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Published online 5 September 2006
Published in Agron J 98:1367-1374 (2006)
DOI: 10.2134/agronj2006.0065
© 2006 American Society of Agronomy
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Production Papers

Irrigated Corn and Soybean Response to Nitrogen under No-Till in Northern Colorado

Ardell D. Halvorson* and Curtis A. Reule

USDA-ARS, 2150 Centre Ave, Bldg. D, Suite 100, Fort Collins, CO 80526

* Corresponding author (ardell.halvorson{at}ars.usda.gov)

Received for publication March 2, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Irrigated, no-till (NT) production systems can potentially reduce soil erosion, fossil fuel consumption, and greenhouse gas emissions compared with conventional till (CT) systems. Including a legume in the rotation may also reduce N fertilizer requirements. Nitrogen fertilization (6 N rates) effects on irrigated, corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields in a corn–soybean rotation were evaluated for 5 yr on a clay loam soil to determine the viability of an irrigated NT system and N needs for optimum crop yield. Corn grain yields were significantly increased by N fertilization each of 3 yr in the rotation, but soybean grain yields (2 yr) did not respond to N fertilization, averaging 2.79 Mg ha–1. Three year average corn grain yields were near maximum with an available N (AN) (soil + fertilizer + irrigation water N) level of 257 kg N ha–1. Nitrogen use efficiency (NUE) by corn and soybean, based on grain N removal, decreased with increasing AN level and ranged from 155 to 46 and 88 to 18 kg grain kg–1 AN for the low and high N treatments for corn and soybean, respectively. Estimated total N required to produce one Mg grain at maximum yield averaged 20 kg N for corn and 54 kg N for soybean. Corn residue increased with increasing N rate, but soybean residue was constant across N rates. Excellent irrigated, NT corn yields were obtained in this corn–soybean rotation for northern Colorado, but soybean yields were only marginally acceptable. Short soybean plant height (30–40 cm) and shattering made combine harvest difficult resulting in significant grain loss. Improved soybean cultivars are needed for this area to make a corn–soybean rotation a viable production system.

Abbreviations: AN, available nitrogen (soil + fertilizer + irrigation water nitrogen) • CT, conventional-till • CT-CC, conventional-till continuous corn • NT, no-till • NT-CB, no-till corn–bean • NT-CC, no-till continuous corn • NUE, nitrogen use efficiency • SOC, soil organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
IRRIGATED farmers often use intensive tillage practices, particularly the moldboard plow, to manage the large quantity of crop residue returned to the soil surface when preparing a seedbed for the next crop. As a result of the numerous tillage operations, most of the crop residue is incorporated into the soil, subjecting the soil to wind and water erosion. Conversion from a CT irrigated crop production system to a conservation tillage system, like NT, would reduce soil erosion potential and soil organic matter loss (Lal, 2004; Olson et al., 2004). Lal (2004) points out the need to use conservation tillage systems on croplands to enhance soil organic carbon (SOC) storage and reduce total greenhouse gas emissions from agricultural lands.

Limited information is available on use of NT systems on irrigated lands in the semiarid central Great Plains (Cahoon et al., 1999; Halvorson et al., 2006; Sims et al., 1998). Data available from the more humid Western-Corn Belt and eastern Nebraska suggest that NT systems may produce lower continuous corn grain yields than CT systems (Wilhelm and Wortmann, 2004; Vetsch and Randall, 2002, 2004; Sims et al., 1998). Halvorson et al. (2006) reported 16% lower continuous corn yields with NT compared to CT at maximum yield in northern Colorado. Cooler soil temperatures at planting time in NT systems delay early crop development and final yield (Halvorson et al., 2006; Sims et al., 1998; Swan et al., 1987). However, Olson et al. (2004) showed that NT corn yields equaled or exceeded CT corn yields after the initial year of NT in a corn–soybean rotation. Including a low-residue crop like soybean in rotation with corn could reduce surface residue levels at corn planting, resulting in higher soil temperatures in early spring than with a NT continuous corn system (Swan et al., 1987). Including soybeans in rotation with corn has resulted in greater corn yields than when corn is grown annually in a monoculture in both CT and NT systems (Varvel and Wilhelm, 2003; Pedersen and Lauer, 2002). Varvel and Peterson (1990) and Varvel and Wilhelm (2003) demonstrated the value of soybean in contributing N to the succeeding corn crop in a soybean-corn rotation, reducing N fertilizer requirements for optimizing corn yields.

