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a Isle of Wight County Extension Office, 17100 Monument Circle, Suite B, Isle of Wight, VA 23397
b Dow AgroSciences, 1799 Percy Place, Collierville, TN 38017
c Clemson University, Pee Dee Research and Education Center, 2200 Pocket Road, Florence, SC 29506
d Virginia Polytechnic Institute and State University, Tidewater Agricultural Research and Extension Center, 6321 Holland Road, Suffolk, VA 23437
e Crop and Soil Environmental Science Dep., Campus Box 0404, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
f BASF Corporation, 5104 Indigo Moon Way, Raleigh, NC 27613
g Professor Emerita, North Carolina State University, 3309 Horton Street, Raleigh, NC 27607
* Corresponding author (noberry{at}vt.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AMS, apical main-stem • DP, Deltapine • FM, Fibermax • HNR, height-to-node ratio • MC, mepiquat chloride • MPB, mepiquat pentaborate • NAWF, nodes above white flower • PGR, plant growth regulator
Received for publication August 15, 2008.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Cotton is a perennial plant with an indeterminate fruiting pattern, which may vary in length by cultivar and management practices employed (Zhao and Oosterhuis, 2000). While vegetative growth continues throughout the growing season, plant resources begin to shift toward reproductive growth approximately 35 d after planting, as the plant begins forming fruiting structures (Gwathmey and Craig, 2003). Excessive vegetative growth may occur due to late planting (Cathey and Meredith, 1988), high plant populations (York, 1983b), abundant N (York, 1983b), and sufficient moisture. This excessive vegetative growth may lead to a delay in reproductive growth (York, 1983a, 1983b; Cathey and Meredith, 1988), increase in late-season fruit shedding (Guinn, 1974; Bednarz et al., 2000) and boll rot (York, 1983a) due to shading, increase in insect damage (York, 1983a), decrease in defoliation and harvest efficiency (York, 1983a, Cathey and Meredith, 1988; Gwathmey and Craig, 2003), and a reduction in yield (Guinn, 1974; York, 1983b; Cathey and Meredith, 1988; Gwathmey and Craig, 2003).
Managing cotton for earliness is a key consideration in the northern region of the Cotton Belt due to limited heat unit accumulation (Gwathmey and Craig, 2003). For this reason, most cultivars planted in the northern regions of the Cotton Belt are relatively early maturing to allow timely removal from the field before losses due to inclement fall weather (Faircloth, 2007). These early maturing cultivars must also have a high yield potential and acceptable fiber quality. In South Carolina, where more heat units are accumulated relative to the northern region of the Cotton Belt, producers may be able to use later maturing cultivars as the growing season is extended.
Plant growth regulators are frequently applied to cotton to hasten maturity through a reduction in vegetative growth, causing a shift in plant resources to reproductive growth (Cathey and Meredith, 1988; Zhao and Oosterhuis, 2000; Gwathmey and Craig, 2003). Plant growth regulator application decreases gibberellic acid concentration within plant cells, thereby reducing cell wall plasticity (Behringer et al., 1990; Potter and Fry, 1993; Yang et al., 1996). This ultimately reduces vegetative growth in cotton by decreasing main stem and sympodial branch internode length, as well as leaf area of expanding leaves, resulting in a shorter, more compact plant (Kerby, 1985; Cathey and Meredith, 1988; Gwathmey and Craig, 2003).
Factors that should be considered before PGR application include fertility, moisture level, cultivar, field history, and previous PGR applications (Gwathmey and Craig, 2003). Reported responses of cotton to PGR application include decreased plant height (Gwathmey and Craig, 2003; Nuti et al., 2006), earlier flowering and physiological cutout (Gwathmey and Craig, 2003), and increased boll retention lower on the plant (Nuti et al., 2006). Physiological cutout is defined as the date of the last effective flowering that will allow adequate time for bolls to mature, which was determined to occur at an average of 5.0 nodes above white flower (NAWF = 5) in Arkansas (Bourland et al., 2001). The determination of physiological cutout can be used as a basis for measuring plant maturity across different environments and cultivars (Bourland et al., 2001; Gwathmey and Craig, 2003). Cotton growth responses to PGR application have been consistent, while yield responses have been inconsistent (Kerby, 1985; Cathey and Meredith, 1988; Zhao and Oosterhuis, 2000; Nuti et al., 2006).
