Biological activity, field persistence and safe recropping intervals for imazethapyr and rimsulfuron on a silty-clay soil

Greenhouse and field trials were carried out from 1990 to 1993 on a silty-clay soil in central Italy to assess the risk of carry-over of imazethapyr and rimsulfuron residues. Turnip and sunflower were the most sensitive species to rimsulfuron residues (damaging thresholds lower than l ng a.i. g^-1), whereas sugar beel. turnip, oilseed rape and grain sorghum proved to be the most sensitive species to imazethapyr (damaging thresholds ranging from 0.5 to 6 n g a.i. g^-1). The time required for 50% imazethapyr disappearance (DT₅₀) from the upper soil layer (0-0.1 m)ranged from 18 to 21 days, whereas the DT₅₀ for rimsulfuron ranged from 5 to 6 days. Safe recropping intervals on soil treated with imazethapyr (rate of 35 g a.i. ha^-1) ranged from 3–4 weeks for the least sensitive species (maize, sunflower and mustard) up to 5–6 months for the most sensitive ones (sugar beet and turnip). For rimsulfuron (rate of 15 g a.i. ha^-1), recropping intervals of 2–3 weeks proved to be long enough to avoid injuries to any crops, except for the most sensitive species (turnip), which required 6 weeks before it could be safely sown in treated fields. Simple greenhouse bioassays allowed reliable predictions to be obtained about injuries observed in the field, confirming their usefulness to forecast the risk of carry-over of residues.