Published online 18 May 2006
Published in Crop Sci 46:1570-1575 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP ECOLOGY, MANAGEMENT & QUALITY
Changes in Pickling Cucumber Yield and Economic Value in Response to Planting Density
Mathieu Ngouajioa,*,
Guangyao Wangb and
Mary K. Hausbeckc
a Michigan State Univ., Dep. Horticulture, 428 Plant and Soil Sciences Building, East Lansing, MI 48824
b Michigan State Univ., Dep. Horticulture, 432 Plant and Soil Sciences Building, East Lansing, MI 48824
c Michigan State Univ., Dep. Plant Pathol., 140 Plant Biology Building, East Lansing, MI 48824
* Corresponding author (ngouajio{at}msu.edu)
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ABSTRACT
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Studies on density-dependent variations in yield and economic value of crops could help identify optimum plant density. Field experiments were conducted in 2003 and 2004 using a wide range of densities (from 88 000 to 330 000 plants ha–1) of pickling cucumber (Cucumis sativus L.) grown for once-over machine harvest. Fruit set was identified as a major yield-limiting factor at low densities. When plant density decreased by 73%, fruit number only increased by 50% within the range of densities used. Total marketable yield increased with density. However, the highest yield observed at the highest density did not translate into the highest economic value. This was mainly due to the added cost of the seed under high densities. Optimum density required to maximize economic value, was between 220 000 and 245 000 plants ha–1 and depended on the selling price of the fruits. Our results support the hypothesis that the density of 330 000 plants ha–1 currently used by many growers could be significantly reduced without losses in economic value of the crop. Finally we propose that seed cost be included in studies designed to identify optimum planting density of crops.
Abbreviations: DAP, days after planting
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INTRODUCTION
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MICHIGAN is a leader in pickling cucumber production with over 14 000 ha planted in 2004 and a total value of over $35 million (Ngouajio and Mennan, 2005; USDA, 2005). Most of the cucumber in Michigan is grown for once-over mechanical harvest. To optimize yield of commercial size fruits, most growers use high planting densities with over 300 000 plants ha–1 in many cases (Cantliffe and Phatak, 1975). In a survey conducted in Michigan in 2003, we found that some growers used row spacing as narrow as 28 cm with 10 cm between plants inside the row, which corresponds to a density of about 355 000 plants ha–1. High plant density may increase relative humidity within the canopy and increase the duration of leaf wetness by reducing air movement and sun light penetration (Burdon and Chilvers, 1982; Tu, 1997). Thus plant density could have significant impact on plant disease incidence (Burdon and Chilvers, 1982; Copes and Scherm, 2005). Legard et al. (2000) reported a significant increase in the incidence of Botrytis fruit rot (caused by Botrytis cinerea Pers.:Fr.) in annual strawberry when planting density was increased. In recent years, the incidence of cucumber fruit rot caused by Phytophthora capcisi Leonian has increased tremendously, forcing growers to rethink their production techniques. In Michigan, pickling cucumber growers are interested in reducing their planting density as a part of an integrated approach to reduce the incidence of P. capcisi.
Several studies have been conducted on the effect of plant density on yield and quality of pickling cucumber (Cantliffe and Phatak, 1975, Hogue and Heeney, 1974; Nerson, 1998; Schultheis et al., 1998; Staub et al., 1992; Widders and Price, 1989). In studies conducted in Michigan using densities ranging from 44 000 to 194 000 plants ha–1, Widders and Price (1989) showed that optimum planting density for once-over harvest of cultivars Tamor and Castlepik was 77 000 plants ha–1. In North Carolina, Schultheis et al. (1998) found optimum densities of 200 000, 240 000, and 330 000 plants ha–1 for cultivars Sumter, Regal, and H-19, respectively. These results suggest that optimum planting density may vary greatly among cultivars and growing conditions. Until 2004, the cultivar Vlaspik was the common cultivar used by most pickling cucumber growers in Michigan, and no detailed study has documented its optimum plant density. Moreover, most published studies evaluating the effect of plant density on cucumber yield have traditionally used analysis of variance and means separation techniques to test differences among discrete levels of plant density (Cantliffe and Phatak, 1975; Hogue and Heeney, 1974; Staub et al., 1992; Widders and Price, 1989). Analysis of continuous variation in yield component using regression models could supplement those previous studies and allow for easy comparison among experiments (Duthie et al., 1999a, 1999b; Schultheis et al., 1998).
