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a Dep. of Crop and Soil Environ. Sci., Virginia Tech, Blacksburg, VA 24061-0404
b Dep. of Soil and Crop Sci., Colorado State Univ., Fort Collins, CO
* Corresponding author (malley{at}vt.edu)
Received for publication July 20, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: AWHC, available water-holding capacity • DOY, day of year • ETa, actual evapotranspiration • ETo, reference evapotranspiration • EWU, efficiency of water use • TDR, time domain reflectometry • WUE, water use efficiency
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Many studies have shown corn grain yields to be especially sensitive to moisture stress at a period beginning approximately at tasseling and continuing through grain filling (Robins and Domingo, 1953; Denmead and Shaw, 1960; Musik and Dusek, 1980; NeSmith and Ritchie, 1992). Robins and Domingo (1953) at Prosser, WA, USA, observed that depletion of soil water to wilting point for 1 or 2 d during tasseling or pollination reduced yields by 22%. Six to eight days of stress at this stage reduced yield by 50%. Musik and Dusek (1980) working in Bushland, TX, USA, found that stress during tasseling and silking was most harmful and that stress during grain filling was more harmful than that during vegetative growth. In areas of erratic precipitation distribution, soil water-holding capacity is a critical factor in maximizing yield potentials through minimizing water stress during these critical growth phases.
Minimizing water stress during critical corn growth periods on these soils may enhance crop productivity. It may be possible to tailor soil-specific management practices to enhance yield potentials through planting dates, plant populations, crop rotations, tillage practices, and fertilizer regimes to maximize water use efficiencies (WUEs). Determining water use of corn grown with present management practices in the mid-Atlantic coastal plain is a first step to developing management practices to enhance corn production by minimizing yield losses through water deficits during critical growth periods of corn.
Total precipitation in the mid-Atlantic region during the corn growing season is generally adequate for rainfed corn production (Table 1). However, precipitation is highly variable between years and can be poorly distributed, with much of the summer precipitation delivered in thunderstorms. The ability of the coarse-textured soils in this region to store water from these large, high-intensity precipitation events for later use is essential for increasing the efficiency of water use (EWU) and, therefore, yields.
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Crop models used to predict yields, irrigation requirements, and N leaching all require some knowledge of crop water use and stress for specific soils. Models that do not explicitly simulate crop growth, calculate crop water use [actual evapotranspiration (ETa)] using the crop coefficient concept (Doorenbos and Pruitt, 1977; Allen et al., 1998) where ETa is calculated by multiplying reference evapotranspiration (ETo) by a crop coefficient (Kc) and a soil water stress factor (Ks) (Allen et al., 1998). The crop coefficient method is now the standard and recommended procedure for calculating irrigation requirements of crops worldwide (Allen et al., 1998). Models that calculate water use using the crop coefficient method include CROPWAT (Smith, 1992; Cavero et al., 2000), an irrigation model, and NLEAP (Shaffer et al., 1991; Delgado et al., 1998a, 1998b), a NO3–leaching model.
Defining soil water stress factors and crop coefficients for corn grown on selected soils of the mid-Atlantic coastal plain will provide a baseline record of measured values of these common inputs to various models of crop growth, crop water stress, irrigation requirements, and NO3 leaching. Comparison of the crop stress factors, crop coefficients, and evapotranspiration for the various corn growth stages can be utilized to determine at which crop growth stages the growing corn is under the greatest stress and how this compares between years. This data, when combined with long-term rainfall records in models of corn water use and yields, can be used to investigate various aspects of management, such as planting dates and maturity group selection to determine how these impact crop water stress and yields.
The first objective of this study was to calculate a daily water balance for rainfed no-till corn grown on three soil types of varying water-holding capacity in the mid-Atlantic coastal plain. The second objective was to compare the water balance components of crop stress factors, crop coefficients, evapotranspiration, and drainage for the three main growth phases of corn—vegetative, tasseling, and grain fill—in 2 yr. The third objective was to measure and compare yields and EWU of the corn grown on these soils.
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METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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An order-1 soil survey by Natural Resource Conservation Service scientists mapped the boundaries of the soil types. The soils studied were two Bojac soils (a sandy loam and a loamy sand) and a Wickham sandy loam. The first Bojac soil (Bojac1) is a sandy loam to 0.86 m, with a sandy-clay loam texture from 0.86 to 1.2 m. The second Bojac soil (Bojac2) is a loamy-sand to 0.18 m, with a sandy-loam texture to 0.18 to 1.2 m. The Wickham soil has a sandy-loam surface layer to 0.3 m, below which is a red-clay loam layer to 0.64 m, with an underlying layer of sandy-clay loam to 1.2 m.
