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Aboveground Dissolution of Tebuthiuron Particles Placed Beneath the Crowns of Small Pinyon Trees

The Nature Conservancy, P.O. Box 11486, Salt Lake City, Utah 84147;

Neil E. West^*

Dep. of Range Science, Utah State Univ., Logan UT 84322-5230.

^* Corresponding author.

ABSTRACT

Placement of large tebuthiuron {N-[5-(1,1 dimethylethyl)-1,3,4-thiadiazol-2yl]-N,N'-dimethylurea} particles (briquettes) beneath tree canopies inhibits rainfall-driven mobilization of the active ingredient. Briquette displacement and dissolution patterns were studied at two central Utah sites after 8-month (cool seasons) and 4-month (summer) exposure periods. Thirty-two 1.8 g (13% a.i.) Graslan Brush Bullets per tree were placed at the dripline, midcrown, and stem base of 24 similar pinyon pine (Pinus edulis Engelm. and P. monophylla Torr. & Frem) saplings. The effects of site, placement, and direction from the stem were analyzed via a balanced, nested factorial experiment. Residues were entirely relocatable, indicating that overland flow, wind, and animals did not move briquettes. The influence of individual trees, placement, and direction strongly depended on season. Summer thunderstorms were three times more effective per unit of precipitation than winter rain and snow. Subcanopy placements permitted 65% as much dissolution as the exposed interspace, with stemflow more effective than throughfall in summer. Directional effects caused by storm paths were mild, and only evident at the dripline after summer exposure. Stem-base applications or smaller particles should minimize site, season, and tree effects and ensure rapid, uniform control of woody-weed regrowth with minimal undesired impacts.

Approved as Journal Paper 3671 of the Utah Agric. Exp. Stn. This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been registered. All uses of pesticides must be registered by appropriate state and/or federal agencies.

Received for publication August 30, 1988.