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Published online 11 April 2006
Published in Agron J 98:529-535 (2006)
DOI: 10.2134/agronj2005.0137
© 2006 American Society of Agronomy
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Production Papers

Corn Response to Nitrogen Rate, Row Spacing, and Plant Density in Eastern Nebraska

Charles A. Shapiroa and Charles S. Wortmannb,*

a Northeast Research and Extension Center–Haskell Agricultural Lab., Univ. of Nebraska, 57905 866 Rd., Concord, NE 68728
b 279 Plant Science, Univ. of Nebraska, Lincoln, NE 68583-0915

* Corresponding author (cwortmann2{at}unl.edu)

Received for publication May 10, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Efficient use of N by corn (Zea mays L.) is financially and environmentally important, and may be improved with higher plant density and reduced row spacing. Hypotheses were tested that irrigated corn yield in northeast Nebraska is increased by reducing row spacing from 0.76 m and increasing plant density above 61 800 plants ha–1, and that grain yield response to applied N is greater with reduced row spacing and increased plant density. Field experiments were conducted for 3 yr comparing the effects 0.76- vs. 0.51-m row spacing, three plant densities, and four N rates on crop performance. The soil was a silty clay loam (mesic Cumulic Haplustoll). Nitrogen rates ranged from 0 to 252 kg N ha–1. Plant N concentration and biomass and grain yield were not affected by plant density. Decreasing row spacing from 0.76 to 0.51 m resulted in 4% more grain yield. Grain yield response to applied N and N rates for optimum yield were not affected by row spacing. Nitrogen application resulted in mean increases of 22% more biomass production and 24% more grain yield. The N response function was linear in 1996, quadratic in 1997, and quadratic with decreased yields at the high N rate (252 kg N ha–1) in 1998. Grain yield was not affected by increasing plant density above 61 800 plants ha–1 but was greater with narrow row spacing. Yield response to applied N was similar for all planting arrangements. Optimal N rate cannot be better predicted by considering plant density and row spacing.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROGEN fertilizer is a major input for corn production in the Midwest. Ideal N management optimizes grain yield, farm profit, and N use efficiency while it minimizes the potential for leaching of N beyond the crop rooting zone. Efficiency of use of applied N is variable, with a mean of only 33% of applied N recovered by cereal crops (Raun and Johnson, 1999).

Demand for N increases with biomass yield, which may be enhanced by reduced row spacing and greater plant density (Jordan et al., 1950). The effect of decreasing corn row spacing from a mean of 1.07 to 0.90 m was estimated to result in an overall mean yield increase of 175 kg ha–1 (Cardwell, 1982), while most farmers have reduced corn row spacing to 0.76 m or less. Corn yields may be further increased by reducing row spacing from 0.76 to 0.38 m (Nielsen, 1988; Widdicombe and Thelen, 2002), but there may be little advantage to further reduction (Porter et al., 1997).

Corn grain yield typically exhibits a quadratic response to plant density, with a near-linear increase across a range of low densities, a gradually decreasing rate of yield increase relative to density increase, and finally a yield plateau at some relatively high plant density (Duncan, 1984; Ottman and Welch, 1989; Thomison and Jordan, 1995). Nitrogen demand also increases as plant density increases (Penning de Vries et al., 1993). Higher plant density combined with narrower row spacing results in a more equidistant planting pattern that is expected to delay initiation of intraspecific competition (Duncan, 1984) while early crop growth is increased (Bullock et al., 1988).

Soil NO3–N, soil organic matter content, and historic yields might be considered in estimation of fertilizer N requirements for corn (Oberle and Keeney, 1990; Vanotti and Bundy, 1994a, 1994b), as well as legume credits, manure application, and irrigation water N (Peterson and Varvel, 1989; Hergert et al., 1995). Nitrogen response models might be further improved by considering the effects of row spacing and plant density.