Crop Management Practices
Corn, cultivar Pioneer Brand 33G26, which has a 112-d maturity and is gray leaf-spot (Cercospora zeae-maydis Tehon & E.Y. Daniels) resistant, was planted no-till in 0.75-m rows using a John Deere 1780 Max Emerge II planter (Deere and Co., Moline, IL). Plant populations were varied by soil type at 49, 59, and 67 thousand plants ha-1 for Bojac2, Bojac1, and Wickham soils, respectively. Plant populations were selected based on Virginia Cooperative Extension recommendations (D.E. Brann, personal communication, 1998) and grower experience.
Corn was planted on 13 Apr. 1998 [day of year (DOY) 104] and harvested on 2 September (DOY 246). The 1999 corn crop was planted on 4 April (DOY 95). Minimum temperatures were below 10°C for most of April, with at least 4 d below 4°C, and the average temperature for the month was 13°C. These conditions combined with low soil moisture availability resulted in low and erratic plant populations. Corn was replanted on 4 May (DOY 125) and harvested on 28 Sept. 1999 (DOY 272).
Potassium was applied as a preplant broadcast application at 112 kg K ha-1. Starter fertilizer consisting of 56 and 14 kg ha-1 N and P, respectively, was applied in a band 5 cm below and 5 cm away from the seed at planting. Sidedress N in the form of urea ammonium nitrate solution was applied on 21 May (DOY 142) in 1998 and 1 June (DOY 153) in 1999. Sidedress N rates were 67, 112, and 140 kg ha-1 for Bojac2, Bojac1, and Wickham soils. These N rates were selected based on long-term yields for similar soils in the region. Weed pressure was minimal on all soil types, and chemical weed control was the same for all soil types.
The corn was harvested using a John Deere 9610 combine (Deere and Co., Moline, IL) equipped with a yield monitor and global positioning system with satellite differential correction. The yield monitor was calibrated using a weigh wagon, with yields consistently measured within 2% error. Yields were determined for the sites by overlaying georeferenced locations of probes used for soil moisture measurements, yield maps, and order-1 soil survey maps. Yield data were collected from the location of probes that were located well within the soil boundaries.
Management of crop residue was similar for all three soil types; however, the amount of residue was greater on the higher-productivity soils. This reflects the nature of production on these soils. In 1998, the corn was planted no-till into the corn residue from that shredded following harvest in the previous year. In 1999, the corn was planted no-till into the residue of soybean.
Point Measurements of Soil Profile Water Content
Soil profile water content was measured with time domain reflectometry (TDR) probes (Moisturepoint, Environmental Sensors, Victoria, BC, Canada). Measurements of volumetric water content were made using a Moisturepoint instrument for measuring soil moisture (Model MP-917, Environmental Sensors, Victoria, BC, Canada). Measurements were made along five increments of the probes: 0 to 0.15, 0.15 to 0.30, 0.30 to 0.60, 0.60 to 0.90, and 0.90 to 1.20 m. The maximum depth was used as it was expected to be beyond the maximum depth from which the rainfed corn would extract water. This assumption was supported by examination of rooting depth and the small percentage of water extraction measured in the 0.90- to 1.20-m depth.
In each year, two sites in each of the three soil types had TDR probes inserted vertically to a depth of 1.2 m. Sites were in different locations in the 2 yr of the study. The Bojac1 sites were referred to as Bojac1A and Bojac1B in 1998 and Bojac1C and Bojac1D in 1999. Likewise, the Bojac2 sites were referred to as Bojac2A and Bojac2B in 1998 and Bojac2C and Bojac2D in 1999. Similarly, the Wickham sites were referred to as WickhamA and WickhamB in 1998 and WickhamC and WickhamD in 1999.
Soil moisture measurements were made once a week during the growing season. In 1998, the first measurements were collected 2 d after planting (DOY 106) and the last measurement 1 wk before harvest (DOY 236). In 1999, TDR measurements commenced 2 wk after emergence (DOY 151), and the final TDR measurement was collected 4 wk before harvest (DOY 234). Corn was physiologically mature on all three soil types on the date of the last TDR measurements in each year.
Soil Water Balance
Total stored water of the 1.2-m soil profile was calculated as the summation of stored water for each depth as measured by the TDR. The water balance of the soil profile throughout the growing season was determined from the simple water balance equation
 | [1] |
Time series of water content at each site (0–1.2 m) were interpolated to a daily basis using the procedure described below (Schwab et al., 1993; Sadler et al., 2000). The procedure used the same basic equation in two ways, dependent on the precipitation intensity and likelihood of drainage between TDR measurements.
