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© 2003 Plant Management Network.
Accepted for publication 12 December 2003. Published 22 December 2003.

Managing Earliness in Cotton with Mepiquat-type Growth Regulators

C. Owen Gwathmey, Associate Professor, and C. Chism Craig, Jr., Assistant Professor, Department of Plant Sciences, University of Tennessee, Jackson 38301

Corresponding author: Owen Gwathmey. cogwathmey@utk.edu

Gwathmey, C. O., and Craig, C. C., Jr. 2003. Managing earliness in cotton with mepiquat-type growth regulators. Online. Crop Management doi:10.1094/CM-2003-1222-01-RS.

Abstract

The plant growth regulator mepiquat chloride (MC or Pix) is an important earliness management tool in cotton (Gossypium hirsutum L.) production, but producers lack clear information about differences in cultivar response to MC, and about newer mepiquat-type products. In a 3-year field study in Tennessee, MC promoted earliness during flowering more in a later, indeterminate cultivar than in two earlier maturing cultivars. The progress of flowering was hastened more consistently in different cultivars by four quarter-rate applications each than with one full-rate application at early bloom, but yield of one cultivar was reduced by multiple applications. A 2-year field study in Tennessee provided no evidence that some newer mepiquat-type products (Pix Plus, Pix Ultra, Pentia) are more effective than MC for earliness management. These products were similar to Pix in controlling vegetative growth and hastening the progress of flowering in an early-maturing cultivar. None of the newer products differed from MC in effects on yield or earliness at harvest. Mepiquat-type growth regulators may be particularly useful for controlling growth and promoting earliness of later maturing cultivars in short-season environments.

Introduction

One challenge of producing cotton (Gossypium hirsutum L.) in the northern U.S. cotton belt is to mature the crop within a relatively short season. In this region, producers seek to shift from vegetative to reproductive growth in mid-season to assure adequate time for bolls to mature before inclement weather or killing freeze occurs. Managing for earliness involves limiting cotton’s vegetative growth with cultivar selection, control of N fertility, timely irrigation management, and use of plant growth regulators (15). Of these, growth regulators containing mepiquat chloride (MC) have become a mainstay of earliness management. Among producers, the brand name "Pix" has become synonymous with mepiquat chloride. Although Pix has been widely used since the 1980s for growth control, producers still lack clear information on cultivar differences in response to MC, and on response to newer mepiquat-based and mepiquat-related products.

The importance of earliness. Reduction of vegetative growth with applications of mepiquat chloride shifts nutrient resources to developing bolls, and a greater proportion of boll production is shifted to lower nodal positions than in untreated cotton (9). This shift can lead to earlier cessation of flowering, which is often beneficial in short-season environments. The gradual shift from vegetative to reproductive growth is reflected by the progression of flowering towards the apex of the plant, from bottom to top. The termination of this process is termed "cutout" (5). A commonly used reference point for this process is when there are five nodes above the upper-most white flower nearest the stem (5). Cutout serves as a useful mid-season indicator of crop maturity for crop managers. In most MC studies, however, crop maturity has been measured at harvest, either as percent open bolls, bolls set by nodes, or bolls harvested with time (9). Studies in Arizona and California, for instance, showed higher boll numbers in early weeks of harvest, and lower numbers in later harvests, when treated with MC than in untreated cotton (6,9). Under longer season conditions, premature cutout of MC-treated cotton may reduce yields (8). Additional research is needed to determine yield and earliness responses to mepiquat chloride in short-season cultivars and environments.

Evidence for cultivar responses to mepiquat chloride. Cultivar traits such as growth habit, canopy size, and maturity, may alter mepiquat chloride effects on earliness. Cultivars that are relatively determinate in the physiological sense typically produce less vegetative growth during boll development than more indeterminate cultivars, suggesting that they may be less responsive to MC. Few studies relating MC response to varietal growth habit have been published. In a Texas study, a single application of MC was applied to short-season and full-season cultivars (2). Plant height was reduced and earliness at harvest increased with MC relative to untreated cotton, but these responses were smaller in the short-season cultivar than in the full-season cultivar. In a Mississippi study, five test cultivars with a wide range of maturity responded similarly to MC (7). In North Carolina, response to MC of 14 cultivars was studied in eight environments over three years (17). Earlier maturity was usually observed with mepiquat chloride as higher percent open bolls, but no cultivar-by-Pix interaction for maturity was found.

