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Environmental Fate of Picloram Used for Roadside Weed Control

Div. of Biological Sciences/Environ. Studies, Univ. of Montana, Missoula, MT 59812;

E. C. Monnig

USDA Forest Serv., Northern Regional Office, Missoula, MT 59802.

^* Corresponding author.

ABSTRACT

The herbicide picloram (4-amino-3,5,6- trichloro-2-pyridine carboxylic acid) was applied to control spotted knapweed (Centaurea maculosa Lam.) in the northern Rockies to determine persistence in soils and vegetation, losses by photodegradation, rainfall induced migration, and potential contamination of surface and groundwater. Two sites were selected to represent best case and worst case conditions (within label restrictions) for on-site retention of picloram. A valley bottom terrace was treated with 0.28 kg/ha of picloram in the spring of 1985, and sampled over the following 445 d. In the spring of 1986, 1.12 kg/ha of picloram was applied to both sides of a minimal construction logging road extending 4 km along a stream (102 to 815 m³) that drains agranitic upper mountain watershed. Of the 17.1 km² watershed, 0.15% (2.5 ha) was sprayed. Vegetation, soils, surface water, and groundwater near the road were sampled during the 90 d following application. At the valley bottom site, 36, 13, and 10.5% of the picloram applied persisted after 90, 365, and 445 d, respectively. At the mountain watershed site, 78% persisted after 90 d, and picloram was not detected in the surface or groundwaters during the 90 d following application. Depending on the timing of delivery, as little as 1% or less of the application could have been detected after delivery to the stream. Loss by photodegradation during the first 7 d after treatment was important at both sites.

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