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|Title:||Laboratory and mesocosm evaluations of controlled-release matrices as potential herbicide delivery systems|
|Authors:||United States. Bureau of Reclamation.|
Aquatic Plant Control Research Program (U.S.)
Netherland, Michael D.
Stewart, R. Michael.
Turner, E. Glenn.
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: Formulations for the controlled release of aquatic herbicides were tested in the laboratory and in flowing water hydraulic flumes. Protein- and gypsum-based matrices were formulated with bensulfuron methyl (2 percent active ingredient (ai)), fluridone (2 percent ai), 2,4-D (2 and 15 percent acid equivalent (ae)), and triclopyr (2 and 15 percent ae). These formulations were tested in the laboratory for release properties over a 7-day period. In addition, conventional granular formulations of fluridone (Sonar® SRP 5 percent ai) and 2,4-D (Aquakleen® 19 percent ae) were tested to provide a basis for comparison of release rates. Results showed the protein matrices produced inconsistent release profiles; whereas, the gypsum matrices resulted in consistent release rates during the comse of the study. Triclopyr and 2,4-D were identified as excellent candidates for hydraulic flume testing. Triclopyr was chosen due to its experimental status and the lack of field efficacy and dissipation information for this compound. Three formulations of endothall were also tested for controlled-release properties and included the conventional granule (Aquathol® 10.1 percent ai), a 27-percent ai clay granule, and a 45-percent ai supersorbent polymer. Results showed all matrices released >90 percent of the active ingredient within 2 hr posttreattnent. Although no controlled-release properties were demonstrated, the 45-percent ai polymer is a good candidate for field testing due to the high percent ai load and the lack of dusting. Hydraulic flumes were planted with the exotic target species Eurasian watennilfoil 4 weeks prior to release rate testing. During the summer of 1992, release testing was conducted with gypsum/triclopyr matrices targeted to achieve 100 and 300 μg/L in two flumes for a 6-day exposure. Results showed that both loading rates delivered consistent amounts of triclopyr during the course of the study; however, release rates were only one-third to one-half of the target rates. As a result of failing to achieve target rates, milfoil control was very poor. Although injury symptoms were visible following these treatments, biomass increased twofold to fourfold during the 6-week posttreatment period. Plant tissue was also sampled and analyzed for triclopyr content. Data showed that uptake of triclopyr was rapid as near maximal levels occurred within 24 hr posttreatment. Although aqueous concentrations remained constant, further tissue accumulation of triclopyr did not occur past 24 hr posttreatment. The rapid uptake and lack of triclopyr accumulation over time was unexpected based on other herbicide uptake studies. Efficacy data indicate that tissue levels in the range of 2,000 μg/kg DWT for 5 days provided poor milfoil control; whereas, tissue levels in the range of 9,000 μg/kg DWT for 3 days provided excellent milfoil control. Tissue levels in the range of 9,000 μg/kg DWT for 1 and 2 days resulted in 0- and 60-percent control. These results suggest that concentration and exposure interact to produce plant control. In 1993, release testing was conducted with gypsum/triclopyr matrices targeted to achieve 300 and 500 μg/L in four flumes for a 5-day exposure. Furthermore, a gypsum/endothall matrix targeted to achieve 500 μg/L for a 4-day exposure was also tested. Results showed that matrices delivered consistent amounts of triclopyr (with some exceptions) and endothall during the course of the study. Some spike release was noted in both triclopyr treatments and was attributed to matrix agitation, higher loading rates and cracking. Failure of the pumps supplying water flow to the flumes (115 hr), resulted in static triclopyr exposures which forced removal of the matrices at 117 hr posttreatment. Residue analyses indicated that following loss of water flow, an increase in triclopyr residues was noted at 120 hr posttreatment; however, triclopyr levels had significantly dropped by 144 hr posttreatment. The 8-week posttreatment harvest indicated that 100-percent milfoil control was achieved following all controlled release matrix treatments. Following triclopyr treatment, thick stands of naiads and pondweeds were abundant in areas once dominated by milfoil. Plant tissue analyses following liquid static treatments (1,500 and 3,000 μg/L for 48 hr) and liquid flowing treatments (3,000 μg/L) again indicated that triclopyr loading was rapid as near maximal levels occurred within 6 hr posttreatment. No accumulation of triclopyr was noted past 6 hr posttreatment even though most aqueous exposures remained quite stable for much longer periods of time. Release of triclopyr from plant tissues was closely correlated to aqueous dissipation. Comparison of results from 2 years of sampling indicated variability existed in bioconcentration factors. Preliminary results indicate that use of tissue burden information for efficacy prediction will be difficult due to the interaction of tissue concentration, exposure period (increased number of samples required), and variability that is likely to exist when sampling plants in the field. Furthermore, the role of adsorption was not evaluated in these studies and will likely require laboratory testing to determine its significance.
|Appears in Collections:||Technical Report|