The objective of the current study was to quantity density-dependant variations in pickling cucumber fruit number and yield under a broad range of planting densities and to determine optimum planting density for maximum economic value.
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MATERIALS AND METHODS
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Experimental site and experiment set-up
Field experiments were conducted at the Michigan State University, Horticulture Teaching and Research Center (HTRC 42°44' N, 82°28' W) at East Lansing, MI, in 2003 and 2004. The soil was a Marlette fine sandy loam with pH 6.1 and 20 g kg–1 organic matter. Weekly precipitation and average temperature for the growing season (June and July) are presented in Fig. 1. The experiments were established on land that had been previously planted with a cereal rye (Hordeum vulgare L.) cover crop.
A total of 12 densities were obtained by combinations of row spacing (30, 46, 61, and 76 cm) and plant spacing inside the row (10, 13, and 15 cm). Row spacing and plant spacing inside the row were selected on the basis of current practices in Michigan and to achieve specific plant populations. Specific treatments and corresponding plant populations are presented in Table 1. The experimental design was a randomized complete block with four replications. Individual plots were 10 m long and consisted of four rows (two guard rows and two data rows). Plot width varied depending on row spacing.
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Table 1. Cucumber planning densities resulting from different combinations of row spacing and plant spacing inside the row.
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Pickling cucumber variety Vlaspik was sown on 3 June 2003 and 8 June 2004. Immediately after planting, fertilizers were applied at a rate of 450 kg/ha of 19–19–19 (N-P2O5- K2O) in a 10- to 15-cm band on each seed line. Because of the differences in the number of rows per hectare in the treatments, the amount of fertilizer was adjusted for each row spacing so that final rate per hectare was similar in all treatments, regardless of row spacing. Overhead irrigation was used during the growing season to supplement rainfall.
Data Collection
Cucumbers were harvested manually on 28 July [55 d after planting (DAP)] and 29 July (51 DAP) in 2003 and 2004, respectively. Plots were harvested when about 10% of the fruits were oversize as recommended by Schultheis et al. (1998). Whole plants were destructively harvested by hand in the two middle rows of each plot to simulate machine harvest. Fruits were separated from the vines and sorted into grade 1, 2, 3, and oversize (OS) according to market standards (USDA, 1997). Grades 1, 2, 3, and OS consisted of fruits with diameter of 0 to 27, 28 to, 39 to 51 mm, and greater than 51 mm, respectively. Fruit number and weight was recorded for each grade.
Statistical Analysis
Marketable yield was calculated as the sum of grade 1, 2, and 3 fruits. Total yield included oversize fruits. Fruit number and weight were converted to values per hectare to allow comparisons among treatments. All data were subjected to ANOVA and data for both years were combined when there was no year x treatment interaction. Density-dependant variations in yield and economic value were analyzed by nonlinear regressions. Fruit number per plant was fitted to density by the reciprocal yield law (Firbank and Watkinson, 1990):
 | [1] |
where N is fruit number per plant, D is plant density, Nm and a are parameters to estimate, with Nm being the maximum fruit number per plant, and a is area required to achieve maximum fruit number per plant. Fruit weight was fitted to density using a similar equation:
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where W is fruit weight per plant. Total cucumber fruit number and final yield were then fitted to the following equations:
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where Y is cucumber yield and NTOT is total fruit number in a given area.
