The persistence and efficacy of acetamide herbicides at application timings from fall to preemergence (PRE) were studied in 1998 and 1999 on mollisols (1.1 to 2.8% organic carbon). Metolachlor, s-metolachlor, acetochlor (as an emulsifiable concentrate [EC] formulation and two encapsulated formulations, capsule suspension [CS] and microencapsulated [ME]), and the combination of flufenacet + metribuzin were evaluated at five application times including late fall, 60 and 30 d early preplant (EPP), preplant incorporated, and PRE. Soil bioassays 180 d after application indicated flufenacet + metribuzin, metolachlor, s-metolachlor, and the acetochlor CS had 62 to 74% giant foxtail control, whereas acetochlor EC and ME had 43 to 46% control. Applications at 60 EPP of metolachlor, s-metolachlor, and acetochlor CS provided 70 to 75% giant foxtail control in greenhouse bioassays, whereas flufenacet + metribuzin, acetochlor ME, and acetochlor EC provided 38 to 57% control. At the 30 EPP timing, metolachlor and acetochlor CS had 80 to 82% control, whereas acetochlor EC provided 46% control, and acetochlor ME, flufenacet + metribuzin, and s-metolachlor had 65 to 74% control. Quantitative soil analysis (0 to 6 cm) 10 d after planting (DAP) indicated metolachlor, s-metolachlor, and acetochlor CS concentrations ranged from 12 to 16% and 32 to 47% of applied herbicide for the fall and PRE application timings, respectively, whereas acetochlor (ME and EC) were from 1 to 3% and 16 to 21% of applied for the fall and PRE application timings, respectively. Bioassay reduction was correlated (R
2 = 0.68) with soil-herbicide concentrations at 10 DAP.