Published in Agron. J. 96:608-614 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
NITROGEN MANAGEMENT
Diagnostic Tests for Site-Specific Nitrogen Recommendations for Winter Wheat
Larry G. Bundy* and
Todd W. Andraski
Dep. of Soil Sci., 1525 Observatory Drive, Univ. of Wisconsin, Madison, WI 53706-1299
* Corresponding author (lgbundy{at}wisc.edu).
Received for publication September 16, 2003.
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ABSTRACT
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Wheat (Triticum aestivum L.) yields can be limited by both inadequate and excessive N availability. This study evaluated several diagnostic tests for predicting the economic optimum N rate (EONR) for winter wheat at 21 site-years. Tests included soil NO3 (90 cm) at three sampling times [preplant, predormant, and at Zadoks Growth Stage (GS) 25—five tillers], UV absorbance of NaHCO3 soil extracts (30 cm) at preplant, and plant N concentration, N uptake, and chlorophyll meter measurements taken at GS 30 (pseudo stem initiation). All samples were obtained from the control plots (no N fertilizer). Yield response to applied N fertilizer was positive for 13 site-years (EONR of 34 to 168 kg ha–1), negative for three site-years, and not significant for five site-years. Nitrogen additions to sites with high soil NO3 levels resulted in yield reductions up to 30%. The strongest relationship among diagnostic tests and EONR was preplant soil NO3 content at the 90-cm depth (R2 = 0.53). The combination of soil NO3–N content at GS 25 plus N uptake at GS 30 was also strongly related to EONR (R2 = 0.58). Nitrogen rate recommendations for wheat adjusted for preplant soil NO3 contents >56 kg ha–1 avoided excessive N applications at 11 of 21 sites, thereby increasing economic gains by $1 to $110 ha–1 (median $17) due to reduced fertilizer costs and avoiding yield reductions due to excessive N availability. Wheat N recommendations based on preplant soil NO3–N measurements in the 0- to 60-cm depth have potential for avoiding reduced profits, lower yields, and environmental risks associated with excess N use.
Abbreviations: EONR, economic optimum nitrogen rate • GS, Zadoks growth stage
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INTRODUCTION
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SOIL NO3 tests are an effective method for identifying optimum N rates for corn (Zea mays L.) in several cropping systems commonly used in Wisconsin (Bundy and Andraski, 1995). Use of these tests often allows a reduction in the N rate applied for corn and thereby provides economic benefits to producers as well as reduced potential for loss of NO3 to groundwater. The success of this approach has stimulated producer's interest in using similar methods for other N-demanding cereal crops such as winter wheat. Winter wheat acreage in Wisconsin has increased by 340% since the 1970s due expanded crop rotation options, low production costs, good grain and straw yields, and ground cover protection during periods of potentially high soil erosion. Diagnostic tests for predicting wheat N requirements have been evaluated in the eastern USA and in the Great Plains wheat-producing areas. The substantial soil and climatic differences between these regions and Wisconsin prevent direct transfer of N test methods for use in humid northern climates where little or no evaluation of N tests for predicting wheat N needs has been done.
The N test approaches that have been found most successful in previous work are (i) soil inorganic N measured at various times and soil depths early in the wheat-growing season, (ii) N concentrations in wheat plants at specific growth stages, and (iii) chlorophyll meter readings on wheat plants at specific growth stages. Wheat stem NO3 concentrations have also been used to determine N needs in some areas (Papastylianou et al., 1982, 1984; Roth et al., 1989; Knowles et al., 1991). However, Vaughan et al. (1990a) found that stem and whole-plant NO3 concentrations were too variable for reliable use in wheat N recommendations. Goos et al. (1982) reported that grain protein concentration was an effective postharvest indicator of N sufficiency in wheat.