Where precipitation had not exceeded 10 mm in any one day between measurements and was not likely to exceed soil water-holding capacity, daily ETa could be calculated using Eq. [1], assuming no drainage or runoff. Using this value of ETa, a calculated ETo (Allen et al., 1998), and a soil water stress factor (Ks) (see below, Eq. [2]), we solved for the crop factor (Kc). When precipitation events had exceeded 10 mm in any one day or were likely to bring soil water content to the upper drained limit, an interpolated value of crop factor was utilized. Precipitation events <10 mm were assumed to be unlikely to have exceeded soil infiltration rates of these coarse-textured soils that had been managed in no-till for 2 yr. The soil water stress factor (Ks) and crop factors (Kc) are defined from the equation
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 | [3] |
Available water-holding capacity (AWHC) was calculated using best estimates of drained upper and lower limit of plant-available water for each layer at each TDR probe site (Sadler et al., 2000). These estimates were obtained by inspecting records of TDR measurements of soil water contents at the sites supplemented by lab measurements of water retention using intact soil cores (USDA-NRCS, 1996). For the four TDR sites in each soil type to a depth of 1.2 m, AWHC values (mean and standard deviation) were 89 ± 8, 149 ± 12, and 194 ± 15 mm for Bojac2, Bojac1, and Wickham, respectively.
Efficiency of Water Use
Pierce and Rice (1988) define EWU as yield divided by water available. The concept of EWU as defined by Pierce and Rice (1988) is modified in this paper to
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Efficiency of water use thus represents the main growth period of corn (from just after emergence to physiological maturity).
Statistics
Statistical analyses were carried out using one-way ANOVA methods of the Minitab 13 software (Minitab, 1999). Tukey's test (P 
0.05) was used for mean separation. Statistical comparisons of Ks values used a mean value of the calculated Ks values from the weekly TDR measurements for each probe during each crop growth stage. Thus, each soil type had two values of Ks (one for each probe) for each crop growth stage. Statistical comparisons of Kc and ETa were conducted similarly, having two mean values (one for each probe) for corn growing on each soil type in each growth stage. Comparisons of yield data used the average of 30 yield points around the location of the TDR probe to represent the yield for each probe location. Linear regression analyses were completed using Sigmaplot 6.0 (SPSS, 2000).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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To separate the effects of ETa, runoff, and drainage, the measured water content was combined with Eq. [3] to calculate Ks values. Measured water content was converted to percentage of available water using upper drained and lower limit of plant-available water (Fig. 2)
. Water stress in the three growth stages—vegetative, tasseling, and grain fill—has been shown to have varying impacts on yield. To determine if water stress differed between the soils during these growth stages in this experiment, the averages of the Ks values calculated from each of the TDR probes during each growth stage were compared. Mean values and statistical comparisons are shown in Table 2. In 1998, no significant differences were observed between the mean values of Ks for the soil types during the crop growth stages. However, in 1999, the Wickham soil maintained higher Ks values than the Bojac2 soil during the vegetative growth stage, indicating less water stress for the corn growing on the Wickham soil during this growth phase. In both years, and in all three soil types, Ks values in the grain-fill growth stage were lower than Ks values of the tasseling and vegetative growth phases (P 0.08), indicating the greatest water stress occurred during grain fill in both years. The observed differences in water stress between the soils would be expected to influence yields on these soils.
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The calculated Kc values varied between soil types, crop growth stage, precipitation distributions, and years (Fig. 4 and Table 3). The Kc values were lower than the recommended value of 1.20 presented by Allen et al. (1998) for calculating crop water use during the main growth period of corn. Values of Kc on the Bojac1 and Wickham soils did exceed 1.20 for some periods during both seasons (Fig. 4). The Kc values in this study represent years of below-average precipitation; in years of greater precipitation, Kc values may be closer to those of Allen et al. (1998) through lower water stress and increased growth. However, the differences between our observed values and those suggested by Allen et al. (1998) would result in different calculated water use for dry periods. The variation in the precipitation data in Table 1 shows that dry periods can be expected to occur in the mid-Atlantic coastal plain. Thus, when using computer models to simulate water balances for corn during these dry periods, the use of lower Kc values is recommended.
