Runoff of Hexazinone and Diuron in Green Cane Systems

Sugarcane is a major crop in Brazil as well as other tropical areas. The rise of green cane systems that maintain straw on the soil surface after mechanical harvesting alongside extreme precipitation has changed the use and environmental fate of pesticides, mainly herbicides. The goal of this research was to evaluate the effects of straw amounts (0, 7, and 14 t ha−1), soil water contents (10 and 18%, volumetric basis), and herbicide incubation time (0 and 3 days) on the runoff of hexazinone and diuron in green cane systems, under a heavy rainfall event of 120 mm that is becoming more frequent over the decades in tropical areas. A rainfall event of 80 mm h−1 during 1.5 h was simulated over a 1 m2 area, using a rainfall simulator with a structure designed to collect runoff. Herbicides in water runoff were determined by ultra performance liquid chromatography with mass spectrometry (UPLC ESI QTOF/MS), while herbicides attached to sediments were estimated using Kd values. Sugarcane straw on the soil surface decreased water, sediments, and diuron runoffs, but barely affected hexazinone losses. Crop residues cannot prevent runoff of highly soluble molecules, such as hexazinone. Herbicides’ runoffs were much higher in the aqueous phase and at higher soil moisture content. Maintaining 7 t ha−1 of sugarcane straw on the soil surface was enough to mitigate water, sediments, and diuron runoff, but 3-day herbicide incubation did not affect both herbicides runoffs. Diuron and hexazinone are heavily used herbicides that can reach concerning concentrations in the runoff and contaminate surface waters in vulnerable areas if no control measures are taken.