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a USDA-ARS Soil Dynamics Research Unit, 411 S. Donahue Dr., Auburn, AL 36832
b Dep. of Agronomy and Soils, 202 Funchess Hall, Auburn University, Auburn, AL 36849
* Corresponding author (wreeves{at}acesag.auburn.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DAP, days after planting • RCB, randomized complete block design
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Degradation of soil quality and increasing governmental regulations on the 50 to 60% of cropland classified as highly erodible land in the region resulted in some farmers turning to conservation tillage systems in the early 1990s. The predominant system implemented was to plant without tillage directly into existing cotton stubble with no winter cover crop. Although equivalent or greater yields have been reported for cotton grown with conservation tillage compared with conventional tillage on loessial soils in northern Mississippi and western Tennessee (Stevens et al., 1992; Bradley, 1993; Triplett et al., 1996), conservation tillage practiced on silty clay soils in northern Alabama resulted in 8 to 15% yield reductions compared with conventional tillage (Brown et al., 1985; Burmester et al., 1993). Slow accumulation of growing degree day-units (base 15.5°C) in the spring and the potential for early fall freezes complicates management decisions in conservation tillage systems for the region (Norfleet et al., 1997). Consequently, many farmers were reluctant to adopt conservation tillage on a large scale, despite possible long term benefits of improved soil quality.
Specific problems with conservation tillage must be overcome before widespread adoption of such systems will occur in the region. Conservation tillage systems that produce large amounts of crop residue can moderate soil temperature because residue acts as insulation (Lal, 1976; NeSmith et al., 1987). Planting cotton on ridges or removing residue from the soil surface may alleviate soil temperature problems. Ridges have been found to provide better aeration and a warmer seedbed, which allows for earlier planting and enhanced cotton development (Boquet and Coco, 1993). Shinners et al. (1994) found that a residue free band (i.e., strip tillage) increased soil temperatures for corn (Zea mays L.) growth in southern Wisconsin.
An increase in soil compaction has also been implicated for poor cotton performance with conservation tillage in the region (Burmester et al., 1993). In-row subsoiling at planting is frequently used to alleviate soil compaction for cotton grown on sandy coastal plain soils (Vepraskas and Guthrie, 1992; Raper et al., 1994; Reeves and Mullins, 1995; Mullins et al., 1997). However, in a conservation tillage system, Touchton et al. (1986) reported no cotton yield response to spring in-row subsoiling in the Tennessee River Valley. Spring tillage in the silty clay soils of this region forms clods, leaving a rough seed bed that is frequently difficult to plant into, and which may suppress yields. The objective of our research was to develop a conservation tillage system for cotton on Tennessee Valley soils that would manage soil compaction, maintain competitive yields, and facilitate widespread adoption of conservation tillage in the region.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The experimental design was a randomized complete block design (RCB) of four replications, with a two by three augmented factorial treatment arrangement. Plots consisted of eight 102-cm wide rows which were 15.2 m long. Treatments were a factorial combination of fall ridging (ridged and nonridged) in combination with non-inversion fall deep tillage (none, in-row subsoiling, and paratilling). The augmented treatments were spring strip tillage and conventional tillage. Nonridging without deep tillage, that is, strict no-tillage, is considered the no-tillage control treatment. All treatments were accomplished with four-row equipment. Subsoiling was implemented under the row with a KMC (Kelley Manufacturing Co., Tifton, GA)1 ripper bedder to a depth of 43 cm. Paratilling was completed with a Paratill (Bigham Brothers, Inc., Lubbock, TX) to a depth of 45 cm. In the fall of 1994, all ridging operations were accomplished using a KMC ripper bedder equipped with disk bedders. The ripper subsoiler shanks were removed for implementation of fall ridging without deep tillage and ridging with paratilling. Data from the fall ridging with subsoiling treatment is not available for 1995 because of difficulties implementing this treatment in the fall of 1994; however, in fall of 1995 and consecutive years, all ridged plots were successfully created with ridging listers rather than disk bedders. Spring strip tillage in 1995 was implemented with an experimental Yetter (Yetter Farm Equipment, Colchester, IL) implement. This implement has an in-row subsoiler that ran 20 to 25 cm deep and has a series of in-row disks, coulters, and spider tines to create a disturbed zone 25 to 35 cm wide. In all other years (1996–1999) a specially designed KMC implement was used for the spring strip tillage treatment. This implement has a shorter subsoil shank that ran 15 to 17 cm deep in the row, and a series of in-row disks and coulters that disturbed a zone 25 to 30 cm wide. Conventional tillage consisted of fall disking and chiseling (22 to 28 cm deep) followed by disking and field cultivating in the spring.