This study was conducted to test three hypotheses for irrigated corn production in eastern Nebraska: (i) increased plant density and reduced row spacing result in increased harvested N and corn grain yield under adequate soil water conditions; (ii) yield increases with applied N are greater with plant densities beyond 61 800 plants ha–1 and row spacing <0.76 m; and (iii) optimum N rate can be more accurately estimated using a model that considers row spacing and plant density.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Characteristics, Experimental Design, and Treatments
The study was conducted at the University of Nebraska Northeast Research and Extension Center–Haskell Agricultural Laboratory near Concord, NE (42°23' N, 96°57' W) during 1996, 1997, and 1998. The soil is classified as an Alcester silty clay loam (mesic Cumulic Haplustoll) with 30 g kg–1 soil organic matter and 6.1 pH in the surface soil. Soil Bray-P1 was 32, 26, and 22 g kg–1 and exchangeable NH4OAc K was 330, 229, and 301 g kg–1 in the 0- to 0.30-m depth for 1996, 1997, and 1998, respectively. In the spring, before N fertilizer application, mean soil NO3–N concentration to a depth of 0 to 0.76 m was 11.4, 2.9, and 7.2 mg kg–1 for the respective years.

The experimental design was a randomized complete block in a split-split-plot treatment arrangement with three replications. The main plot factor was plant density with targeted densities of 61 800 (low), 74 160 (medium), and 86 520 (high) plants ha–1. Intended populations were not achieved in every year; hence the terms low, medium, and high will be used in the discussion. The low planting rate is typical of populations grown under irrigation in the area in which this research was conducted. The main plot size was 12.2 by 24.4 m. Row spacing treatments (0.51 and 0.76m) comprised the split plots, which measured 6.1 by 24.4 m. These treatments aimed for within-row distances between plants of 0.22, 0.18, and 0.15 m and 0.32, 0.26, and 0.23 m for the 0.76- and 0.51-m row spacing at the low, medium, and high plant densities, respectively. The split-split factor was N fertilizer rate (0, 84, 168, and 252 kg N ha–1); N was surface applied (unincorporated) using dry NH4NO3 (34–0–0) with a 3-m-wide Barber spreader (Barber Engineering Co., Spokane, WA) on 11 June 1996, 10 June 1997, and 19 May 1998. Experimental units in the split-split plot measured 3.0 by 12.2 m.

The trial was conducted on a different part of the same field each year, with the whole field planted in corn during the 3 yr. Corn was always the preceding crop. The site was irrigated with 25.4 mm of water using a lateral irrigation system when soil water was <50% of field capacity. There were four, five, and three applications in 1996, 1997, and 1998, respectively. Irrigation water contained 20 mg L–1 NO3–N and 5 kg ha–1 N as NO3 was applied with each irrigation event. Daily weather data were collected at the laboratory (Fig. 1 ).


Figure 1
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  		Fig. 1. Precipitation, and accumulation of growing degree days (GDD) following crop emergence, for the growing seasons of 1996, 1997, and 1998.

 
Crop Management
The land was prepared for planting by disk followed by cultivator tillage. Corn hybrid Pioneer 3394 (matures at 1290 growing degree days) was planted on 29 May 1996, 8 May 1997, and 5 May 1998. The 0.51-m rows were planted with a 12-row John Deere 7300 custom-built planter and the 0.76-m rows were planted with an eight-row John Deere 7100 planter (Deere, Inc., Moline, IL). Weeds were controlled using preplant herbicides.

Field and Plant Measurements
Yield data were collected on 4 Nov. 1996, 22 Oct. 1997, and 24 Sept. 1998. Plants were cut at the surface from the central 6.1 m of the two middle rows in the wide-row plots and the three middle rows in the narrow-row plots. Ears were separated, shelled, weighed, and the moisture content was measured. Grain yields were adjusted to 155 g kg–1 water content. Biomass yield was calculated from stover and grain weights, which were adjusted to oven-dry weights after subsamples of grain and stover were dried at 60°C. Grain and stover subsamples were analyzed for N content using the Dumas dry oxidation procedure (Bremner, 1996). Total plant N content was determined by combining stover N and grain N estimated on the basis of 0 g kg–1 water. Harvest index and N harvest index were calculated by dividing grain dry weight by biomass dry weight, and harvested grain N uptake by total plant N uptake, respectively. Agronomic efficiency of applied N (AEN, kg grain yield increase kg–1 N applied) and apparent recovery efficiency of applied N (REN, kg total N uptake kg–1 N applied) were determined as: AEN = (GY+N – GY0N)/N and REN = (UN+N – UN0N)/N, where GY is the grain yield (kg ha–1), UN is the aboveground plant N accumulated (kg ha–1), N is the amount of applied N (kg ha–1), and +N and 0N refer to treatments with and without N applied, respectively.

