Efficacy of atrazine and simazine applications over harvest residue in Queensland’s subtropical softwood plantations

Abstract

This study investigated the efficacy of atrazine and simazine when spray technique, amount and timing of herbicide, and amount/arrangement of harvest residue were varied. Separate atrazine and simazine field trials were undertaken, soon after site-preparation, in second-rotation native hoop pine and exotic Pinus plantations, respectively, in south-eastern Queensland.

Spray patterns and atrazine recovery were determined when the herbicide was applied manually in a fan- or a sprinkle-spray pattern at a range of delivery rates. Herbicide recovery from soil and overlying residue was quantified following simulated rainfall events, ranging from 5 to 50 mm, over 1 m2 plots with either nil, single- or double-residue cover. A separate trial assessed the effect of the timing of rainfall for periods between 10 and 30 days after herbicide application.

Under ‘ideal’ test conditions, the recovery of atrazine applied at 5 kg a.i. ha−1 from targets laid across the ground surface was 2.7, 4.7 and 4.5 (mean 4.0) kg ha−1 for the fan spray with delivery rates of 60, 120 and 240 L ha−1, respectively, and 2.2, 2.8 and 2.7 (mean 2.6) kg ha−1 for the sprinkle spray with delivery rates of 30,60 and 120 L ha−1, respectively. The poorer recovery from the sprinkle spray was largely attributed to spray being delivered outside the target band width of 3 m.

When residue was retained on site, between 45% and 67% of recovered atrazine and between 27% and 52% of recovered simazine was intercepted by residue cover. Simulated rainfall events of up to 50 mm, and timing of 10 mm events up to 30 days after application, had little effect on the washing of intercepted herbicide from residue to the mineral soil.