Published online 3 May 2006
Published in Agron J 98:760-765 (2006)
DOI: 10.2134/agronj2005.0201
© 2006 American Society of Agronomy
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Production Papers
Plant Population Density and Maturity Effects on Profitability of Short-Season Maize Production in the Midsouthern USA
Michael Poppa,*,
Jeff Edwardsb,
Patrick Manninga and
Larry C. Purcellc
a Dep. of Agric. Econ. and Agribusiness, 217 Agric. Bldg., Univ. of Arkansas, Fayetteville, AR 72701
b Dep. of Plant and Soil Sci., 368 Ag. Hall, Oklahoma State Univ., Stillwater, OK 74078
c Dep. of Crop, Soil, and Environ. Sci., 1366 W. Altheimer Dr., Univ. of Arkansas, Fayetteville, AR 72701
* Corresponding author (mpopp{at}uark.edu)
Received for publication July 2, 2005.
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ABSTRACT
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Maize (Zea mays L.) production in the Midsouthern USA has increased dramatically in recent years, primarily as a function of growing nematode pressure in cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] fields as well as the increasing yield potential of maize hybrids. Traditionally, 112- to 120-d maturity hybrids (full season) have been grown using 76-cm row spacing at population densities of 
6 to 8 plants m–2. These hybrids, however, reach the reproductive phase of development during the period of a typical midseason drought. Shorter-season (<110-d maturity) hybrids, by contrast, avoid a large portion of this drought and require substantially less irrigation. The yield potentials of short-season hybrids are similar to those of full-season hybrids, but they require substantially narrower rows (50 cm) and increased populations (10 to 12 plants m–2). In this report, economic trade-offs among irrigation, seeding rates, yield potential, and seasonal market price trends for production of short-season hybrids are evaluated. Shorter-season hybrids were comparable to longer-season hybrids in terms of yield potential and partial returns. In general, profit-maximizing seeding rates were higher for shorter-season hybrids, and higher seasonal prices were insufficient to offset higher seeding costs and thereby change optimal hybrid choice. While irrigation use was curtailed with shorter-season hybrids, irrigation savings, at current irrigation costs, were insufficient to offset higher seeding costs. Finally, using a simple decision rule of picking maize hybrid by selecting top-yielding hybrids is challenged in this study as lower-yielding hybrids with lower seeding requirements exhibited higher comparative returns than the highest-yielding hybrids at one of the locations.
Abbreviations: PPD, plant population density in plants m–2 • PPD*, plant population density required to maximize partial returns • PR, partial returns in $ ha–1 • PR*, partial returns at profit-maximizing plant population density
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INTRODUCTION
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MAIZE PRODUCTION in the Midsouthern USA (defined here as Arkansas, Louisiana, Mississippi, Southeastern Missouri, and the Delta portion of Tennessee) has increased dramatically in recent years. For example, harvested hectares in the Midsouth increased from an average of 2.47 million ha from 1991–1995 to 4.34 million ha in 2004 (USDA-NASS, 2004). The most dramatic increase occurred in Arkansas where maize production increased 247% over this time period. Changes in the farm bill of 1996, high maize yield potential with new hybrids, and increasing nematode pressure in traditional cotton and soybean production were the primary reasons for this growth in maize production (Cartwright et al., 2003).
Traditionally, in Arkansas, the highest maize yields have been achieved by planting 112- to 120-d maturity hybrids with 76-cm row spacing in late March or early April. Some producers elect to forgo maximum yields by planting 108- to 110-d hybrids to efficiently allocate machinery and labor in a whole-farm plan (Cartwright et al., 2003), yet neither of these planting-date-by-hybrid-maturity combinations avoid the midseason drought typically experienced in the Midsouthern USA (Cartwright et al., 2003; Purcell et al., 2003).
Arkansas maize grain performance tests at Keiser, AR (Dombek et al., 2002, Dombek et al., 2003) on Sharkey clay soils showed that early-to-mid-season hybrids tended to exhibit slightly lower yields (less than 3% on average) compared with mid- to full-season hybrids at population densities of approximately 6.6 plants m–2 using 96.5-cm row spacing.1 Edwards et al. (2005) evaluated the production potential of short-season (<110 d from emergence to maturity) maize hybrids. They reported a 30 to 50% reduction in irrigation requirements by using shorter-season hybrids compared with a 109-d hybrid. Yield potential of short-season hybrids was similar to that of the 109-d hybrid, provided that narrow (
0.5 m) row spacing was used and plant population density (PPD) of short-season hybrids was high enough to ensure adequate cumulative light interception.