The product label for Pix (3) includes an option for low-rate multiple applications starting at matchhead square (early bud) stage. In a two-year Mississippi study, low-rate multiple applications of MC tended to have more effect on plant height and node number, whereas an early bloom application tended to have more effect on boll retention (16). This study did not directly measure earliness, but concluded that low-rate multiple applications allow more management flexibility than a single early bloom application.

Newer mepiquat-type products. Newer mepiquat-related plant growth regulators have recently become available commercially, including mixtures of mepiquat chloride with other active ingredients, and other compounds that incorporate the mepiquat cation. Specimen labels (3) indicate that Pix, Pix Plus, and Pix Ultra all contain at mepiquat chloride at 0.35 lb/gal. In addition, Pix Plus contains 3.1 × 10⁸ colony units/oz Bacillus cereus, and Pix Ultra contains an unspecified quantity of borate solution. Pentia or BAS130 contains mepiquat pentaborate at 0.82 lb/gal; a similar molar concentration of mepiquat cation as Pix.

Claims have been made that some of these products increase yield and enhance earliness relative to MC alone (14). Few data have been published, however, to support these claims. One Texas study found no significant differences between Pix, MepPlus, and BAS130 in plant height, node number, lint yield, and percent first harvest (11). Another Texas study found that Pix, BAS130, Pix Plus and Pix Ultra decreased plant height, but only Pix and Pix Ultra reduced node number relative to untreated cotton (12). Lint yield responses of 3 to 9% higher than untreated cotton were not statistically significant and earliness was not reported in the latter study.

Objectives of these studies were to compare responses of contrasting cultivars to single and multiple applications of mepiquat chloride (MC), and to determine if newer mepiquat-related growth regulators differ from MC in their effect on cotton growth, earliness, and yield in a short-season environment.

Cultivar-by-Mepiquat Chloride Field Studies

To compare responses of contrasting cultivars to mepiquat chloride, a three-year study was conducted on conventionally-tilled Loring-Calloway silt loam at the West Tennessee Experiment Station at Jackson. Except for the application of MC, TN Agricultural Extension guidelines (1) were followed for fertilizer, crop and pest management. Each year, N at 80 lb/acre was broadcast before planting. Supplemental irrigation was applied by traveling sprinkler boom to reduce moisture stress during periods of prolonged drought. Experimental design was a randomized complete block factorial arrangement of cultivars and MC regimes with four replications. Cultivars were Stoneville 132 (ST132), an early, relatively determinate cultivar; Stoneville 453 (ST453), an early, determinate cultivar with compact canopy; and Stoneville LA887 (LA887), a later maturing, indeterminate cultivar. These cultivars were planted in 40-inch rows on 17 May 1993, 28 April 1994, and 16 May 1995. Three regimes of MC were compared: a single application versus multiple applications at the same seasonal rate, and an untreated check. Pix treatments were applied by a high-clearance sprayer to 4-row plots of each cultivar in 1993, and to 6-row plots of each cultivar in 1994 and 1995. Following Pix label recommendations (3), the single application of Pix at 16 oz/acre at early bloom was made 65, 80, or 70 days after planting in respective years. The low-rate multiple regime consisted of four applications of Pix at 4 oz/acre each, starting at 51, 56, or 63 days after planting, respectively, and followed at intervals of 7 to 10 days. Seven flagged plants in each plot were monitored at a 7- to 10-day interval for plant height and nodes above white flower, using methods described by Bourland et al. (5). Days to five nodes above the uppermost white flower (cutout) were calculated for each monitored plant to determine treatment effects on the progress of flowering. Defoliants were applied to all plots, but no ethephon was applied in order for treatments to express earliness as percent of total yield picked at first harvest (13). The two center rows of each plot were mechanically picked twice: 141, 142, or 160 days after planting in 1993, 1994, and 1995, respectively, and again 18 to 21 days later. Seedcotton harvested from each plot was weighed and a sub-sample from each plot was ginned to calculate lint yields. Data were analyzed by GLM procedures using SAS version 8.0.2 (SAS, Inc., Cary, NC). Least-square means were separated by applying Fisher’s protected LSD to all possible pairs of means at P = 0.05.

