INFLUENCE OF APPLICATION TIME ON BIOACTIVITY OF IMAZETHAPYR IN NO-TILLAGE SOYBEAN (GLYCINE-MAX)

Field research was conducted at Arlington, WI, in 1988 and 1989 to determine the influence of application time on weed control and residue carryover with imazethapyr in no-tillage soybean production. Imazethapyr at greater-than-or-equal-to 55 g ai ha-1 applied early preplant controlled > 90% of the common lambsquarters, velvetleaf, and giant foxtail before no-tillage planting of soybean. Early preplant and sequential treatments with an early preplant component controlled greater-than-or-equal-to 88% of all weeds for the entire growing season. Delaying the initial imazethapyr application until immediately after soybean planting reduced weed control compared to the early preplant treatments. Low level of weed control with planting time treatments appeared to be due to a lack of control of common lambsquarters emerged at the time of imazethapyr application and dry weather following imazethapyr application. No soybean injury from imazethapyr was observed and differences in soybean yield appeared to be due to differences in weed control. No significant carryover of imazethapyr was detected through a corn bioassay in the field.