Farmers in the South Platte River Valley in northern Colorado produce alfalfa (Medicago sativa L.), corn, winter wheat (Triticum aestivum L.), barley (Hordeum distichon L.), and dry bean (Phaseolus vulgaris L.) using predominantly intensive tillage (including the moldboard plow) to prepare a seedbed. Soil erosion due to wind and water is a serious problem in this irrigated area. Converting to an irrigated, NT production system could reduce soil erosion, reduce fossil fuel consumption through less tillage and fewer field operations, and potentially reduce greenhouse gas emissions through increased SOC sequestration (Liu et al., 2005; Mosier et al., 2006).

Halvorson et al. (2006) reported reduced corn yields with NT compared to CT within a continuous corn production system, due to cooler soil temperatures and slower plant development in the NT system. We hypothesized that NT corn yields in a corn–bean rotation under irrigation in the central Great Plains would equal or exceed continuous corn yields under CT and NT production systems reported by Halvorson et al. (2006) due to less residue accumulation on the soil surface with the corn–bean rotation, resulting in warmer spring soil temperatures, and the benefits of crop rotation. Our objectives were to determine N fertilizer needs for optimizing corn and soybean grain yields and the influence of N fertilization on crop residue production in an irrigated, NT corn–soybean production system; and to compare corn yields from the corn–soybean rotation with CT and NT continuous corn yields reported by Halvorson et al. (2006).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
This study was conducted on a Fort Collins clay loam soil (fine-loamy, mixed, mesic Aridic Haplustalfs) with a 1 to 2% slope at the Agricultural Research Development and Education Center (ARDEC) (40°39'6'' N, 104° 59'57'' W; 1555 m above sea level) near Fort Collins, CO. The study was initiated in 1999 on a field that had previously been continuously cropped to corn for 6 yr using a CT production system. Dry bean was grown in the plot area in 1999, which received one tillage treatment with a mulch-treader to level out the field ridges resulting from cultivation of the corn in 1998. The dry beans yielded 2.39 Mg ha–1, and had an average residue level of 2063 kg ha–1 with a C/N ratio of 38, and 24 kg ha–1 of residue N. Because dry beans need to be knifed and windrowed to facilitate combine harvest, we decided to experiment with soybean in 2001 and 2003 because they were more adaptable to a NT production system, although soybean is not normally grown in this production area.

Using a NT system, corn and soybean were directly planted into the previous year's crop residue each spring without any other field operations for seedbed preparation, followed by application of herbicides for weed control, and harvest. Triple super phosphate (0–46–0) was applied (56 kg P ha–1) to the entire plot area before dry bean planting in 1999 and 28 kg P ha–1 before corn in 2004. The soil had a pH of 7.8 (soil/0.01 M CaCl2 solution ratio 1:2), organic matter content of 20 g kg–1, electrical conductivity of 0.9 dS m–1 (soil/water ratio 1:1), sodium bicarbonate extractable P content of 10 mg kg–1, and a clay and silt content of 330 and 260 g kg–1, respectively, in the 0- to 15-cm depth.

Six N rates (0, 34, 67, 101, 134, and 202 kg N ha–1 referred to as N1, N2, N3, N4, N5, N6, respectively) were used in 2000 and 2002 for corn production. The N6 rate of N application was increased to 224 kg N ha–1 in 2004 to assure that adequate N was available to maximize grain yields, based on NT corn yields reported by Halvorson et al. (2006). In 2001, the N rates for soybean were 0, 17, 34, 50, 67, and 84 kg N ha–1. Due to no response to N in 2001, the N rates for soybean were reduced to 0, 11, 22, 34, 45, and 56 kg N ha–1 in 2003 for the N1, N2, N3, N4, N5, and N6 treatments, respectively. The reduction in N rate for soybean was implemented to reduce the buildup of residual N in the soil. The above rates of N were applied to the same plots (N treatments) each year depending on crop grown. The N source was urea ammonium nitrate (UAN; 32–0–0) which was applied with a liquid fertilizer applicator which banded the N about 5 cm below the soil surface in bands spaced 33 cm apart (parallel to the corn or soybean row, but at varying distance from the row) the day before corn or soybean planting. Liquid starter P and K fertilizer was applied directly with the seed at planting, except in 2001 when no starter fertilizer was applied. The experimental design was a randomized complete block with three replications with each N treatment 10.7 by 15.2 m in size.

