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a Univ. of Georgia, Dep. of Crop and Soil Sciences, P.O. Box 8112 Georgia Southern Univ., Statesboro, GA 30460
b Texas A&M Univ., Dep. of Soil and Crop Sciences, College Station, TX 77843-2474
* Corresponding author (pjost{at}arches.uga.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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Abbreviations: UNRC, ultra-narrow row cotton • DAP, days after planting • LAI, leaf area index • PPFD, photosynthetic photon flux density
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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The effects of high plant densities grown in narrow row spacings have been examined extensively in other crops. Such studies have been conducted on soybean [Glycine max (L.) Merr.] as early as 1939 (Wiggans). Planting soybean in narrow row spacings can be advantageous. Costa et al. (1980) cited that, if available water and nutrients are adequate, then the factor that limits production is solar radiation. Thus, one reason for altering row spacings and plant arrangement is to improve light interception (Shaw and Weber, 1967). Improved light interception occurs because leaf area index (LAI) increases more rapidly in narrow row spacings, compared to wide row spacings (Weber et al., 1966). In addition, at similar plant densities, the time to reach critical LAI is reduced as row spacings decrease (Weber et al., 1966).
Although yield response to narrow row soybean production has been erratic, the general trend is that, as row spacing decreases, soybean yield increases (Costa et al., 1980; Ablett et al., 1984; Beuerlein, 1988). This trend, however, is affected by annual water supplies. In years with a high water supply, narrow rows consistently increased soybean yields; but in years with limited moisture, row spacings had no effect on yield (Taylor, 1980).
Cotton is an indeterminate plant that flowers at regular intervals, and the flowering period normally lasts 60 d (Mauney, 1986). Lewis (1971) stated that UNRC could reduce production costs by shortening the growing season. He reasoned that with increased densities, fewer bolls per plant would be necessary to maintain yields at current levels. Therefore, if fewer bolls were required to maintain yields, the corresponding time to set a crop would be reduced. Researchers currently examining this topic state that a shortened fruiting window could reduce production costs by decreasing insecticide applications needed for fruit protection (Allen et al., 1998). Lewis (1971) contended that, since fewer fruit per plant should be necessary to maintain yields in UNRC the fruiting period could be shortened. Therefore, the fruiting structures would be at similar developmental stages throughout the season. This growth characteristic is in contrast to conventionally spaced cotton that sets fruit over several different developmental stages and, thus, has fruit of varying ages at any given time during the season (Mauney, 1986). A more synchronized fruiting pattern in UNRC could also lead to more effective growth regulation and irrigation scheduling and possibly enhance the ability to increase yields through such production practices. In fact, some early studies have demonstrated that yield increases can be obtained with ultra-narrow row production, compared to conventional row spacings (Briggs et al., 1967; Hoskinson et al., 1974).
Many studies have evaluated density variations of cotton in conventional row spacings. The general conclusions from these studies were that, as plant densities increased, plant height, boll size, bolls per plant, and nodes per plant decreased (Tavernetti and Ewing, 1951; Ray et al., 1959; Bilbro and Quisenberry, 1973; Fowler and Ray, 1977). While these studies have shown that modifications in plant structure occur with differing densities because of the plasticity of the cotton plant, only slight effects were observed on yield.