Numerous studies have evaluated the use of residual soil profile NO3 tests in the Great Plains region as a guide for N fertilization of wheat and other small grains. Early work (Soper et al., 1971; Olson et al., 1976) showed that yield response to applied N and crop N uptake were strongly affected by soil residual NO3. Olson et al. (1976) reported that grain yield response to added N was unlikely if soil profile inorganic N exceeded 120 kg ha–1. Gelderman et al. (1988) found that soil NO3 (0 to 30 cm) was the best predictor of N uptake by wheat in South Dakota, and consideration of several other N availability indices (soil organic matter, UV absorbance of NaHCO3 soil extracts, and NH3 released by autoclaving soil with CaCl2) improved the prediction of wheat N uptake. The UV260 test indicates the amount of dissolved organic matter contained in the soil, and the UV200 test reflects the amount of dissolved organic matter plus NO3 content in the soil (Hong et al., 1990). In humid regions, Scharf and Alley (1994) showed that winter wheat yield response to added N was strongly influenced by the amount of inorganic N in the top 120 cm of several soils in Virginia. Vaughan et al. (1990b) found that soil NH4 levels in spring explained more wheat yield variation than did NO3, and sampling to depths greater than 60 cm did not improve the test. However, the procedure providing the best prediction of wheat yield response in this eastern Colorado study included soil NO3 plus NH4–N to a 60-cm depth.
Relationships between plant N concentrations at various growth stages and wheat yields or response to added N have been extensively studied. Most of these studies have found good predictive value from plant N concentrations or plant N uptake measured at or near GS 30 (Donohue and Brann, 1984; Baethgen and Alley, 1989a, 1989b; Roth et al., 1989; Vaughan et al., 1990a; Scharf and Alley, 1993; Scharf et al., 1993). This time of plant sampling represents a compromise between good test predictive value and time availability after sampling and analysis to apply additional N if needed. For example, samples taken at GS 25 would allow more time for analysis and subsequent N application, but the predictive value of samples taken at GS 25 is not as good as that from samples taken at GS 30 (Vaughan et al., 1990a). Vaughan et al. (1990b) found that a combination of spring soil inorganic N and plant N concentrations provided a better prediction of wheat N response than either soil or plant N measurements alone. Similarly, Scharf and Alley (1993) developed an N-testing strategy involving both a soil NO3 test (90 cm) and plant N uptake at GS 30, depending on the availability of tiller density counts at GS 25 and grower interest in making split N applications.
Chlorophyll meter readings at GS 30 have been studied as a method of predicting wheat response to N fertilization (Follett et al., 1992; Fox et al., 1994). This work indicates that chlorophyll meter readings are well related to yield, leaf N concentration, and soil inorganic N content (Follett et al., 1992). Fox et al. (1994) found that chlorophyll meter readings were more accurate than plant N concentration for predicting wheat response to N fertilization, but too few N responsive treatments were included in their study to determine if chlorophyll meter readings can be used to predict optimum N rates for wheat. Schepers et al. (1992) suggested normalization of chlorophyll meter readings on corn ear leaves by comparison with readings from adequately fertilized treatments. This approach was used by Fox et al. (1994) and recommended by Follett et al. (1992).
The sites selected in our study represent a range of production conditions including variables such as soil characteristics, previous crop and N management, and annual climatic differences. The objective of this study was to evaluate several N diagnostic tests (soil NO3 in the root zone at three sampling times, UV absorbance at two wavelengths of NaHCO3 soil extracts in the top 30 cm at preplant, and early-season plant N concentration, N uptake, and chlorophyll meter measurements) for site-specific prediction of optimum N rates for winter wheat on a site-specific basis.
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MATERIALS AND METHODS
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Experiments to evaluate several diagnostic tests for site-specific prediction of optimum N rates for winter wheat were established at the University of Wisconsin Research Stations at Lancaster and Arlington and on private farms at Racine and Chilton. Soil textures were loam, silt loam, and silty clay loam, and previous crops included corn silage, oat (Avena sativa L.), cabbage (Brassica oleracea var. capitata L.), winter wheat, and soybean [Glycine max (L.) Merr.] (Table 1). At Arlington, corn silage was grown with four N fertilizer rates to establish a range of residual soil NO3–N levels before planting wheat.