Corn hybrid, Pioneer Brand 38B22 Bt LL, was planted with a 76 cm row spacing on 27 Apr. 2000; Pioneer Brand 37H26 LL on 24 Apr. 2002; and Pioneer Brand 38P04 LL on 27 Apr. 2004 in the plot area. Corn hybrid varied between years due to availability of seed and the desire to optimize yield potential as new hybrids became available. Planting rates were approximately 84000 seeds ha–1 in 2000 and 2004 and 96000 seeds ha–1 in 2002. All corn hybrids had about a 94-d relative maturity. Corn plant populations were estimated each year by counting the number of plants in two rows 7.6 m long that were used for grain harvest. Soybean hybrid, Pioneer Brand 90B72, was planted in the plot area on 11 May 2001 using a John Deere model 1560 NT-grain drill with 19-cm row spacing, and Pioneer Brand 90B51/RR on 15 May 2003 using a John Deere MaxiMerge model 1780 vacuum planter with 38-cm row spacing. Soybean cultivar was changed in an attempt to optimize yield potential and take advantage of glyphosate tolerant soybeans for improved weed control in the plot area. Soybean planting rates were approximately 555000 seeds ha–1 in 2001 and 2003, but established plant populations were not determined. The vacuum planter was equipped with residue managers or trash whippers to facilitate planting in the NT system. Herbicides were applied for weed control and the plots were relatively weed free during the study period.

The corn and soybean crops were sprinkler irrigated with a linear move system as needed [determined weekly by the feel method (Klocke and Fischbach, 1998)] during the growing season each year. Irrigation amounts each month are shown in Table 1. The irrigation water contained an average of 2.8 mg NO3–N L–1 in 2002, 3.6 mg NO3–N L–1 in 2003, and 2.1 mg NO3–N L–1 in 2004. In 2000 and 2001, N level in the water was not monitored, but was assumed to be similar to that in 2002–2004. The total N contribution from the irrigation water to the plot area was estimated to be 10 kg N ha–1 in 2000, 12 kg N ha–1 in 2001, 14 kg N ha–1 in 2002, 15 kg N ha–1 in 2003, and 8 kg N ha–1 in 2004 during irrigation with no observed runoff from the plot area.


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  		Table 1. Monthly precipitation received and irrigation water applied to plot area each year.

 
Annual precipitation at ARDEC for the study period is shown in Table 1. Annual precipitation for the site for the May through September growing season varied from a low of 122 mm in 2001 to a high of 181 mm in 2004. All growing seasons had below average precipitation. Due to severe drought conditions (Table 1) from 2000 through 2003, irrigation water availability was questionable in 2003 and 2004; therefore, corn populations in 2004 were reduced in case irrigation water became unavailable late in the growing season.

Crop residue cover on the soil surface was estimated each spring just before planting using the line-transect method. Soil NO3–N levels in the 0- to 180-cm depth were monitored from 2000 through 2004, and were measured before spring fertilization. One soil core sample was collected from near the center of each plot (0- to 180-cm depth) before planting each crop year in increments of 0 to 7.6, 7.6 to 15.2, 15.2 to 30.4 cm, then 30.4 cm increments to a depth of 180 cm for determination of gravimetric soil water and soil NO3–N content. Soil NO3–N was determined by cadmium reduction (Lachat Instruments, 2001) using a continuous flow analyzer (Lachat QuickChem FIA+8000 Series, Lachat Instruments, Loveland, CO) after extraction with 1 M KCl (1:5 soil/solution ratio). Soil bulk density was determined on the soil cores from the 0- to 30-cm depths from each plot during the fall sampling. Soil bulk density was determined for the 30- to 180-cm soil depths of each replication and an average value calculated for the entire plot area. The soil bulk density was used to calculate soil NO3–N mass on an area basis.

