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a Texas Tech Univ., Box 42122, Lubbock, TX 79409
b Texas Tech Univ. and the Texas Agric. Exp. Stn., Box 42122, Lubbock, TX 79409
c P.O. Box 748, Univ. of Georgia, Tifton, GA 31794
d 4401 Williams Hall, Box 7620, North Carolina State Univ., Raleigh, NC 27695. Journal article no. T-4-579
* Corresponding author (cory.mills{at}ttu.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AFIS, advanced fiber information system • BG, Bollgard • BGII, Bollgard II • DAP, days after planting • FR, flower removal • NAFS/WF, nodes above first square/white flower • NAWF, nodes above white flower • RR, Roundup Ready • RRF, Roundup Ready Flex
a Texas Tech Univ., Box 42122, Lubbock, TX 79409
b Texas Tech Univ. and the Texas Agric. Exp. Stn., Box 42122, Lubbock, TX 79409
c P.O. Box 748, Univ. of Georgia, Tifton, GA 31794
d 4401 Williams Hall, Box 7620, North Carolina State Univ., Raleigh, NC 27695. Journal article no. T-4-579
* Corresponding author (cory.mills{at}ttu.edu).
Received for publication October 25, 2006.
New transgenic cotton (Gossypium hirsutum L.) technologies Bollgard II/Roundup Ready Flex (BGII/RRF) provide additional mechanisms for the cotton crop to retain early initiated fruiting structures positioned in the lower canopy. It may be possible, therefore, for early fruit retention to become too high with these new technologies resulting in early cutout and reduced yield. The objective of this investigation was to determine if glyphosate-induced differences in early season retention occur between BGII/RRF and the older Bollgard/Roundup Ready (BG/RR) technologies and if so, to test if these differences in retention impact crop maturity, yield, or quality under irrigated and nonirrigated conditions. A study was conducted in 2004 and 2005 at two locations in southwestern Georgia to compare the two technologies under dryland vs. irrigation with or without flower removal. The BG/RR matured later than BGII/RRF when glyphosate was applied late at the seventh and 11th leaf stages. The BG/RR compensated for fruit loss by producing heavier remaining bolls. The BGII/RRF maturity was unaffected by the late glyphosate applications and produced a higher percentage of plants having a harvestable boll in the lower canopy than BG/RR. The BGII/RRF cotton had increased boll number and weight at the first sympodial position at lower main stem nodes while BG/RR produced more and heavier bolls on upper main stem nodes. Flower removal did not negatively affect BGII/RRF or BG/RR yields. Few differences in fiber quality were observed. The BGII/RRF retained more early reproductive structures than BG/RR but also cutout earlier. Yield differences between the two technologies may be due to agronomic performance of the variety backgrounds used.
Abbreviations: AFIS, advanced fiber information system • BG, Bollgard • BGII, Bollgard II • DAP, days after planting • FR, flower removal • NAFS/WF, nodes above first square/white flower • NAWF, nodes above white flower • RR, Roundup Ready • RRF, Roundup Ready Flex
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In 2006, cotton with two new transgenic technologies, Roundup Ready Flex (Flex) and Bollgard II, became commercially available. In contrast to Roundup Ready (RR) cotton, the new Flex cultivars have increased reproductive glyphosate tolerance within flowers throughout the growing season. The new Bollgard II technology has a second Bt toxin in addition to that found in Bollgard with an additional mode of action for increased lepidopteran activity and for insect resistance management (Tabashnik et al., 2003).
Transgenic crops that are engineered to produce the Bt toxin are widely used to control key lepidopteran pests in cotton. Bollgard, which contains the Cry1Ac 
-endotoxin and Bollgard II, which contains both the Cry1Ac -endotoxin and the Cry2Ab -endotoxin (Federici, 2003), have been introduced to provide an alternative integrated management program for larval pests. The addition of these genes has reduced the use of insecticides in the field (Jackson et al., 2003) thus, reducing chemical and equipment costs for growers.