The number of seeds planted at each row spacing to achieve each target plant density differed due to planter differences (Table 1) but the data were analyzed and results interpreted under the assumption that row spacing and plant density treatments were independent of each other (Teasdale, 1998).


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  		Table 1. Seed drop to achieve intended plant densities for two row spacings.

 
Data Analysis
Crop performance data were analyzed using the SAS PROC GLM procedure to develop the ANOVA for a split-split-plot design. The PROC MIXED procedure was used to make tests of simple effects (Little et al., 1996). Row spacing was treated as a qualitative variable, and plant density and N rate as quantitative variables. For PROC MIXED, plant density, row spacing, and N rates were treated as fixed effects, and year and replication were treated as random effects.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Plant Nitrogen
Grain N concentration and uptake were affected by the N x year interaction (Table 2). There was a positive linear relationship between N rate and grain N concentration in 1997 and 1998, but with no effect of N rate on grain N concentration in 1996 (Fig. 2a ).


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  		Table 2. Analysis of variance of the effect of plant density (PD), row spacing (RS), and N fertilizer rate (N) across 3 yr (YR).

 

Figure 2
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  		Fig. 2. The effect of N rate during three growing seasons on (a) N concentration in grain and stover and (b) biomass yield. All linear functions are significant except for grain N concentration in 1996 and biomass yield in 1998. All quadratic functions were significant except for grain N concentration in 1996 and biomass yield in 1998.

 
Grain N uptake increased with N rate in 1997 and 1998 with significant linear and quadratic responses, but uptake reached a plateau at the 84 kg ha–1 N rate in 1998 while uptake increased with higher N rates in 1996 and 1997 (Fig. 3 ). Grain N uptake was also affected by the row spacing x year interaction, as N uptake was less with 0.76- than with 0.51-m row spacing in 1996 and 1997, but unaffected by row spacing in 1998 (Table 3).


Figure 3
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  		Fig. 3. The effect of N rate on N uptake and N harvest index (HI) during three growing seasons. All linear and quadratic functions were significant except for N HI in 1997.

 

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  		Table 3. Row spacing effect on corn, averaged across three plant densities and four N rates.{dagger}

 
At the highest plant density, the simple effect for the row spacing x N rate x year interaction was significant for grain N content for the 168 and 252 kg ha–1 N rates (Table 4). Grain N uptake was further increased at the highest plant density by increasing the N rate to 252 kg ha–1 with 0.51-m row spacing but decreased with 0.76-m spacing in 1996 and 1997, while grain N uptake increased at all N rates for both row spacings in 1998 (Table 4).


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  		Table 4. Row spacing x N rate x year interactions induced by high N rate at the 86 520 plants ha–1 density.{dagger}

 
Grain N concentration and uptake were not affected by plant density (Table 2). Stover N was not affected by interaction effects or by the main effects of plant density and row spacing. Stover N concentration increased with N application with linear and quadratic responses (Fig. 3).

Harvest Index and Nitrogen Recovery
Nitrogen rate and row spacing did not affect harvest index (HI; Table 2). There was a plant density effect on HI in 1997. Harvest index at the low population was 55% and decreased to 50% at the medium and 51% at the high populations; however, populations were reduced in 1997 and the low population was below the target population by 7233 plants ha–1. In other years, when the low population was closer to the target population, HI was similar at all populations (Table 5).


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  		Table 5. Plant density effects on corn, averaged across two row spacings and four N rates.{dagger}

 
Nitrogen HI decreased as linear and quadratic functions of N rate in 1996 and 1998, but was not affected by N rate in 1997 (Fig. 3). Harvested grain N increased less with increased N rates relative to accumulated stover N, resulting in the decreased N HI values at higher N rates. Row spacing had no effect on N HI (Table 3). The N HI was highest at the lowest plant density in 1997, but was not affected by plant density in the other years (Table 4).

The apparent recovery efficiency of applied N in the aboveground crop ranged from 0.23 to 0.58, 0.21 to 0.37, and 0.16 and 0.30 kg accumulated N kg–1 applied N at the first, second, and third increment of applied N, respectively. Agronomic efficiency of applied N ranged from 5.8 to 22.1, 4.8 to 14.6, and 3.8 to 9.6 kg grain yield increase kg–1 N applied at the first, second, and third increment of applied N, respectively. Differences in N use efficiency were related to grain and biomass yield response to applied N. Apparent recovery efficiency and agronomic efficiency, as well as yield increase, were greater in 1997 than in the other years. The values for these efficiency traits were generally less and declined more with increased N rate than observed by Binder et al. (2002) elsewhere in eastern Nebraska; this increased efficiency was due to high yield response to applied N at three of four trials.

