A three-year study was conducted from 1986-1988 in which the effects of timing of application of a tank mixture of Prep (ethephon) + Dropp (thidiazuron) were studied. Deltapine 41 cotton was planted each year on April 25± 1 day at a rate of 4-6 seeds per linear foot of row in 40-inch rows on Commerce silt loam soil with 90 pounds nitrogen applied preplant. The test area was subdivided into 8-row by 50 foot plots in a randomized complete block design with timing of application being the treatment variable. In each year of the study a foliar spray of Prep + Dropp (1.5 + 0.1) pounds active ingredient per acre was first applied shortly after natural boll opening began and subsequent applications were made at approximately 60 heat unit (DD-60) intervals. Treatments were mechanically harvested twice-at 240 and at 480 heat units following application. Hand harvested subplots were employed to establish various treatment parameters.

Initial treatment dates were as follows: August 12, 1986, August 25, 1987, and August 23, 1988. Final treatment dates for the three years were: September 4, 24, and 20, respectively. Heat unit accumulations following planting in moist soil for the various treatment timings ranged from 2250-2720 in 1986, 2520-3030 in 1987, and 2360-2830 in 1988. Cotton ranged from an average of 6 to 98% open (weight basis) in 1986, 6 to 93% open in 1987, and 25 to 98% open in 1988.

In general, in each of the three years, seedcotton yield tended to increase as harvest aid treatment timing was delayed when yield of treated cotton was compared to that of cotton that was allowed to mature without harvest aid treatment. Significant yield loss was encountered in 1986 when cotton was treated prior to accumulation of 2540 heat units and 42% open. In 1987 significant seedcotton yield loss occurred when treatments were applied at 2620 heat units and earlier (20% or less open) and near significant losses occurred at 2740 heat units and 45% open, although no loss occurred at 2670 heat units. At 2800 heat units and 56% open and beyond no reduction in seedcotton yield occurred. In 1988 seedcotton yield was significantly reduced only when cotton was treated at 2390 and 2530 heat units (52 and 55% open) with near significant reductions occurred at the 2360 heat unit timing (25% open). No reductions occurred when cotton was treated at 2570 heat units and 69% open and beyond.

Lint percentage of seedcotton in the first harvest was not affected by harvest aid treatment in 1986 or 1987; however, significant reductions in lint percent occurred at four of five earliest timings of application in 1988. Lint percentage data for second harvest seedcotton (available for 1987 and 1988 only) revealed a significant reduction in lint percent at four of the five earliest timings of harvest aid treatment in 1987 and a significant reduction at all treatment timings except two of the final three in 1988.

Fiber property determinations have at this time been completed only for 1986 and 1987. Fiber strength, length, and uniformity of lint harvested at the first mechanical harvest were unaffected by harvest aid treatment both years. Micronaire was reduced by approximately 10% in the first harvest sample in 1986 at the 2250 and 2470 heat unit treatment timings (6 and 19% open). In 1987 micronaire in the first harvest sample was reduced at the first three timings of treatment-2520, 2590 and 2620 heat units (6, 15 and 20% open).

In the second harvest, only slight and inconsistent effects on fiber strength, length, and uniformity were observed each year. Effects on micronaire were, however, more consistent and more pronounced. In 1986 micronaire of all treated cotton obtained at the second harvest was significantly lower (by approximately 25%) than that of untreated cotton. In 1987 micronaire of cotton obtained in the second harvest was reduced at the first four timings of application (2520, 2590, 2620, and 2670 heat units)--all treated at 20% or less open bolls.