Two field studies were conducted in 1985 to evaluate the effects of the harvest aid chemical ethephon [(2-chloroethyl)phosphonic acid] on yield and fiber quality of cotton (Gossypium hirsutum L.) when applied at relatively early stages of boll development. One test employed the DES 422 cultivar with 2 lb a.i./A. ethephon (PREP) applied when either 30% or 60% of the bolls were open. One hundred unopened bolls were tagged in each plot on the day of treatment and these checked for dehiscence at 3 to 4 day intervals. Yield data were taken by hand harvest of 26 ft. from each plot and boll component data taken on the tagged bolls.

The Stoneville 112 cultivar was used in a second test where 2 lb a.i./A. PREP were applied at either the 40%, 60%, or 80% open boll stage. All cotton was hand harvested from 13 ft of row in each plot at treatment time and at 3-4 day intervals thereafter. The cotton from each harvest was used for yield and boll component determinations. The fiber from both studies were analyzed at the USDA Cotton Quality Research Laboratory, Clemson, South Carolina.

The 30% PREP treatment in the first test caused a significant reduction in lint yield and the boll components boll weight, seed index, lint index, micronaire, Rd color and 50/2.5 U.R. No adverse effects occurred when PREP was applied at the 60% open stage. The percentage of the total yield gathered at first harvest was increased in both PREP treatments in this test.

In the second test PREP applied at 40% open bolls increased the percent of total yield gathered 10 days after application, but no increase occurred in either the 60% or 80% PREP treatments. None of the PREP treatments had a significant effect on total yield or any of the boll or fiber properties measured. There was a trend, however, toward reductions in micronaire, boll size, seed index and lint index in plots treated at the 40% stage. Because of management practices and environmental factors the cotton in the second test was more determinate than that in the first test. So when the cotton was only 40% open the unopened bolls were relatively mature and less susceptible to deleterious effects of forced boll dehiscence.

The data from these studies indicate that early applications of PREP can have adverse effects on both yield and quality, and that the relative maturity of the unopened bolls rather than percentage of open bolls would be a more reliable indicator for the proper time to apply this chemical. The fiber maturity and length array determinations for these studies are being made and will be reported later.