USING A HYDROLOGICAL MODEL TO DETERMINE ENVIRONMENTALLY SAFER WINDOWS FOR HERBICIDE APPLICATION

A modification of the GLEAMS model was used to determine application windows which would optimise efficacy and environmental safety for herbicide application to a forest site. Herbicide/soil partition coefficients were determined using soil samples collected from the study site for two herbicides (imazapyr, Koc=46; triclopyr ester, Koc = 103 8) and published values for two other herbicides (hexazinone, Koc=54; triclopyr amine, Koc=20) were used in the model. Other site-specific characteristics were taken from catchment topographic maps and soil data. Long-term climatic records for the region were then used to provide meteorological data for use in the hydrology component of the model. The model was run with herbicide application for each day of the manufacturers ' recommended growing-season application windows. Average surface run-off losses, expressed as a percentage of applied, were low for all herbicides modelled (hexazinone, 0.37%; imazapyr, 0.34%; triclopyr amine, 0.21%; triclopyr ester, 1.85%). Model predictions of herbicide loss for each application day were then summarised and the application days with the lowest predicted loss within the manufacturers ' applicatio