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a USDA-ARS, Crop Genet. and Prod. Res. Unit, P.O. Box 345, Stoneville, MS 38776
b Clemson Univ., Pee Dee Res. and Educ. Cent., 2200 Pocket Rd., Florence, SC 29506-9706
* Corresponding author (bpettigrew{at}arsusda.gov)
Received for publication March 29, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: DAP, days after planting • LAI, leaf area index • NAWB, nodes above white bloom • PPFD, photosynthetic photon flux density
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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Declining profit margins have forced Mississippi Delta cotton producers to reassess all of their cultural practices. Producers have been motivated to consider conservation tillage as a means of reducing inputs for many cotton production systems. Input savings from conservation tillage come primarily from reduced use of tillage equipment. These savings are manifested as decreased fuel consumption, longer machinery life, reduced horsepower requirement, and lower labor requirements. These may be offset, however, to a degree by increased chemical weed-control costs (Harmon et al., 1989; Segarra et al., 1991).
Yield responses of cotton to conservation tillage reported in the literature have been inconsistent. In some studies, reduced or equivalent cotton yields have been produced under no-till compared with conventional systems (Brown et al., 1985; Stevens et al., 1992; Bauer and Busscher, 1996; Wheeler et al., 1997). However, others have reported enhanced yields of cotton from conservation tillage systems compared with conventional systems (Wiese et al., 1994; Clark et al., 1996; Karlen et al., 1996; Hunt et al., 1997). Research indicates that any yield benefit derived from conservation tillage may not be seen until after multiple years of using the system (Triplett et al., 1996). Yield increases have been attributed to improved soil moisture under conservation tillage (Harmon et al., 1989; Baumhardt et al., 1993; Daniel et al., 1999). Lascano et al. (1994) showed that while both conventional and strip-tilled cotton [planting in a narrow-tilled strip in herbicide-killed wheat (Triticum aestivum L.) residue] had similar levels of evapotranspiration, the strip-tilled cotton had reduced cumulative soil evaporation, higher cumulative crop transpiration, and increased yields compared with cotton grown under conventional tillage. Stand establishment problems with no-till systems have been shown to be associated with no yield response or a negative yield response (Hicks et al., 1989; Stevens et al., 1992; Colyer and Vernon, 1993; Wheeler et al., 1997).
Work devoted to characterizing the physiological or growth and development response of cotton to conservation tillage has been limited compared with the yield question. Hicks et al. (1989) found no differences among tillage treatments for the yield components of bolls per plant or lint per plant during one year. In another 1-yr study, Lascano et al. (1994) reported that strip-tilled cotton had greater height and leaf area index (LAI) than conventionally tilled cotton early in the growing season but that these differences dissipated by season's end. Stevens et al. (1992) showed that no-till cotton had fewer fruiting sites per plant but lower rates of fruit abscission than conventionally tilled cotton. Their data also showed that the conventionally tilled cotton produced more bolls on the lower fruiting nodes than did the no-till cotton. In contrast, Triplett et al. (1996) reported that no-till cotton had more nodes, fruiting sites, and bolls.
Most of the aforementioned studies utilized only one genotype. Hunt et al. (1997), however, used several genotypes in a conventional vs. conservation tillage comparison and detected a significant genotype x tillage interaction for seed cotton yield. Second generation lines, generated from a half-diallel design using five genotypes, also had superior yield performance under both conservation and conventional tillage compared with parent lines (Bauer and Green, 1996). To date, none of these studies has utilized okra-leaf genotypes that possess traits that possibly allow for better utilization of soil moisture (Karami et al., 1980; Pettigrew et al., 1993). In addition, whether or not transgenic lines behave differently in conservation tillage compared with conventional tillage systems has not been addressed.