Economic (dollar) value was calculated using an average $1.65 per 1000 seeds and selling price of 3.00, 3.25, 3.50, 3.75, and $4.00 for 25 kg (bushel). The value was obtained by multiplying marketable yield by selling price and subtracting the cost of the seed. The economic grades for machine-harvested pickling cucumber in Michigan are mainly grade 2 and 3. Grade 1 fruits are either lost in the field or damaged during harvesting and handling operations and oversize fruits are not sold. In 2005, price varied with grade and growers but average price was about $3.25 for 25 kg (bushel). Grade 2 fruits had a higher economic value than grade 3 fruits but the average price accounted for the proportion of each grade in the total yield (about 60% grade 2 and 40% grade 3). Seed cost and selling price estimates were averages used by the industry in 2005 (J. Swanson, 2005, Swanson Pickle C., Ravenna, MI, personal communication, 2005). Data on economic value were fitted to the following quadratic equation:
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where Y is economic value and D is cucumber density, a, b, and c are regression parameters. Optimum density was calculated as the density producing the highest economic value.
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RESULTS
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Fruit Number
As plant density increased, total number of fruits per plant decreased while total number of fruits per unit area increased (Fig. 2). Average number of fruits per plant decreased from 2.4 to about 1.2 fruits per plant as density increased from 88 000 to 330 000 plants ha–1. The rate of decrease was greater at lower densities than at high densities as indicated by the curvilinear trend of the data. Changes in total fruit number per unit area followed a trend similar to that of the number of fruits per plant, but the responses were in opposite directions (Fig. 2). The number of fruits ha–1 was lowest at 88 000 plants ha–1 and greatest at 330 000 plants ha–1.
Cucumber fruit number was adequately described by the nonlinear regression models in Eq. [1] and Eq. [3] with better fits when all four grades were included (Table 2). The coefficient of determination for the regressions (R2) ranged from 0.55 to 0.80 for data combined over the 2 yr. Theoretical value of maximal fruit number was 3.39 fruits per plant when planting density was very low.
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Table 2. Parameter estimates for the regression between cucumber plant density and yield or fruit number for 2003 and 2004 seasons combined.
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Fruit Yield
The effect of plant density on cucumber yield was similar to that on fruit number (Fig. 3). As density increased, fruit weight per plant declined while total weight per unit area increased. The response was observed for cumulative yield of all marketable grades (grade 1, 2, and 3) as well as for total yield. Total marketable yield increased from about 13 Mg ha–1 at 88 000 plants ha–1 to 22 Mg ha–1 at 330 000 plants ha–1. However, the rate of yield increase was progressively lower as density increased. The coefficients of determination from the regressions using Eq. [2] an Eq. [4] varied from 0.52 to 0.83, with total yield having largest R2 (Table 2). Theoretical maximum yield ranged from 8.89 to 480.77 g per plant.
Economic Value
The relation between planting density and economic value was well described by Eq. [5]. R2 values ranged from 0.78 to 0.81 as selling price changed from $2.75 to $3.75/25 kg for data combined over 2 yr (Table 3). Optimum planting density was moderately affected by the selling price of the fruits. The quadratic response of economic value to planting density is presented in Fig. 4 A. Economic value increased as planting density increased when planting density was smaller than 240 000 plants ha–1. Higher densities failed to produce the higher economic value, irrespective of the selling price used for the calculations. Within a broad range of cucumber fruits selling price (from 2 to $5/25 kg), optimum planting density was minimally affected and ranged from 220 000 to 245 000 plants ha–1 (Fig. 4 B). Therefore there was no apparent benefit of increasing plant density beyond 245 000 plants ha–1, regardless of the selling price.
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Fig. 4. Economic value of cucumber as affected by planting density (A) and impact of selling price on optimum planting density (B). Seed cost was estimated at $1.65/1000 seeds.
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DISCUSSION
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Results of this study provide important information that could help better manage plant populations of pickling cucumber grown for once-over machine harvest. Several factors directly linked to plant density were shown to affect the performance of individual plants, total yield, and economic value of production. Increasing plant density reduced fruit set per plant, as well fruit weight per plant. However, the reduction on a per plant basis was more than offset by the total number of fruits and total yield per unit area. Thus, the major limitation of pickling cucumber productivity at low densities is likely the inability of individual plants to fully compensate for the small plant number by producing multiple fruits that mature simultaneously. In this study, decreasing the density from 330 000 to 88 000 plants ha–1 (73% reduction) only increased fruit number form 1.2 to 2.4 fruits per plant (50%).
