Current weed control strategies employed by North Carolina cotton producers include early season applications of fluometuron and/or MSMA. Overtop (OT) applications with these herbicides, while undesirable, are sometimes unavoidable, particularly when rainfall is insufficient to activate preemergence treatments. Reported yield responses to OT treatments are inconsistent. Yield reductions following OT applications are not always observed. Cotton may compensate for early-season developmental retardation if environmental conditions are favorable later in the season. Yield information a one does not provide sufficient detail of the developmental response of cotton to these herbicides. The objective of this study was to describe the impact of fluometuron and MSMA OT applications on the fruiting profile of cotton. This information should enable managers to modify their production practices if OT treatments are required.

Overtop treatments of 1.5 lb ai/A of fluometuron alone and in combination with 1.5 lb ai/A of MSMA were made on 5-6 inch tall cotton in weed-free plots measuring 4 rows x 40 ft. Prior to bloom, 10 row feet were tagged on Mondays, Wednesdays, and Fridays, beginning at first bloom and continuing through August 20 (the end of the effective bloom period). All retained bolls were collected at harvest. Flowers, bolls and percent fruit set were calculated on a weekly basis for each plot.

Fluometuron alone and in combination with MSMA retarded the onset of flowering by 3-7 days. Peak bloom was also delayed approximately 7 days by the OT treatments. During all three years of the study, compensation later in the bloom period tended to reduce the impact of these applications on cotton lint yields. Yield reductions over the 3 years averaged 3% when fluometuron was used alone and 5% when MSMA was added. Yield reductions were greatest when environmental conditions following application resulted in a short effective bloom period.

These results suggest that production practices need to be modified following OT applications to reduce the impact of this early-season developmental retardation on subsequent lint yields.