The grain and crop residue samples collected for N analysis were ground to pass a 150-µm screen and analyzed for N content using a Carlo Erba C/N analyzer (Haake Buchler Instruments, Saddle Brook, NJ). Because of the long-term nature of this study and declining residual soil N levels with time in the zero fertilizer N plot, an accurate estimation of nitrogen fertilizer use efficiency could not be made for any of the N fertilizer treatments. Therefore, NUE was estimated by dividing grain yield by AN [soil N (0–90 cm depth) + fertilizer N + irrigation water N] similar to that reported by Moll et al. (1982) and Sabata and Mason (1992). Nitrogen use efficiency was also estimated by dividing the grain N uptake by AN multiplied by 100 to obtain percent NUE similar to that reported by Halvorson et al. (2005).

Grain yields were generally determined in mid-September (soybean) and mid-October to early November (corn) each year by hand harvesting a 3-m2 area in 2001 and a 4-m2 area in 2003 of each soybean plot and the ears from an 11.6-m2 area of each corn plot. The corn ears were shelled with a corn sheller to determine grain and cob weights. Corn grain yields were measured at physiological maturity at about 170 to 180 g kg–1 moisture content and final grain yield was expressed at 155 g kg–1 moisture content. Aboveground corn biomass was determined in mid-September in 2000 and 2002 and mid-October in 2004 by hand harvesting 15 whole corn plants from a 1.5 m2 or larger area from each plot. Except for 2000, the plants were separated into grain, cobs, and stover for total biomass determination. In 2000, the corn plants in a significant portion of the plot area were damaged by herbicide drift (glyphosate) as we controlled weeds between the corn rows with a hooded-band sprayer following the 1999 dry bean crop. The best parts of each plot (little or no damage to corn stand or plants) that had good corn stands in adjacent rows to provide competition to the harvest rows were used for grain harvest in 2000. However, corn plants collected for total biomass from other parts of the plot area seemed to be larger than normal due to lack of sufficient competition. Therefore, corn residue production and residue N uptake data in 2000 were not used in the analysis of residue response to N fertilization and total N uptake. Soybean grain yields were expressed at 130 g kg–1 moisture content. Above-ground biomass of soybean was determined near physiological maturity (growth stage R7, leaves had yellowed and started to drop) from a 2 m2 area of each plot on 28 Aug. 2001 and 8 Sept. 2003. The grain was separated from the total biomass before analysis for N content.

Analyses of variance (ANOVA) were performed using Analytical Software Statistix8 program (Analytical Software, 2003) to determine treatment effects. A split-plot ANOVA was used with N as main treatment and years treated as a subplot fixed effect. All statistical comparisons were made at {alpha} = 0.05 probability level unless otherwise stated using the least significant difference method for mean separation. If the ANOVA indicated a significant F value for N rate, either a linear or quadratic function (using equation with best r2 value) was fit to the N response data using regression functions present in the graphics program (SigmaPlot version 9.0, SPSS, Chicago, IL).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Crop Residue Cover on Soil Surface
The percent of crop residue cover on the soil surface each spring at corn or soybean planting is shown in Table 2 for each treatment. At corn planting in 2000, residue cover was very low following the 1999 dry bean crop. In 2001, residue cover at soybean planting was much greater than in 2000, reflecting the influence of the NT system and large quantity of corn residue added to the soil surface in 2000. Residue cover at corn planting in 2002 did not vary from that at soybean planting in 2001, but residue cover was greater at soybean planting in 2003. Following the 2003 soybean crop, surface residue cover was lower in 2004 than in 2003, reflecting the low residue addition of the soybean crop. Overall, residue cover varied significantly with N rate in 2001 and 2004, with the highest N treatment having the lowest amount of residue cover. The trend for a lower level of residue cover at the highest N rate (N6) could be the result of more rapid residue decomposition due to a higher level of N in the residue of the N6 treatment than in the other N treatments. Residue cover at planting in the NT corn–bean rotation was similar to that of the NT continuous corn system reported by Halvorson et al. (2006). Except for 2000, the level of surface residue cover in this corn-bean system was sufficient to visually reduce soil erosion by wind and water during the study period, demonstrating a benefit of an irrigated, NT production system for this area.