In the production of machine-harvested pickling cucumbers, fruit weight is usually optimized by selecting a harvest time that would maximize yield of marketable grades (Schultheis et al., 1998). It is usually recommended to harvest when about 10% of fruits are oversize (Schultheis et al., 1998). Moreover, fruit weight is highly dependent on harvest time and under warm condition, fruit weight could easily double in 2 to 4 d. Therefore, fruit number per plant is a better indicator of the yield potential of machine-harvested pickling cucumber at a given density than is fruit weight. For hand picked cucumber using indeterminate cultivars, this is not the case as new fruits are formed when old ones have been harvested. For once-over machine-harvested cucumber, the number of fruits per plant quickly becomes a limiting factor for yield increase at low plant densities. The low productivity at low plant densities in pickling cucumber was attributed to low fruit set (Cantliffe and Phatak, 1975; Widders and Price, 1989). Generally, low fruit set could result from limited net photosynthesis leading to source-sink limitation (Schapendonk and Brouwer, 1984) and/or first fruit monopolizing the pool of photosynthates by expanding rapidly (Widders and Price, 1989). The above hypotheses are in support of the use of high plant densities to compensate for the low performance of individual plants. In fact, those hypotheses have been the driving force for the high plant populations currently used by many growers in Michigan.
Other factors that should be taken into account while selecting pickling cucumber plant density are varieties and growing conditions. Widders and Price (1989) showed that under Michigan conditions, the optimum density of cultivars Tamor and Castlepik was about 77 000 plants ha–1. That is in contrast with results of the present study that showed yield of cultivar Vlaspik could be maximized at significantly higher densities. Our results are similar to those reported by Cantliffe and Phatak (1975) for work conducted in Ontario, Canada.
Profitability of pickling cucumber is not just a function of total fruit weight but is also highly dependent on seed cost and selling price. With the average cost of 1.44 to $1.85 per 1000 seeds observed in 2005, seed cost could significantly affect total revenue to the farmer. Therefore, we propose that seed cost be included in analyses of studies designed to identify optimum pickling cucumber plant density because it has a direct effect on the economic value of the crop. In this study, the optimum density required to maximize yield was different than the density needed to maximize the value of production. If maximum yield were the only objective, then 330 000 plants ha–1 would be more appropriate to meet the objective. On the basis of a wide range of selling prices, optimum density for highest economic value was around 220 000 to 245 000 plants ha–1. Schultheis et al. (1998) reported similar densities for cultivars Regal and Sumter in North Carolina. Moreover, they showed that optimum density depended on the growth habit of the cultivar used. A cultivar with a smaller canopy, H19, required 330 000 plants ha–1 for maximum economic value in the same study.
Results of this study suggest that growers who are currently planting cultivar Vlaspik at 330 000 plants ha–1 could significantly reduce their planting density without any loss in crop value. This could be achieved by widening the rows from 30 to 46 or even 61 cm as well as plant spacing inside the row to fine tune their final density. Such a strategy could also improve the microclimate in the plant canopy and potentially reduce the incidence of soilborne diseases like Phytophthora spp. However, those benefits would need to be demonstrated in research trials before making any practical recommendation to farmers.
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ACKNOWLEDGMENTS
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This research was funded in part by GREEEN (Generating Research and Extension to meet Environmental and Economic Needs) Project # GR03-023 and GR04-023, PPRC MSU (Pickle and Pepper Research Committee for Michigan State University), PPI (Pickle Packers International Inc), and the Michigan Vegetable Council. Pickling Cucumber seeds were provided by Seminis Inc. We acknowledge technical help from William Chase, Gary Winchell, Erin Hill, Kevin Charles, Mohan Selvaraj, and Heather Johnson. Thanks to Dr. Rebecca Grumet for her critical review of an early version of this manuscript.
Received for publication October 16, 2005.
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REFERENCES
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ABSTRACT
INTRODUCTION
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