Daily Soil Water Balance
The Kc curves enabled calculation of a daily water balance. The daily values of Kc over the season were established using measured Kc values, where drainage and runoff were unlikely, and values from the interpolated Kc curves (Fig. 4) where drainage and runoff may have occurred. The daily water balance was solved using Eq. [1]. The first measured TDR values were used as starting points for total water content and Ks. In the daily calculation, soil water content was increased through inputs from precipitation and decreased by daily ETa. The daily calculation of ETa used a calculated Ks based on the previous day's available water content and a daily record of Kc and ETo. Predictions of total soil water content were limited to the upper drained limit. Any water predicted in excess of the upper drained limit was considered lost through runoff and drainage. The predictions of this daily interpolation of the available water content fitted well with the measured values of water content of the sites throughout both years (Fig. 1).
Differences were observed in daily ETa on the three soil types during the three growth stages (Table 3). In 1998, all three soil types had significantly greater daily ETa during the tasseling period than the vegetative or grain-fill period. The tasseling period of 1998 was also the time of greatest precipitation during the two growing seasons. Water stress would have been minimal during this period, and the ETo was high (5.1 mm d-1). In 1999, daily ETa values in the tasseling and grain-fill periods were significantly higher than those of the vegetative growth stage on all three soil types. In 1999, ETa rates on the Wickham soil followed a pattern of increasing ETa throughout the growth stages, with each stage being significantly higher than the previous, from vegetative to grain-fill growth stages. As for the Wickham soils, the Bojac soils showed increased ETa rates from vegetative to tasseling. In contrast to the Wickham soils, tasseling and grain-fill ETa rates were similar for the Bojac soils.
In 1998, differences in ETa between the soils reflected the combination of precipitation distribution and water-holding capacity of the soils. The greater-than-average precipitation in June 1998 (Table 1) resulted in similar water use from all three soils during the vegetative phase. Significantly lower ETa was observed on the Bojac2 soil during tasseling–silking. The lower plant populations on this soil type may have contributed to lower water-use rates as water availability was high during this time. During grain fill in 1998, however, precipitation was limited to 53 mm over the 50 d till physiological maturity. During this grain-fill period in 1998, differences in average ETa rates were observed for the three soil types. The differences reflected the water-holding capacities of the soils, with ETa values of Wickham greater on average than those of Bojac1, which were greater than those of Bojac2 (Table 3). In 1999, corn growing on all three soils had similar ETa rates during the vegetative and tasseling growth stages. In the grain-fill period of 1999, ETa values on the Wickham soil were significantly greater than those on the Bojac soils. Again, this reflected the capacity of the soils to retain precipitation for later use.
The cumulative ETa of the six sites (Fig. 5) shows the additive effects of the differences in water use at the six sites during the 1998 and 1999 corn growing season. In both 1998 and 1999, the corn grown on the Bojac1 and Wickham soils had significantly higher cumulative ETa than the corn on the Bojac2 soil. Corn growing on Bojac1 soil in 1998 had significantly higher cumulative ETa than corn grown on the Bojac1 soil in 1999. Similarly, corn growing on Wickham soil in 1998 had significantly higher (P 0.05) cumulative ETa than corn grown on the Wickham soil in 1999.
Cumulative ETa values in this experiment (Fig. 5) ranged from 218 to 336 mm in 1998 and 162 to 241 mm in 1999 with precipitation of 400 and 220 mm, respectively. These values are lower than cumulative evapotranspiration reported for corn grown in southern Virginia, 434 to 454 mm (Khosla and Persaud, 1997). The values of the present study are likely lower than those of Khosla and Persuad (1997) due the greater precipitation received by the corn in their study (480 mm). However, the cumulative ETa values in the present study are similar to those reported for corn grown in the South Carolina (183–301 mm; Sadler et al., 2000).
Cumulative Losses
The daily interpolation of water balance predicted the combined value of drainage and surface runoff from the sites when the precipitation additions to the soil exceeded the upper drained limit. The combined total of calculated surface runoff and drainage will be referred to as losses. Precipitation runoff on these coarse-textured soils is likely to be minimal given infiltration rates of >20 mm h-1 for all three soil types (USDA, 1985). Losses of water from the soil were only observed in May and June of 1998 before DOY 174, following several large precipitation events (Fig. 6)
. Total cumulative losses ranged from 61 mm for site WickhamB to 105 mm for Bojac 2B and were not significantly different between soil types. Cumulative losses were more closely related to the cumulative ETa to DOY 174 (R2 = 0.93) than water-holding capacity of the soil types (R2 = 0.68). This is expected as the rainfall on DOY 119 to 130 had recharged all soil profiles to the upper drained limit. From DOY 130 to 174, ETa was the only source of water removal from the soil profile. Thus, the amount of ETa was directly related to the amount of soil water that the profile could store in the event of a large precipitation event before drainage occurred again.
