Agronomy Journal Journal of Natural Resources and Life Sciences Education
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Published online 31 October 2006
Published in Agron J 98:1634-1639 (2006)
DOI: 10.2134/agronj2006.0083
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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Production Papers

Influence of Plant Density on Cotton Response to Mepiquat Chloride Application

Jonathan D. Sieberta,* and Alexander M. Stewartb

a Monsanto Co., Leland Agronomy Center, 4006 Old Leland Rd., Leland, MS 38756
b LSU AgCenter, Dean Lee Research Station, 8105 Tom Bowman Dr., Alexandria, LA 71302

* Corresponding author (jonathan.d.siebert{at}monsanto.com)

Received for publication March 21, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Increased yield potential associated with full-season cotton (Gossypium hirsutum L.) varieties can be offset by excessive vegetative growth that leads to undesirable fruit shed and boll rot. Producers often make multiple mepiquat chloride (1,1-dimethyl-piperidinium chloride) applications with excessive seasonal use rates to combat this problem, often with inconsistent results. Field studies were conducted in 2004 and 2005 to evaluate reduced plant populations as a potential management tool to be used in conjunction with mepiquat chloride application strategies for managing plant height. Plant populations of 152883, 101929, and 50958 plants ha–1 received a single application at 12 nodes (15.2 g a.i. ha–1) or early bloom (45.8 g a.i. ha–1), sequential applications at 12 nodes (15.2 g a.i. ha–1) and early bloom (30.6 g a.i. ha–1), or the modified early bloom schedule (a plant growth regulator application decision aid that recommends rates and timing based on plant growth parameters). Mepiquat chloride application reduced final plant height at least 15 cm compared with the nontreated in both years but also reduced the number of main stem nodes in 2005. In 2005, lint yield was inversely related to plant population, and a significant yield response occurred with the modified early bloom mepiquat chloride application strategy. Mepiquat chloride application increased lint yield in 1 of 2 yr but was useful for reducing plant height regardless of population. Reducing plant population had no adverse effects on lint yield or fiber properties and may be valuable in achieving a desired plant stature with less intensive plant growth regulator management.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
SUCCESSFUL boll weevil (Anthonomous grandis grandis [Boheman]) eradication and the introduction of transgenic Bt (Bacillus thuringiensis) cotton has dramatically reduced problems with late-season insect infestations. The opportunity to take full advantage of the long growing season in the lower Mississippi Delta has led to the increased adoption of more indeterminate mid–full-season cotton varieties with increased yield potential. In 2005, DP 555 BG/RR (Delta and Pine Land Company, Scott, MS), a variety documented as having a very tall growth habit (Lege and Leske, 2003), accounted for 30% of the hectareage (349240 ha) in Arkansas, Louisiana, and Mississippi (Anonymous, 2005a). Producers rely on plant growth regulators such as mepiquat chloride to combat excessive vegetative growth, which if left uncontrolled can lead to undesirable fruit shed (Baker, 1976), boll rot (Snow et al., 1981), and yield reductions (Fowler and Ray, 1977).

Application of mepiquat chloride to cotton usually results in a more compact plant (Reddy et al., 1992) resulting from reduced stem elongation, main stem node formation, leaf expansion, and leaf area (Reddy et al., 1990). Consistent reductions in plant height can be expected in conventional (Zhao and Oosterhuis, 2000; Pettigrew and Johnson, 2005) and ultra-narrow row cotton production (Nichols et al., 2003), but any potential yield benefit (Biles and Cothren, 2001) is contradicted by research showing a negative impact or no response at all (Nichols et al., 2003; Zhao and Oosterhuis, 2000; Pettigrew and Johnson, 2005). Research suggests that a beneficial response from mepiquat chloride application will most likely occur under conditions that favor excessive vegetative growth, such as a late planting date (Cathey and Meredith, 1988), access to surplus nitrogen (Kerby et al., 1982), or high plant population (York, 1983).

Several strategies and guidelines for applying mepiquat chloride to control plant vegetative growth have been developed. The modified early bloom schedule is a plant monitoring program developed as a plant growth regulator application decision aid that uses cotton growth measurements at specific developmental stages to trigger applications at recommended rates (Stewart, 2005). Mepiquat chloride application with the modified early bloom method would typically begin 10 to 14 d after the first square is present and continue, based on plant growth, until 2 wk after first bloom. This places the modified early bloom strategy between the aggressive (low rate multiple strategy with application beginning at pinhead square) and conservative (early bloom strategy with application beginning at first bloom) plant growth regulator application methods. The modified early bloom strategy, which was developed in North Carolina and is applicable to much of the Mid-South and Southeast United States cotton growing area, has been readily adopted by growers who have historically used the early bloom strategy but consistently have the problem of needing to spray more cotton than can be sprayed in a timely manner (Faircloth, 2005).