There are numerous PGR's commercially available, most being slight variations of or additives to the mepiquat chloride (MC) molecule (1,1-dimethylpiperidinium chloride) (Gwathmey and Craig, 2003). Mepiquat pentaborate (MPB) (Pentia, BASF, Research Triangle Park, NC) is a PGR applied to cotton which contains the active ingredient MPB (N,N- dimethylpiperidinium pentaborate) at 98.3 g a.i. L–1. This PGR combines five boron molecules along with mepiquat (BASF Ag Products, 2007). Mepiquat pentaborate has been reported to effect cotton growth and development similar to MC products, including a reduction in plant height and number of fruiting branches, plus hastened maturity, while not altering boll retention or lint yield compared to untreated cotton (Coccaro et al., 2003, 2004; Gwathmey and Craig, 2003, 2004).
Various cultivars may respond differently to PGR application as there are distinctions in growth habits and maturity associated with cultivar genetics. Gwathmey and Craig (2003) reported that single MC applications hastened the maturity of later maturing cultivars causing earlier physiological cutout, but had no effect on the flowering of earlier cultivars. Multiple MC applications have been reported to also hasten flowering and reduce plant height, with the greatest response occurring with late-maturing cultivars (Gwathmey and Craig, 2003). Gwathmey and Craig (2003) concluded that higher total season rates of MC resulted in excessive growth control of early maturing cultivars, causing a reduction in yield, while MC application benefited late-maturing cultivars through a reduction in vegetative growth and increased yield.
There is limited research examining the response of cultivars with different growth habits and maturities to PGR application (Gwathmey and Craig, 2003). Furthermore, no research has examined the impact of MPB application on cultivars in multiple environments. The objective of this research was to determine the response of several cotton cultivars to MPB application in environments accumulating different levels of heat units.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Twelve treatment combinations were examined in a split-plot design with four replications, with cultivar as the main-plot and plant growth regulator application regimes as the subplot. Plots consisted of four 12.2-m long rows spaced on 91.4-cm centers in Virginia and 96.5-cm centers in South Carolina. Three cotton cultivars were evaluated: Deltapine (DP) 444 BG/RR, an "early-maturing" cultivar; Fibermax (FM) 960 BR, a "medium-maturing" cultivar; and DP 555 BG/RR, a "late-maturing" cultivar (Faircloth et al., 2005b; Bowman, 2007). Although the cultivars chosen for this experiment were selected based on differing maturity classes, differences in the results could not be attributed to maturity class as there were no other cultivars from similar classes to compare against. Seed were planted 5 May 2005 and 16 May 2006 in Virginia, and 10 May 2005 and 2006 in South Carolina. An untreated control and three PGR treatments were examined. The first PGR treatment was a 12.2 g a.i. ha–1 application of MPB at pin-head square, 18.3 g a.i. ha–1 at match-head square, and 24.4 g a.i. ha–1 at early bloom. This was modeled to be similar to a producer's typical usage pattern in nonirrigated cotton in the upper southeastern United States (Edmisten, 2005). Higher rates applied at the same growth stages were used for treatment two, where 24.4 g a.i. ha–1 MPB was applied at pin-head square, 24.4 g a.i. ha–1 at match-head square, and 36.5 g a.i. ha–1 at early bloom. Lastly, a PGR approach was used where the producer initiates an earlier (five-leaf stage) application strategy and is aggressive throughout the growing season. This strategy is sometimes used by producers that want the first PGR application to coincide with their last labeled over the top application of glyphosate in Roundup Ready cotton. In this approach 12.2 g a.i. ha–1 MPB is applied at the five-leaf stage, 24.4 g a.i. ha–1 at pin-head square, 36.5 g a.i. ha–1 at match-head square, and 48.8 g a.i. ha–1 at early bloom. These plant growth regulator applications were made using MPB with a tractor mounted carbon dioxide pressurized sprayer, calibrated to deliver 140.3 L ha–1. Decisions on fertility, weed control, and insect control were made according to respective state cooperative extension recommendations (Jones et al., 2005; Faircloth et al., 2005a).