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Table 1. Site characteristics of winter wheat studies and previous crop management history from 21 site-years, 1996–1999.
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Winter wheat was planted in 19-cm rows at a seeding rate of 320 to 380 seeds m–2 in mid- to late September. Winter wheat varieties included ‘Cardinal’, ‘Dynasty’, ‘Kaskaskia’, and ‘Pioneer 25R26’. A separate experimental area was used each year to avoid residual N treatment effects. The experimental design was a randomized complete block consisting of five to six N fertilizer rate treatments and four replications. Plot dimensions were 7.6 m long by 3 m wide. Soil test P and K levels were in the excessively high category for winter wheat (Kelling et al., 1998); therefore, no additional P and K fertilizer was applied. Nitrogen treatments included rates ranging from 0 to 136 or 170 kg N ha–1 in 34-kg increments that were surface broadcast-applied in early spring at about GS 25 for wheat at Chilton, Racine, and Lancaster using NH4NO3 to avoid potential N losses through NH3 volatilization. At Arlington, N treatments included rates ranging from 0 to 136 kg N ha–1 in 34-kg increments using urea broadcast-applied in September just before seedbed preparation.
Soil samples were collected from the 0 kg N ha–1 treatment before wheat planting in September (preplant), before winter freeze-up in November (predormant), and in April following soil profile thaw at GS 25 for wheat. Samples included three cores from each plot taken to a depth of 90 cm in 30-cm-depth increments. Soil samples were dried at 33°C in a forced-draft dryer and ground to pass a 2-mm screen. Nitrate in the soil samples was determined by automated analysis of 2 M KCl extracts (Bundy and Meisinger, 1994). The UV absorbance of NaHCO3 preplant soil extracts (0 to 30 cm) were also determined at the 200- and 260-nm wavelengths (Hong et al., 1990). Dry matter yield was determined at GS 30 (May) by collecting aboveground plant samples from a 0.5-m2 area from the control (0 kg N ha–1). Plant samples were dried at 60°C and ground to pass a 1-mm screen. Total N concentration in wheat tissue was determined by automated analysis of NH4 in digested samples (Nelson and Sommers, 1973). Nitrogen uptake was calculated from tissue N concentrations and corresponding dry matter yields. Chlorophyll meter readings were taken on the uppermost fully expanded true leaf of about 20 representative plants at GS 30 from the control (0 kg N ha–1) using a Minolta SPAD 502 chlorophyll meter. At maturity, grain yields were determined by harvesting each plot with a plot combine. Grain yields are reported at a grain H2O concentration of 135 g kg–1. Lodging was evaluated at Chilton and Racine using the Belgian system (Szoke et al., 1979).
An analysis of variance was performed to determine the effect of N rate on grain yield (SAS Inst., 1992). Economic optimum N rate and grain yield at EONR were determined by regression analysis and consisted of comparing linear-response plateau and quadratic-response plateau models using PROC NLIN and linear and quadratic regression models using PROC REG. Economic optimum N rates reflect a fertilizer/wheat price ratio calculated from prices of $0.55 kg–1 of N fertilizer and $117 Mg–1 of winter wheat. A standardized method was used to determine the EONR due to the variability of EONR typically determined by the various models (Bundy and Andraski, 1995). Where the effect of N rate was significant (P < 0.10), the EONR was identified using the regression model with the highest R2 value if that value was 
0.25. If the R2 value was <0.25, mean separation analysis was used to identify the optimum N rate as the lowest N rate treatment in the highest t grouping for yield using PROC ANOVA. If N rate was not significant at the 0.10 probability level, the EONR equals zero.