Questions remain about conservation tillage for cotton because of inconsistencies in growth and development and in yield performance. The primary objective of this research was to determine whether or not planting cotton using conservation tillage altered the dry matter partitioning, flower production, canopy light interception, lint yield, and components of yield compared with conventionally grown cotton in the Mississippi Delta. A secondary objective was to evaluate the performance of two okra–normal leaf isoline pairs, a popular conventional cotton genotype, and a transgenic cotton genotype expressing a Bacillus thuringiensis endotoxin under conventional and conservation tillage practices.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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,,-trifluoro-m-tolyl)-3-(2H)-pyridazinone] were soil-applied but not incorporated to all plots. In late March, the conventionally tilled plots were bedded on 1.02-m centers. Two weeks before planting in 1997 and 4 wk before planting in 1998, paraquat dichloride (1,1'-dimethyl-4,4'-bipyridinium dichloride) was preplant, foliar applied to the entire experimental area to fully terminate the wheat in the no-till plots and any remaining winter weeds in the conventional tillage plots.
Cotton was planted on 7 May 1997 and 5 May 1998. Both tillage treatments were planted using a JD 7300 vacuum planter (John Deere, East Moline, IL1) equipped for no-till planting (added fluted coulters, increased pressure on springs, and a 15% increased seeding rate). At planting, 0.84 kg ha-1 aldicarb [2-methyl-2-(methylthio) propionaldehyde O-(methylcarbamoyl) oxime] was applied in-furrow to aid early season insect control. Later, insecticide applications were made as needed. Also, at planting, 0.28 kg ha-1 norflurazon and 0.56 kg ha-1 fluometuron [1,1-dimethyl-3-(,,-trifluoro-m-tolyl)urea] were soil applied pre-emergence to all plots. Postemergence weed control was accomplished using postdirected applications of cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl]amino]-2-methylpropionitrile} at 0.56 kg ha-1 and loctofen {1-(carboethoxy) ethyl 5-[2-chloro-4-(tri-fluoromethyl) phenoxy]-2-nitrobenzoate} at 0.175 kg ha-1 when the cotton was at least 15 cm tall. In mid to late May of each year, 134 kg N ha-1 as a urea ammonium nitrate solution was knifed in beside each row. The plots were not irrigated in either year of the study.
The experimental design was a randomized complete block with six replicates. Two tillage treatments and six genotypes were arranged factorially within each replicate. Tillage treatments were a no-till treatment and a conventional tillage treatment consisting of preplant disking and bedding, followed by two postemergence cultivations in late May and early June as part of the weed control program. Genotypes used in the study were ‘DeltaPine NuCot 35B’, ‘MD 51 ne normal leaf’, ‘MD 51 ne okra leaf’, ‘SureGrow 125’, ‘Stoneville 6413 normal leaf’, and ‘Stoneville 6413 okra leaf’. Plot size was four 1-m-wide by 12.2-m-long rows. Individual genotype–tillage combinations were assigned to the same experimental units each year.
Dry matter harvests were taken at 47, 61, and 90 d after planting (DAP) in 1997 and 48, 63, and 90 DAP in 1998. One of the two inner plot rows was designated for use in the dry matter harvests. On each harvest date, the aboveground portions of plants from 0.3 m of row were harvested and separated into leaves, stems and petioles, squares, and blooms and bolls. Leaf area index was determined using a LI-3100 leaf area meter (LI-COR, Lincoln, NE), and main-stem nodes were counted. Samples were dried for 48 h at 65°C, and dry weights recorded.
The percentage of photosynthetic photon flux density (PPFD) intercepted by the canopies was determined with a LI 190SB point quantum sensor (LI-COR, Lincoln, NE) positioned above the canopy and a 1-m-long LI 191SB line quantum sensor placed on the ground perpendicular to and centered on the row. Two measurements were taken per plot, and the mean of those two measurements was used for statistical analyses. These measurements were taken under clear skies at 42, 54, and 82 DAP in 1997 and 45, 62, and 91 DAP in 1998.
Beginning at the initial sign of blooming, weekly counts of white blooms (blooms at anthesis) per plot were conducted to document the blooming rate throughout the growing season. The number of main-stem nodes above a sympodial branch that had a white bloom at the first branch fruiting position (NAWB) were also counted weekly on three plants per plot to document the progressive reproductive development up the stem and crop maturity.