In an earlier effort to determine optimum plant density and row spacing, Fowler and Ray (1977) evaluated two different cotton cultivars in a wide range of equidistant spacing configurations. In this study, the spacing between plants was equal to that between rows; plant spacings of 12.7, 17.8, 25.4, 35.6, and 50.8 cm were examined. These spacings had plant densities ranging from 3.9 to 62 plants m-2. One cultivar was an early maturing cultivar (Paymaster 101A) that was well adapted to the Texas High Plains where the study was conducted. The other cultivar was a shorter and more compact cultivar (C.A. 491) that was thought to be more suitable for high-density production. Yield in this 1-yr study was greatest for the 35.6- to 50.8-cm spacings and least for the 12.7- and 17.8-cm spacings in both cultivars. However, some interesting trends in plant growth were observed. This work showed, as have others (Tavernetti and Ewing, 1951; Ray et al., 1959; Bilbro and Quisenberry, 1973), that as plant density increased, plant height, node numbers, and plant dry weight decreased. They also demonstrated that LAI accumulated more rapidly at higher plant densities, but a greater proportion of photoassimilates was directed to vegetative growth rather than reproductive growth. These observations led these researchers to conclude that plant density would affect yield both positively with LAI accumulated early in the season, and negatively through lowering the fruiting-vegetative ratio. A high fruiting-vegetative ratio has been shown to be desirable in cotton (Meredith and Wells, 1989).
Fowler and Ray (1977) found significantly more fruiting structures per unit land area in narrow spacings, compared to wide spacings. According to Pearce et al. (1965) and Williams et al. (1965), more fruiting structures per unit land area should enhance earliness. However, no differences were detected for earliness in the Fowler and Ray (1977) study. The lack of earliness was attributed to the height of the first fruiting branch being greater at high densities and to lower fruiting-vegetative ratios, and they suggested that a high fruiting-vegetative ratio may be a key factor in breeding cotton for a high-density, narrow-row culture.
Closer row spacings and elevated plant densities in UNRC also led to more rapid canopy closure, compared to conventionally spaced cotton (Jost and Cothren, 2000). Rapid canopy closure could reduce weed competition (Snipes, 1996), increase light interception (Krieg, 1996), and possibly decrease soil water evaporation. Krieg (1996) determined that up to 40% of the available water supply is lost to evaporation from the soil in traditional row spacings. A greater proportion of the total water supply may be accessible to the plant in UNRC rather than being lost to evaporation.
One concern with the UNRC system is that fiber quality may be sacrificed. Heitholt et al. (1992) showed that narrow rows resulted in earlier canopy closure; while Buxton et al. (1979) showed that narrow row spacings caused a greater percentage of fruit to be set earlier. Both of these factors, along with reduced boll size observed in high densities (Fowler and Ray, 1977), have the potential to negatively affect fiber quality. However, studies with current cultivars have failed to show any detectable influence of narrow or ultra-narrow row spacings on fiber quality (Smith et al., 1989; Heitholt et al., 1993; Gerik et al., 1998).
The availability of transgenic herbicide-resistant cotton varieties has reduced the weed control problems in UNRC production that were encountered in the past, and has been the primary impetus for renewed interest in this production strategy. Moreover, studies of UNRC in the past were conducted with older cultivars that possessed low fruiting-vegetative ratios. Because of these factors, this study was conducted to compare the effects of ultra-narrow and conventional row spacings on various vegetative and reproductive growth parameters of a recently available transgenic cotton variety. The major objective of this study was to determine optimum plant densities in ultra-narrow row spacings, compared to a conventional system of cotton production, and to delineate differences in crop maturity as influenced by these row spacings and plant densities. In addition, another objective of the study was to examine the performance of these treatments on two different soil types.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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The 76- and 101-cm row spacings were planted to achieve a stand of 13.6 and 9.9 plants m-2, respectively. These populations were chosen for the 76- and 101-cm spacings because they typify populations for these row spacings in conventional cotton production in this region. For ease of discussion, the 101- and 76-cm row spacings are collectively referred to as the conventional row spacings. The 19-cm row spacings were planted at a seeding rate >45 seeds m-2. When all plants had emerged, the 19-cm row plots were thinned to plant densities of 12.2, 18.5, and 40.5 plants m-2. A seeding rate of 22 plants m-2 was used in the 38-cm row spacing; these plots were then thinned to 11.3 or 19.5 plants m-2. Due to insect and disease pressures, natural plant mortality, and delayed emergence, stand counts were taken at crop maturity to ascertain the final plant density.