Field Testing of Newer Products

To evaluate responses to newer mepiquat-related products, a 2-year study was conducted on a Calloway silt loam at the West Tennessee Experiment Station. Methods were very similar to those described above for the cultivar response study except for the following differences. The cultivar Stoneville 474 (STV474) was planted in 38-inch rows on 14 May 1998 and 11 May 1999. STV474 was selected from a cross with STV453 and has similar maturity, but a more open canopy. In 1998, four products (Pix, Pix Ultra, BAS 130 01W, and BAS 130 00W) were applied to 6-row plots arranged in randomized complete blocks with 4 replications. In 1999, four products (Pix, Pix Plus, BAS 130 01W, and BAS 130 UAW) were applied to 4-row plots arranged in randomized complete blocks with 6 replications. The BAS 130 materials (BASF Corp., Research Triangle Park, NC) were various experimental formulations of mepiquat pentaborate tested prior to commercial release of Pentia (3). In both years, rates of application were determined by SysCot (BASF Corp., Research Triangle Park, NC), a MC-rate calculator based on research by Landivar (10). SysCot uses data on plant size, population density, moisture availability, and internode length to calculate the MC rate required to control plant growth. Two applications of the materials were made each year at rates determined by the MC-rate calculator (10).

Ten plants per plot were monitored at a 7- to 10-day interval for plant height and nodes above white flower or date of cutout (5). For ease of interpolation, 5 nodes above white flower were used in 1998, and days to 4 nodes above white flower were used in 1999, to determine treatment effects on cutout. The two center rows of each plot were spindle-picked 131 and 129 days after planting in 1998 and 1999, respectively, and again 17 days later. Samples were collected and ginned and data were analyzed as in the cultivar response study described above.

Cultivar-by-Mepiquat Chloride Trial Results

Combined analyses of variance (not shown) indicated no significant Pix-by-year interaction, nor a three-way interaction of Pix, cultivar, and year for any response variable in this study. There were significant cultivar-by-year interactions, but since this study focused on Pix and Pix-cultivar interactions, data were combined across years. Cultivar-by-Pix interactions varied by response variable as discussed below.

Mepiquat chloride effects on flowering. Mepiquat chloride significantly hastened the progress of flowering (days to cutout), but cultivars differed in this response (Table 1). Across cultivars, cutout occurred four to six days sooner with MC than in untreated cotton. The single MC application did not hasten flowering progress in the earliest cultivar, STV132, relative to untreated, but the low-rate multiple regime hastened flowering progress by four days relative to untreated or the single application. Mepiquat chloride effects on flowering progress in STV453 were similar with single or multiple applications. The greatest influence on flowering progress (-8 days) occurred with low-rate multiple Pix application to the relatively indeterminate cultivar, LA887. To the extent that these cultivars represent growth types, results suggest that the growth habit of later, more indeterminate cultivars may be shifted more by MC than earlier, more determinate types. This finding is consistent with that of Bader and Niles (1986).

Table 1. Effect of mepiquat chloride (MC) applied once or multiple times on the progress of flowering (days to five nodes above white flower; NAWF 5) and plant height of three cotton cultivars at Jackson, TN^v.

 ^v 1993-95 least-square means. Main effects means (MC mean, in bold) or interaction means followed by the same letter do not differ significantly according to Fisher's protected LSD at P = 0.05. Letters do not separate main effects of Mepiquat Chloride from main effects of cultivars, nor main effects means from interaction means.

 ^w Single 16 oz/acre application of mepiquat chloride at early bloom.

 ^x Four 4 oz/acre applications of mepiquat chloride between matchhead square and early bloom.

