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Dep. of Agron., Iowa State Univ., Ames, IA 50011-1010
* Corresponding author (malkaisi{at}iastate.edu).
Received for publication January 18, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CAMS, computer-aided management system • ET, evapotranspiration • WUE, water use efficiency
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Like many other regions in the Great Plains, northeastern Colorado mainly depends on the Ogallala Aquifer for irrigation. The traditional irrigation amount and N fertility rate (64 cm of water, including rainfall and 250 kg N ha-1, including N from soil, fertilizer, and irrigation water during the growing season) for corn production are probably related to water quality problems (nitrate contamination of groundwater). Furthermore, depletion of the Ogallala Aquifer has been accelerated by the extensive use of irrigation in this region. Therefore, protecting both quality and quantity of the aquifer is crucial for the survival of agriculture in northeastern Colorado.
The relationships of corn yield and nitrate leaching with irrigation, N fertilization, and plant population have been extensively investigated. Irrigation effectively increases corn yield although WUE expressed as corn yield to total water use ratio decreases as irrigation rate increases (Stone et al., 1987, 1993; Hergert et al., 1993). Nitrogen fertilization also increases corn yield when N supply by soil is low (Wienhold et al., 1995; Sexton et al., 1996). Nitrogen fertilizer applied at rates higher than the optimum requirement for crop production may cause an increase in nitrate accumulation below the root zone and pose a risk of nitrate leaching (Linville and Smith, 1971; Lund et al., 1974; Ludwick et al., 1976). Otherwise, little nitrate will be available for leaching (Pratt et al., 1972). Increasing plant population usually increases corn yield until an optimum number of plants per unit area is reached (Lang et al., 1956; Holt and Timmons, 1968; Lutz et al., 1970; Thomison et al., 1992). Optimum plant population is related to soil moisture availability (Holt and Timmons, 1968; Karlen and Camp, 1985), N fertility, and other environmental factors. Optimum plant population for corn production increases with the increase of available N until plant population at approximately 50000 plants ha-1 (Lang et al., 1956; Colyer and Kroth, 1968; Thomison et al., 1992; Eckert and Martin, 1994).
Several studies have investigated the effect of irrigation and N interactions on corn production, WUE, and N leaching (Russelle et al., 1981; Martin et al., 1982; Eck, 1984; Sexton et al., 1996). In general, increase in soil moisture enhances corn yield response to N fertilization, especially when high N rates are applied (Burman et al., 1962; Eck, 1984). In addition, N uptake was strongly influenced by water supply (Martin et al., 1982). Russelle et al. (1981) reported that the optimum N rate for maximum corn yield was the same under different irrigation treatments. However, N fertilization increases WUE on N-deficient soils where water is adequate (Viets, 1962; Olsen et al., 1964).
Irrigation and N fertilization effects or their interactions are usually evaluated in terms of corn yield (Eck, 1984; Norwood, 2000); only a few studies (Sexton et al., 1996) used both corn yield and nitrate leaching to identify an appropriate N application rate, recommending N rate to obtain 95% of optimum yield. Little information is available about the integrated best management practices of irrigation, N rates, and plant populations to achieve high corn yield and high WUE as well as low N loss.
The primary objective of this study was to evaluate the impacts of N fertility rates, irrigation rates, and plant populations on corn yield and WUE to develop a best management system for high corn yield and WUE simultaneously.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Design
A randomized complete block split-split plot design with three replicates was used each season. Irrigation treatments were randomly assigned to the main plots, N fertility rates were assigned to the split plots, and plant population rates were assigned to the split-split plots.
Irrigation scheduling was conducted using a center-pivot sprinkler system equipped with a computer-aided management system (CAMS) based on the daily water use (ET) developed from on-farm weather station data. The irrigation circle was divided into three sectors, and the three irrigation treatments were randomly assigned to the three sectors where they were kept in the same place each year. Within each sector, there were three replicates of the same irrigation treatment. Because of the center-pivot sprinkler system, it was too complicated to replicate the irrigation treatments as for a normal randomized complete block design. Because the field used for this experiment was uniform, this unique arrangement of irrigation replicates would not result in any significant additional errors to the results. The three irrigation treatments (rainfall included) were 0.60, 0.80, and 1.00 of the estimated ET, which represented 38, 51, and 64 cm of total water applied during the growing season.