Bollgard II technology, containing the two Bt endotoxins, was introduced to provide better control of bollworms (Helicoverpa zea Boddie), other foliage feeders, and to enhance resistance management. Recent studies have shown the pyramiding effect of these endotoxins has significantly increased protection against beet armyworms (Spodoptera exigua), soybean loopers (Pseudoplusia includens Walker), and bollworms (Gore et al., 2001). Bollgard II cotton provides a wider spectrum of control than Bollgard and provides superior control of larvae that feed on reproductive structures.
The glyphosate resistant 5-enolpyruvylshikimate-3-phosphate synthase (CP4-EPSPS) gene provides resistance to glyphosate. The current RR technology provides vegetative glyphosate tolerance throughout plant life but limited reproductive tolerance (May et al., 2004). The CP4-EPSPS is expressed in different levels throughout the various tissues in the plant. Pline et al. (2002) reported the stigma, floral bud, anther, apical meristem, petal, and fruiting branch had significantly less concentration of CP4-EPSPS than the leaf and ovary. Therefore, the expressed location of CP4-EPSPS in plant parts, if sprayed with glyphosate topically during reproductive stages, could affect boll retention, which may impact cotton yield and quality.
The new technology (tradename Roundup Ready Flex) that has been introduced provides growers with a wider glyphosate application window because this technology is equipped with the same gene but a new promoter allowing the glyphosate resistant gene (CP4-EPSPS) to be expressed in reproductive parts of the pant. May et al. (2004) confirmed the new Flex technology does have extended resistance to later and higher dosages of glyphosate than the current Roundup Ready technology. With extended glyphosate protection, more fruit were produced on the first five, first position, fruiting sites of Flex cotton (May et al., 2004) than Roundup Ready cotton.
As the new Bollgard II/Roundup Ready Flex (BGII/RRF) technology provides additional mechanisms for the plant to retain earlier fruiting structures positioned in the lower canopy, the question arises, does the increased early fruit retention affect maturity rate and result in earlier cutout and changes in yield distribution? Ehlig and LeMert (1973), for example, concluded that high early boll load leads to low boll retention later in the season and could result in midseason cutout. Also, would increased early fruit retention increase the risk of early cutout when the crop is exposed to stress such as high temperature or water-deficit stress? Therefore, the objective of this investigation was to determine if differences in early season retention occur between Bollgard/Roundup Ready (BG/RR) and BGII/RRF technologies and if so, to determine how these differences in retention impact crop maturity, yield, or quality when the crop is water-stressed.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experimental design at each location and year was a split split plot with four (Gibbs 2005 and Stripling 2005) or five replications (Gibbs 2004) where irrigations were the main plot, technology was a subplot, and flower removal was a sub-subplot. Specific guidelines were observed for new technology cotton trials in 2004. Plots were 21 m long and 3.66 m wide (four rows) with a 9 m buffer region between irrigation treatments in addition to a 12.19 m border region around the entire perimeter. The design, at both locations in 2005, was a split split plot with four dryland and four irrigated replicates. Plots were 21 m long and 1.83 m wide (two rows) with 9 m buffer regions between irrigation treatments. Three sets of watermark sensors were buried at depths of 20, 40, and 60 cm and irrigation triggers were set at –40, –50, and –50 kPa to minimize drought stress for each study. The plots were irrigated using a linear overhead sprinkler system. Gibbs 2004 location received five irrigation events with a total of 114 mm, Gibbs 2005 location received three irrigation events with a total of 50 mm, and Stripling 2005 location received three irrigation events with a total of 76 mm. The nonirrigated studies received rainfall from time of planting to harvest; Gibbs 2004 received 716 mm, Gibbs 2005 received 889 mm, and Stripling 2005 received 662 mm.
Glyphosate Applications
Glyphosate applications were made to impose differences in fruit retention between the two technologies. Glyphosate (Roundup WeatherMax) was applied over-the-top at the third leaf (4.3 kg a.i. ha–1) and seventh leaf (6.4 kg a.i. ha–1) stages plus directed at the bottom 53.3 cm of the plants (6.4 kg a.i. ha–1) at the 11th leaf stage to provide adequate ground cover for weed control.