Seed premiums, technology fees associated with transgenic cotton cultivars, and the use of seed treatments for insect and disease control have increased at-planting variable costs and have inspired renewed interest in reduced seeding rates. Establishing an acceptable stand of cotton seedlings is paramount to obtaining high yields (Christiansen and Rowland, 1981). Current plant density recommendations in Louisiana are 10 to 13 plants m–1 of row for conventionally spaced cotton (96.5–101.6 cm row widths) (Siebert et al., 2006). However, recent studies have shown no differences in cotton yield due to plant population (Jones and Wells, 1998; Bednarz et al., 2000; Franklin et al., 2000; Siebert et al., 2006); this is due in part to an increased number of main stem nodes (Jones and Wells, 1997; Siebert et al., 2006) and distally located sympodial and monopodial bolls (Jones and Wells, 1998; Siebert et al., 2006) on plants grown under low plant densities. These findings in the current plant population literature coupled with widespread adoption of vacuum seed metering, which allows uniform plant spacing with lower seed requirements (Wanjura, 1980), will likely lead to a decline in seeding rates. Within currently recommended seeding rates, plant height-to-node ratio generally decreases as plant density decreases (Siebert et al., 2006). York (1983) found that higher plant populations led to cotton with excessive vegetative growth that was more responsive to mepiquat chloride. This suggests that in addition to reducing at-planting variable costs, reduced plant populations could be a practical tool to be used in conjunction with mepiquat chloride for controlling plant height in areas with a history of rank growth. The objective of this research was to evaluate the influence of plant density on cotton response to mepiquat chloride application in an attempt to isolate a specific combination of plant density and mepiquat chloride application strategy for optimizing plant stature and yield.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Field experiments evaluating the influence of plant density on cotton response to mepiquat chloride application were conducted during 2004 and 2005 on a Norwood silt loam soil (fine-silty loam, mixed calcareous, thermic Typic Udifluvent) at the Dean Lee Research Station near Alexandria, LA. The experimental design was a randomized complete block with a factorial treatment arrangement (Factor A: plant density; Factor B: mepiquat chloride application) and four replications. Plot size was four rows that were 96.5 cm wide and 12.15 m long. All treatments were planted in a drill-seeded configuration with cotton (cv. DP 555 BG/RR) seed using a four-row John Deere Max Emerge II vacuum planter (Moline, IL) with 5.1 cm between seeds on 24 May 2004 and 17 May 2005. Plots were hand thinned with uniform plant spacing 3 wk after emergence to densities of 152833, 101929, or 50958 plants ha–1.

Mepiquat chloride (41.98 g a.i. L–1) (Pix Plus; BASF, Research Triangle Park, NC) treatments consisted of a nontreated, a single 15.2 g a.i. ha–1 application at 12 nodes, sequential applications of 15.2 g a.i. ha–1 at 12 nodes followed by 30.6 g a.i. ha–1 at early bloom, a single 45.8 g a.i. ha–1 application at early bloom, or application based on the modified early bloom schedule. Mepiquat chloride applications for modified early bloom treatments on each population in both years are listed in Table 2. Twelve node treatments were applied when the twelfth mainstem leaf was >25 mm wide (cotyledons counted as node zero) (18 June 2004 and 27 June 2005), and early bloom treatments were applied when 50% of plants in the experimental area started flowering (29 June 2004 and 9 July 2005). Treatments were applied with a tractor-mounted CO2 sprayer calibrated to deliver a carrier volume of 140 L ha–1 at 330 kPa and 5.81 km h–1 through a four-row boom equipped with ConeJet (TeeJet Spraying Systems, Wheaton, IL) nozzles.


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  		Table 2. Effect of plant density on parameters measured for making mepiquat chloride decisions when using the modified early bloom strategy and the corresponding cotton final height reduction in 2004 and 2005 in Alexandria, LA.{dagger}

 
The experimental area was maintained using standard cultural practices based on extension service recommendations by the Louisiana State University AgCenter. Cotton was nonirrigated, and 32% UAN nitrogen (urea and ammonium nitrate solution) was applied at 101 kg ha–1 both years. At approximately 60% open bolls, the experimental area was chemically defoliated with a tankmix of thidiazuron (N-phenyl-N'-1,2,3-thiadiazol-5-ylurea), tribufos (S,S,S-tributyl phosphorotrithioate), and ethephon (2-chloroethyl phosphonic acid).

