Herbicide transport in surface runoff from conventional and zero-tillage fields.

During the past four decades of crop production in the prairie region of Canada, there has been a dramatic shift from conventional management (CM) to conservation tillage management in which one or more tillage operations has been replaced by herbicide application. To determine whether this management shift has affected the quality of snowmelt runoff, field-scale side-by-side runoff watersheds were used in a 6-yr study. Herbicide concentrations and fluxes in snowmelt runoff water from CM and zero-till management (ZTM) systems were compared relative to an organic production system used as the control. Snowmelt runoff volume from the ZTM watershed exceeded that from the CM watershed in 5 yr of the 6-yr study. Frequencies of detection, concentrations and mass loss (mg ha) of the fall-applied herbicides were generally higher in snowmelt runoff relative to those of the spring-applied herbicides. On days when multiple consecutive samples were collected, herbicide concentrations generally decreased as runoff flow increased. Incorporation of trifluralin and triallate reduced their losses in snowmelt runoff by approximately 5 and 8 times, respectively. Regarding the amount of herbicide applied during the 6-yr study, percent loss varied from 0.002% for trifluralin to 0.15% for 2,4-D. Edge-of-field concentrations of 2,4-D, trifluralin, and triallate in snowmelt runoff frequently exceeded Canadian aquatic life water quality guidelines. The adoption of conservation tillage strategies for crop production resulted in increased (∼20%) herbicide use and an increased amount (∼25%) of herbicide transported in snowmelt runoff (8.6 versus 6.9 g ha).