Glyphosate efficacy on eastern Canadian forest weeds. II. Deposit-response relationships and crop tolerance

First-season herbicide efficacy and crop tolerance results are presented for a range of glyphosate (VISION) rates (0-1.00 kg acid equivalent per hectare) aerially applied with two different dispersal systems (AU5000 Micronairs and conventional hydraulic nozzles). Derivative-free nonlinear regression was used to model post-treatment raspberry cover (Rubus idaeus L. var. strigosus (Michx.) Maxim.) as an exponentially decreasing function of chemical deposit (R2 = 0.87). Similar methods were used to model post-treatment crown area as a function of chemical deposit and pretreatment crown area for pin cherry (Prunus pensylvanica L.f.) (R2 = 0.92), red maple (Acer rubrum L.) (R2 = 0.79), white birch (Betula papyrifera Marsh.) (R2 = 0.93), elderberry (Sambucus pubens Michx.) (R2 = 0.85), and aspen (Populus tremuloides Michx.) (R2 = 0.96). Acceptable first-season control (>60% cover reduction) of raspberry, pin cherry, elderberry, and aspen was achieved with rates of deposit greater than or equal to O.50 kg acid equivalent per hectare. Red maple and white birch crown area reduction averaged 60% at the highest rate tested (1 kg acid equivalent per hectare). Significant differences in efficacy attributable to the different dispersal systems were found only for pin cherry: the Micronair system resulted in 30% greater efficacy on one of the four blocks tested (P <0.0001). On this particular block, characterized by unstable weather conditions during treatment, both systems produced significantly greater efficacy than observed on the other blocks for all species studied except aspen (P < 0.01). No significant differences were detected in black spruce (Picea mariana (Mill.) B.S.P.) health patterns in areas treated with the two different application systems (P = 0.780). Crop trees in untreated areas declined in health relative to trees in treated areas (P < 0.001).