SPRAY BOOM FOR SENSING AND SELECTIVELY SPRAYING SMALL MESQUITE ON HIGHWAY RIGHTS–OF–WAY

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The program currently used for mesquite (Prosopis glandulosa Torr. var. glandulosa) control on highwayrightsofway in Texas, mowing and hand spraying of all basal stems by ground crews, is extremely labor intensive and theacreage that can be treated annually is limited. The objective of this work was to develop a spray boom that would sensemesquite plants and selectively apply an effective highvolume foliar spray containing 0.5% clopyralid + 0.5% triclopyr tothe mesquite plants. Sprayers were designed for a tractor and an allterrain vehicle (ATV) that utilized beams of pulsedinfrared light to sense mesquite canopies. Controllers were designed to send fixedduration pulses of voltage to solenoidvalves for spray release through flatfan nozzles when mesquite canopies interrupted the light beams. Height of the spraybooms was adjusted onthego by the operator to minimize the release of excessive amounts of spray by tall grass or weeds.The tractormounted sprayer performed at 1.73, 1.48, and 1.15 ha/h in field plots with an average of 276, 985, and970 mesquite/ha, respectively, and within each test site, performance was independent of mesquite density because groundspeed remained constant. The tractormounted sprayer missed 13 to 31% of the target plants. The ATVmounted sprayerperformed at 1.72 ha/h in field trials where mesquite densities averaged 625/ha and missed only 16% of the target plants.Predicted performance for the ATV unit in densities of 200, 500, and 1000 mesquite/ha was 2.51, 1.76, and 1.11 ha/h. Thelevels of mesquite mortality achieved, 38 to 67% for the tractormounted sprayer and 44% for the ATVmounted sprayer,were equivalent to those mortality levels that have been achieved by ground crews hand spraying the same herbicide solutionto mesquite regrowth in highway rightsofway. These mesquitesensing sprayers offer a viable alternative to hand sprayingwhere labor is limiting and where vast areas must be treated. Mesquite canopies must be about 30 cm taller than associatedvegetation to minimize offtarget deposition of herbicide solution.