Experiments were conducted from 1986 to 1988 at the Delta Branch Experiment Station, Stoneville, MS and in 1987 and 1988 at the Plant Science Research Center, Starkville, MS to evaluate the effectiveness of various chemical defoliants when used in combination at one-half the standard use rate.

Defoliants and their standard rates were as follows: Def^® 6 at 1.5 pts/A, Dropp^® 5OWP at 0.25 lb/A, Harvade^® 5F at 0.5 pt/A + X-77^® at 0.25% v/v, and Prep^® 6 at 2.66 pts/A. When used in two-way combination treatments, each defoliant rate was reduced by one-half. Surfactant was included in all Harvade treatments. Treatments were arranged in a randomized complete blockwith three replications in 1986 and four replications in 1987 and 1988 for all locations. Plots were four 40-inch rows wide. Plot length varied from year to year between 30 and 55 ft.

All treatments were applied with high clearance ground equipment calibrated to deliver a carrier volume of 20 gallons per acre. Treatments were initiated each year when cotton was 50 to 75% open. Percent defoliation was determined by visual ratings at 5, 7, and 14 days after treatment (DAT).

In 1986, combination treatments at one-half the standard rate were as good or better than either component alone at the full use rate. Def + Dropp was better than Dropp alone at 5 and 7 DAT, and at 7 and 14 DAT was better than Def alone. At 14 DAT, Dropp was equal to the Def + Dropp combination. By 14 DAT, all combinations except Prep + Harvade were comparable. Prep + Harvade provided less than 60% defoliation for all evaluation dates in 1986.

In 1987, rainfall within 6 hours of application at both locations altered the response of most treatments. Rainfall delayed maximum response until 14 DAT where Def, and all combinations including Def, provided equal percent defoliation at Stoneville. However, at Starkville, the Def + Prep combination was significantly less than Def, Def + Harvade or Def + Dropp. All other treatments were significantly less than Def treatments where rainfall was subsequent to application.

In 1988, at Stoneville, all combination treatments were equal or superior to the corresponding single full use rates. At 14 DAT, Dropp, Def + Dropp, Dropp + Harvade, and Dropp + Prep all provided better than 92% defoliation.

In 1988, at Starkville, cold temperatures during and subsequent to application reduced defoliant activity to less than 60% for all treatments. However, at 14 DAT, Prep + Harvade was better than all other combination treatments. Dropp, and combinations including Dropp, were least effective. In conclusion, two defoliants tank-mixed at one-half the standard use rate were equal to or superior than either single component applied at the full use rate. In addition to equal effectiveness, combinations offered additional safeguards against adverse environmental conditions.