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RESULTS AND DISCUSSION
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Grain Yield and Economic Optimum Nitrogen Rate
The effect of N rate on wheat grain yield was significant at 16 of 21 sites (Table 2). Yield response to applied N was positive for 13 sites, negative for three sites, and not significantly different for five sites. The EONR ranged from 34 to 168 kg ha–1 (median = 78 kg ha–1) among the 13 sites with a positive yield response and 0 kg ha–1 for the remaining eight sites. Yields ranged from 1.21 to 5.13 Mg ha–1 where no N was applied and from 2.29 to 5.78 Mg ha–1 at the EONR. The correlation between EONR and yield at the EONR among sites was weak (r = 0.43; p = 0.05), indicating the level of N response was not well related to observed yields. The EONR and yield at the EONR was highest at the Chilton location (Sites 9, 16, and 19) for any given year, apparently due to soil textural and organic matter differences compared with the other sites.
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Table 2. Effect of N rate on wheat grain yield, economic optimum N rate (EONR), and yield at EONR for 21 site-years.
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Soil and Plant Nitrogen Diagnostic Tests
The relationship between soil N diagnostic tests and EONR for three sampling times is shown in Table 3. The relationship between the UV200 test and EONR resulted in a higher R2 value (0.28) than the UV260 test (0.09). The critical test level (absorbance) for the UV200 test was 0.721. Hong et al. (1990) reported higher correlation coefficients between soil N-supplying capability (0 to 20 cm) and UV200 (r = 0.73) than UV260 (r = 0.41). Likewise, Gelderman et al. (1988) found a poor correlation between UV260 (0 to 15 cm) and N uptake in spring wheat (r = 0.21).
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Table 3. Relationships between soil N diagnostic tests and economic optimum N rate for winter wheat, and critical test level (CTL) for three sampling times using a linear-response plateau (LRP) regression model from 21 site-years.
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The strongest relationship between soil NO3–N content and EONR occurred at preplant (R2 = 0.36 to 0.53) and declined with successive sampling times (R2 = 0.20 to 0.44 and 0.12 to 0.21 for predormant and GS 25, respectively). The R2 values increased as soil sampling depths increased for all sampling times. The strongest relationship between soil NO3–N content and EONR was for the preplant 90-cm depth (R2 = 0.53), with a critical soil NO3–N test level of 121 kg ha–1. These results are similar to the relationships between preplant soil NO3–N contents and EONR for corn reported by Bundy and Andraski (1995) except for the lower N needs of wheat compared with corn (Fig. 1)
. Similarly, Olson et al. (1976) found yield response to applied N depended on preplant soil NO3–N contents (180 cm) with critical test levels of 120 kg ha–1 for winter wheat and 240 kg ha–1 for irrigated corn in Nebraska.
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Fig. 1. Relationship between preplant soil NO3–N content (0 to 90 cm) and economic optimum N rate (EONR) for winter wheat from 21 site-years (solid line).
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A wide range of EONRs was observed where preplant soil NO3–N contents were below 121 kg ha–1. The two highest EONRs (134 and 168 kg ha–1) occurred at Chilton (Sites 19 and 16) where the soil organic matter content was 19 g kg–1. The current Wisconsin base N rate recommendation for winter wheat is 68 and 90 kg ha–1 for soils with organic matter contents >20 and <21 g kg–1, respectively (Kelling et al., 1998). The EONRs for these two sites were 48 and 87% higher than base N recommendations. Climatic conditions and changes in soil NO3–N content did not explain the high EONR (116 kg ha–1) at Racine (Site 11) or the low EONR (0 kg ha–1) at Arlington (Sites 2 and 14) when soil NO3–N contents were below the critical test value. The remaining N-responsive sites at low preplant soil NO3–N levels (<50 kg ha–1) had EONRs near current base N rate recommendations for wheat.