Biomass and Grain Yield
Corn biomass increased with applied N in a quadratic manner in 1996 and 1997, but the linear and quadratic functions of N rate were not significant in 1998 (Fig. 2b). In all years, the main effect of increasing N rate from 168 to 252 kg ha–1 did not result in an increase in biomass. Corn biomass production was less in 1997 with the 0.76- than the 0.51-m row spacing (Tables 2 and 3), but unaffected in 1996 and 1998. Biomass yield was not affected by plant density, except in 1998 when biomass was decreased at low plant density (Table 5).

Mean grain yields were 8.67, 7.51, and 10.40 Mg ha–1 in 1996, 1997, and 1998, respectively. The year x N rate interaction effect for grain yield was due to differing responses to applied N for rates above 84 kg N ha–1 (Table 2). In 1998, grain yield was increased with 84 kg N ha–1 applied but did not respond to additional N, with the effect that neither the linear nor the quadratic functions of N rate were significant (Fig. 4 ). The response of grain yield to increased N application rate was linear and quadratic in 1996 and 1997, agreeing with the results of others (Blackmer and Sanchez, 1988; Jokela, 1992; Liang et al., 1996). The reason for the N rate x year interaction effect is not clear but possibly more N was released from mineralization of soil organic matter in 1998 due to more precipitation in March and April than in other years.


Figure 4
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  		Fig. 4. Corn grain yield (Y) as a linear function of N fertilizer rate in 1996 and a quadratic function of N fertilizer rate in 1997 and 1998.

 
Interaction effects of row spacing with plant density and N rate were not significant for grain yield. The simple effect (Little et al., 1996), however, for the row spacing x N rate interaction, at high plant density, was significant. At the high plant density, grain yield was less at the 252 kg N ha–1 rate than with the 168 kg N ha–1 rate for the 0.76-m row spacing in 1996 and 1997 (Table 4) while yield was increased or unchanged for the 0.51-m row spacing.

Grain yield was higher with 0.51- vs. 0.76-m row spacing in 1996 and 1997, but not in 1998. The increased yield with 0.51-m spacing is supported by the findings of others who have reported yield increases of up to 10% with reduced row spacing (Hodges and Evans, 1990; Polito and Voss, 1991; Porter et al., 1997; Ulger et al., 1997; Widdicombe and Thelen, 2002). High yields can be achieved with 0.76-m row spacing, however, as was the case in 1998 when grain yields were higher than in previous years and the main effect of row spacing was not significant. The advantage of 0.51- over 0.76-m row spacing may therefore be related to management and specific growing conditions. A notable difference in the 1998 season, compared with the other seasons, is the greater rate of growing degree day accumulation following planting in May and subsequent earlier physiological maturity, but this does not offer an explanation for the lack of response to reduced row spacing. The absence of a row-spacing effect on yield in 1998 may be due to the great plant growth, resulting in canopy interception of a very large proportion of the incident incoming photosynthetically active radiation at both row spacings (Ottman and Welch, 1989). Andrade et al. (2002) found that yield response to decreased row spacing was negatively correlated to radiation interception at pollination time with the wider spacing. Radiation interception was not measured in this study, but it is likely that the proportion of incoming radiation intercepted was very high with both row spacings in 1998 due to the great amount of plant growth.

The main effect of plant density was not significant for grain yield (Tables 2 and 5). These results differ with findings in Maryland where grain yield increased as plant density was increased from 56 000 to 128 000 plants ha–1 (Teasdale, 1998), but crop competition with weeds was a factor in that study. Porter et al. (1997) reported maximum corn grain yield at 82 000 to 89 000 plants ha–1. Cox and Cherney (2001) reported increased corn silage yield by changing plant density from 80000 to 116000 plants ha–1. There was no plant density x row spacing interaction, although theory based on plant crowding alone suggests that such an interaction should occur with a greater advantage with narrower row spacing at high plant density than at lower plant density (Duncan, 1984). Porter et al. (1997) also found that the row spacing x plant density interaction did not significantly affect yield.