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  		Table 2. Soil residue cover at planting each year in the corn-bean rotation for each N treatment from 2000 through 2004.

 
Grain Yield
Estimated corn plant population did not vary between N treatments but were significantly different in 2000, 2002, and 2004 with respective plant populations of 81400, 92000, and 80000 plants ha–1, which reflects the higher seeding rate in 2002 compared with 2000 and 2004. Corn grain yields increased each year with increasing N fertilizer rates with a significant N treatment x year interaction (Fig. 1 ). There was only a small grain yield response to N fertilization in 2000, but a greater response in 2002 and 2004, which contributed to the significant interaction. Varvel and Wilhelm (2003) also reported small responses of irrigated corn to N fertilization in some years following soybean. In 2000, soil NO3–N levels (Table 5) for all N rates at corn planting were slightly higher than in 2002 and 2004, which could have reduced the response to N fertilization. Although the 2000 corn crop received less precipitation and irrigation water (Table 1) than the 2002 and 2004 corn crops, the 2000 corn crop showed few visual signs of water stress that would have limited grain yield. Cumulative growing degree day (GDD) heat units calculated on Celsius basis from planting until June 16th was similar for all years (335, 321, and 321 for 2000, 2002, and 2004, respectively). However, by July 24th when tasseling was occurring, cumulative GDD units were 713, 769, and 647 for 2000, 2002, and 2004, respectively. Close to maturity, September 30th, cumulative GDD units were 1345, 1363, and 1192 for 2000, 2002, and 2004, respectively. The GDD units indicate that early spring development was similar for all years, but heat stress during tasseling and pollination was in the order 2002 > 2000 > 2004. Corn yield potential in 2004 was enhanced by cooler temperatures during pollination and grain fill and slightly greater May–September growing season precipitation compared to 2000 and 2002. Corn response to N fertilization was similar in 2002 and 2004, with highest grain yields in 2004.


Figure 1
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  		Fig. 1. Corn grain yields in an irrigated, no-till corn–bean rotation as a function of N fertilizer application rate for three crop years near Fort Collins, CO.

 

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  		Table 5. Spring soil NO3–N levels for each N treatment in the corn-bean rotation from 2000 through 2004.

 
The average 3-yr corn yield (Fig. 1) in this corn–soybean rotation exceeded the grain yields reported by Halvorson et al. (2006) for CT and NT continuous corn production systems at this site for the same years (2000, 2002, and 2004) (Fig. 2 ). In this NT corn–soybean rotation, soil temperatures (Mosier et al., 2006) were several degrees warmer than in the NT continuous corn system, so early spring development of the corn plants was similar to that in the CT continuous corn system. Thus the delayed corn seedling emergence and development in early May observed in the NT continuous corn plots each year (Halvorson et al., 2006), did not occur with the corn in this NT corn–soybean rotation. This probably results from less surface crop residue following soybean in the NT corn–soybean rotation than following corn in a NT continuous corn system.


Figure 2
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  		Fig. 2. Corn grain yields in an irrigated, no-till corn–soybean rotation (NT-CB) compared to no-till and conventional-till continuous corn (NT-CC and CT-CC, respectively) (Halvorson et al., 2006) as a function of N fertilizer rate averaged over 3 yr near Fort Collins, CO.

 
Using the approach of Varvel and Wilhelm (2003), the estimated N fertilizer rate required to achieve the 9.04 Mg ha–1 3-yr average corn grain yield obtained in the NT corn–soybean rotation with no N fertilizer applied (Fig. 2) was about 119 kg N ha–1 in the no-till continuous corn (NT-CC) and 55 kg N ha–1 in the conventional-till continuous corn (CT-CC) production system. This clearly demonstrates the value of crop rotation, especially with a legume, and the contribution of N from the bean crops to the following corn crop.