All plots except the conventional-tilled plots were seeded in rye with a grain drill immediately after fall tillage. The cover crop was terminated prior to spring planting with an application of glyphosate [N-(phosphonomethyl) glycine]. A four-row John Deere Maxi-Emerge (Deere & Company, Moline, IL) planter equipped with Martin (Martin & Company, Elkton, KY) row cleaners was used to plant ‘DP 51’ cotton on 12 May 1995, ‘NuCOTN 33B’ on 1 May 1996, ‘DP 20B’ on 7 May 1997, and ‘PM 1220 BG/RR’ on 6 and 5 May in 1998 and 1999, respectively. Seeding rate for all treatments and years was 145000 seed ha-1. Rapidly changing technologies with transgenic varieties, heavy insect pressure from the tobacco budworm [Heliothis virescens (Fabricius)] in 1995, and mass adoption of newer varieties by farms in the region resulted in the use of different cotton varieties from year to year in this study. Consequently, any variety effects are confounded with environmental factors (e.g., differences in rainfall distribution and amounts, temperature patterns, cloud cover, and insect and disease pressures) that are normally confounded in year effects. Following planting, 17 kg N and 7 kg P ha-1 was applied in a band over the row. Nitrogen was also sidedressed at a rate of 100 kg ha-1 in all years. An additional 34 kg N ha-1 was applied in 1996 as a result of visual N deficiency at first bloom. Auburn University Extension recommendations were used to apply all insecticides and defoliants. Preemergence weeds were controlled by the application of [1,1-dimethyl-3-(a,a,a-trifluoro-m-tolyl)] urea and paraquat dichloride [1,1'-dimethyl-4,4'-bypridinium dichloride]. Cyanazine {2[[4-chloro-6-(ethylamino)-s-triazin-2-yl]amono]-2-methylpropionitrile} and MSMA (monosodium acid methanearsonate) were applied for postemergence weed control in all years. In 1998 and 1999, labeled applications of glyphosate were applied over-the-top of the glyphosate-resistant cultivar PM 1220 BG/RR.
Soil temperature was measured hourly in-row at a depth of 10 cm for the first 14 DAP in two replications in 1995 and 1996. Soil temperature readings were measured with thermocouple wires and recorded with a CR 10 measurement and control module data logger (Campbell Scientific, Inc., Logan, UT). Average daily soil temperature and the daily soil temperature range (daily maximum - daily minimum) were subjected to ANOVA.
Average volumetric water content was determined in the top 38 cm of soil approximately twice a week from squaring to 10% open bolls in 1995 and 1996, and from early bloom to 10% open bolls in 1997. This determination was performed in-row, in the nontrafficked middle, and in the trafficked middle at one location in each plot. A Tektronix 1502B (Tektronix, Inc., Beaverton, OR) cable tester was used for soil water determination using time-domain reflectometry (Topp, 1980). Two stainless steel guide rods (0.64-cm diameter) spaced 5.1 cm apart were placed into the soil and connected to the cable tester with coaxial cable. The volumetric water content was subjected to ANOVA. Row position and measurement days (as DAP) were analyzed as an expansion of the original ANOVA RCB model to a split-plot (row position as subplots) and split-split plot model (DAP as sub-subplots), respectively.
A tractor-mounted, hydraulically-driven, soil cone penetrometer was used for determination of soil strength after planting in 1995, 1996, and 1997 (Raper et al., 1999). The tractor-mounted penetrometer determined soil strength in five positions simultaneously: (i) in-row, (ii) 25 cm from the row in the trafficked middle, (iii) 50 cm (midway) from the row in the trafficked middle, (iv) 25 cm from the row in the nontrafficked middle, and (v) 50 cm (midway) from the row in the nontrafficked middle. A cone with a base area of 323 mm2 was used on each of the penetrometers (American Society of Agricultural Engineers, 1998). Readings were taken continuously throughout the soil profile to a depth of 40 cm and were averaged every 5 cm.