Grain yield response to row spacing and plant density might have been different for other hybrids. Porter et al. (1997), however, found in a study with six adapted, high-yielding hybrids, that corn hybrids were similarly affected by plant density and row spacing.

Grain Yield Response Models
In 1996, a linear model, with row spacing as a factor (Bullock and Bullock, 1994), accounted for >90% of the grain yield response to applied N (Fig. 4a). The quadratic model was also significant, but the linear model is presented due to the small quadratic coefficient. The model predicted more grain production at 252 kg N ha–1 for both row spacings (9.40 and 8.84 Mg ha–1 for 0.51- and 0.76-m row spacing, respectively) than for lower N levels. It also predicted 6.9% more grain yield across N rates for 0.51- vs. 0.76-m spacing, with a yield gain of 2.2 kg ha–1 mm–1 for reduction in row spacing. The linear model did not allow prediction of potential maximum yield.

In 1997, yield data were fitted to a quadratic yield model that predicted optimum grain yields of 8.23 and 8.60 Mg ha–1 at 240 and 200 kg N ha–1 for the 0.76- and 0.51-m row spacings, respectively (Fig. 4b). The row-spacing effect on grain yield was similar to that in the 1996 model. The 1997 model predicted an average yield advantage of 7.8% with 0.51- vs. 0.76-m row spacing across all N rates, with a yield gain of 2.2 kg ha–1 mm–1 for reduction in row spacing.

In 1998, the yield response to N rate was quadratic and the maximum predicted yield was 11.12 Mg ha–1 at 150 kg ha–1 of N (Fig. 4c). Row spacing effects did not account for significant variation in yield response to N rates.

Plant density did not affect response to applied N, but N use efficiency was increased with narrower row spacing when plant density was at the highest level. The results, therefore, suggest that it is not necessary to consider plant density (>62 000 plants ha–1) in determination of N rate. Optimal N rate was affected by row spacing in 1 yr only and row spacing (<0.76 m) is not a major determinate of N fertilizer requirements. Treatment x corn hybrid interactions were not addressed in this study. Corn hybrid x N rate interactions can be important (O'Niell et al., 2004), but the effects of row spacing and plant density on this two-way interaction are not known. The lack of hybrid interactions with row spacing (Ottman and Welch, 1989) and plant density (Porter et al., 1997) suggests that the three-way interactions may not be very important and that our information on N-rate interactions with row spacing and plant density are applicable to other adapted, high-yielding hybrids.

The optimal N rate for grain year differed across years. The N rates to achieve 98 and 99% of maximum yield, averaged across years and row spacing, were 123 and 143 kg ha–1. Nitrogen uptake with no fertilizer N applied, however, was 124, 86, and 165 kg ha–1 with 132, 53, and 101 kg ha–1 of N available to the crop in 1996, 1997, and 1998, respectively, from soil and irrigation water NO3–N.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This research tested three hypotheses. The results are: (i) reduced row spacing, but not increased plant density, resulted in more crop N uptake and more grain yield; (ii) yield response to applied N was not greater with increased plant density and reduced row spacing; and (iii) optimum N rate cannot be better predicted by considering plant density and row spacing. Maximum grain yield and yield response to applied N can be achieved with 62 000 plants ha–1 when weeds are well controlled. High grain yield can be achieved with 0.76-m row spacing but yield was 6 to 8% higher with 0.51- than with 0.76-m row spacing in 2 of 3 yr. Grain yield was regularly increased with application of 84 kg N ha–1 but response to higher rates was inconsistent. Grain yield response to applied N and the optimum N application rates were generally similar for 0.51- and 0.76-m row spacing and for the range of plant densities tested, but response to applied N was greater with 0.51-m rows when plant density was high. Row spacing and plant density need not be considered in a model for estimation of optimum N rate.


   ACKNOWLEDGMENTS
 
We are grateful to Dr. Robert Caldwell, Dr. David Holshouser, and Dr. John Lindquist for their insights and advice at various stages of this research, and to Mark Langrud, whose Master's thesis includes part of this research. Lisa Lunz and Ray Brentlinger provided valuable assistance to implement the fieldwork. We acknowledge the contribution of Logan Valley Implement (now Northeast Implement), who provided the narrow-row planter and combine used in this research.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution of the Univ. of Nebraska Agric. Res. Div., Lincoln, NE, as Journal Series no. 14529.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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