Flower Removal
To ensure differences in fruit retention that occurred within a technology, a fruit removal treatment was imposed. In flower removal plots, flower shedding was simulated by removing flowers by hand. After flowering began, all flowers were removed daily for 1 wk and counted. Late glyphosate applications have been shown to induce pollen sterility (Pline et al., 2002) and flower shed (Jones and Snipes, 1999). Thus, this treatment was included to simulate glyphosate-induced flower shed.
Yield Data and Plant Development
Plant development was monitored weekly by counting nodes above first square/white flower (NAFS/WF) beginning at first square. Before harvest, a section of 3.1 m from the middle two rows in each plot was removed for hand harvest. Plants were removed from this area and bolls harvested by fruiting position to determine the total boll weight, percentage of plants having a harvestable boll (ratio of the number of bolls at a node by the number of plants harvested multiplied by 100), and mean boll weight from each fruiting position.
Statistical Analyses
The analyzed variables were: NAFS/WF, total boll weight (g seed cotton m–2), percentage of plants having a harvestable boll, and the mean boll weight (g seed cotton boll–1). These variables were analyzed using proc MIXED where irrigation, technology, and fruit removal were fixed effects and rep, main plot error, and subplot error were random effects. Variables were also analyzed considering nodes as a fixed effect, using a split split plot design. Since node could not be randomized it is considered as an effect in space for the total boll weight, percentage of plants having a harvestable boll (expressed in %), and mean boll weight. All locations were combined and analyzed. There was an environmental interaction present. Therefore, the data was not combined.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Boll Weight
At all locations mean boll weight was greater for BG/RR than for BGII/RRF (Table 1). These observations support the hypothesis that the reduced percentage of plants having a harvestable boll in BG/RR resulted in compensation through production of heavier fruit.
Lint Yield
Significant yield differences were observed at Gibbs 2005 with BGII/RRF yielding 1355 kg ha–1 and BG/RR yielding 1425 kg ha–1 and at Stripling 2005 with BGII/RRF yielding 1140 kg ha–1 and BG/RR yielding 1221 kg ha–1 (Table 2
). These yield differences could be the result of favorable growing conditions that would allow compensation for early-season fruit loss of BG/RR. The agronomic performances of the two technologies may have differed due to the variety backgrounds that were used.
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Fiber Quality
The AFIS fiber length by weight [L(w)] and the upper quartile length by weight [UQL(w)] was greater in the BGII/RRF than in BG/RR in all three locations (Table 3
). However, length by weight coefficient of variation [L(w) CV] was also higher for the BGII/RRF in all environments (Table 3) and short fiber content by weight [SFC(w)] was higher in two of the three locations (Table 4
). Finally, AFIS fineness (Fine) was lower in the BGII/RRF in all locations and maturity ratio was lower in two of the three locations (Table 4). These data suggest while both technologies had similar genetic background, backcrossing and individual variety selection may have resulted in inherently different fiber properties.
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Percentage of Plants Having a Harvestable Boll
Flower removal significantly affected the percentage of plants having a harvestable boll at the first and third sympodial positions at Gibbs 2005 (Table 5). At this location, the percentage of plants having a harvestable boll was greater for the NoFR treatment in the first sympodial position but greater for the FR treatment in the third sympodial position. In addition, at Gibbs 2004 (Table 5) the percentage of plants having a harvestable boll in the second and third sympodial positions was greater in the FR treatment. Bednarz and Roberts (2001) observed under intense early-season floral bud removal the percentage of plants having a harvestable boll was reduced in the lower plant canopy and in the first sympodial position. However, floral bud removal increased the percentage of plants having a harvestable boll in the upper canopy and in the third sympodial position. Bednarz and Roberts (2001) attributed these changes in yield distribution to the hypothesis originally proposed by Sadras (1995) that greater retention of reproductive structures in the upper canopy and third sympodial position occurred under early season fruit loss.
Mean Boll Weight
Flower removal treatments affected mean boll weight at Gibbs 2005 and Stripling 2005 (Table 5). At both locations, mean boll weight of the FR treatment at the first and third sympodial positions was greater in the FR treatment than that in the NoFR treatment. These observations also support the hypothesis that the reduced percentage of plants having a harvestable boll in the flower removal (FR) treatment resulted in compensation through production of heavier fruit.