A soil compaction index was also determined for the evaluation of soil strength. Data were plotted to give scaled contour graphs using Surfer for Windows (Golden Software Inc., Golden, CO). Using this software, the area of the graph (cm2) occupied by each incremental 0.5 MPa of soil strength was determined. This procedure results in a separate value of area for each of the 0 to 0.5, 0.5 to 1.0, 1.0 to 1.5, 1.5 to 2.0, 2.0 to 2.5, and so on MPa cone index ranges. Each of these area values was multiplied by the cone index at the upper end of each increment and summed for all increments according to the following formula:
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In-row bulk density was determined in 1996 at 12 DAP. Three undisturbed soil samples (5.3-cm diameter) were taken from the top 6 cm of soil in each plot with a double cylinder, hammer driven core sampler. These undisturbed soil cores were dried in a forced air oven for 72 h at 105°C and bulk density was calculated (Blake and Hartge, 1986).
Cover crop dry matter production was determined prior to termination within a 0.25-m2 area from each plot except conventional tillage. Cotton populations were determined in 1995, 1996, 1997, and 1998 by counting the number of plants in two 1.5-m sections of row from each plot prior to harvest. From 1995 to 1998, the number of bolls and the percentage open bolls were determined before defoliation from 3 m of row in each plot. In all years, the middle four rows were harvested with a spindle picker for determination of seed cotton yield.
Data were subjected to ANOVA using the Statistical Analysis System (SAS Institute, 1988). Where year x treatment interactions occurred for response variables, data were analyzed and are presented by year. Preplanned single degree of freedom contrasts (Table 1) and Fisher's protected LSD were used for mean comparisons. A significance level of P < 0.100 was established a priori.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Cotton Population
Contrary to previously reported research from the Tennessee Valley Region (Touchton et al., 1984; Brown et al., 1985) conventional tillage did not produce greater cotton populations compared with any of the conservation tillage treatments in any year, with the exception of 1997, when conventional tillage resulted in significantly greater plant population than all nonridged conservation tillage treatments (Table 4). A similar trend was found in 1995, when the no-tillage control (nonridged without deep tillage) had lower plant population compared with conventional tillage (78200 vs. 97700 plants ha-1, P < 0.123). Single degree of freedom contrasts in 1996 showed paratilling (88000 plants ha-1) and subsoiling (81400 plants ha-1) resulted in greater plant stands than treatments without deep tillage (64800 plants ha-1, P < 0.025 and 0.098, respectively). In 1998, fall subsoiling had lower plant populations than no fall deep tillage (66600 vs. 90800 plants ha-1, P < 0.016). In 1997, fall ridging resulted in greater plant population compared with nonridged treatments (118200 vs. 88400 plants ha-1 P < 0.001). In 1995 and 1996 (when soil temperatures were measured), fall ridging maintained a greater average soil temperature than the nonridged treatments, which could be related to the differences in plant population (Table 3). Wanjura et al. (1967) reported a direct relationship between cotton emergence and soil temperature. However, despite inconsequential and inconsistent differences in plant populations, adequate stands were obtained in all treatments for all years. Delaying planting until 1 May or later and removing residue in the seeding zone with planter-equipped row cleaners likely minimized the soil temperature effects on cotton stands. In coordinated research using long-term climatological data, we determined that 50 degree day-units (base 15.5°C), the optimum required for rapid cotton emergence, are normally not accumulated until May 1 in the Tennessee Valley (Norfleet et al., 1997).
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Soil surface bulk density, taken in 1996 within the seedbed, indicated a nonsignificant trend for reduced bulk density in the row with fall ridged, conventional tillage, and spring strip-tilled systems (Table 6). However, contrasts indicated increased soil surface compaction in the no-tillage control treatment (1.44 Mg m-3) compared with conventional tillage (1.33 Mg m-3, P < 0.06). Fall ridging (with or without deep tillage) had significantly lower bulk density compared with nonridged treatments (with or without deep tillage) (1.34 vs. 1.42 Mg m-3, P < 0.01). There was no clear relationship between bulk density and plant populations or yield in the 1 yr (1996) that bulk density was determined in the seed zone.