All data were collected from the center two rows of the plots, including plant stand, plant height (late July and at harvest), number of apical main-stem (AMS) nodes (at harvest), height-to-node ratio (HNR) (at harvest), nodes above white flower (NAWF), lint yield, and fiber quality. Plant stands were collected randomly after emergence from a 3.05 m long section of each row and were not different when pooled for all environments, with a mean of approximately 100 plants per row. All other growth characteristic measurements were collected on five randomly selected plants in each row. Plant heights were measured from the cotyledonary node to the terminal of the plant, while the number of AMS nodes were evaluated beginning with the node above the cotyledonary node and ending at the node with the uppermost leaf larger than 2.5 cm in diameter. The HNR was calculated after these measurements had been collected. Nodes above white flower measurements were collected on 25 July 2005 and 2006 in Virginia, and on 17 July 2005 and 27 July 2006 in South Carolina. The NAWF measurement was evaluated using a common technique recommended for making PGR application decisions, which included counting the number of nodes from the uppermost first-position white flower to the uppermost node with a leaf larger than 2.5 cm in diameter. Plant mapping data were collected at harvest in Virginia during 2005 and 2006 to determine the total number of monopodial branches, monopodial bolls, first and second position sympodial bolls, first sympodial node, and the percent retention of sympodial bolls plant–1. Plant mapping data were not collected in South Carolina.
Harvest-aid applications were applied uniformly across all treatments, and the center two rows of each four-row plot were harvested using a two-row commercial spindle cotton harvester. Harvest dates were 1 Nov. 2005 and 3 Nov. 2006 in Virginia, and 10 Oct. 2005 and 1 Nov. 2006 in South Carolina. Seed-cotton samples from each plot were retained and ginned on a 10-saw gin to determine lint yield. A subsample was sent to the USDA Classing Office in Florence, SC to determine physical fiber properties using high volume instrument analysis (USDA AMS Cotton Program Florence South Carolina Classing Office, Florence, SC).
Analysis of variance was performed using PROC GLM (SAS Institute, 2003). Means were separated using Fisher's Protected LSD test and statistical significance was evaluated at P = 0.05. Initially, the combined data from the four trials (two locations in each of 2 yr) were subjected to an ANOVA in which the trial effects were regarded as levels of a single random factor "environment." If any components of the treatment x environment interaction (i.e., environment x cultivar, environment x MPB or environment x cultivar x MPB) were significant (Table 1 ), additional analyses were performed separately by environment. As a result, comparisons of cultivar means are presented separately by environment for late-July plant height, late-July plant height reduction percentage, harvest plant height, actual harvest plant height reduction compared to the untreated of each cultivar, harvest plant height reduction percentage, apical main-stem nodes, height-to-node ratio, lint percentage, lint yield, micronaire, fiber length, fiber strength, and fiber length uniformity. For simplicity of presentation, cultivar means for NAWF and harvest plant height reduction were reported by environment even though the environment x cultivar interaction was not significant (Table 1). Means for MPB are presented by environment for plant heights, HNR, and fiber length uniformity, and averaged over environments for NAWF, AMS nodes, lint percentage, lint yield, micronaire, fiber length, and fiber strength. The few significant environment x cultivar x MPB interactions (Table 1) were not considered important in interpreting treatment responses.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Growth Characteristics
In Virginia in 2005, cultivar had no influence on plant height in late July, while in South Carolina in 2005 and Virginia in 2006, DP 444 BG/RR and DP 555 BG/RR plants were taller than FM 960 BR (Table 3
). Deltapine 444 BG/RR plants were the tallest at 48 cm in late July in South Carolina in 2006. Deltapine 444 BG/RR and DP 555 BG/RR plants were the tallest at harvest during both years in Virginia.