Four N fertility rates (30, 140, 250, and 360 kg N ha-1) and three plant populations (57000, 69000, and 81000 plants ha–1) were used each season. Four N rates were randomly nested within each main plot of irrigation treatment, and three plant populations were randomly nested within each split plot of N rates. Soil testing showed high variability in initial soil N concentrations in the field before experiment in 1998 and in residual soil N concentrations before corn planting in 1999 and 2000. To achieve a genuine response to different N fertility rates, N treatments were allocated to the plots that had initial soil N or residual soil N levels close to the experiment-designed N rates each season. Nitrogen fertilizer rates were calculated based on the designated N fertility rates and by crediting soil NO3–N content in the top 1.22 m. Nitrogen in the irrigation water also was credited to N fertility rate. Nitrogen fertilizer rates were split into two applications: 60% was applied during ripping, and the remainder was applied through the center-pivot sprinkler system before tasseling by using a CAMS. The CAMS was installed on the center pivot, allowing application depth to be as shallow as 0.25 cm. In addition, a starter of 30 kg N ha-1 plus equal P, K, and micronutrients was applied to all treatments each season. Irrigation scheduling for the three irrigation treatments was conducted based on the maximum ET rate (1.00ET) where CAMS was used to achieve the application rate. The other two irrigation treatments (0.80ET and 0.60ET) were applied at the same time and in proportion to 1.00ET.
A split-split plot consisted of 12 rows of corn with a length of 15 m. Corn (cv. Midwest 7620) was planted in 76-cm row widths on 28 Apr. 1998, 19 May 1999, and 28 Apr. 2000. The plants were planted at the same distance from each other for each plant population. Weed, pest, and disease control were done in a timely manner.
Plant Sampling
Corn plant samples were collected at six-leaf (V6), 12-leaf (V12), tasseling (VT), and physiological maturity (R6) growth stages as defined by Ritchie et al. (1997) on a split-split plot basis in all 3 yr. Plant sampling area for each season consisted of two adjacent rows, 4.6 m in length for each split-split plot. This 4.6-m length of corn was divided into three 1.53-m subsections. At the V6 growth stage, two plants were cut from each subsection at ground level, giving a total of six plants per split-split plot. For the V12 and VT growth stages, one plant per subsection was cut, totaling up to three plants per split-split plot. At the R6 growth stage, all remaining plants from each subsection were cut, and each sample was partitioned into plant (stalk + leaves) and grain. The plant sampling at the R6 growth stage was conducted only in 1999 and 2000. The edge plants resulting from previous sampling activities for all leaf stages were excluded from subsequent samplings. We made sure that each plant collected at each growth stage was growing between two original plants to eliminate uneven competition. Plant samples were dried in a forced-air oven at 65°C for at least 3 d before weighing. After physiological maturity, grain yield was determined by hand harvesting an area of two rows, 6.1 m in length, each on a split-split plot basis. Grain samples were collected from the yield samples for the determination of moisture content. Corn yield was adjusted to moisture content of 155 g kg-1.
Moisture Measurement and Water Management
Soil moisture was monitored on a weekly basis by using a neutron probe (Depth Moisture Gauge model 4300; Troxler Electronics Lab., Research Triangle Park, NC) based on a volumetric basis. Access tubes were installed in the center of each split-split plot of each plant population at 250 kg N ha-1 under each irrigation treatment. Three access tubes were installed for each irrigation treatment, one tube per plant population. Soil moisture was monitored weekly, starting immediately after corn emergence until harvest at the soil depth intervals of 0 to 30, 30 to 60, 60 to 90, 90 to 120, and 120 to 150 cm. During the growing season, soil moisture measurements were taken before and immediately after each irrigation event (1- to 2-d lag period).
Irrigation water was applied using a center-pivot system equipped with a CAMS to achieve the desired application rate. The irrigation scheduling was done with the Crop Flex program (King et al., 1991) by using data collected from the local weather station to estimate daily crop water use (ET). The Crop Flex program estimates crop water use (ET) by using the modified Penman equation (Jensen et al., 1971) along with polynomial functions to generate crop coefficients based on the fraction of time from planting to full cover and days after full cover (Duke, 1987). Highest rate was applied using the manageable or allowable water depletion, which refers to the acceptable soil moisture depletion (40–50%) of available soil moisture at different growth stages by using the water balance approach, which includes estimated ET, soil moisture changes, precipitation, and drainage to determine irrigation amount. The following water balance relationship was used in determining the irrigation amount:
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Water use efficiency was calculated based on the following relationship:
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Laboratory Analysis
Total N concentrations in the plant (including whole plant and grain) samples at each growth stage were determined using automated combustion (Miller and Kotuby-Amacher, 1996, p. 71–72). Soil NO3–N content before the experiment in spring 1998, for depth 0 to 150 cm in increments of 30 cm, was determined using Colorado State University Soil Test Laboratory standard procedures (Keeney and Nelson, 1982, p. 649 and 676). Nitrogen uptake by plant was calculated based on the total N concentrations in plant or grain multiplied by their respective mass (dry matter or grain weight).
