Imazaquin spray retention, foliar washoff and runoff losses under simulated rainfall

Spray retention and foliar washoff of imazaquin in smooth pigweed (Amaranthus hybridus L.) and sicklepod (Senna obtusifolia (L.) Irwin and Barneby) were investigated. Imazaquin (70 g AI ha-1) was applied alone, with nonionic surfactant 'X-77' or organosilicone-based nonionic surfactant 'Kinetic' to plants at two- to five-leaf stage and subjected to 2.5 cm rainfall in 20 min either 1 or 24 h after application. Imazaquin spray retention was higher with adjuvants than without. Retention was similar between adjuvants in smooth pigweed but 'Kinetic' retained twice as much imazaquin as 'X-77' in sicklepod. Rainfall 1 h after application washed off three-quarters of foliar residues regardless of plant species or adjuvant. However, at 24 h after application, foliar washoff was lowest with 'Kinetic' followed by 'X-77' in both species. Imazaquin washoff ranged from 33 to 88% in the two species at 24 h after application. Overall, imazaquin activity was similar with either adjuvant in smooth pigweed but 'Kinetic' was more effective than 'X-77' in sicklepod. Runoff losses from the surface of a Bosket sandy loam (Mollic Hapludalfs) soil in runoff trays (1.2% slope) were also studied. Imazaquin was applied as above to trays with and without smooth pigweed canopy. A 2.5-cm rainfall was applied in 20 min at 24 h after application. Runoff samples collected in one-litre fractions were analyzed by enzyme-linked immunosorbent assay. Sediment (but not water) in runoff was greatly reduced (56%) by pigweed cover as compared to bare trays. Imazaquin in the first litre of runoff was higher than in subsequent runoff fractions regardless of pigweed cover. Total imazaquin lost in runoff was higher in pigweed cover (23%) than bare trays (16% of applied). Imazaquin concentration in 10-20 cm soil depth in pigweed cover trays was higher than in bare trays. These results suggest that imazaquin is vulnerable to foliar washoff and the herbicide washed off could move in the aqueous phase due to shorter contact time with soil for sorption.