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Mepiquat pentaborate applications decreased plant height of all cultivars compared to the untreated in both Virginia and South Carolina, except for the measurement at harvest in South Carolina in 2006, in which the untreated and typical usage rate of MPB were statistically the same (Table 4 ). This is similar to other studies where plant growth regulator applications consistently reduced plant height in comparison to untreated cotton (Zhao and Oosterhuis, 2000; Gwathmey and Craig, 2003; Nichols et al., 2003; Siebert and Stewart, 2006).
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Cultivar influenced the HNR measurements in all environments, with DP 444 BG/RR resulting in a higher HNR, compared to both the FM 960 BR and DP 555 BG/RR cultivars (Table 3). Cotton treated with MPB produced a lower HNR compared to the untreated cotton in all environments (Table 4). Similar results have been found when PGR's are applied to cotton and were attributed to shortened internode lengths (Zhao and Oosterhuis, 2000; Nichols et al., 2003; Nuti et al., 2006; Siebert and Stewart, 2006).
Virginia plant mapping data were combined for both years and resulted in no significant differences for the number of monopodial branches, total sympodial bolls, first position sympodial bolls, first sympodial node, and the percent retention of first and second sympodial bolls plant–1 for either cultivar or MPB applications (data not shown). In Virginia, there were approximately 458 to 561 heat units accumulated during August and September, which may have allowed full-season maturing varieties to develop similarly to the shorter-season maturing varieties.
Lint Percentage and Yield
Lint percentage differed among cultivars in both Virginia and South Carolina in 2005, with DP 555 BG/RR resulting in a higher lint percentage than either DP 444 BG/RR or FM 960 BR (Table 3). In Virginia and South Carolina in 2006 the DP 555 BG/RR and DP 444 BG/RR cultivars produced the highest lint percentage, although DP 444 BG/RR was not different from FM 960 BR. Deltapine 555 BG/RR contains small seeds and has consistently produced a numerically higher lint percentage in university cultivar trials relative to DP 444 BG/RR and FM 960 BR (Bowman, 2007; Faircloth, 2007).
Averaged over both environments, MPB application decreased the lint percentage when compared to the untreated control (Table 5). Plant growth regulator application has been reported to reduce lint percentage (Kerby, 1985; Cathey and Meredith, 1988; Zhao and Oosterhuis, 2000; Biles and Cothren, 2001). Although seed weight was not recorded in this experiment, previous research has attributed this reduction in lint percentage to an increase in seed weight following PGR application (York, 1983b; Oosterhuis and Egilla, 1996; Biles and Cothren, 2001).
In this experiment yield was not influenced by cultivar in any environment (Table 3). However, when averaged over environments, the higher total season rates of MPB decreased yield compared to the untreated, although the standard MPB application rate did not differ significantly from the untreated (Table 5). This decrease in lint yield ranged from 3.7 to 8.5%, and may have occurred because of oversuppression of vegetative growth due to increased rates and earlier initiation of MPB application. Similarly, Gwathmey and Craig (2003) reported that excessive control of vegetative growth on early maturing cultivars with limited growth potential also resulted in reduced lint yields. The variability associated with yield differences has been attributed to various factors including vegetative growth, seasonal heat unit accumulation, and precipitation (Kerby, 1985; Cathey and Meredith, 1988; Gwathmey and Craig, 2003; Siebert and Stewart, 2006). Plant growth regulator application can result in increased yields by reducing vegetative growth under conditions that are conducive to rank growth, and can also increase yields by hastening maturity in environments with limited heat units or reduced growing seasons (York, 1983b; Kerby, 1985; Kerby et al., 1986; Cathey and Meredith, 1988).