Statistical Analysis
Plant N uptake data at the V6, V12, and VT growth stages were analyzed using an analysis of variance (ANOVA) appropriate for a randomized complete block split-plot design with irrigation treatment as the main-plot factor and plant population as the split-plot factor. Separate ANOVAs were performed for each dependent variable. Plant N uptake at the R6 growth stage and grain yield at harvest were analyzed using a randomized complete block split-split plot model with irrigation as the main factor, N rate as the split factor, and plant population as the split-split factor. All data for each measurement were combined across years if possible. Mean separation of treatment effects in this study was accomplished using Fisher's protected least significant difference (LSD) test. Probability levels lower than 0.05 were categorized as significant. All data analyses in this study were accomplished using the SAS System for Windows, version 8 (2) (SAS Inst., 2001).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Plant population effects on N uptake were more evident early in the season (Tables 2 and 3). Nitrogen uptake at the V6 growth stage increased as plant population increased up to 81000 plants ha-1. Similarly, N uptake under 57000 plants ha-1 at the V12 and VT growth stages was significantly lower than that at 69000 and 81000 plants ha-1. Plant N uptake at the R6 growth stage was significantly reduced by changing the population from 81000 to 57000 plants ha-1 whereas no differences were observed between 69000 and 81000 plants ha-1 in 1999 or 2000. In contrast, plant population did not affect grain N uptake in either year.
In general, plant N uptake response to irrigation treatments, N rate, and plant population at various growth stages seemed to be more related to an increased dry matter production than to an increase in N concentrations in plants.
Grain Yield
The effect of interaction between irrigation treatments and N rates on grain yield was significant and varied by year (Table 4). In 1998, 0.80ET and 0.60ET consistently resulted in lower yield than 1.00ET treatment, regardless of N rate. Average yield decreases were 43% with 0.60ET and 25% for 0.80ET relative to 1.00ET. In 1999, yield differences between 0.80ET and 1.00ET were not significant. However, 0.60ET produced yield 45% lower than the 1.00ET averaged over the four N rates. In 2000, grain yield under 0.80ET was similar at low N rates of 30 and 140 kg ha-1 but 15 to 23% lower at high N rates of 250 and 360 kg ha-1 compared with 1.00ET. Irrigation treatment 0.60ET resulted in a significant yield decrease that averaged 47% compared with 1.00ET.
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The 3-yr results, in general, showed that 250 kg N ha-1, which is the common N fertility rate currently used in northeastern Colorado, can be replaced by 140 kg N ha-1 for corn production under the three irrigation treatments. Nitrogen rate impacts on yield were not significantly influenced by plant populations. Our results agreed to some degree with those of Russelle et al. (1981) that the optimum N rate for maximum corn yield was the same under different irrigation conditions. The same optimum N rate for maximum yield under different irrigation conditions could be explained by the decrease in N use efficiency as soil moisture content decreased.
Although irrigation effects on yield were influenced by plant population (Table 5), yield showed a strong decreasing trend, with decreases in irrigation in 1998 and 2000 no matter which plant population was used. In 1999, however, the yields of 0.80ET and 1.00ET did not differ at any population. Yield response to plant population varied with irrigation treatment and growing season (Table 5) but was not affected by N fertilization rate (data not shown). In 1998 and 2000, there were generally no significant yield differences among the three plant populations at 0.60ET or 0.80ET. Under 1.00ET, however, plants with population of 69000 plants ha-1 produced comparable yield, but plants under 57000 plants ha-1 had a significant yield reduction of 16% in 1998 and 10% in 2000 compared with 81000 plants ha-1. In contrast, no yield differences due to plant population were observed regardless of irrigation treatment in 1999. Karlen and Camp (1985) also reported that plant population significantly responded to water management for corn yield. It seemed that yield response in 1999 was mainly due to irrigation treatment rather than N rate or plant population changes.
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Water Use Efficiency
There was a significant interaction between irrigation treatment and N rate for WUE, and it varied with growing season (Table 6). In 1998, WUE did not differ among the three irrigation treatments at any N rate except 30 kg N ha-1. However, in the 1999 season, 0.80ET had WUE 25% greater than 1.00ET averaged over all N rates. The 0.60ET treatment had WUE similar to 1.00ET at the 140 kg N ha-1 rate. In 2000, there were no significant WUE differences among the three irrigation treatments regardless of N rates.
