Isoxaben dissipation in field soil as affected by application timing

Timing of preemergence (PRE) herbicide application is critical for effective weed management in turfgrass and ornamentals. Information on soil dissipation, as affected by application timing and rate, is needed when using preemergence herbicides for weed control. Field studies were established at Blacksburg, VA, to determine the effect of spring, fall, and a double (spring followed by fall) application on the dissipation of isoxaben applied at 0.56, 0.84, and 1.12 kg a.i. ha(-1). The half-life of isoxaben was estimated to be 2.7 mo following spring application, 5.7 mo following fall application, and 6.1 mo following spring plus fall application. Application rate did not affect the degradation pattern of isoxaben following spring or fall application. Residues of isoxaben at 3 and 6 months after treatment (MAT) following fall application at 1.12 kg ha(-1) were higher than that following spring application. A positive correlation (gamma = 0.62) was determined between daily isoxaben breakdown rate and mean ambient daily temperatures following the two application timings. Degradation following Pall application may be stower than that following spring application due to lower microbial activity during winter months. Negligible levels of the herbicide remained in the top 3.8 cm of soil 9 mo following spring or fall application. Double application of isoxaben resulted in higher mean levels of isoxaben residues than single spring or fall application at 0, 1, 6, and 9 MAT.