Expressing corn grain yield as a function of AN [soil NO3–N (0- to 90-cm depth) + fertilizer N + irrigation water N] for all 3 yr (Fig. 3 ) shows grain yield increasing with increasing N availability. Based on the regression equation for the 3-yr average, the amount of total AN needed to maximize grain yield was 257 kg N ha–1, or 285 kg N ha–1 if only the more responsive years (2002 and 2004) were used. This range in AN level for maximum corn yield in this corn–soybean rotation is similar to that reported by Halvorson et al. (2006) for the NT and CT continuous corn systems. Halvorson et al. (2005) also reported maximum yields in furrow irrigated continuous corn occurred at 265 kg N ha–1 of AN (soil + fertilizer N) in southeastern Colorado.


Figure 3
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  		Fig. 3. Corn grain yields in an irrigated, no-till corn–bean rotation as a function of plant available N [soil N (0–90 cm depth) + fertilizer N + irrigation water N] for three crop years near Fort Collins, CO.

 
Using the algorithms for N fertilizer recommendations published by Bauder and Waskom (2003) for CT corn production following dry bean in Colorado, we estimated that about 148, 173, and 167 kg N ha–1 of fertilizer N would have been needed to attain a 12.5 Mg ha–1 corn grain yield in 2000, 2002, and 2004, respectively. Corn grain yields in this corn–bean rotation (Fig. 1) were near maximum with 127 kg N ha–1 in 2000, approaching maximum yield in 2002 with 202 kg N ha–1, and exceeding 12.5 Mg ha–1 grain yield in 2004 with 134 kg N ha–1. This would indicate that N fertilizer recommendations used for CT corn production following dry bean in Colorado (Bauder and Waskom, 2003) can be used to estimate the N fertilizer needs of NT corn in a corn–bean production system, until sufficient irrigated, NT corn production data becomes available to develop algorithms for predicting N needs for NT corn systems in Colorado.

Soybean grain yields were not influenced by N fertilization in 2001 and 2003, averaging 3.46 and 2.12 Mg ha–1 in 2001 and 2003, respectively. Hand-harvest soybean grain yields in 2001 were acceptable; but due to shattering during combine harvest as a result of low humidity conditions, it is estimated that <60% of the grain made it into the combine grain tank. In 2003, the soybean plants were only about 30- to 40-cm tall, making combine harvest losses even greater than in 2001. Based on this study, producing a harvestable and economical crop of soybean in this area appears to have limitations. Soybean would fit well into a corn-bean rotation in this area, but until suitable soybean cultivars become available for this production area, soybean does not appear to be a viable option for irrigated crop rotations.

Grain Nitrogen Removal
The amount of N removed with each Mg grain (oven dry basis) tended to increase with increasing N rate (Table 3) with a significant N treatment x year interaction. The interaction resulted from the fact that grain N content did not vary significantly with N treatment in 2000, the N6 treatment had a significantly greater grain N content than the other N treatments in 2002, and there was a linear increase in grain N content with increasing N rate in 2004 (Table 3). The decrease in grain N content at the highest N rate in 2000 seems unusual, but the authors do not have an explanation for this result. At the higher N rate, the average amount of N in each Mg grain (about 14 kg N Mg–1 grain) was similar to that reported by Heckman et al. (2003) for various corn production systems in five states and Halvorson et al. (2005, 2006) for irrigated continuous corn produced within CT and NT systems.


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  		Table 3. Corn grain N content each year as a function of N treatment.

 
Corn grain N removal (Fig. 4 ) increased with increasing N rate with differences between years (significant N treatment x year interaction) reflecting grain yield (Fig. 1). The significant interaction resulted because of limited grain yield response to N application in 2000 and little difference in grain N content with added N (Table 3) which resulted in a curvilinear N uptake response function in 2000 versus a linear increase in N uptake in 2002 and 2004 with increasing fertilizer N rate. Averaged over years, grain N removal was in the order N6 > N5 > N4 > N3 > N2 > N1 or 155, 139, 130, 118, 110, and 90 kg N ha–1.


Figure 4
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  		Fig. 4. Total N removed by corn grain in an irrigated, no-till corn–bean rotation with increasing N rate for 3 yr.