To address the economic trade-offs among yield potential, irrigation requirements, seeding rate, and differential in harvest dates associated with the choice of maize maturity, the objectives of this study were to: (i) develop profit-maximizing seeding rate recommendations for maize hybrids with maturity defined by changes in days from seedling emergence to black layer and (ii) use sensitivity analysis on irrigation cost and seasonal sale price changes to help select maize hybrids that differ in maturity.
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MATERIALS AND METHODS
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Experimental Description
Field experiments evaluating the response of maize yield and biomass to increased PPD were conducted at Fayetteville, AR (36°05' N, 94°10' W) on Captina silt loam (fine-silty, siliceous, active, mesic Typic Fragiudults) and Keiser, AR (35°40' N, 90°5' W) on Convent silt loam (coarse-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquepts) in 2001, 2002, and 2003. All experiments were conventionally tilled, irrigated, and fertilized according to Arkansas Cooperative Extension recommendations. Plot size was four 0.5-m rows that were 7.5 m long at Fayetteville and eight 0.4-m bedded rows that were 10 m long at Keiser. A randomized complete block design was used with maize hybrid (Table 1) as the main plots and maize seeding density (5, 9, 12, 16, or 20 seed m–2) as the subplots. Year, location, maize hybrids planted, maturity, harvest dates, and average week-of-year prices at harvest are shown in Table 1.
Timing of irrigation was determined using the University of Arkansas' Irrigation Scheduling Program, which is available for download (www.aragriculture.org/computer/schedule/default.asp; verified 22 Feb. 2006). This program subtracts daily estimates of evapotranspiration from daily inputs of irrigation or rainfall (Cahoon et al., 1990). Irrigation is recommended once the cumulative amount of evapotranspiration exceeds the cumulative water input (rainfall plus irrigation) by an amount determined by soil characteristics. At Fayetteville, a sprinkler irrigation system was used to apply water when the soil-moisture deficit reached a threshold of 30 mm. At Keiser, furrow irrigation was used to irrigate when the soil moisture deficit reached a threshold of 50 mm. Further details on irrigation methods are reported by Edwards et al. (2005).
It was assumed that between emergence and black-layer formation that all hybrids had similar irrigation requirements and responses. Although plant population may have had slight effects on irrigation early in the growing season (Howell et al., 1998), these differences were small and occurred during a portion of the season when rainfall was more plentiful (Purcell et al., 2003). Therefore, it was assumed that differences in irrigation amounts among hybrids and maturity classes were driven by the duration of their growing cycle.
Yield response to PPD (plants m–2) was hypothesized to change by hybrid maturity and production year but not hybrid variety (i.e., hybrids 39R34 and 39W54 would respond to PPD changes the same in 2002 at Fayetteville, but the same hybrid 39W54 would respond to PPD changes differently in 2001 compared with 2002). Specific production practices and more detailed location information can be found in Edwards et al. (2005).
Model Estimation
A separate yield response function to PPD was estimated for each location, year, and maturity category. Hybrids with maturity differences less than or equal to 2 d for a specific location and year were modeled as one maturity category to have sufficient degrees of freedom for estimation. For example, at Fayetteville, 2002 data for hybrids of 79 and 81 d to maturity were used to estimate yield response to PPD for the 79- to 81-d maturity category. In total, 15 yield response equations were estimated using the Mitscherlich–Spillman functional form (Edwards et al., 2005) with the software package EViews (Hall et al., 1995):
 | [1] |
where Y represents predicted yield in g m–2, 
is the yield potential, and ß measures the sensitivity of yield to PPD. As shown by the arrow in Fig. 1
, the larger the ß coefficient, ceteris paribus, the greater the yield response to PPD and therefore the lower the PPD required to attain a particular yield (in Fig. 1, approximately four fewer plants m–2 are required to achieve a yield of 600 g m–2).
Economic Analysis
Profit-Maximizing PPD
Economic theory dictates that an input should be applied up to the level where the marginal cost of adding one more unit of input equals the additional value derived from its use (Beattie and Taylor, 1985). In this case, the input was seed (or indirectly PPD), and the benefit of the seed was expressed through the value of yield produced. Profit-maximizing PPD, holding all other crop-growing inputs constant, was thus calculated by solving for the PPD where the cost of having one more established seedling was equal to the value of the extra yield this seedling produced for each maturity, year, and location combination.
With the following profit function,
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where P is the price of maize in $ g–1, using the 1992–2003 long-run average price of $0.0933 kg–1, which is the net of hauling and transportation charges of $0.0055 kg–1 (Windham and Marshall, 2004), and w is the cost of one extra established plant per square meter ($0.00175), which was derived by adjusting the cost of an extra kernel of seed, $0.00131, by its survival rate (75% in this study). Setting the first derivative of profit with respect to PPD equal to zero yields the following economically optimal yield (Y*) and plant population density (PPD*):
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and
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This is also shown in Fig. 1 by points A and B where the slope of the response functions is equal to w/P (as indicated by the lines tangent to the yield response functions) for the two hybrids with different PPD yield response and similar yield potential. Hybrids with higher ß values, ceteris paribus, thus require lower PPD to achieve higher profit-maximizing yields.