Data were collected from the center two rows of the four-row plot. Final plant height and total number of mainstem nodes were recorded from 10 randomly selected plants per plot. Cotyledons were counted as node zero, and the uppermost node with a mainstem leaf > 25 mm wide was considered the terminal node. Plant height and number of mainstem nodes were used to calculated height-to-node ratio. After defoliation, the center two rows of each plot were harvested on 27 Oct. 2004 and 11 Oct. 2005 with a commercial two-row spindle picker equipped with a weigh cell capable of being tared between plots. A 0.9-kg subsample of seedcotton was retained from each plot and ginned on a 12-saw research gin to determine lint percentage (gin turnout). Physical fiber properties were determined using a high-volume instrumentation method at the Louisiana State University AgCenter Fiber Laboratory, Department of Agronomy and Environmental Management, Baton Rouge, LA (Sasser, 1981).

Plant height, number of mainstem nodes, height-to-node ratio, lint yield, gin turnout, and fiber properties were subjected to ANOVA (SAS PROC GLM), and means were separated with Fisher's Protected LSD ({alpha} = 0.05) (SAS Institute, 1998). Dunnett's t test (P = 0.05) was used to compare final plant height of modified early bloom mepiquat chloride treatments at each population to their respective nontreated (SAS Institute, 1998).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Significant year-by-treatment interactions existed for all variables measured; therefore, data are presented by year. Differences were attributed to variations in environmental conditions during the 2004 and 2005 growing seasons. Average monthly rainfall in 2004 was twice that of 2005; however, higher air temperatures in 2005 accounted for an additional accumulation of 73 heat units (using °F, 45 heat units °C) per month on average (Table 1). Plant population by mepiquat chloride application interactions did not occur for final vegetative growth parameters, lint yield, gin turnout, or physical fiber properties in either year; however, differences were attributed to each main effect. Previous research by Briggs (1982), York (1983), and Pettigrew and Johnson (2005) also noted the lack of a significant plant growth regulator by plant population interaction for rates and populations evaluated.


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  		Table 1. Minimum and maximum air temperature, precipitation, and cumulative heat units recorded at Dean Lee Research Station, Alexandria, LA, 2004–2005.

 
Modified Early Bloom Strategy
Management of plots using the modified early bloom strategy was identical for plant densities of 101929 and 50958 plants ha–1 both years; they required one less mepiquat chloride application and had seasonal use rates 24.4 and 12.2 g a.i. ha–1 below that of 152883 plants ha–1 in 2004 and 2005, respectively (Table 2). In 2005, a significant height reduction, compared with the respective nontreated at the same plant density, was achieved with mepiquat chloride application using the modified early bloom strategy for all populations. However, in 2004, a significant reduction in final plant height was not achieved at 152883 plants ha–1 despite sequential plant growth regulator applications and seasonal use rate of 73.2 g a.i. ha–1 (Table 2). Research conducted internally by Delta and Pine Land Company (Kerby et al., 2004) indicated that a mepiquat chloride response with DP 555 BG/RR is most likely when field growth conditions result in a fully expanded internode length of 6.89 cm at the time of late squaring to early bloom. This research differs from earlier work because mepiquat chloride was not only applied at a predetermined application rate but was also applied based on and varying with plant growth parameters, whereas in earlier research a predetermined rate was applied (York, 1983; Kerby et al., 1986; Cathey and Meredith, 1988; Biles and Cothren, 2001) or mepiquat chloride was applied to maintain a certain concentration in the plant (Kerby et al., 2004).

Final Vegetative Growth Parameters
Final plant height was not influenced by plant population in either year. In 2004, an increase in the number of main stem nodes was associated with populations of 101929 and 50958 plants ha–1 compared with 152883 plants ha–1; however, the final height-to-node ratio was not influenced (Table 3). Bednarz et al. (2000), Jones and Wells (1997), and Heitholt (1995) also showed that plant population and number of main stem nodes are inversely related. Although not significant, the same relationship occurred with respect to plant population and main stem nodes in 2005. In 2005, plant population did not influence plant height, but height-to-node ratio was lower for 50 958 plants ha–1 (Table 4).


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  		Table 3. Effect of plant population and mepiquat chloride application on final height, mainstem nodes, height/node, lint yield, lint percentage, and physical fiber properties in 2004 in Alexandria, LA.

 

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  		Table 4. Effect of plant population and mepiquat chloride application on final height, mainstem nodes, height/node, lint yield, lint percentage, and physical fiber properties in 2005, Alexandria, LA.