Total N uptake, tissue N concentration, and SPAD chlorophyll meter readings at GS 30 were poorly related to EONR, with R2 values of 0.26, 0.13, and 0.12, respectively (Table 4). Critical test levels were 68 kg N ha–1 for N uptake, 49 g kg–1 for tissue N, and 53 for SPAD. Other studies indicated critical test levels ranging from 48 to 100 kg ha–1 for N uptake and 32 to 50 g kg–1 for tissue N (Donohue and Brann, 1984; Baethgen and Alley, 1989b; Roth et al., 1989; Vaughan et al., 1990a; Scharf et al., 1993). The critical SPAD test level identified in our study is slightly higher than ranges (42 to 46) reported in earlier studies (Follett et al., 1992; Fox et al., 1994; Singh et al., 2002). A strong correlation between SPAD and tissue N at GS 30 (r = 0.78; p < 0.01) shows that the SPAD meter provides a good indication of N concentration in aboveground tissue.
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Table 4. Relationships between plant N diagnostic tests and economic optimum N rate for winter wheat, and critical test level (CTL) at Zadoks Growth Stage 30 (GS 30) using a linear-response plateau (LRP) regression model from 21 site-years.
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The poor predictability of EONR using a single early-season plant or postpreplant soil test suggests that both accumulated (plant) and potentially available (soil) N need to be considered in identifying optimum N rates. Total N uptake at GS 30 accounted for an average of 54, 44, and 26% of total plant N uptake at physiological maturity in the control treatment relative to the N uptake at EONR where the EONR was 0, 1 to 100, and >100 kg ha–1, respectively. Previous studies confirm significant N uptake in early stages of wheat development across a range of climatic regions (Karlen and Whitney, 1980; Christensen and Brett, 1988; Baethgen and Alley, 1989a; McGuire et al., 1998) and wheat cultivars (Costa et al., 2000).
The combination of GS 25 soil NO3–N content plus GS 30 plant N uptake was strongly related to EONR, with R2 values ranging from 0.55 to 0.60 (Table 5). However, two spring N diagnostic tests would need to be performed to provide an estimate of N availability to winter wheat as reliable as that with a single preplant soil NO3–N test. In Virginia, Scharf and Alley (1993) developed an N-testing strategy involving both a soil NO3 test (90 cm) and plant N uptake at GS 30, depending on the availability of tiller density counts at GS 25 and grower interest in making split N applications.
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Table 5. Relationships between soil NO3–N content at Zadoks Growth Stage 25 (GS 25) plus wheat N uptake at GS 30 and economic optimum N rate for winter wheat and critical test level (CTL) at three soil sampling depths using a linearresponse plateau (LRP) regression model from 21 site-years.
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Excess Nitrogen Effects on Wheat Yield
Excessive N availability can significantly reduce wheat yield (Fig. 2)
. The decline in wheat yield was frequently observed where N fertilizer was added to soils with high residual NO3 contents. Relative yields declined by 15% as preplant soil NO3–N content plus N fertilizer rate increased from about 200 to 450 kg N ha–1 at Arlington, Chilton, and Racine. This relationship was more pronounced at Lancaster where relative yield declined by 30% as soil NO3–N plus N fertilizer rate increased from about 150 to 300 kg N ha–1. One reason for yield reductions due to excessive N availability is increased lodging. This is illustrated by the strong relationship (r2 = 0.87) between lodging and preplant soil NO3–N content plus N fertilizer rate (Fig. 3)
. A sharp yield decline was apparent where lodging values were >4 (Fig. 4)
. Another potential contributor to yield reductions was lower grain test weights. A highly significant (p < 0.01) inverse correlation between soil NO3–N plus N fertilizer rate and grain test weight occurred at Arlington (r = –0.72) but not at the other sites.
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Fig. 2. Relationship between preplant soil NO3–N content (0 to 90 cm) plus N fertilizer rate and relative grain yield from 21 site-years.
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Fig. 3. Relationship between preplant soil NO3–N content (0 to 90 cm) plus N fertilizer rate and winter wheat lodging from six site-years. Belgian lodging scale 0.2, none and 9.0, severe.