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Water use efficiency response to N rate was influenced by irrigation treatment and year (Table 6). In 1998, 140 and 360 kg N ha-1 had comparable WUE as 250 kg N ha-1. In 1999, WUE did not differ among the four N rates at 0.80ET and 1.00ET. In 2000, 30, 140, and 360 kg N ha-1 all had similar WUE as 250 kg N ha-1. The results of this study generally agreed with the observations that N fertilization increases WUE on N-deficient soils where water is adequate (Viets, 1962; Olsen et al., 1964). Therefore, 140 kg N ha-1 can be used as an alternative to 250 kg N ha-1 for corn production in terms of WUE.
Similar to yield, WUE response to plant population varied with irrigation treatment and year (Table 7). In 1998 and 2000, when grain yield was within low to medium ranges, significant WUE response to plant population was generally not observed under 0.60ET and 0.80ET. However, 57000 plants ha-1 had WUE 10 to 19% lower than 81000 and 69000 plants ha-1 under 1.00ET in both years. In 1999, a very high-yielding season, WUE was generally identical among the three populations, no matter which irrigation treatment was applied.
In general, 0.80ET accompanied by 140 to 250 kg N ha-1 and plant population of 57000 to 69000 plants ha-1 was the best combination for corn production aimed at maximum WUE in this study. This recommendation is slightly different in irrigation from our recommendation aiming at optimum grain yield. Because the decline of the Ogallala Aquifer in the Great Plains occurred rapidly due to extensive irrigation, the best management system aimed at maximum WUE should be used in corn production. In addition, efforts should be made to implement the best management system for maximum WUE in the entire area and for the long term. Therefore, the adoption of 0.80ET will be superior to 1.00ET in northeastern Colorado to maintain stable irrigated corn production for the long term.
Our recommendations for both optimum yield and WUE were based on the minimum levels of all of the variables to reach our goal. Some may argue that 1.00ET is not harmful to the Ogallala because any excess water will recharge the aquifer and be available for pumping again. However, 1.00ET increases energy costs for pumping water from wells. In addition, 1.00ET may increase the potential of underground water contamination by nitrate, pesticides, and other pollutants. Because the majority of farmers are not able to implement best management practices in irrigation scheduling to the level that were implemented in this study during irrigation events, 1.00ET will probably cause some overirrigation under current growers' irrigation management. Similarly, the use of a higher N rate and plant population than our recommendations also may lead to maximum yield or WUE, but the production costs of additional fertilizer and seed will be increased, and the potential of Ogallala Aquifer contamination by nitrate may be increased. The effects of N rate, irrigation rate, and plant population on nitrate leaching were evaluated in this study and will be presented in another publication.
Soil Moisture Extraction
Initial soil moisture status was uniform across the field before this experiment began. Soil moisture extraction during the entire season averaged over 1998 to 2000 showed that the influences of irrigation on moisture extraction varied with soil depth (Fig. 1)
. The 0.60ET treatment had moisture extraction 19% lower on average within the top 60 cm of soil than 1.00ET (Fig. 1). The differences in moisture extraction were not observed between 0.80ET and 1.00ET treatments within the top 90 cm (Fig. 1). In addition, moisture extraction was the greatest from the top 30 cm compared with other soil depths regardless of irrigation treatment. The moisture extraction from the 0- to 30-cm depth accounted for 34 to 38% of the total moisture extraction from the soil profile down to 150 cm in depth. Plant population showed no significant effects on moisture extraction in any soil depth (data not shown).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The 0.80 irrigation treatment had the same or even greater WUE than 1.00ET and 0.60ET regardless of N fertility rate. Treatment 0.80ET accompanied by 140 to 250 kg N ha-1 and plant population of 57000 to 69000 plants ha-1 is the best management system for optimum WUE. This recommendation is slightly different in irrigation from the suggestion for optimum yield. To preserve the Ogallala Aquifer, the best management system aimed at maximum WUE should be recommended for corn production in this region.
Influences of irrigation on soil moisture extraction during the entire season and soil moisture at harvest varied with soil depth. Irrigation treatment 0.60ET resulted in lower moisture extraction within the top 60 cm and lower soil moisture content at harvest within the top 90 cm compared with 1.00ET. Some residual effects of irrigation on crop production could occur in the following season due to the differences in moisture content at harvest. No significant differences in water extraction from the soil profile for the entire season and soil moisture content at harvest between 0.80ET and 1.00ET treatments were additional indications that 0.80ET is superior to 1.00ET.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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