AGRONOMY Wick Applicator for Applying Mepiquat Chloride on Cotton: II. Use in Existing Mepiquat Chloride Management Strategies

Abstract

Mepiquat chloride (1,1-dimethylpiperidinium chloride) is widely used in cotton (Gossypium hirsutum L.) to control excessive vegetative growth. One way to apply mepiquat chloride is with a wick applicator mounted at a specific height on implements or sprayers. Two experiments conducted in 11 environments compared wick application of mepiquat chloride to a conventional spray application in three mepiquat chloride management strategies and investigated the effect of setting the wick at two heights for cotton. In one experiment, mepiquat chloride did not affect yield regardless of application system when applied at 9 to10 nodes and/or early bloom. Plant height was reduced by mepiquat chloride applied with a wick at 9 to10 nodes more than by a spray application or a two-step approach of a spray followed by a wick application at early bloom. Height-to-node ratio, main-stem nodes, and nodes above white flower were reduced by mepiquat chloride irrespective of delivery system. In the second experiment, setting the height of the wick to touch the top 8 cm of the plant reduced yield compared with a spray in one of six environments. Setting the wick to brush the top 24 cm reduced yield in two environments. There was no difference in yield between wicking the top 8 cm and the top 24 cm. These data suggest that reduced plant height is the major difference that occurs between wick and spray delivery systems, and the wick should be set to treat only the top 8 cm of the plant. C is a perennial plant that can grow vegetatively at the expense of reproductive growth. In cultivating cotton as an annual crop, producers attempt to manage excessive vegetative growth through judicious use of N and irrigation, and sometimes with the application of mepiquat chloride (Silvertooth et al., 1999). Use of mepiquat chloride on cotton has become widespread since the 1980s, and its ability to create a more compact plant has been well documented (Kerby, 1985; Reddy et al., 1990; Stuart et al., 1984; York, 1983). Because a favorable response to mepiquat chloride depends on environmental conditions at and following application, studies have investigated effects of application timing on cotton response. Guthrie (1989) reported that at nine North Carolina sites mepiquat chloride increased yield, compared with nontreated cotton when used in low-rate, multiple applications starting at match-head square or when applied once at early bloom. Weir et al. (1991) reported that multiple, low-dose applications of mepiquat chloride slightly increased yields over a single application at early bloom in 76-cm row cotton. However, one early bloom application performed better in 97 or 101 cm rows. Other research suggests that mepiquat chloride can be scheduled better by using plant monitoring techniques rather than automatic application based 17 JOURNAL OF COTTON SCIENCE, Volume 5, Issue 1, 2001 on growth stage. Fletcher et al. (1994) based applications on height-to-node ratio; Edmisten et al. (1995) used plant height, height-to-node ratio, and square retention as guidelines. Cotton yields with mepiquat chloride applications based on plant monitoring were equal to or greater than yields where mepiquat chloride was applied automatically. Also, less mepiquat chloride was used. Weir (1993) separated average plant heights from 17 California tests into short, medium, and tall (96, 116, and 165 cm, respectively) categories and reported a more consistent response to mepiquat chloride when it was applied to tall cotton. In tests conducted on fields with variable plant height, Munier et al. (1995) applied mepiquat chloride at variable rates based on the tractor driver’s perception of relative plant height, whereby larger plants received more mepiquat chloride. When pooled over nine locations, variable rate application was slightly less effective in increasing yield than broadcast application was, despite a high percentage of short and medium plants in