In both years, the cotton was flat-planted. The 101-cm row spacing was obtained with a planter set for that row spacing, and the 19-, 38-, and 76-cm row spacings were established with a planter set on 38-cm row spacings. Alternate planter units were disengaged to achieve the 76-cm row spacing, and the 19-cm spacing was established by splitting the first 38-cm pass with a second pass of the planter.
In both years, the cotton was grown under irrigated conditions, with a pivot sprinkler as the irrigation source in 1998 and an overhead line source sprinkler as the irrigation source in 1999. Insecticides and herbicides were applied consistent with local agronomic practices. Because the study was conducted in a grower's field in 1998, the irrigation schedule and amount were at the discretion of the producer. In 1999, one irrigation of 50 mm was made to the study at mid-bloom.
Data collected consisted of several growth and development parameters including height and node counts at harvest. All height measurements were taken from the cotyledonary node to the terminal of the plant. Node counts were made from the cotyledonary node to the terminal of the plant with the cotyledonary node being designated as node zero.
Leaf area index (LAI) was determined at 49, 77, and 91 days after planting (DAP). Leaf area was calculated by removing all the leaves from five plants per plot and then passing them through a LI-COR LI-3100 Area Meter (LI-COR Inc., 1979). The resulting leaf area was then divided by five and multiplied by the number of plants m-2 in the corresponding treatment to obtain LAI.
Biomass partitioning into leaves, stems, and bolls was determined at 91 DAP. Dry weights were obtained after drying samples at 60°C for 7 d. Biomass partitioning was calculated by dividing the weight of leaves, stems, or bolls by the weight of the total biomass.
A laboratory saw gin was used to separate a seed cotton sample into lint and seed. Percent lint was then calculated by dividing the weight of the lint by the weight of the seed cotton sample and multiplying by 100. Seed weight was determined by weighing 100 acid-delinted seed. To calculate seed density, the weighed seeds were submerged in a known volume of water. Once the seeds were submerged, the initial volume of water was subtracted from the volume of the water containing the seeds. This value was then the volume (cm3) of the seed. Seed density (g cm-3), was then obtained by dividing the weight of the 100 seed by the volume that the seed occupied.
Lint yields and time to crop maturity were ascertained by hand-harvesting the plots. In the conventional row spacings of 76- and 101-cm, two bordered rows 6 m long were harvested. In the 19-cm row spacing 10 bordered rows 6 m long were harvested; in the 38-cm row spacing, five bordered rows 6 m long were harvested. Time to crop maturity was estimated by a method similar to that used by Kerby et al. (1990), where plots were hand-harvested in 4- to 8-d increments beginning as soon as the first bolls opened and continuing until all harvestable bolls had been collected. Differences in crop maturity rates were then determined by calculating the accumulated percent of the total yield harvested at a given time. Intervals between harvest time were similar in both years.
Harvestable boll distribution patterns were determined by documenting boll set by node and fruiting position. All distribution percentages were calculated by dividing the number of harvestable bolls at a given position, on a given range of sympodial nodes or on monopodial nodes, by the total number of harvestable bolls and multiplying by 100.
Lint quality was determined by subjecting 50-g fiber samples to high volume instrument (HVI) testing at the International Textile Center located at Texas Tech University in Lubbock, TX. Fiber characteristics reported include micronaire, length, and strength.
In both years, treatments were arranged in a randomized complete block design with six replications. All data analysis was performed using the General Linear Model Procedure in the SAS software package (SAS Institute, 1990). Means were separated using Fisher's Protected LSD Test at a significance level of 5%. When significant row spacing x year interactions were detected for a measured parameter, those means were presented separately by year. However, when possible, means were combined over both years. Homogeneity of error variances were tested as described in Gomez and Gomez (1984). When the test for homogeneity of error variances was not significant, the pooled error term was used to test the spacing x year interaction and the spacing factor. When the test for homogeneity of error variances was significant, the spacing x year term was used to test the spacing factor. All percentage type data was subjected to the arcsine square root transformation. Actual percentage means are reported, but statistics reflect the transformed data.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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In 1998, the medium and high densities in the 19-cm row spacing and the low and medium densities in the 38-cm row spacing had fewer main-stem nodes per plant than both conventional row spacings. Final main-stem node counts in 1998 were consistent with the data of Kerby et al.(1990) and Vories et al. (1999), who also observed fewer total nodes per plant at higher plant densities. In 1999, there were no differences in the number of main-stem nodes between treatments at harvest.
