen and 0.002 ug/ml pendimethalin in nursery irrigation water that was recycled from runoff collection ponds. Although they speculated that these concentrations are unlikely to cause plant injury, seedling and mature creeping bentgrass (Agrostis palustris) are injured by irrigation water containing 0.05 and 0.08 ug/ml atrazine respectively.

The objectives of this study were: 1) to determine the movement of isoxaben in runoff water following application of Snapshot TG (DowElanco), 2) to monitor the dissipation of isoxaben in runoff collection pond water, and 3) to determine the effect of residual concentrations of isoxaben in irrigation water on the growth of container-grown plants.

Materials and Methods

Runoff Events. A 4 acre containerized plant nursery production area containing a diversity of plant species was treated with Snapshot TG (0.5% isoxaben + 2.0% trifluralin, DowElanco) in August 1992 and May 1993 at a rate of 100 lb product/A. The treated area received 3.0 lb ai in 1992 and 2.4 lb ai in 1993. The entire area drains into a 0.75 A runoff collection pond through a single storm drain. Overhead irrigation (0.5 in - 1992 and 0.75 in - 1993) was applied following herbicide application and water samples were collected from the runoff water before it entered into the collection pond. Sample collection times were 0.25, 0.5, 1.5, 2.5, and 3.5 hours after runoff water began to enter into the pond on the day of treatment and at 2 and 5 days after treatment (DAT). Measurements of water depth within the runoff discharge pipe, pipe diameter, and slope of the pipe were used to determine the runoff water volume entering the pond during sampling periods.

Pond water samples were collected from the top 12 in of water at a site near the runoff entry point and at the site where water exits the pond to monitor isoxaben dissipation. Pond sediment sampleswere collected at the same sites, and at one other site chosen at random, during the 1993 study using an Eckman dredge. Water and sediment samples were collected before herbicide application, after the first runoff event, and at 2, 5, 7, 14, 29, and 60 days after treatment. Isoxaben in the collected samples was analyzed by liquid chromatography.

Irrigation experiment.Spring-rooted liners of Snow Azalea (Rhododendron obtusum 'Snow'), Bucaneer Azalea (Rhododendron obtusum 'Bucaneer'), and Heller's Japanese Holly (Ilex crenata Thumb. 'Helleri') were potted in 1 qt. plastic containers and freshly harvested root divisions of Daylily (Hemerocallis hyperion) and Dwarf Gardenia (Gardenia jasminoides) liners were potted in 1 gal containers using 100% fine pine bark. Fountain Grass (Pennisitum rupelli) was seeded into the same potting media. Liners were fertilized twice after potting with 16-4-8 fertilizer and placed in a glasshouse for the duration of the experiment.

Plants in 1 qt. containers received 120 ml and plants in 1 gal containers received 240 ml of isoxaben-fortified irrigation water two to three times per week as needed. These quantities are equivalent to 0.5 in of irrigation water. Treatments included 1.0 ppm and 10.0 ppm isoxaben prepared in water from the commercial formulation of Gallery 75 DF (DowElanco). Although these treatment levels were much higher than ever detected in the runoff water collection pond, preliminary unpublished studies showed no species response to levels detected in the pond. Due to low solubility in water, isoxaben was first dissolved in acetone and then diluted with water to maintain 1% acetone by volume. Other treatments included a control with 1% acetone and a water control. The experiment began the last week of March and continued for six weeks. Daylilies were irrigated for an additional five weeks for a total experiment duration of 11 weeks. Total active ingredient received by plants varied by species according to the irrigation schedule listed in (Table 1). All experiments used a randomized complete block design with six replications.

Growth measurements taken at the beginning and end of the experiment allowed the determination of growth differences using a calculated growth index and shoot fresh weights. The growth index was calculated as the average of the height and two perpendicular widths. Root fresh weights of Dwarf Gardenia and Heller's Japanese Holly were used to determine the effects on root growth. Herbicide effects on Daylily and Fountain Grass roots were quantified using visual ratings on a scale of 0 to 10 (0 = senescent roots, 10 = normal root growth).

Results and Discussion

Herbicide Movement. Cumulative isoxaben loss from the nursery site was greatest during the first irrigation following herbicide application. In 1992, 53.6 g (7.8%) of the applied isoxaben moved from the nursery site in irrigation runoff water during the first runoff event following application (Figure 1). Only 2.5 (0.36%) and 5.6 g (0.82%) were detected in runoff water during the second and third runoff events, respectively. In 1993, the greatest isoxaben loss again occurred during the first runoff event following herbicide application. Approximately 42 g (8.2%) of the applied isoxaben moved from the application site during in first irrigation runoff water (Figure 1). Losses during the second and third runoff events were 15 (2.9%) and 7 g (1.4%), respectively. A total of 9.0 and 12.5% of the applied isoxaben moved from the application site in runoff water within 5 DAT during 1992 and 1993, respectively. These data agree with a 4.7% loss of oryzalin reported following application of Rout [oxyfluorfen (2%) + oryzalin (1%)] we found at the same nursery site in 1991. Runoff samples in their study were only collected during the first runoff event following herbicide application. Similar water solubilities of isoxaben (2 ug/ml) and oryzalin (2 to 3 ug/ml) may account for the similar losses observed between the two studies.

