Four field experiments were carried out in several representative cotton producing areas of Arizona to evaluate the effectiveness of a number of defoliation treatments on Pima (and Upland) cotton. Somewhat variable but generally hot and dry conditions were encountered among the experimental locations in 1992 for treatment comparisons. It appears that consistencies in the effectiveness of several treatments for Pima defoliation offer a basis for further refinement of recommendations across the state.

This study was conducted to determine the effect of various defoliation treatments on the HVI quality of short staple cotton. We compared two applications of sodium chlorate and Accelerate to single applications of Dropp plus crop oil with and without Accelerate. We measured significant differences in percent defoliation and desiccation. We also measured significant differences in the HVI parameters of staple, micronaire, and length.

A field study was conducted at the Maricopa Agricultural Center to determine the influence of plant water status at the time of defoliation on the effectiveness of defoliants and yield of Pima cotton. Irrigation termination dates of 4 and 18 September and 2 October were used to achieve different levels of plant water stress at the time defoliants were applied (16 October). Irrigation termination dates tested had no significant effect on lint yield or the effectiveness of defoliants. All irrigation termination dates resulted in defoliation of 80% or higher using a single application of defoliants. At the time of defoliation, CWSI values ranged front 0.59 to 0.80 and plant water potential from -2.0 to -2.9 MPa for the three irrigation termination treatments.

Field studies were conducted at the Maricopa Agricultural Center to evaluate the effectiveness of selected defoliation treatments on Pima and Upland cotton under warm and cool weather conditions. Weather conditions during September and October defoliation tests were warm and dry. Several cool weather tests in November were terminated by an early frost. Dropp used alone was effective as a single application in September tests, but not in October. In October, Dropp used with the adjuvant Sylgard was superior to Dropp with Agridex and resulted in defoliation similar to Dropp combination treatments. An experimental defoliant, SN597 NA300, was as effective as the Dropp+ Def +Accelerate treatment in October tests with botlu Pima and DPL 90 cotton.

Three defoliation studies were performed on Pima cotton at the Safford Agricultural Center in 1992. One tested two different formulations of Dropp with various adjuvants. The second compared experimental material SN597 with other common defoliants. The third compared SN597 at different rates in a late application. At 14 days after application SN597 dropped 97% of the leaves compared to 37% for sodium chlorate. All of the combinations dropped leaves much more efficiently than chlorate.

Chemical defoliation of cotton is a cultural practice that induces abscission of cotton foliage earlier than normal leaf shedding. Early cotton defoliation is an integral part of short- season cotton production because it facilitates the mechanical harvesting of cotton and helps reduce insect population in late summer. Effectiveness of the chemical defoliants depends upon the environmental conditions at the time of application. Research was conducted to access the impact of temperature, relative humidity (RH), and water stress at time of defoliation on effectiveness of thidiazuron (Dropp) a chemical defoliant to defoliate cotton grown under field and growth chamber conditions. Humidity was increased by spraying a fine mist over the cotton canopy. Plant moisture stress was achieved by terminating irrigation of cotton at selected times before defoliation. In growth chambers, the greatest defoliation of Pima S -7 and Deltapines 5415 cotton treated with Dropp occurred at day /night temperatures of 32C/18C with 80% RH whereas, lowest defoliation occurred at day /night temperatures of 40C /19C and 22C/9C with 80% RH. In the field, irrigation termination dates of Sept 4, Sept 18, and Oct 2, 1992 were evaluated for their effect on defoliation of Pima S -7 cotton defoliation on Oct 16, 1992 with Dropp. The Sept 4 irrigation termination date resulted in higher percent defoliation than Oct 2 irrigation termination date 14 days after Dropp was applied.

Arizona Cooperative Extension generates and distributes weather -based Planting Date and Cotton Development Advisories for 11 cotton production areas (Marana, Laveen, Paloma, Litchfield Pk., Pinal Co., Parker, Mohave Valley, Queen Creek, Safford, Yuma Valley, and Aguila). Planting Date Advisories are distributed from mid -February through the end of April and stress 1) planting cotton varieties according to heat unit accumulations rather than calendar date and 2) the importance of soil temperature to good germination. Cotton Development Advisories are distributed from early May through mid -September and provide updates on crop development, insects, weather and agronomy. The Cotton Advisory Program will continue in 1994 and growers may obtain the advisories by mail (fax only in Yuma County only) from the local county extension office or by computer from the AZMET computer bulletin board.

A single field experiment was conducted in 1993 at Marana, Arizona (2,000 ft. elevation) to evaluate the response of three Upland cotton varieties to three dates of planting. Planting dates ranged from as early as 6 April to 11 May. Planting date was a significant effect for all varieties and revealed a substantial drop in yield with delays past 20 April in 1993, which corresponded to 568 heat units (HU, 86/55 °F thresholds) accumulated since 1 January.

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