 ^y Days from planting to five nodes above highest first-position white flower (nearest the stem). Main effects of MC regime and cultivar were significant at P < 0.001. Interaction effects were significant at P = 0.038. Model R-square = 0.915.

 ^z Main effects of MC regime and cultivar on plant height were significant at P < 0.001. Interaction effects were significant at P = 0.032. Model R-square = 0.866.

Plant height. Either mepiquat chloride regime significantly reduced final plant height 6 to 10 inches relative to untreated cultivars, but cultivars differed in height response to MC (Table 1). The low-rate multiple regime decreased plant height more than the single MC application in two of the three cultivars. Height of the compact cultivar, STV453, was similar with either the single or multiple regime. The extent of growth reduction by low-rate multiple MC was greatest in the more indeterminate cultivar, LA887 (-26%). Results suggest that if growth control is the main objective of MC application, then low-rate multiple applications may be more effective than a single application in managing height, especially in a relatively indeterminate cultivar.

Yields. The regime of growth regulator applications did not significantly affect lint yields across cultivars, but cultivars differed in lint yield response to MC in two ways (Table 2). Untreated LA887 yielded less than it did in either MC regime, unlike the other two cultivars. Under the low-rate multiple-application regime, STV132 produced less total lint than it did untreated or treated with a single MC application. Early cutout and very short stature of STV132 under the low-rate multiple regime suggests that excessive growth control resulted in reduction of total yield potential. This result is consistent with Kerby (8).

Table 2. Lint yield from first harvest, total lint yield, and percentage of total yield contributed by first harvest of three cotton cultivars under three mepiquat chloride (MC) regimes at Jackson, TN^u.

 ^u 1993-95 least-square means. Main effects means (MC mean, in bold) or interaction means followed by the same letter do not differ significantly according to Fisher's protected LSD at P = 0.05. Letters do not separate main effects of MC from main effects of cultivars, nor main effects means from interaction means. Absence of letters indicates a non-significant F-test (P > 0.05).

 ^v Single 16 oz/acre application of mepiquat chloride at early bloom.

 ^w Four 4 oz/acre applications of mepiquat chloride between matchhead square and early bloom.

 ^x Main effects of MC regime on first harvest lint yield were not significant (P = 0.142). Main effects of cultivar were significant at P < 0.001. Interaction effects were significant at P = 0.032. Model R-square = 0.935.

 ^y Main effects of MC regime and cultivar on total yield were not significant (P = 0.906 and 0.099, respectively). Interaction effects were significant at P = 0.007. Model R-square = 0.928.

 ^z Main effects of MC regime and cultivar on percent first harvest were significant at P = 0.019 and < 0.001, respectively. Interaction effects were not significant at P = 0.717. Model R-square = 0.858.

Contribution to first harvest. The cotton was harvested twice, and across cultivars MC increased first harvest percentage relative to untreated cotton (Table 2). Single and multiple MC applications had similar effects on earliness at harvest. Although cultivars differed in first harvest percentage, they did not react differently to MC regime in earliness at harvest (P = 0.717). The lack of growth regulator-by-cultivar interactions in harvest percentage is consistent with Cathey and Meredith (7). This result implies that cultivar differences in earliness response to MC expressed during flowering did not persist until harvest. The gain in first harvest percentage is, however, especially important for later maturing cultivars in the northern cotton belt. The MC-induced earliness shift at harvest of LA887 corresponded to about 3 to 5 days (P. E. Hoskinson, 1996, personal communication). With Pix, about 100 lb/acre more lint was produced at first harvest by this indeterminate cultivar than without MC. Results are consistent with Boman and Westerman (4), who found that MC increased percent first harvest significantly (2.6 to 5.5%) in a less determinate cultivar in a short-season Oklahoma environment.