In both years, the majority of isoxaben was detected in runoff water during the first runoff event following herbicide application. The amount of runoff increased until irrigation was stopped at 1.5 hours after it began and decreased after 1.5 hours. Maximum and minimum losses correspondedto runoff rates. Minimum losses occurred at the 0.25 and 3.5 hr. sampling periods when runoff rates were lowest and maximum losses occurred at 1.5 hr. when runoff rate was the highest (Figure 2).

The data for the two years were presented separately because statistical differences were identified. Several factors may have contributed to these differences. In 1992, 3.0 lb a.i. was applied to the nursery area; whereas, only 2.4 lb a.i. was applied to the area in 1993. In addition, the 1992 study received 0.5 in irrigation following herbicide application, while the 1993 study received 0.75 in irrigation. In 1992, the herbicide was applied to fully spaced plants; whereas not all plants had been fully spaced when the application was made in 1993. Pots are normally pushed together during winter months for cold protection and are spaced apart during the active growing season to allow maximum plant development. Thus, non-target area herbicide interception may have differed between the two years. Seasonal differences (August vs. May) may have also played a role in differentiating the two studies.

Herbicide Dissipation. Isoxaben concentrations were highest immediately following the first irrigated runoff event following herbicide application and decreased to below the detection limit 60 DAT (data not shown). Pond water contained 32 ppb (parts per billion) isoxaben immediately following the first runoff event in August 1992 (Figure 4). Residue levels decreased to below 10 ppb within the first 14 DAT and approached the detection limit (1 ppb) within 30 DAT. No isoxaben was detected in pond water 60 DAT (data not shown). Initial isoxaben levels were nearly two times higher following treatment in the 1993 study (Figure 4). Isoxaben concentrations were 55 ppb following the first runoff event, decreased to below 5 ppb 14 DAT, approached the detection limit (1 ppb) 30 DAT, and were not detected 60 DAT as in 1992. No isoxaben was detected in pond sediment samples taken in 1993 and analysis of filters used in the extraction process contained no detectable isoxaben residues indicating limited adsorption to small particulate matter filtered from the pond water samples prior to extraction.

The decline in isoxaben concentration may have resulted from the continuous dilution by successive irrigation runoff events. In addition, the pond is not a closed system. There is a daily influx and outflow of water in the pond. Herbicide binding to algal populations may also account for part of the isoxaben disappearance in pond water. In 1992, algal populations within the collection pond appeared much larger than in 1993, probably due to seasonal differences (spring vs. summer). Light and microbial degradation of isoxaben may have also resulted in a decrease in herbicide concentrations.

Irrigation Experiment. Ornamental species varied in their responses to irrigation with water containing isoxaben. Irrigation with isoxaben-fortified water induced no effects on the growth index of Dwarf Gardenia, Heller's Holly, or Bucaneer Azalea (Table 2). However, isoxaben reduced the growth index of Snow Azalea and Daylily at both 1 and 10 ppm isoxaben.

No differences were observed in shoot growth for Heller's Holly, Dwarf Gardenia, or Buccanear Azalea (Table 2). However, the 1.0 and 10.0 ppm treatments reduced the shoot fresh weights of Fountain Grass by 14 and 19% respectively. Isoxaben-fortified irrigation water produced no observable reductions in root fresh weight for Dwarf Gardenia (Figure 5). However, the 10 ppm treatment reduced the root fresh weight of Heller's Holly 25% when compared to the control (Figure 5).

Root quality, assessed using a rating scale (0 - 10, scenescent - normal), was reduced for both Daylily and Fountain Grass . After 11 weeks of irrigation with isoxaben-fortified water, Daylily root quality was reduced to 6.3 by the 1.0 ppm treatment and 4.4 at the 10 ppm treatment level.Root quality of Fountain grass was reduced to 8.8 and 4.8 after 6 weeks of irrigation at the 1.0 and 10.0 ppm treatment levels, respectively.

The ornamental species evaluated in this study fell into three catagories of isoxaben-residue sensitivity. Daylily and Fountaingrass were most sensitive to low levels of isoxaben in irrigation water; Snow Azalea and Heller's Holly were intermediate in sensitivity; and Bucaneer Azalea and Dwarf Gardenia were the least sensitive. Herbaceous species appear to be more sensitive than woody species to low levels of isoxaben in irrigation water. However, woody species may require more time for responses to appear.

Summary and Conclusions

Results indicate that isoxaben from Snapshot TG was lost from the nursery site of application in runoff water. These losses are higher than those previously reported for other herbicides, possibly due to differences in water solubility, adsorption to carrier particles, sediments, etc., and stability in the environment. Approximately 10% of the applied isoxaben moved from the commercial nursery site in irrigation runoff water. The remaining isoxaben, not detected in runoff water, was possibly bound to ground cover and ditch surfaces, or subjected to degradative reactions before movement could occur. Isoxaben concentrations dissipated in the runoff collection pond indicating that isoxaben does not accumulate. Isoxaben concentrations in the runoff collection pond were highest following the first runoff event and decreased to below 1 ug/ml 60 days after treatment. Ornamental species varied in their sensitivity to irrigation water containing 1 and 10 ug/ml isoxaben. However, it should be noted that in preliminary studies, ornamental species did not respond to isoxaben concentrations comparable to those detected in the runoff collection pond.

Acknowledgment

Special thanks to Dow Elanco and Gilbert's Nursery for supporting this research.

Reference to Figure 3

Last Updated 2/1/97