 
Mepiquat chloride application reduced plant height at least 14.6 and 10.0% compared with the nontreated in 2004 and 2005, respectively, but also resulted in a reduction in main stem nodes in 2004. Similar height reductions of 9% (Pettigrew and Johnson, 2005) and 11% (Cook and Kennedy, 2000) associated with plant growth regulator application have been reported. A reduction in the number of main stem nodes and fruiting positions with mepiquat chloride use was previously documented (Kerby, 1985; Kerby et al., 1982). The greatest reduction in plant height, which resulted in the most compact plants as was evident from height-to-node ratio, occurred with sequential and early bloom treatments in 2004 (Table 3) and sequential, early bloom, and modified early bloom treatments in 2005 (Table 4). These results are similar to those of Craig and Gwathmey (2005) who found that height reductions were greatest with low- and high-rate multiple application strategies beginning before bloom.

Lint Yield and Quality
Lint yield, gin turnout, upper half mean staple length, fiber strength, uniformity, and elongation were not influenced by plant population or mepiquat chloride application in 2004. Fiber micronaire was not affected by plant population, but mepiquat chloride application increased micronaire at least 0.12 units (Table 3). However, micronaire values were within accepted ranges and would not warrant premiums or discounts for the grade (Anonymous, 2005b). In 2005, lint yield was inversely related to plant population (Table 4). Mepiquat chloride application using the modified early bloom strategy increased lint yields 164 kg ha–1 over the nontreated. Although not significant, all other mepiquat chloride treatments increased yield over the nontreated in 2005, whereas mepiquat chloride treatment caused a numerical yield decrease in 2004. Gin turnout, micronaire, upper half mean, strength, uniformity, and elongation were not influenced by treatments in 2005. Recent cotton plant density studies (Bednarz et al., 2000; Franklin et al., 2000; Siebert et al., 2006) that investigated plant populations within the range of those in this study have shown no lint yield response to plant population, which is similar to our findings in 2004. Additionally, research addressing growth and fruiting response of DP 555 BG/RR showed that plant spacing of 7.6, 15.2, and 30.5 cm had no effect on yield (Miller et al., 2004). Ample rainfall combined with favorable weather in September 2005 (Table 1) did not limit heat unit accumulation resulting in continuation of boll development. A combination of factors, such as a mid-full season variety with increased yield potential (Lege and Leske, 2003), a greater number of potential fruiting sites because of increased number of main stem nodes (Jones and Wells, 1997), sympodial branch length (Kerby et al., 1990), nodes per monopodial branch (Buxton et al., 1977), increased boll retention (Galanopoulou-Sendouka et al., 1980), and boll size (Bednarz et al., 2000) associated with decreasing population density may have allowed 50958 plants ha–1 to out yield higher plant populations. Kerby et al. (1996) reported that the optimal plant density is greater under conditions of severe stress, and it is likely the opposite is true under very favorable growing conditions.

Yield response to mepiquat chloride application was inconsistent. A yield response did not occur in 2004 when excessive early season rainfall led to poor root development and plant stress (Table 1); however, in 2005 adequate rainfall and favorable temperatures well into September are attributed to the yield response. Nichols et al. (2003) reported a yield response to mepiquat chloride application in 1 of 3 yr, yield enhancement with mepiquat chloride application in 1999, corresponded to average monthly rainfall 6.1 and 4.1 cm below that of 1998 and 2000 when no response occurred. These data support reports that conclude that the variability of response to mepiquat chloride application is due to environmental conditions and that a response is most likely when the environment favors excessive growth (Briggs, 1982; Kerby, 1985; Kerby et al., 1986).


   SUMMARY
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Mepiquat chloride is a useful tool for controlling plant height, especially when growing conditions favor rank growth; however, a consistent yield response should not be expected. The lack of a plant population by mepiquat chloride application interaction dictates that the response of cotton to mepiquat chloride for this tall, full-season variety is independent of plant density. Plant monitoring and application based on the modified early bloom strategy suggests that lower plant populations may require less intensive management, and a desired plant height reduction can be achieved with lower seasonal use rates and fewer applications. In all cases, plant stature was maintained without excessive use rates, which suggests that application based on plant developmental stage and growing conditions is more favorable than application based on calendar date or other time points. These data support current findings that suggest that cotton plant populations can be lowered without adversely affecting yield, given that planting conditions and the prevailing weather are conducive to achieving uniform plant distribution and that delayed maturity offset by environmental conditions allows maturation of distally located bolls.


   ACKNOWLEDGMENTS
 
The authors thank the faculty and staff at the Dean Lee Research Station for assistance in conducting these studies, Dr. David Blouin for assistance with statistical analyses, and Cotton Incorporated for financial assistance.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 





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