Defoliation Tests with Ginstar at the Maricopa Agricultural Center in 1997

Abstract

Defoliation tests were conducted on Upland and Pima cotton at the Maricopa Agricultural Center to evaluate rates of Ginstar, tank mixes of Ginstar with other defoliants and Ginstar used as a preconditioner. The cotton used in these tests was generally difficult to defoliate, probably because of an excessive supply of nitrogen and cool temperatures in October and November. In early October, only the Ginstar + Def treatment gave good defoliation of upland while all treatments gave good defoliation of Pima cotton. In late October tests, temperatures were cool and only Ginstar + Def and Ginstar followed by Ginstar treatments gave effective defoliation in 14 days. In October and November tests, Ginstar used as a preconditioner was effective in helping to defoliate cotton under cool weather conditions.

Introduction

Research conducted the past several seasons has indicated that the defoliant Ginstar is as effective or more effective than the Dropp + Def treatment for defoliating cotton in a single application (Nelson and Hart, 1993; Nelson and Hart, 1994; Nelson and Hart, 1995; Nelson and Hart, 1996; Nelson and Hart, 1997).

However, information is needed on the rates of Ginstar to use under different environmental conditions. In central Arizona, cotton is defoliated and harvested under a wide range of weather conditions. For example, when harvest is in September air temperatures can be over 100° F at the time defoliants are applied, while for October harvests, temperatures can be much cooler during defoliation. In addition, research is needed to determine if other defoliants and adjuvants will enhance the activity of Ginstar. In tests reported here, we evaluated different rates of Ginstar used alone and tank mixes of Ginstar with other defoliants.

Preliminary tests have indicated that acceptable defoliation of cotton may be obtained under adverse conditions using Ginstar as a preconditioning treatment. In this season, we evaluated the use of Ginstar at several rates as a preconditioning treatment on hard to defoliate cotton under cool weather conditions.

Materials and Methods

Defoliation tests were conducted on 1 October and 31 October 1997 using DP5415 and Pima S-7 cotton. Separate tests evaluating Ginstar preconditioning treatments on DP5415 cotton were conducted on 1 October and 4 November 1997. The cotton for all tests was planted in moist soil on 10 April 1997. Standard practices for irrigation, cultivation and pest control were followed. The cotton received a total of 140 lbs of N per acre during the growing season and 10 furrow irrigations with the final irrigation on 5 September.

Weather conditions and heat units (HU) for all defoliation tests are shown in Table 1. Descriptions of the defoliation treatments are shown in Tables 2, 3, 4, 5, 6 and 7. Preconditioning treatments of Ginstar were applied 5 to 7 days prior to application of standard defoliation treatments. In all tests, defoliation treatments were applied with a Hi Boy sprayer with a 7 nozzles per row spray boom at 9:00 - 11:00 a.m. A 25 GPA application rate was achieved using 40 psi pressure, disc-core type cone spray tips (Disc No. 1.5 and Core No. 13) and a ground speed of 3 MPH. Plots were 4, 40-inch rows wide by 38 ft. long. Each test utilized a randomized complete block experimental design with 4 replications. Plots were rated for leaf drop by 2 to 3 person 7 and 14 days after defoliants were applied.

Petioles collected from cotton used for defoliation tests contained 2,800 ppm NO3-N for DP5415 and 1,800 ppm for Pima S-7 on 1 October, 1997. At the time of the first test, 1 October, DP5415 cotton had 2-3 NAWB and Pima S-7 had 1-2 NAWB. A September infestation of whitefly resulted in honeydew deposits on cotton leaves in early October.

Results

The defoliation tests reported here were conducted under a wide range of temperature conditions. Heat units decreased from 241 for the 14 day period after defoliants were applied on 1 October to 111 for the 4 November test (Table 1). It is generally thought that 200 or more HU are needed in the 14 day period after application of defoliants to achieve acceptable defoliation (70-75% leaf drop).

In the 1 October upland test (Table 2), only one treatment (Ginstar + Def) gave acceptable defoliation, even though the temperatures were favorable for defoliation (241 HU). In the Pima test, using Ginstar as a preconditioner gave almost complete defoliation in 8 days (Table 3). All defoliation treatments gave excellent defoliation of Pima cotton 14 days after application. In the upland test, the cotton was still growing and contained a dense, green canopy when defoliants were applied. Excessive N in cotton at the end of the season can delay senescence and make defoliation difficult. Our research has shown that the effectiveness of defoliants generally decreases as the petiole NO3-N content increases (Nelson and Hart, 1992). The 2,800 ppm NO3-N content of petioles in the upland cotton in early October indicates an excessive supply late in the season that probably contributed to the vegetative condition of the cotton and the lack of response to defoliants.

In the 31 October upland and Pima tests, temperatures were cool and only the Ginstar + Def and Ginstar followed by Ginstar treatments were effective in defoliating cotton in 14 days (Tables 4 and 5). In both of the 31 October defoliation tests, tankmixing Def with Ginstar improved defoliation compared to using Ginstar alone. When defoliating cotton under cool conditions late in the season, considerable patience is required since very little leaf drop occurs in the first week after defoliants are applied.

Separate tests to evaluate Ginstar as a preconditioning treatment for hard-to-defoliate upland cotton were conducted on 1 October and 4 November ( Tables 6 and 7). In the October test, Ginstar used as a preconditioner was equally effective at rates of .047 and .071 lb a.i./A (Table 6). Both preconditioning treatments resulted in almost 100% leaf drop 8 days after a regular treatment (.071 lb a.i.) of Ginstar was applied. In the November test, there were no differences in percent defoliation among rates of Ginstar used for preconditioning cotton ranging from .047 to .071 lb a.i./A. All preconditioning treatments gave acceptable defoliation in 14 days whereas a single application of .094 or .117 lb a.i./A of Ginstar did not.

References

1. Nelson, J.M. and G. Hart. 1992. Effect of plant nitrogen status on defoliation of short-season upland cotton. Cotton, A College of Agriculture Report. The University of Arizona, Series P-91: 317-319.
2. Nelson, J.M. and G. Hart. 1993. Defoliation research on Pima and upland cotton at the Maricopa Agricultural Center in 1992. Cotton, A College of Agriculture Report. The University of Arizona, Series P-94:56-60.
3. Nelson, J.M. and G. Hart. 1994. Defoliation research on Pima and upland cotton at the Maricopa Agricultural Center in 1993. Cotton, A College of Agriculture Report. The University of Arizona, Series P-96:57-63.
4. Nelson, J.M. and G.L. Hart. 1995. Defoliation research on upland and pima cotton at the Maricopa Agricultural Center in 1994. Cotton, A College of Agriculture Report. The University of Arizona, Series P-99:40-45.
5. Nelson, J.M. and G.L. Hart. 1996. Defoliation tests with Ginstar at the Maricopa Agricultural Center in 1995. Cotton, A College of Agriculture Report. The University of Arizona, Series P-103: 46-52.
6. Nelson, J.M. and G. Hart. 1997. Defoliation tests with Ginstar at the Maricopa Agricultural Center in 1996. Cotton, A College of Agriculture Report. The University of Arizona, Series P-108: 67-75.