Lint Yield
The FR treatment, which was imposed during the first week of blooming, significantly affected yield at Stripling 2005 (Table 2). At this location FR resulted in increased yields relative to the NoFR treatment when averaged over the two technologies (1243 vs. 1118 kg ha–1, respectively). This response, however, was not observed in the other environments. The inconsistency in the response of lint yield to FR in this investigation is in agreement with Jones and Snipes (1999) who suggested the amount of reproductive compensation after fruit loss is generally dependent on the stage of crop development when the loss occurred, growing season length, and environmental conditions.
Irrigation
Total Boll Weight
Irrigation effects for total boll weight were observed at Gibbs 2004. At this location total boll weight was greater in the irrigated treatment at the first sympodial position (Table 6
).
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Mean Boll Weight
Irrigation had a significant effect on mean boll weight at the second sympodial position at Gibbs 2004 (Table 6). At this location and sympodial position mean boll weight was greater in the nonirrigated treatment. In the nonirrigated treatments at Gibbs 2005 and Stripling 2005, greater mean boll weights were generally observed in the BG/RR (4.55 g boll–1) at main stem nodes eight and above compared to BGII/RRF (4.24 g boll–1). At Stripling 2005, mean boll weight in the irrigated BG/RR treatments (4.39 g boll–1) was also greater at main stem nodes eight and above compared to BGII/RRF (3.88 g boll–1). As discussed previously, the probability of harvesting mature boll was greater for the irrigated treatment at this location (Table 6) and position. Thus, while the nonirrigated treatment resulted in a lower percentage of plants having a harvestable boll, the nonirrigated crop was attempting to compensate by producing greater mean boll weights, in this case, at the second sympodial position.
Lint Yield
Lint yield was significantly affected by irrigation at Gibbs 2004 and Gibbs 2005 (Table 2). Irrigation resulted in higher lint yield in these two environments while the third environment did not but followed the same trend.
The irrigation by technology interaction was significant at Gibbs 2004. At this location under irrigated conditions, lint yield of BG/RR was 1033 kg ha–1 and the yield of BGII/RRF was 944 kg ha–1as opposed to lint yield of BGII/RRF was 745 kg ha–1 and the yield of BG/RR was 723 kg ha–1 under nonirrigated conditions. The lack of water could possibly have limited the plant's ability to compensate for fruit loss in the BG/RR technology.
Physiological Cutout
Cutout was significantly affected by irrigation in this study (Table 2). At Gibbs in 2004, NAWF in the nonirrigated at 63 d after planting (DAP) was lower than the irrigated treatment (planted 2 June 2004). Stripling 2005 (Table 2) showed similar trends at 84 DAP (planted 20 Apr. 2005). Pettigrew (2004a) observed that NAWF differences due to irrigation were not seen until later in the growing season. Humid, temperate environments can cause slower developing and less severe drought stress which delays physiological response to moisture deficit stress (Pettigrew, 2004b). This may be attributed to the amount of available water in the beginning of the season providing adequate water for sustainable growth which is subsequently depleted later in the growing season. In two of the three environments in this study, cut out occurred only approximately 3 d earlier in the nonirrigated treatment. Thus, water deficit stress in this environment likely developed slower and was less severe than is generally observed in arid environments.
Fiber Quality
Advanced fiber information system (AFIS) fiber lengths and fineness at Gibbs 2004 were greater in the nonirrigated treatment (Tables 3 and 4). The AFIS [L(w) CV] was also lower in this environment (Table 3). However, AFIS fiber lengths at Gibbs 2005 were lower in the nonirrigated while [L(w) CV] was greater (Table 4). Irrigated treatments at Gibbs 2004 and Stripling 2005 resulted in lower fineness and maturity ratio ratings when compared to nonirrigated treatments (Table 4). Thus, there were no clear results found.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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5) and lint yield (kg ha–1) were examined among technology, flower removal (FR), and irrigation treatments in studies conducted at the University of Georgia Gibbs Farm at Tifton in 2004 and 2005 and the C.M. Stripling Irrigation Research Park at Camilla in 2005.