Field studies were conducted in 1999 to 2000 on a clay soil and a sandy-loam soil in Londrina and Palmeira, PR, Brazil, respectively, to determine the persistence and carryover effect of a mixture of imazapic and imazapyr, applied to imidazolinone-tolerant corn, on rotational crops of soybean, edible bean, wheat, and corn in two different planting systems (no till and tillage). Main plots were herbicide treatments (0, 52.5 17.5, and 105 35 g ai/ha for imazapic and imazapyr, respectively) and subplots were five intervals (0, 30, 60, 90, and 120 d) between the herbicide application and rotational crop planting. Soil samples were collected for a cucumber bioassay and chemical residues analysis at each time interval. The dissipation time (DT50) of the herbicides in the soil was greater in Londrina than Palmeira, for both imazapic (54 d vs. 27 d, respectively) and imazapyr (40 d vs. 33 d, respectively), probably due to the lower pH and greater clay content of the soil in Londrina compared with Palmeira. The DT50 for both herbicides tended to increase slightly in no-till compared with conventional tillage but the differences were not great. Soybean was the least sensitive rotational crop, with a period for no yield drag (PINYD) of 87 d in Londrina and 88 d in Palmeira. Wheat and edible bean showed intermediate sensitivity. The PINYD for wheat and edible bean was 99 and 98 d for Londrina and 91 and 97 d for Palmeira, respectively. Corn was the most sensitive, with a PINYD of 117 d in Londrina and 97 d in Palmeira. Cucumber was more sensitive to imazapic and imazapyr residues than the rotational crops and should be an effective bioassay to indicate when rotational crops can be safely planted.
Nomenclature: Corn, Zea mays L.; cucumber, Cucumis sativus L.; edible bean, Phaseolus vulgaris L., soybean, Glycine max (L.) Merr.; wheat, Triticum aestivum L.
Additional index words: Crop tolerance, imidazolinone, herbicide carryover, dissipation time, planting interval, soil bioassay.
Abbreviations: ALS, acetolactate synthase; CPINI, cucumber planting interval with no injury; DAP, days after planting; DAT, days after treatment; DT50, interval (in days) to degrade 50% of the herbicide rate applied in the soil; LCHS, lowest concentration of the herbicide in the soil which causes crop injury; PINYD, the period between herbicide application and rotational crop planting with no yield drag.