The effects of soil properties on cotton growth have been documented (Smith, 1995). Varying rates of growth regulators have been applied to cotton to compensate for plant height differences in a large field study where plant height varied with soil type (Landivar and Searcy, 1999). The current study was conducted on a heavy clay soil (Ships clay) in 1998, and on a lighter soil (Weswood silt loam) in 1999. While significantly more rainfall was received in 1999 than in 1998 (Table 2), a previous study (Jost and Cothren, 2000) demonstrated that on a Ships clay soil in contrasting years of rainfall, row spacings and plant densities similarly affected plant height and node counts under irrigated conditions. As discussed previously, no treatment effect existed for plant height or main-stem nodes in 1999. Collectively, the data suggest that a reduction in plant height and main-stem nodes may not be realized in UNRC when grown on lighter soils that tend to promote more vegetative growth. Since this study was grown under irrigated conditions, even in a dry year similar results would be expected.
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The greater LAIs in 1999, compared to 1998, are attributable primarily to soil type but may have also been mediated by weather conditions. In the time period from 63 to 77 DAP, fewer heat units were accumulated in 1999 than in 1998 (Table 2), and these conditions in 1999 may have been more suitable for rapid growth (Mauney, 1986). In addition, from 63 to 77 DAP there was only 0.2 mm of rainfall in 1998, compared to 70.1 mm in 1999 (Table 2). It is doubtful however, that these factors alone could cause such a vast difference in LAIs between the years, especially since the study was irrigated in both years. The causative factor appears to be soil type. In 1998 the soil was a Ships clay, a very heavy soil, that typically does not produce extremely vegetative plants. In 1999, the soil type was a Weswood silt loam, a lighter soil that has a history of producing excessive vegetative growth.
In 1998 at 91 DAP, there was no significant difference in LAI between the treatments, with measurements ranging from 2.36 to 3.23 (Table 3). In 1999 at this time, the LAI in the 19-cm rows with the high plant density, and the 38-cm rows with the medium plant density, was significantly greater than for all other treatments. The LAI in the 19-cm rows with the high plant density was 8.36, which surpassed the 38-cm rows with the medium plant density by an LAI of 1.72. While rainfall was significantly greater in the time period of 77 to 91 DAP in 1998 compared to 1999, the heat unit accumulation was nearly identical during this growth period in both years (Table 2). Cotton normally intercepts 90% of the incident PPFD at an LAI of 4 to 5, exaggerated LAIs may occur under high plant densities in years and environments conducive to excessive vegetative growth (Heitholt, 1994). These elevated LAIs can be detrimental in that the lower canopy is so shaded that boll development may be hindered (Hake et al., 1996).
Biomass Partitioning
In 1998 at 91 DAP, the percent of biomass partitioned into leaves and stems tended to decrease across all row spacings as plant density increased, and the percent of biomass partitioned to bolls increased (Table 4). In 1999 at 91 DAP, no differences were detected in biomass partitioning between the treatments. These data support the 1998 LAI data that showed the higher plant densities in the 19- and 38-cm row spacings had reached a maximum LAI at 77 DAP, causing more biomass to be partitioned into reproductive structures. In contrast, in 1999, LAI continued to accumulate from 77 to 91 DAP, which caused an overall lower percentage of biomass to be partitioned into bolls.
