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Cotton Flowering and Fruiting Responses to Application Timing of Chemical Growth Retardants¹

The influence of several chemical growth modifiers on suppression of undesirable late fruiting and on yield of cotton (Gossypium hirsutum, L.) was studied in 1972 and 1973. Plant responses were critically dependent on timing of application. With application at first flower, two compounds had no effect on rate of flowering, but reduced boll retention from 3rd and 4th week flowers. A third compound reduced both flowering and boll retention during weeks 4 and 5. Yield was significantly reduced in each case. A fourth compound was evaluated at three dates of application: Jul. 23, Aug. 8 (peak of flowering), and Aug 23. Yield was reduced by treatment at the early and intermediate dates. Treatment Aug. 23 did not reduce yield significantly but effectively suppressed late boll set.

Comparisons of yield responses to growth retardants for both years revealed that the 1973 crop with generally higher yield, was the more sensitive to yield reduction. Application in early to mid-August at peak of flowering reduced yield significantly in 1973, whereas application 2 weeks later did not. Boll weight was reduced somewhat in late opening bolls from plots sprayed at peak of flowering. Thus chemical growth retardants that suppress late boll set may also impair full development of the last bolls that mature. Therefore, application timing is a compromise between the desired earlier termination of fruit set and the possible loss in yield and quality of late-opening bolls.

Key Words: Gossypium hirsutum L. • Growth retardant • Boll set • Cotton harvesting

² Plant physiologist, ARS, USDA, Stoneville, MS 38776.

Received for publication July 15, 1974.