the field. Possible explanations for this response may have been the inherent plant-to-plant variability in the field as well as the perspective of the operator. A system that applies mepiquat chloride only to the tallest plants, at the most appropriate growth stage, may hold promise for maximizing efficacy of mepiquat chloride. Development of a wick delivery system for mepiquat chloride may allow applications that affect only the tallest plants. Research comparing wick application to spray application has revealed lower use rates were possible in the wick delivery system (Stewart et al., 2001). As a practical matter, the wick applicator can be easily mounted on other implements as well as on sprayers for simultaneous application with other crop protection chemicals. Optimizing performance of mepiquat chloride would be useful in cotton production systems. Therefore, the objectives of these experiments were: (i) to compare cotton response to mepiquat chloride applied by a wick and by a broadcast spray within the context of prevalent mepiquat chloride management systems; (ii) to establish a wickmounting height requirement for cotton for effective application of mepiquat chloride through the wick delivery system. MATERIALS AND METHODS Method and Timing Experiment The experiment was conducted at five locations in North Carolina in 1998 and 1999. Soils and locations in 1998 were Norfolk sandy loam (fineloamy, siliceous, thermic Typic Paleudult) near Rocky Mount; Aycock sandy loam (fine-silty, siliceous, thermic Typic Paleudult) near Snow Hill; and Perquimans silt loam (fine-loamy, mixed, thermic Typic Ochraquult) near Edenton. The1999 sites were Goldsboro sandy loam (fine-loamy, siliceous, thermic Aquic Paleudult) near Lewiston, and Nahunta loam (fine-silty, siliceous, thermic Aeric Paleaquults) near Trenton. Cotton was planted between 25 April and 12 May and standard production practices for eastern North Carolina, with the exception of mepiquat chloride applications, were followed. Cotton was not irrigated. Treatments included a nontreated control of 25 g a.i. ha-1 mepiquat chloride sprayed at the early bloom stage (three white blooms per 3 m of row), 25 g ha-1 mepiquat chloride wicked at early bloom stage, 12 g ha-1 mepiquat chloride sprayed on 9 to 10node cotton, 12 g ha-1 mepiquat chloride wicked on 9 to 10 node cotton, and 12 g ha-1 wicked when cotton had 9 to10 nodes, followed by 25 g ha-1 sprayed at early bloom. An additional treatment of 12 g ha-1 mepiquat chloride sprayed at 9 to10 nodes followed by 25 g ha-1 sprayed at early bloom was included in 1999. The experimental design was a randomized complete block with treatments replicated four times. The wick applicator described by Stewart et al. (2000) was set to wipe the top 8 cm of the cotton and calibrated to deliver 5 L ha-1. Broadcast sprays were applied with a CO2pressurized backpack sprayer delivering 150 L ha-1 at 207 kPa through two flat-fan nozzles per row. Plots were 15 m long by four 92 or 97 cm rows. Wick Height Experiment The experiment was conducted at six locations in North Carolina in 1998 and 1999. Soils and locations in 1998 included: Torhunta loam (coarseloamy, siliceous, thermic Acid Typic Humaquept) near Goldsboro; Stallings loamy fine sand (coarseloamy, siliceous, thermic Aeric Paleaquult) near 18 ALEXANDER ET AL: WICKED AND SPRAYED MEPIQUAT CHLORIDE ON COTTON II. USE IN MANAGEMENT Table 1. Effects of mepiquat chloride application systems, rates, and timings on cotton growth in 1998 and 1999. Applicator Timing Height Nodes Ht:node† NAWF‡ Rate 1998 1999 1998 1999 1998 1999 1998 1999 g ha-1 cm no. cm node-1 no. Untreated 95 a§ 111 a 17.3 a 18.0 a 5.5 a 6.2 a 4.5 a 6.1 a Spray 9-10 nodes 12.25 82 b 95 b 16.3 b 16.1 bc 5.0 a 6.1 ab 3.8 bc 5.3 abc Wick 9-10 nodes 12.25 82 b 88 c 16.4 b 15.9 bc 5.0 a 5.5 abc 4.2 bc 5.0 bc Spray Early bloom 24.50 87 b 96 b 16.6 ab 16.9 b 5.3 a 5.7 abc 4.0 abc 5.3 abc Wick Early bloom 24.50 83 b 94 b 15.8 b 15.9 bc 5.3 a 6.1 ab 4.0 abc 5.5 abc Wick then spray 9-10 nodes;

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