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To identify the factor associated with the increased percent lint, seed weight and density were calculated. The medium and high plant densities in the 19-cm rows and the medium density in the 38-cm rows had lower seed weights than both conventional row spacings (Table 5). Seed weight was reduced by as much as 9% in these treatments, compared to the conventional row spacings. However, despite significant reductions in seed weight, no differences in seed density were noted between the treatments. Therefore, the increased percent lint associated with the 19-cm row spacing at the high plant density is attributed to the production of smaller, lighter weight seeds, compared to the conventional row spacings. In addition, these data also suggest that the 19-cm row spacing at the high plant density produced lighter weight seed yet generated as much lint per seed as the conventional row spacings.
The percent lint data also contrasts that of Fowler and Ray (1977), who found that as plant density increased percent lint decreased. Meredith and Wells (1989) demonstrated that much of the increased yield obtained through breeding programs was due to current cultivars having reduced vegetative to reproductive ratios. A possible explanation of the contrast in data between the Fowler and Ray (1977) study and the current study may be attributable to the decreased vegetative to reproductive ratios present in current cultivars influencing the percent lint response between the treatments.
Lint Yield
In 1998, all plant densities within the 19- and 38-cm row spacings yielded significantly more lint than the 101-cm row spacing (Table 5). In addition, the medium and high plant densities in the 19-cm row spacing had greater yields than both conventional row spacings. There was also a trend in 1998 for lint yield within the 19-cm row spacing to increase with plant density. Lint yields within the row spacing and plant density treatments in 1999 were much greater than those observed in 1998 and ranged from 1564 to 1816 kg ha-1. These elevated yields in 1999 serve as evidence that the study was conducted on a more productive soil type in that year. However, no significant treatment effect on lint yield was observed between treatments in 1999. Also, unlike 1998, as plant density increased within the 19-cm row spacing, lint yield tended to decrease in 1999, although this decrease was not statistically significant.
These data demonstrate that planting cotton in ultra-narrow spacings at elevated plant densities does not adversely affect yield on either of the soil types evaluated. These data, in fact, directly contradict Culp et al. (1974), who projected that, in a hot and dry year, such as 1998, yield would be more adversely affected in UNRC than in conventional row spacings. Studies in which varying densities of cotton have been evaluated in conventional row spacings suggest that, in general, plant height, boll size, bolls per plant, and nodes per plant decrease as plant densities increase (Tavernetti and Ewing, 1951; Ray et al., 1959; Bilbro and Quisenberry, 1973; Fowler and Ray, 1977). Although these previous studies have shown modifications in plant structure with differing densities, due to the plasticity of the cotton plant, only slight effects were observed for yield.
Time to Crop Maturity
One of the main advantages thought to be attained by producing UNRC is a reduction in the amount of time required for the crop to mature (George, 1971; Lewis, 1971; Cawley et al., 1998; Allen et al., 1998). In 1998 at 112 DAP, 53.9 and 59.6% of the final yield had been amassed from the medium and high plant density in the 19-cm row spacing, respectively (Table 6). In comparison, <36% of the yield had been harvested by 112 DAP in all other treatments. At 116 DAP, at least 70% of the yield had been gathered from the medium and high plant densities in the 19-cm row spacing; while 60.6% or less had been collected from all other treatments. The 101-cm row spacing was the only treatment with <60% of its final yield harvested at 122 DAP.
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In 1999, no differences in crop maturity were detected between the treatments (Table 6). A total of 137 d were required to achieve 60% open boll, compared to 122 d in 1998. However, in both years heat unit requirements from planting to 60% open boll in all treatments were 1469.
The effects of plant density and row spacings on time to crop maturity from previous studies have been as erratic and inconsistent as the results from the current study. Kerby et al. (1990) demonstrated that crop maturity was delayed by densities of 15 plants m-2 compared to 5 and 10 plant m-2 in 76-cm rows. In a much earlier study, Fowler and Ray (1977) also showed delayed maturity with elevated plant densities. In contrast, Smith et al. (1989) showed that low plant densities in 101-cm row spacings delayed crop maturity. As with the plant height and node count data, crop maturity differences in UNRC appears to be a function, at least in part, of soil type. A decrease in the time to crop maturity is less likely to occur on soils historically promoting more vegetative growth.
