Pyrithiobac and imazaquin are persistent herbicides used in the midsouthern region of the United States. Certain rotational crops are extremely sensitive to each herbicide. Field studies were established near Brooksville, MS, and St. Joseph, LA, to examine crop injury from the previous year's application of pyrithiobac and imazaquin. In Mississippi, 690 g ai ha−1 of each herbicide was applied as a preplant-incorporated carryover treatment. The subsequent year, pyrithiobac-treated areas were planted with soybean, corn, and grain sorghum. Imazaquin-treated areas were planted with cotton, corn, and grain sorghum. In plots adjacent to the carryover treatment, pyrithiobac or imazaquin was applied at rates ranging from 0 to 173 g ha−1 as in-field bioassay plots. In Louisiana, pyrithiobac was applied at 70 and 240 g ha−1 as a postdirected broadcast layby treatment in cotton. The subsequent spring, pyrithiobac-treated areas were planted to corn. Assay rates of pyrithiobac ranging from 0 to 140 g ha−1 were established in areas adjacent to the carryover treatment. Grain sorghum was most sensitive to pyrithiobac soil residues, followed by corn and then soybean. Cotton was the most sensitive crop to imazaquin soil residues, followed by corn and then grain sorghum. Residual amounts of pyrithiobac and imazaquin were approximately 15 and 24 g ha−1, respectively. The approximated half-life of pyrithiobac and imazaquin was 61 and 71 d, respectively. Pyrithiobac applied at 280 g ha−1 as a layby treatment the preceding year injured corn and reduced yields in 1 of 2 yr. Pyrithiobac at 70 g ha−1 did not affect corn in either year. The difference in persistence between the 2 yr was attributed to reduced precipitation and lower soil temperatures in the year with observed carryover. Injury and yield reduction observed in the second year of the study correspond to an approximate pyrithiobac half-life of 60 d.
Nomenclature: Imazaquin; pyrithiobac; corn, Zea mays L. ‘Pioneer 3223’, ‘Pioneer 3222’; cotton, Gossypium hirsutum L. ‘Coker 312X1445’, ‘Stoneville 474’; grain sorghum, Sorghum bicolor (L.) Moench ‘Triumph 55’; soybean, Glycine max (L.) Merr. ‘Asgrow 4701RR’, ‘Hartz 5088RR’.