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Yields
In 1998, corn grain yields were significantly different between the three soil types (Table 4). Yield differences followed the same pattern as the AWHCs, with the highest yields observed for the Wickham soil (Table 4), which has an AWHC of 194 mm. The Bojac1 soil, with the AWHC of 149 mm, produced lower yields than the Wickham soil but greater yields than the Bojac2 soil (Table 4), which has an AWHC of only 89 mm.
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Corn grain yield can be reduced by moisture deficit at any growth stage (Howe and Rhoades, 1955; Denmead and Shaw, 1960). The full yield potentials of these soils were probably not observed in this study due to the below-average precipitation and water stress observed in the grain-fill growth phase of both years. The variation in yield for these soils highlights the need for soil specific management. Crop fertilization in particular needs to be managed individually for the soil types with the varying yield potentials. Corn stalk NO3 tests (Binford et al., 1992) were completed for corn grown on all soils in both years to determine N rates sufficient for maximum productivity. Phosphorus, K, and other essential nutrients were determined to be adequate for the corn on all soils in both years, and the yield differences could be attributed to variations in water availability.
Efficiency of Water Use
To compare crops and management practices in terms of the production of grain in relation to water availability, a standardized measurement technique is required. A concept used in many studies (Norwood, 1999; Trooien et al., 1999; Owesis et al., 2000; Sadler et al., 2000) to measure the relationship between soil water and yields is WUE, which is defined as grain yield divided by ETa (Pierce and Rice, 1988). In some studies (Copeland et al., 1993; Norwood and Currie, 1996; Khosla and Persaud, 1997; Norwood, 1999), runoff and drainage have been assumed to be negligible, and WUE has been calculated by using the water balance equation (Eq. [1]) to solve for ETa as shown by Eq. [5].
 | [5] |
However, in the mid-Atlantic coastal plain, runoff and drainage losses can be substantial during the growing season (Fig. 6). Including these drainage losses in the calculation is important when determining and comparing how efficient a cropping system or management practice is with regard to yields and water availability. Both the WUE and EWU concepts can be calculated as {grain yield divided by [growing season precipitation + (soil water at planting - soil water at harvest)]}. The distinction between WUE and EWU depends on whether the calculation of EWU includes runoff and drainage losses as shown in Eq. [6].
 | [6] |
When EWU and WUE are calculated by this method {grain yield divided by [growing season precipitation + (soil water at planting - soil water at harvest)]}, they are directly comparable as a measure of production efficiency in relation to water availability. The EWU and WUE measures are an advance on the precipitation use efficiency concept, which relates yields only to precipitation. Efficiency of water use includes both the initial and final water content. In years of low precipitation, water availability at the start of the season can be a major source of water the crop uses for initial growth. Furthermore, the retention of water by a crop in rotation for the next crop is important to providing water availability for early growth of the next crop. This is likely to be particularly important where double cropping occurs and there is only a small chance for profile recharge between crops. For example, double-cropped soybean is planted within a few days of harvest of small grains, and wheat can be planted within a few weeks of corn harvest.
The dry corn growing seasons of 1998 and 1999 have shown the importance of soils with higher water-holding capacity on yield and EWU (Table 4). In 1998, the EWU of corn growing on the three soils was directly related to the soil water-holding capacity. Increasing water-holding capacity improved EWU. Efficiency of water use in the 1999 season followed a similar pattern; however, as with yields, there was no significant difference between the Bojac1 and Bojac2 soils (Table 4). The EWU values of corn grown on each of the soils were consistent between the years.
The EWU values of no-till corn in this study ranged from 1.37 to 3.64 kg m-3 and are similar to those calculated by the same method (Eq. [5]) but termed WUE in other studies. Copeland et al. (1993) measured WUE of 2.6 kg m-3 for rainfed corn grown in Minnesota, which is within the range measured in this study. By comparison, Norwood (1999) measured WUE values ranging from 0.69 to 1.89 kg m-3 for dryland no-till corn in Kansas. The greater precipitation received by rainfed corn of the mid-Atlantic coastal plain in this study resulted in generally greater yields and EWU than obtained by Norwood (1999). Similarly, greater yields and EWU were observed in the present study than the 0.75 to 0.83 kg m-3 observed for corn grown in southern Virginia by Khosla and Persuad (1997). Sadler et al. (2000) reported WUE values ranging from 0.36 to 1.57 kg m-3. In the present study, the values for the Bojac soil in both years are in the range of those from Sadler et al. (2000). The EWU values of both the Wickham and Bojac1 soils are above this level of efficiency. This is likely due to the greater precipitation received in this study compared with that received by the corn in the study by Sadler et al. (2000).
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMETHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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