Canopy temperature measurements were taken under clear skies during the afternoon at 68 and 83 DAP in 1997 and 76 DAP in 1998 using a Telatemp Model AG-42 infrared thermometer (Telatemp Corp., Fullerton, CA). This instrument recorded both canopy surface temperature and the difference between canopy surface temperature and ambient air temperature. Two instantaneous measurements were taken per plot, and the mean of those two measurements were used for statistical analyses.
Yield was determined by hand-harvesting 4.6 m of row from the inner plot row that was not used in the dry matter harvest, avoiding the ends of the row. Harvests were made on 127, 142, 155, and 169 DAP in 1997 and on 114, 128, 142, and 156 DAP in 1998. The number of bolls harvested per plot was counted on each harvest date. Individual seed cotton harvests for each year were combined together and ginned to determine the lint yield and lint percentage of each plot. Boll mass was determined by dividing the total seed cotton weight per plot by the total number of bolls per plot. Average seed mass was determined from 100 nondelinted seeds per plot. After ginning, the fiber quality of the lint from each plot was determined by Starlab (Knoxville, TN). Fiber strength was determined with a stelometer. Span lengths were measured with a digital fibrograph. Fiber maturity, wall thickness, and perimeter were calculated from arealometer measurements.
At the end of the 1997 growing season, plants from 1 m of row in the yield harvest row of each plot were mapped for boll location. Plant height, number of main-stem nodes, node number of first sympodial branch, and the main-stem node and sympodial branch position of all bolls were recorded.
Statistical analyses were performed using analysis of variance. Tillage means were averaged across genotypes when the tillage x genotype interaction was not significant. Overall tillage means or tillage–genotype means were separated using a protected LSD at P 
0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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Seedling emergence was 2 to 3 d slower in no-till than in conventional tillage for both growing seasons (data not shown). Stand counts at 40 DAP showed that the conventional tillage had a significantly higher population density (16 plants m-2) than the no-till plots (12 plants m-2) in 1997. Similar population density differences were recorded at 22 DAP in 1998 when plots of conventional tillage had 15 plants m-2, whereas those of no-till had 8 plants m-2. The 8 plants m-2 is a borderline low density for optimum yield of the okra-leaf genotypes, and the 15 to 16 plants m-2 is a borderline high density for the normal-leaf genotypes (Heitholt, 1994). Hicks et al. (1989) and Wheeler et al. (1997) reported similar stand reductions when cotton was planted with conservation tillage into wheat stubble. Stevens et al. (1992) reported stand reductions for no-till cotton with hairy vetch (Vicia villosa Roth) but not wheat as a cover crop. Hicks et al. (1989) further reported that aqueous extracts from wheat straw demonstrated allelopathic effects and inhibited cotton seedling development. Colyer and Vernon (1993) documented an increase in seedling disease pressure for 1 out of 3 yr with conservation tillage. Both greater seedling disease pressure and wheat stubble allelopathic effects could have contributed to the reduced stand establishment seen in the no-till plots of our study.
Slower emergence of the no-till plants retarded their growth and development compared with plants in conventional tillage plots (Table 1). Dry matter harvests taken during the pre-bloom stage of growth (47 and 48 DAP in 1997 and 1998, respectively) demonstrated that no-till plants were 23% shorter, had 42% less LAI, and had a 16% lower height/node ratio than the conventionally tilled plants. Plants in the no-till treatment continued to demonstrate significantly reduced growth during midbloom (61 and 64 DAP in 1997 and 1998, respectively) in 1997 when plants were 16% shorter, had 27% lower LAI, and had a 14% lower height/node ratio in the no-till treatment. These vegetative growth parameters were not statistically different from those in conventional tillage in 1998. In both 1997 and 1998, plants in no-till plots averaged 44% less reproductive growth, which contributed to a 29% lower harvest index (reproductive dry weight/total dry weight) than in conventional tillage plots during midbloom. When dry matter harvests were taken during cutout (a period of slowing vegetative growth and flowering due to strong demand for assimilates by the existing boll load) (90 and 91 DAP in 1997 and 1998, respectively), the vegetative growth of no-till plants had caught up to that of conventionally tilled plants. No differences were detected between tillage treatments for plant height, LAI, height/node ratio, or any other vegetative parameter for either year at that time. In 1997, plants in conventional tillage continued to demonstrate increased reproductive growth and a greater harvest index at cutout than did plants in no-till. At cutout in 1998, no differences in reproductive growth were detected between tillage treatments.