 
Soybean grain N removal did not vary with N rate, but was greater in 2001 (150 kg N ha–1) than in 2003 (86 kg N ha–1) due to the greater grain yield in 2001. The amount of N removed per Mg soybean grain yield did not vary with N rate, but was similar in 2001 (50 kg N Mg–1 grain) and 2003 (47 kg N Mg–1 grain).

Nitrogen use efficiency by corn and soybean decreased similarly each year with increasing AN level with no significant N treatment x year interactions. The average NUE estimates for corn and soybean are shown in Table 4 for each N treatment. For corn and soybean, NUEs > 100% indicate that more N was removed in the grain than was present as AN. This would indicate that mineralization of soil organic matter and N fixation by soybean were contributing to the AN pool. Based on grain yield per unit of AN, NUE efficiency declined as AN level increased. The lower NUEs associated with the N5 and N6 treatments were also associated with the highest corn grain yields in this study.


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  		Table 4. Nitrogen use efficiency (NUE) by corn and soybean based on grain N removal (%) or grain yield as a function of available N (AN) (soil + fertilizer + irrigation water N) for each N treatment.

 
Crop Residue
Corn residue (leaves, stalks, and cobs) production (2002 and 2004) increased with increasing N rate (Fig. 5 ) similarly each year. The N rate x year interaction was not significant. Averaged over N rates, corn residue production was 7657 and 7921 kg ha–1 for 2002 and 2004, respectively.


Figure 5
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  		Fig. 5. Corn residue production and residue N uptake in an irrigated, no-till corn–bean rotation as a function of N rate averaged over years (2002 and 2004).

 
The amount of N in the corn residue at harvest increased with increasing N rate (Fig. 5), with no significant differences between years and no significant N rate x year interaction. Thus the higher N rates had more N cycling back to the soil with the residue than the lower N rates. The greater amount of residue N cycling back to the soil surface with the higher residue amounts should contribute to greater SOC sequestration with time in this corn-bean system (Varvel, 1994). The highest N rate contributed about 1642 kg ha–1 more corn residue than the zero-N rate (Fig. 5) when averaged over 2002 and 2004. Corn residue production with increasing N rate in this corn–soybean rotation was similar to those reported for the CT and NT continuous corn production systems by Halvorson et al. (2006).

Soybean residue production did not vary significantly with N rate or year, averaging 2911 kg ha–1 over N rates and years. Residue N uptake did not vary significantly with N rate or year, averaging 14 kg N ha–1. The N rate x year interaction was not significant for either residue production or residue N uptake. There was considerably less residue N being cycled back to the soil with soybean than with corn.

Total Nitrogen Uptake and Total Nitrogen Requirements
The total N uptake (grain + residue) for the 2002 and 2004 corn crops is shown in Fig. 6 . Total N uptake for the 2000 corn crop was not included because accurate residue production and residue N uptake data were not available. Total N uptake increased with increasing N rate and biomass production each year. The increases in total N uptake with increasing N rate were linear in 2002 and 2004. The N rate x year interaction was significant, so each year is shown separately in Fig. 6. At the lowest N rate, difference in total N uptake between 2002 and 2004 was 20 kg N ha–1. The difference between the 2 yr increased as the N rate increased, with 2004 having the greatest level of total N uptake by corn. Averaged over years and N rates (Fig. 6), this no-till corn–bean (NT-CB) rotation had a greater total N uptake than that reported by Halvorson et al. (2006) for the CT and NT continuous corn systems. This would indicate a significant contribution of AN to the corn crop from the previous year's soybean crop.


Figure 6
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  		Fig. 6. Total N uptake (grain N + residue N) by corn as a function of N rate and year in an irrigated, no-till corn–bean rotation.

 
Averaged over 2002 and 2004, the total amount of N required to produce one Mg corn grain (oven dry basis) increased with increasing N rate (15.2, 15.5, 16.2, 17.5, 17.5, and 20.1 kg N Mg–1 grain for the N1, N2, N3, N4, N5, and N6 treatments, respectively). Averaged over N rates, 16.6 kg N Mg–1 grain was required to produce the 2002 corn crop, and 17.4 kg N Mg–1 grain in 2004 with no significant difference between years with a 2-yr average of 17 kg N Mg–1 grain. The 2-yr average total N requirement to produce one Mg grain at near maximum yield potential was 20 kg N. The total N requirement to produce one Mg grain for corn in this NT corn–soybean rotation is similar to that reported by Halvorson et al. (2006) for NT and CT continuous grain corn. The total N requirement values from this study at maximum yield were in the range of total N needs for irrigated, CT corn of 19.6 to 21.4 kg N Mg–1 grain yield reported by others (Bauder and Waskom, 2003; Karlen et al., 1998).

