AbstractOff‐target deposit has been quantified from various silvicultural glyphosate application methods and an estimate made of the buffer‐zone widths required around water to protect fish and their invertebrate food species from possible toxicological effects. To overcome the difficulty of estimating different buffer widths to meet the various use conditions encountered, a realistic worst‐case scenario was chosen for small‐drop drift and data were collected accordingly. Three glyphosate application methods were tested, employing a helicopter equipped with a ‘Microfoil’ boom, a ‘Thru Valve Boom’ and D8‐46 hydraulic nozzles respectively. Airborne glyphosate and off‐target glyphosate deposits on ground sheets and foliar surfaces were measured at downwind distances between 50 and 200 m from multiple overlaid crosswind swaths. Over this distance airborne glyphosate decreased by factors of 3‐130, ground deposits by factors of 4‐2200, and foliar deposits by factors of 5‐100. In general, airborne glyphosate and off‐target deposit was highest from the D8‐46 application, and lowest from the ‘Microfoil’ boom application. Using these measurements, mathematical equations were formulated to predict glyphosate deposits on water surfaces downwind of multiple swath applications, and calculations made for 100‐ha applications. Large‐drop drift was also calculated using a ballistic model. An estimate was then made of buffer widths required around water bodies to prevent toxicological effects from small‐ and large‐drop drift, using reported glyphosate toxicities to salmon, rainbow trout and various aquatic invertebrates. A buffer width of 25 m around water bodies limits mortality in populations of salmon, rainbow trout and aquatic invertebrates to less than 10 for the application methods employing the ‘Microfoil’ and ‘Thru Valve’ Boom. For the third ap
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