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0.07. The number of bolls per square meter was the most consistent yield component contributing to the increased lint yield seen with conventional tillage, with 10% more bolls per square meter in conventional tillage than in no-till. Reduced lint percentage and boll mass also contributed to the lower lint yields measured in the 1997 no-till treatment. The delayed plant development in no-till during both growing seasons (Table 1; Fig. 1–3) was maintained through harvest. Of the total cotton harvested, no-till had 32% less picked on the first harvest than did conventional tillage (Table 3). Plant mapping data collected in 1997 showed that conventional tillage plants produced more bolls on the lower main-stem nodes (Nodes 1–5) than did no-till (Table 4). The mapping data are further evidence of the delayed development in no-till plots.
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0.07 level), are not encouraging for the use of no-till in the Mississippi Delta. In all fairness, 2 yr may be too short a time period for the benefits from conservation tillage to be realized. Triplett et al. (1996) did not find increased cotton yields from using no-till until after 3 yr in a no-till production system. Yield benefits attributed to conservation tillage after multiple years in the system have also been reported in soybean [Glycine max (L.) Merr.] (McGregor et al., 1992) and corn (Zea mays L.) (Griffith et al., 1988). While the short period of this study did not allow it to address any possible long-term benefits derived from conservation tillage, the data dramatically illustrate some short-term pitfalls that producers will encounter when initially converting an existing conventional tillage production system to a no-till system. The most obvious tillage response seen in this study is delayed development in LAI (Table 1), canopy closure (Fig. 1), and reproductive growth (Fig. 2 and 3) in no-till. This delayed development of plants in no-till culminated with less of the total yield being obtained on the first harvest compared with that of conventional tillage and is similar to the first-harvest data reported by Stevens et al. (1992) and Brown et al. (1985). Triplett et al. (1996) reported earlier maturity for conventionally tilled cotton during the first 2 yr of their study, but in Years 3 to 5, no-till plants were earlier in maturity than those in conventional tillage. Stevens et al. (1992) reported cooler soil temperatures shortly after emergence under no-till conditions compared with conventional tillage, which may be the underlying reason for the delayed development seen in the no-till treatments. Triplett et al. (1996) offered no explanation for no-till plants exhibiting accelerated maturity during the later years of their study after having delayed development during the first 2 yr of the study.
Delayed development under the no-till conditions of our study meant that the blooming period was shifted to later in the growing season; therefore, no-till plants encountered different weather patterns during bloom than did those in conventional tillage (Table 6). Differences observed in lint yield and fiber quality between tillage treatments can probably be attributed to the different weather conditions experienced during the blooming period by plants in the two tillage treatments. Development of cotton cultivars with enhanced germination and seedling growth under cooler temperature regimes might allow a no-till cotton production system to avoid the developmental delays seen in this study.
Despite the delayed development and reduced lint yields associated with no-till in this study, the concept should not be disregarded. This study did not address economic issues. Thus, if Delta producers are able to reduce production inputs necessary for growing cotton under no-till as has been done by Texas high-plains producers (Harman et al., 1989; Keeling et al., 1989; Segarra et al., 1991), this production system may find a niche among Mississippi Delta cotton producers. In addition, yield benefits (Triplett et al., 1996) and improvements in soil fertility and erodibility-related properties have been reported after multiple years in a conservation tillage system. Furthermore, the selection of genotypes to grow can be made based on the same yield, maturity, and fiber quality criteria for either tillage system due to the fact that tillage systems did not interact with genotypes to alter genotypic performance. Producers considering no-till, however, should realize that they may encounter delayed plant development, reduced yields, and altered fiber quality traits early in the adoption period compared with a conventional tillage system. After multiple years in a conservation tillage system, these short-term problems may eventually diminish.
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONREFERENCES |
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