Boll Distribution
Within the 19-cm row spacing in 1998, the percentage of harvestable main-stem bolls set at the first position on nodes below 10 increased with increasing plant density (Table 7). The percentage of harvestable main-stem bolls set at the first position on nodes below 10 in medium and high plant densities in the 19-cm row spacing was 27.2 and 37.8%, respectively, percentages that were greater than all other row spacing and plant density combinations. Similar trends were evident for the percentage of bolls set on main-stem nodes below 10 at all positions combined. There was no treatment effect on the percentage of bolls set at the second or more distal positions on these nodes. At nodes 10 through 12, the high plant density in the 19-cm row spacing set a higher percentage of bolls at the first position and a lower percentage of bolls at the second and greater positions than both conventional row spacings. At nodes above 12 the 19-cm row spacing at the medium and high plant densities set a lower percentage of second position bolls than both conventional row spacings. The same trend was observed in 1998 for bolls set at all positions combined at nodes above 12. The 19-cm row spacing at the high plant density also set fewer bolls on monopodial branches than both conventional row spacings (data not shown).
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In 1999, overall boll set was shifted to higher nodal positions compared to 1998 (Table 8). The lack of early boll set in 1999 was attributed to the lighter soil type which was more conducive to early vegetative growth. The increased vegetative growth in 1999 led to greater LAIs, which can delay and even retard the setting of bolls on lower nodes due to shading of lower leaves (Hake et al., 1996). Collectively, the boll distribution data from 1999 showed that the 19-cm row spacing at the high plant density tended to set a lower percentage of bolls at the second position than did the conventional row spacings (Table 8). There were no differences between the treatments for the percentage of bolls set on monopodial nodes in 1999 (data not shown).
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Fiber length was significantly influenced by row spacing and density treatments in 1998 (Table 9). The high plant density in the 19-cm rows had significantly shorter fibers than the conventional row spacings. The general trend in 1998 was for fiber length to decrease with increasing plant density in the 19- and 38-cm row spacings. This trend was not present in the 1999 data when no differences in fiber length between the treatments were observed. In a past study, row spacing and plant density combinations did not affect fiber length (Baker, 1976). More recently, Vories et al. (1999) also failed to show a significant reduction in fiber length in UNRC production.
Row spacing and plant density treatments in 1999 did not affect fiber strength (Table 9). However, in 1998, the 19-cm row spacing with the high plant density had significantly weaker fibers than all treatments except for the 76-cm row spacing. The tendency in both years of the study was that within the 19- and 38-cm row spacings fiber strength decreased as plant density increased. These results are contrasted to those of Vories et al. (1999) and the early study of Baker (1976), in which fiber strength was unaffected by narrow row spacings at high plant densities. More important, however, is that in both years fiber strength readings were at least average in the classing table (Ramey, 1999).
Collectively, these data suggest that UNRC production does not have a detrimental effect on fiber quality. While fiber length and strength were adversely affected in 1 of the 2 yr, this response was inconsistent for both parameters. Other researchers have shown that the most consistent lint quality factor affected by UNRC is the amount of trash introduced into the seedcotton during harvest (Vories et al., 1999; Bader et al., 1999; Anthony et al., 1999). All cotton was hand-harvested in this study, therefore trash content was not evaluated.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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These data also suggest that minimal differences in plant growth and yield existed between the medium and high plant densities in the 19- and 38-cm rows, and that no conclusive argument could be made to recommend one or more of these row spacing and plant density configurations exclusively. Results from this study indicate, however, that the advantages of UNRC may be realized more readily in growing conditions and soil types that are not historically conducive to excessive vegetative growth.
Received for publication December 10, 1999.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
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