Paclobutrazol is a plant growth regulator belonging to the triazole chemical class (U.S. EPA, 2007B). Paclobutrazol was first registered by U.S. EPA in 1985. At the time of preparation of this review in 2011, PBZ was undergoing registration review by U.S. EPA to determine whether it continues to meet the FIFRA standard for registration . As part of the registration review process, a summary document was issued. This document includes a factsheet describing the Environmental Fate of Paclobutrazol.
Paclobutrazol(CAS NO:76738-62-0) has been characterized as a compound with a moderate potential for mobility in soil and water environments. The summary document for registration review prepared by U.S. EPA documents that laboratory batch equilibrium studies indicated that this chemical has the capacity to be mobile under certain conditions. Studies with nine US soils ranging in texture from sand to silt loam indicated values for the soil adsorption coefficient KD in the range from 1.3 to 23.0 ml/g.
Adsorption appeared to increase with an increase in soil organic matter content and a decrease in soil pH. In the draft assessment report prepared by the United Kingdom adsorption data for 13 soils are summarized that show KD values in the range of 0.8-21.3 ml/g with a geometric mean of 4.3 ml/g. The ketone metabolite showed on average a slightly higher affinity for adsorption to soil with KD values in the range of 2.1-13.5 with a mean of 8.0 across 6 soils.
Results from laboratory soil column leaching experiments summarized in U.S. EPA indicated low mobility in the experiments using methine-labeled Paclobutrazol(CAS NO:76738-62-0) in soils ranging in texture from sand to clay-loam. The experiments using triazole-labeled Paclobutrazol showed low mobility in columns of sand and sandy loam soils, and mobility in loamy sand and clay loam soils. In all cases, the majority (58.6-90.7%) of applied Paclobutrazol aged residue did not leach out of the upper 10 cm of the treated soil columns.
Although characterized as moderately mobile in laboratory studies, no significant movement of this chemical was detected in field studies in agricultural soils. In the orchard studies, Paclobutrazol residues (parent plus degradate) were detected at 10% or less of total applied in soils with depths of 48 inches in the California study, 24 inches in West Virginia study, and 48 inches in the Florida study.
These depths are the maximum depths sampled at each study. No information was provided on the nature or type of soils in the summary document. The Paclobutrazol ketone metabolite was predominately detected in the subsurface soil layers, also at insignificant levels.
A scientific publication by Baris et al. (2010) provided information regarding the potential of Paclobutrazol to impact groundwater from its use on turf areas. Paclobutrazol was included in a comprehensive evaluation of water quality monitoring data and assessment. This evaluation considered water quality data for a large number of turf-related pesticides from 44 studies involving 80 golf courses in the US over a 20-year period. PBZ was found in 3/440 groundwater samples, with the highest detection at 4.2 µg/L.
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