Once yield potential and other costs that are assumed to be independent of changes in PPD, such as irrigation in this study, change, the profit potential will not necessarily correlate with yield potential () and/or yield response (ß), however. Incorporating these other costs, as well as results of Eq. [3] and [4] into Eq. [2], resulted in the following profit-maximizing partial returns (PR*):
 | [5] |
where I is the direct cost of irrigation ($3 ha-cm–1) that varied by maturity, year, and location and other variables were as defined above. Since production costs, other than irrigation and seed, were assumed to be equal across hybrids, PR* were used to determine the most profitable maturity category of maize hybrid for each location–year combination.
Sensitivity Analysis
This study used a long-run average price to determine PPD* and resulting PR for each maturity, year, and location situation. To test for the role of seasonal harvest price variation, PPD* was also calculated using the 1992–2003 average week-of-year harvest cash prices for the appropriate harvest week as reported in Table 1. This was done since Fig. 2
revealed downward pressure on price during the harvest season (see shaded region of Fig. 2) and because output price (P) affects both PPD* and PR* (see Eq. [4] and [5]).
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Fig. 2. Weekly seasonal index for Memphis, TN, cash yellow no. 2 maize prices, 1992–2003. The shaded area represents the harvest window across experiment years and maize hybrid maturity. The seasonal index is calculated as the average of the ratio of weekly to 53-wk centered moving average prices from 1992 to 2003. Prices were not adjusted for inflation.
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By the same token, the effect of changing irrigation costs on maturity choice was analyzed by determining the range of irrigation costs for which a particular maturity choice would remain optimal compared with the other maturity choices in a particular year and location. The cost of irrigation analyzed ranged from free to charges that would drive PR to zero for the optimal alternative for that location–year combination.
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RESULTS
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Model Estimation
The coefficient estimates, standard errors, p values, and coefficients of determination for each regression are shown in Table 2. The and ß coefficient estimates were statistically significantly different from zero for all maturity categories for each of the location and year combinations. In general, the higher the maturity of the hybrid, the higher its ß coefficient and therefore the higher the potential for lower PPD requirements to achieve required yields. Further, yield potential () was comparable across the range of maturity hybrids examined in this study. Generally, coefficients of determination were greatest for earlier-maturing classes and declined for later-maturing classes.
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Table 2. Regression statistics for yield (Y, g m–2) response to plant population density (PPD, plants m–2) [Y = (1 – e–ß·PPD)] estimated for each maturity category, location, and year.
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Economic Analysis
Profit-Maximizing PPD
Optimal PPD, associated estimated yield (Y*), and irrigation amount applied resulted in the PR shown in Table 3. Irrigation applied within a given year increased as days to maturity increased, and PPD* tended to be higher for the shorter-season hybrids, as expected. Highest-yielding maturity categories, however, did not necessarily exhibit the highest PR. For example, at Fayetteville in 2003, the 84-d hybrid attained the highest yield potential () of 1225 g m–2; however the 88- and 109-d hybrids had higher PR of $819 and $828 ha–1, respectively. This was primarily due to the variation in the cost of seed planted, which had a greater effect on PR than the increased cost of irrigation of the longer-season hybrids. At Keiser, the profit-maximizing maturity category also had the highest yield potential in both years. No pattern between highest PR and maturity category was detected across the 2 yr given the limited range of maturity studied. In 2002, the longest-season hybrids (85 and 87) displayed the highest PR of $639 ha–1, while in 2003, the 72-d hybrid had the highest PR at $675 ha–1.
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Table 3. Profit-maximizing plant population density (PPD*), associated yield (Y*), irrigation, partial returns, and irrigation cost sensitivity analysis by location, year, and maturity category (in order of decreasing partial returns).
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Sensitivity Analysis
The effect of substituting P (which was independent of harvest time) with the long-run average, week-of-year harvest prices (Table 1) was minimal and did not affect results substantially. The primary reason was that the selling price did not affect PR, as harvest week-of-year did not change across maturity for either year at Keiser or for Fayetteville in 2002 (Table 1). At Fayetteville in 2003, using long-run average, week-of-year harvest prices would have made the 88-d hybrid the optimal choice based on PR. Again, instead of using P, the minimum price from the 1992–2003 time period ($0.0561 kg–1 net of hauling charges) did not change the results while using the maximum price ($0.2025 kg–1 net of hauling charges) would cause the optimal choice at Fayetteville, AR to be the 86-d hybrid in 2001 and the 84-d hybrid in 2003.