Total N uptake by soybean varied only by year, 164 kg N ha–1 in 2001 and 99 kg N ha–1 in 2003. The higher grain yield and AN levels in 2001 compared to 2003's lower grain yield, reduced N rate, and lower residual soil NO3–N are responsible for the difference in total N uptake between years. Total N uptake by soybean was less than that of corn, reflecting the lower grain and residue yield with soybean. Total N required to produce one Mg of soybean grain was 54 kg N, which did not vary with N rate or year.

Soil Nitrate
Soil NO3–N levels just before planting each spring are reported in Table 5. In 2000, soil NO3–N levels were not significantly different between N treatments. In 2001, soil NO3–N levels had declined in all N treatments, except in N6 which showed a slight increase following the 2000 corn crop. Soil NO3–N levels tended to be higher at corn planting in 2002 than at soybean planting in 2001 in the 0- to 90-cm soil depth, but were not significantly different between N rates. At corn planting in 2004, residual soil NO3–N levels tended to be slightly lower for the N5 and N6 treatments than in previous springs, possibly reflecting the drop in N rates applied to soybean in 2003 and good corn yields in 2002. The residual soil NO3–N levels in 2004 appear to be normal for an irrigated cropping system.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Corn grain yields were significantly increased by N fertilization each year. Grain yields were near maximum with an AN (soil + fertilizer + irrigation water N) level of 257 to 285 kg N ha–1. The corn plants in this irrigated, NT corn–soybean rotation were not affected by slow early spring development and delayed tasseling as reported for the NT continuous corn system (Halvorson et al., 2006). Nitrogen use efficiency (NUE) based on grain N removal decreased significantly with increasing level of AN. Total N required to produce one Mg of corn grain near maximum yield potential averaged 20 kg N in this NT corn–bean rotation. Soybean yields did not vary with N rate, nor did the total N (54 kg N) required to produce one Mg soybean grain. Corn residue returned to the soil increased with increasing N rate, but soybean residue did not. Addition of N fertilizer increased the level of residual soil NO3–N in the 0- to 180-cm profile mainly at the highest N rate.

This work shows that irrigated, NT corn production would benefit from having a low residue crop like soybean or dry bean in the rotation. However, short soybean plant height (30–40 cm) and shattering made combine harvest difficult resulting in significant grain loss. Improved soybean cultivars are needed for this area to make a corn–soybean rotation a viable production system. An irrigated, NT production system has potential for replacing irrigated, CT production systems in the central Great Plains area when high residue crops are rotated with low residue crops and adequate N is applied. The study shows that corn in this NT corn–bean system responded similarly to AN supply when compared with the CT system reported by Halvorson et al. (2006). Corn yields at low fertilizer N rates benefited most from having beans in the rotation. The study shows that soybean did not respond to N fertilization at the yield levels attained in this study. Current Colorado N fertilizer recommendations for irrigated, CT corn–dry bean production systems appears to be useable for estimating N fertilizer needs of corn in NT corn–bean rotations until sufficient irrigated, NT production data becomes available to develop improved algorithms for irrigated, NT corn production systems.


   ACKNOWLEDGMENTS
 
The authors wish to thank Patti Norris, Brad Floyd, and Catherine Cannon for their field assistance and analytical support in processing the soil and plant samples and collecting the data reported herein. We also acknowledge the financial support of USDA-CSREES-NRI (grant no. 2001-35108-10719) and USDA-CSREES-CASMGS (grant Agreement no. 2001-38700-11092).


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Contribution from USDA-ARS. The U.S. Department of Agriculture offers its programs to all eligible persons regardless of race, color, age, sex, or national origin, and is an equal opportunity employer. Trade names and company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment of the product by the authors or the USDA-ARS.


   REFERENCES
TOP
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 




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