Fiber Quality
Micronaire, fiber length, fiber strength, and fiber length uniformity are influenced by both genetics and environmental conditions (Meredith and Bridge, 1973; Meredith et al., 1975; Bednarz et al., 2005). Faircloth et al. (2004) reported differences in micronaire values due to cultivar. Micronaire values were higher for FM 960 BR (4.6 and 4.2 units) and DP 555 BG/RR (4.5 and 4.3 units) than DP 444 BG/RR (4.1 and 4.0 units) in Virginia and South Carolina in 2005 (Table 3), while there were no differences due to cultivar in Virginia or South Carolina in 2006. Applying PGR's to cotton has resulted in inconsistent changes in micronaire (Kerby, 1985; Cathey and Meredith, 1988; Nuti et al., 2006; Siebert and Stewart, 2006). Averaged over environments, cotton treated with MPB at the higher rates had lower micronaire values compared to the untreated control and standard application rate (Table 5).
In Virginia in 2005, DP 444 BG/RR and DP 555 BG/RR resulted in longer fiber length at 2.94 and 2.90 cm, respectively, compared to FM 960 BR at 2.84 cm, while in South Carolina in 2005, DP 444 BG/RR produced the longest fibers at 2.93 cm (Table 3). Cultivar had no influence on fiber length in either environment in 2006. As reported in previous research (Cathey and Meredith, 1988; Nuti et al., 2006), in this experiment averaged over environments, MPB application increased fiber length (Table 5).
Fiber strength was influenced by cultivar as FM 960 BR fiber had the highest strength value in each environment at 32.5, 34.9, 32.1, and 33.7 g tex–1, respectively (Table 3). Bowman (2007) and Faircloth (2007) reported FM 960 BR to consistently have numerically higher fiber strength than either DP 444 BG/RR or DP 555 BG/RR in cultivar trials. Kerby (1985), Cathey and Meredith (1988), and Nuti et al. (2006) observed an increase in fiber strength following application of PGR's to cotton. Consistent with these results, averaged over environments, MPB application increased fiber strength in this study (Table 5).
In both Virginia and South Carolina in 2005, DP 444 BG/RR and FM 960 BR resulted in higher fiber length uniformity values, compared to DP 555 BG/RR, while there were no differences in Virginia or South Carolina in 2006 (Table 3). York (1983a) reported variable fiber length uniformity amongst cultivars. As previously reported by Nichols et al. (2003) and Siebert and Stewart (2006), MPB application had no influence on fiber length uniformity in either year in Virginia or South Carolina (Table 4). The differences recorded in fiber quality properties were limited in this experiment and were likely due to genetic responses and hastened maturity following MPB application.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Based on changes in the growth parameters measured in this experiment, MPB applications can reduce vegetative growth and hasten maturity. The reduction in vegetative growth and increase in earliness following PGR application increases the probability of defoliation and harvest before freezing temperatures and inclement fall weather, which is an annual challenge in Virginia. While there were some responses in fiber quality properties due to MPB application, these differences were limited and may be related to hastened maturity following MPB application.
The decrease in yield (3.7–8.5%) observed in association with increasing total season MPB application rates and early applications examined in this experiment suggests that as strategies become more aggressive in controlling vegetative growth, the potential for yield losses may increase in any environment. In field situations that favor rank vegetative growth (e.g., late-planted cotton, thick stands, high fertility, and excessive moisture) higher MPB rates and earlier application timings may be more appropriate to reduce the potential for rank growth and/or a yield reduction. Documenting the variations in cultivar response to PGR applications could result in refined PGR recommendations. Future research may focus on the differences in cultivar maturity and how PGR applications affect the growth characteristics and yield of numerous early, mid-, and late maturing cultivars.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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