Effect of moisture on quinclorac dissipation in Lethbridge soil

This study examined the dissipation and carry-over of quinclorac residues in a Lethbridge sandy clay loam with 2% organic matter. Experiments were conducted in covered outdoor lysimeters using different simulated rainfall regimes. Quinclorac residues were monitored in the top 10 cm of soil using chemical residue analysis and the activity of carried-over residues assessed by bioassay. Quinclorac dissipation was very slow although, in general, the amount of residue remaining decreased with increased moisture (117–447 mm) applied. Forty-eight weeks after quinclorac application, 85, 66, 52, 49, and 48% of initial residues remained in the very dry, dry, normal, wet and very wet moisture regimes, respectively. Residue persistence could be accurately predicted (r 2 = 0.96) using a simple moisture model (% quinclorac remaining = 101% −0.18 × mm cumulative moisture) for up to 300 mm moisture. Quinclorac dissipation was attributed mostly to the residues leaching beyond the 0- to 10-cm soil layer. A separate laboratory experiment showed that 80% of applied quinclorac leached through 9.7-cm deep soil columns when 304 mm of water was applied. The quinclorac residues remaining after 48 wk were biologically available and caused injury to fababeans (Vicia faba L.) under all moisture regimes. Key words: Quinclorac, persistence, moisture effects, leaching, residue carry-over, recropping