The sensitivity analysis results on irrigation cost are shown in the last two columns of Table 3. For a particular location and year, irrigation cost was changed from the baseline of $3 ha-cm–1 to identify how sensitive the hybrid choice was to changing irrigation cost. The low and high indicate the range of irrigation costs for which a particular maturity choice exhibits the highest PR. For most location-year combinations, irrigation costs would need to change substantially before hybrid choice would be affected.
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CONCLUSIONS
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Producers often choose hybrids on the basis of maximum yield or yield potential. When using the long-run average maize price to assess the profitability of short-season maize hybrids, including seeding rate and irrigation cost, profitability was not necessarily associated with yield potential for conditions presented in this study. At Fayetteville, for example, the profit-maximizing maturity hybrid did not exhibit maximum yield in any of the 3 yr. Seed cost, and to a lesser extent irrigation, varied enough to affect hybrid choice. At Keiser, by contrast, profit-maximizing maturity hybrid choice coincided with the highest yield, but the range of maturity tested was smaller, and only 2 yr of data were available. Irrigation requirements and seasonal harvest price played minor roles in the selection of maize hybrid on the basis of profitability. Finally, further research at more locations and over a wider range of maturity would be desirable to formulate more definitive conclusions regarding the trade-offs with respect to yield potential, irrigation requirements, and profit-maximizing PPD and associated seed cost.
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ACKNOWLEDGMENTS
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The authors thank the Arkansas Corn and Grain Sorghum Board for financial support. The authors also extend appreciation to Pioneer, Garst, and Syngenta seed companies for providing seed and for their help in identifying appropriate hybrids for evaluation.
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NOTES
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1 The hybrids were divided into the two categories at the discretion of the seed companies with no specific maturity information provided. 
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REFERENCES
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- Beattie, B.R., and C.R. Taylor. 1985. The economics of production. John Wiley & Sons, New York.
- Cahoon, J., J. Ferguson, J. Edwards, and P. Tacker. 1990. A microcomputer-based irrigation scheduler for the humid mid-south region. Appl. Eng. Agric. 6:289–295.
- Cartwright, R.D., L. Espinoza, D. Gardisser, G. Huitink, T.L. Kirkpatrick, P. McLeod, J. Ross, B. Scott, K. Smith, G. Studebaker, G. Tacker, D.O. TeBeest, E. Vories, and T. Windham. 2003. Arkansas corn production handbook. MPV 437. Coop. Ext. Serv., Univ. of Arkansas, Little Rock.
- Dombek, D.G., D.K. Ahrent, R.D. Bond, and I.L. Eldridge. 2002. Arkansas corn and grain sorghum performance tests. Res. Ser. 489. Arkansas Agric. Exp. Stn., Fayetteville.
- Dombek, D.G., D.K. Ahrent, R.D. Bond, and I.L. Eldridge. 2003. Arkansas corn and grain sorghum performance tests. Res. Ser. 510. Arkansas Agric. Exp. Stn., Fayetteville.
- Edwards, J.T., L.C. Purcell, and E.D. Vories. 2005. Light interception and yield potential of short-season maize (Zea mays L.) hybrids in a narrow-row production system in the Midsouth. Agron. J. 97:225–234.[Abstract/Free Full Text]
- Hall, R.E., D.M. Lilien, G. Sueyoshi, R. Engle, J. Johnston, and S. Ellsworth. 1995. EViews: User guide. Quantitative Micro Software, Irvine, CA.
- Howell, T.A., J.A. Tolk, A.D. Schneider, and S.R. Evett. 1998. Evapotranspiration, yield, and water use efficiency of corn hybrids differing in maturity. Agron. J. 90:3–9.[Abstract/Free Full Text]
- Purcell, L.C., T.R. Sinclair, and R.W. McNew. 2003. Drought avoidance assessment for summer annual crops using long-term weather data. Agron. J. 95:1566–1576.[Abstract/Free Full Text]
- [USDA-AMS] USDA Agricultural Marketing Service. 2004. Livestock and market news. USDA-AMS, Washington, DC.
- [USDA-NASS] USDA National Agricultural Statistics Service. 2004. Corn: Acreage, yield, production, price and value [Online]. Available at www.nass.usda.gov/ar/histcorn.PDF (verified 22 Feb. 2006). USDA-NASS, Washington, DC.
- Windham, T.E., and J. Marshall. 2004. Corn, center pivot irrigated, loamy soils. Enterprise Budget AG-795–12–03. Univ. of Arkansas Coop. Ext. Serv., Little Rock.
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TOP
ABSTRACT
INTRODUCTION