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As expected, due to rainfall variations, there were tillage system x measurement day (DAP) interactions as well. Presentation of soil water content data by day and row position for all eight tillage treatments would be extensive and confusing. Daily soil water contents for the in-row position during the measurement period are shown in Fig. 3 for four tillage systems: conventional tillage, deep tillage without ridging (the no-tillage control), subsoiling without ridging, and paratilling without ridging. These four treatments for the in-row position are chosen to illustrate soil water content variations during the measurement period, as they represent significant trends in the data and also because these treatments demonstrated variations in seed cotton yields. Throughout the sampling period in 1995, conventional tillage and the no-tillage control treatment maintained a higher daily soil water content compared with nonridging with subsoiling and nonridging with paratilling. A similar pattern was seen in 1996, but because of rainfall distribution, differences in daily soil water content were minor. An extended drought with only one significant rainfall event late in the growing season (when cotton water use would be minimal) (Fig. 2) resulted in similar daily soil water content among treatments in 1997.
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Unlike 1996, 1997 weekly rainfall was near or above normal from 2 wk after planting until the first week in July (Fig. 2). However, in the critical blooming period (July through early August), rainfall was 87% below normal. Subsoiling, with or without fall ridging (2990 kg ha-1) had significantly greater yield than treatments without deep tillage (2700 kg ha-1, P < 0.08) in 1997. Fall subsoiling reduced soil compaction compared with treatments without deep tillage, as indicated by the soil compaction index (Table 5). This reduction in soil compaction is believed to have resulted in increased rooting, allowing plants to cope with drier weather during the critical fruiting period. Although treatments with paratilling also reduced soil compaction, yields were not significantly greater than treatments without deep tillage (2880 kg ha-1 vs. 2700, P < 0.27). Percentage open bolls at defoliation, an indication of cotton maturity, was found to be significantly greater with subsoiling compared with paratilled treatments (62.3 vs. 37.3%, P < 0.02). We believe this delay in maturity with paratilling was responsible for the reduced yield compared with subsoiled treatments.
Similar to 1996, 3 of the first 4 wk of the 1998 season had lower than average rainfall (Fig. 2). This early season drought continued to the middle of July (midway into the critical blooming period), resulting in 67% of the normal rainfall. As in 1996, fall ridging (2250 kg ha-1) significantly reduced yields compared with nonridged treatments (2440 kg ha-1, P < 0.06). Once again, we believe that early season drought stress resulted in lower yields with fall ridged treatments. All conservation tillage treatments, with the exception of fall ridging without deep tillage, had greater yields than conventional tillage (Table 8).
In 1999, there was an extended drought from July through August, the critical fruiting period (Fig. 2). Subsoiling without ridging resulted in greater seed cotton yield than subsoiling with ridging, paratilling without ridging, the no-tillage control (nonridged without deep tillage) and conventional tillage (Table 8). Fall ridging (2550 kg ha-1) also had significantly greater yield than conventional tillage (2270 kg ha-1, P < 0.012). Unlike 1996 and 1998, with drought stress in early June, fall ridged treatments were not significantly disadvantaged compared with other treatments in 1999.
Excluding 1995, a year in which unusually heavy insect pressure disproportionately affected treatments with the greatest yield potential, average seed cotton yields during the study (1996–1999) were greater for all conservation tillage systems compared with conventional tillage. Numerically, the highest yields were obtained with fall non-inversion deep tillage, either subsoiling or paratilling, in the absence of ridging. Without ridging, fall paratilling or subsoiling under the row increased yields compared with ridged treatments (regardless of deep tillage) and strict no-tillage. Shallow strip tillage in spring resulted in yields statistically similar to paratilling or subsoiling but not statistically greater than strict no-tillage. However, this practice is difficult and operationally inefficient on these heavy soils. Also, Raper et al. (2000) found no benefit to strip-tillage in spring on this soil type when a rye cover crop was used, compared with strict no-tillage.
Historically, dry weather during the critical cotton fruiting period in the Tennessee Valley Region is common (Ward et al., 1959). During this period, from the last week of June to the second week of August, a minimum of one-third of the days will be drought days (plant-available soil water is reduced to zero) in 50% of the years (Ward et al., 1959). Three years out of ten, a minimum of 65% of the days during this period will be drought days. For these soils, a conservation system that includes deep tillage under the row in fall, to reduce soil compaction and increase the volume of soil available for rooting and water storage, coupled with a cover crop to produce adequate residue for soil and water conservation, can reduce the risks of drought-induced yield reductions.
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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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1 Reference to a trade or company name is for specific information only and does not imply approval or recommendation of the company by the USDA or Auburn University to the exclusion of others that may be suitable. 
Received for publication June 21, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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