Persistence and Carryover Effect of Imazapic and Imazapyr in Brazilian Cropping Systems1

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.