Biological activity, availability and duration of phytotoxicity for imazamox in four different soils of central Italy

Greenhouse bioassays were carried out from 1999 to 2002 on several types of soils of central Italy to assess the carry-over risk of imazamox residues to non-target crops. No observable effect levels (NOELs) were determined on quartz sand; sugar beet showed the highest sensitivity to imazamox (NOEL 0.4–0.8 ng a.i. mL-1 of substrate), followed by spinach, oilseed rape, fennel, cauliflower and lettuce (NOELs from 1 to 5 ng a.i. mL-1 of substrate). Wheat, sunflower, grain sorghum and maize were not very sensitive to this herbicide. Imazamox availability was greatest on sandy soils and decreased in soils with high clay or organic carbon content, where herbicide efficiency was less than 50%, with respect to non-sorptive media. The decline of herbicide efficiency was quick in sandy soils, where herbicide efficiency dropped to 50% in less than 3 days. In clay-loam or organic soils, 50% relative efficiency was reached in 15–33 days. Such results suggest that imazamox sprayed at normal field application rates can pose slight risks of carry-over of residues, which may damage very sensitive species (sugar beet, oilseed rape and spinach) in sandy soils. In these cases, safe recropping intervals of 1–3 months are required, so current label guidelines for imazamox are adequate to protect rotational vegetable crops in central Italy.