Comparing Mepiquat Chloride Products

All products hastened flowering progress by 7 to 11 days in 1998 and by 2 to 3 days in 1999, relative to untreated cotton (Table 3). Data were not combined across years due to differences in treatments, replications, and reference node for cutout used in each year. All products reduced plant height relative to the untreated check by 19 to 24%. There were no significant differences among products in height reduction or in the progress of flowering (days to cutout) in either year. This result suggests that if growth control is the main objective in using a mepiquat-type growth regulator, then mepiquat chloride is as effective as any of these newer mepiquat-related products in this environment.

Table 3. Progress of flowering (days to nodes above white flower or NAWF) and final plant height of STV474 cotton treated with four plant growth regulator products and an untreated check at Jackson, TN, 1998 and 1999^v.

 ^v Within groups, means followed by the same letter do not differ significantly according to Fisher's protected LSD at P = 0.05.

 ^w Nodes Above White Flower indicates the number of days after planting (DAP) until there were 5 nodes above the top-most white flower in 1998; 4 nodes above white flower in 1999. Treatment effects were significant at P = 0.017 and < 0.001 in respective years.

 ^x Treatment effects on plant height were significant at P < 0.001 in 1998 and 1999.

 ^y 1998 treatments applied 48 and 64 DAP at 7.4 and 10.3 oz product per acre, respectively.

 ^z 1999 treatments applied 52 and 70 DAP at 5.4 and 7.5 oz product per acre, respectively.

Yield and earliness of harvest. These products did not significantly affect total lint yields or percentage contributed by the first harvest in either year (Table 4), suggesting that this group of newer mepiquat-type products performs similarly to mepiquat chloride. This result is consistent with some other reports of newer mepiquat product performance (11,18), but not all (14). Results are consistent with the cultivar response study (Table 2) which found no yield response to Pix in a closely related cultivar, STV453. The earliness response to MC at harvest of ST474 resembled that of STV453 in the cultivar response study, in that earliness at flowering was promoted by MC but gain in first harvest percentage did not reach statistical significance.

Table 4. Lint yield from first harvest, total lint yield, and percentage of total yield contributed by first harvest of STV474 cotton treated with four plant growth regulator products and an untreated check at Jackson, TN, 1998 and 1999^u.

 ^u Within groups, means followed by the same letter do not differ significantly according to Fisher's protected LSD at P = 0.05. Absence of letters indicates non-significant F-test (P > 0.05).

 ^v Treatment effects on lint yield at first harvest were not significant at P = 0.313 and 0.224 in 1998 and 1999, respectively.

 ^w Treatment effects on total yield were not significant at P = 0.677 and 0.584 in 1998 and 1999, respectively.

 ^x Treatment effects on yield at first harvest lint yield were not significant at P = 0.409 and 0.521 in 1998 and 1999, respectively.

 ^y 1998 treatments applied 48 and 64 DAP at 7.4 and 10.3 oz product per acre, respectively.

 ^z 1999 treatments applied 52 and 70 DAP at 5.4 and 7.5 oz product per acre, respectively.

Management Implications

Results of these studies confirm that mepiquat chloride is an important element of an earliness management system for short-season cotton production. These studies provided no evidence, however, that newer mepiquat-related or enhanced products are more effective than mepiquat chloride alone for earliness management. For producers who wish to control vegetative growth chemically, mepiquat chloride appears to be as effective as any of the newer mepiquat-type product tested here. Low-rate multiple applications may provide more effective growth control than a single application at early bloom. Low-rate multiple applications may be practical to the extent that mepiquat chloride applications can be combined with other chemical applications to the crop. Multiple applications to a very early maturing cultivar, however, can result in excessive growth control and reduced total yield. With this exception, however, cultivars responded favorably to the Pix-induced gain in the contributions from the first harvest. The effect of MC on earliness was most apparent during flowering of a later maturing cultivar, altering its indeterminate growth habit to resemble more determinate cultivars. The resulting shift in harvest readiness can help producers avoid inclement harvest weather in short-season environments.

Disclaimer and Acknowledgments

Mention of brand names does not constitute commercial endorsement of the products to the exclusion of others which may be of similar, suitable composition, nor does it guarantee or warrant the standard of the products.

This research was supported in part by BASF Corp. and Stoneville Pedigreed Seed Co.

Literature Cited

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