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Field trials were conducted to determine the effectiveness of shields in reducing off-target droplet drift from ground-rig sprayers. Sprayer booms ranging in width from l0 to 13.5 m and equipped with com--e.ciully avaltitti shields *ere operated along a 150-m swath in a field of approximately 2O-cm-tall spring wheat in wind speeds ranging from lb to 35 km h-r. Airborne drift was measured using aipiriteO air samplers. ihe ure of an 80" flat fan tip (8001) at a pressure of 275 kPa and a ground speed of 8 km h -I resulted in 7 .5% of the 50 L ha -r spray solution drifting off the target area. The ule of protective cones with 8fi)1 tips without lowering the boom reduced airborne drift by 337o at_ a 20 krir h-r wind speed, while a Si-gSn drift reduction was accomplished with the combination of solid or perforated s-hielding and lowering the sprayer boom. Increasing the application rate to 100 L ha -r by using 8002 tips reduced drift of the unshielded sprayer by 65% . Decreasing application rate to 15 i ha-riy using tOOOtZ tips increased drift by 29% despite the use of a shield. Off-target drift increased withincreaiing wind speeds for all sprayers, but the increase was less for shielded sprayers and coarser sprays. The decreased droplet size of spray from 1 10" tips increased drift when the boom height was the same as for 80o tips. High wind speeds, lower carrier volumes and finer sprays, 110" tipJ, and solid shields tended to dicrease on-swath deposit uniformity, whereas a perforated shield or cones did not affect deposit uniformity.

Dissipation of triallate [S‐(2,3,3‐trichloroallyl) diisopropylthiocarbamate] and trifluralin (a,a,a‐trifluoro‐2,6‐dinitro‐N,N‐dipropyl‐p‐toluidine) in air and soil was measured following their application as a pre‐emergence treatment to a wheat (Triticum aestivum L.) field. Drift losses during application and incorporation were less than 1% of the amounts applied. Air samples, collected at six heights ranging from 30 to 200 cm above the soil surface initially and then above the crop canopy following emergence during the 67 d after application, showed distinct gradients of each herbicide in the air, with the highest concentrations in samples closest to the ground. The highest flux rates for triallate and trifluralin were 4 and 3 g ha−1 h−1 during the 4‐ to 6‐h period after application, when the concentrations at 30 cm were 2500 and 1700 ng m−3, respectively. Fluxes of both herbicides decreased with time, but were dependent mainly on soil moisture conditions. The total vapor losses for the 67‐d sampling period were 17.6 and 23.7% triallate and trifluralin, respectively. About half of these losses were in the first week. There were three distinct phases in the dissipation of both herbicides from the soil. The initial rapid phase, with vapor losses as the major route (Phase I), was followed by slow and continual dissipation over the entire growing season (Phase II), with volatilization and degradation as the potential pathways of dissipation. The third phase with little or no dissipation was reflective of the Canadian winter conditions. The gross dissipation of both herbicides during Phases I and II, however, followed the first‐order rate equation, with half‐concentration time of 88 ± 7 and 99 ± 9 d for triallate and trifluralin, respectively, with volatilization as the dominant process during Phase I.

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