Effect of moisture on quinclorac dissipation in Lethbridge soil

This study examined the dissipation and carry-over of quinclorac residues i n a Lethbridge sandy clay loam with 2% organic matter. Experiments were con ducted in covered outdoor lysimeters using different simulated rainfall reg imes. Quinclorac residues were monitored in the: top 10 cm of soil using ch emical 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, a nd 48% of initial residues remained in the very dry, dry, normal, wet and v ery wet moisture regimes, respectively. Residue persistence could be accura tely predicted (r(2) = 0.96) using a simple moisture model (% quinclorac re maining = 101% - 0.18 x mm cumulative moisture) for up to 300 mm moisture. Quinclorac dissipation was attributed mostly to the residues leaching beyon d the 0- to IO-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 wer e biologically available and caused injury to fababeans (Vicia faba L.) und er all moisture regimes.