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Fig. 4. Relationship between winter wheat lodging and relative grain yield where the N fertilizer rate was greater than or equal to the economic optimum N rate from six site-years. Belgian scale 0.2, none and 9.0, severe.
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Economics of Preplant-Based Nitrogen Rate Recommendations for Winter Wheat
The relationship between preplant soil NO3–N content and EONR shown in Fig. 1 indicates yield response to the full base N rate recommendation usually occurs where NO3–N levels are less than 56 kg ha–1. Similarly, corn N rate recommendations using the preplant soil NO3 test in Wisconsin include a background soil NO3–N level of 56 kg ha–1 where yield response to the full base N rate is likely to occur (Bundy and Sturgul, 1994). Using this method, N rate recommendations for wheat based on a preplant soil NO3 test were calculated as:
where NrR is the N rate recommendation (kg ha–1), BNr is the base N rate (68 or 90 kg ha–1 for soil organic matter 20 or <20 g kg–1, respectively), PPNT is the preplant soil NO3–N content in the top 90 cm (kg ha–1), and 56 is the background soil NO3–N level (kg ha–1). Using the regression models for determining EONR for each site-year, yields were calculated for N rates determined using the preplant and base N rate recommendation methods. Economic returns were determined for each site and N rate recommendation method by: (Mg grain ha–1 x $117 Mg–1 grain) – (kg N fertilizer ha–1 x $0.55 kg–1 N fertilizer). Economic gain (or loss) was calculated as the difference in economic returns for the preplant vs. base N rate recommendation methods.
Accounting for preplant soil NO3–N contents >56 kg ha–1 resulted in an economic gain of $0.34 kg–1 of soil NO3–N compared with base N rate recommendations for 11 of 21 sites (Fig. 5)
. Economic gains averaged $32 ha–1 for these 11 sites and increased linearly as soil NO3–N values increased from 56 to 355 kg ha–1 (r2 = 0.96). Gains were as high as $40 ha–1 due to lower N fertilizer costs where soil NO3–N contents were 56 to 200 kg ha–1 at nine sites. Further increases in economic gains (up to $110 ha–1) occurred at two sites where soil NO3–N contents were >200 kg ha–1 due the prevention of yield losses due to excessive N availability. The preplant-based N rate recommendation method resulted in an economic loss ($6.12 ha–1) at one site (11) due to an inadequate N rate (60 kg ha–1) and a 0.09 Mg ha–1 yield loss compared with the base N rate. There was no economic gain from using the preplant soil test at nine sites where soil NO3–N contents were 
56 kg ha–1.
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Fig. 5. Relationships between preplant soil NO3–N contents and economic gains using N rate recommendations adjusted for measured (0 to 90 cm) [solid line] and measured (0 to 60 cm) plus predicted (60 to 90 cm) [dashed line] preplant soil NO3 contents compared with base N rates from 21 site-years.
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Predicting Nitrate in the 60- to 90-cm Soil Depth
Soil sample collection to a 90-cm depth can be difficult for certain soil types. Relationships between preplant soil NO3–N content at several depth increments and at the 60- to 90-cm depth increment were determined to assess whether shallower soil sampling depths could be used to predict soil NO3–N contents at deeper depths (Table 6). Soil NO3–N contents ranged from 10 to 252 kg ha–1 in the 0- to 30-cm depth, 3 to 76 kg ha–1 in the 30- to 60-cm depth, and 4 to 46 kg ha–1 in the 60- to 90-cm depth. Median soil NO3–N values for the aforementioned soil depth increments were 30, 12, and 12 kg ha–1, respectively. Significant linear relationships occurred between soil NO3–N in both the top 30- and 60-cm-depth increments with soil NO3–N contents in the 60- to 90-cm depth (r2 = 0.46 and 0.55). The soil NO3–N content at the 30- to 60-cm-depth increment provided the best prediction of soil NO3–N content at the 60- to 90-cm depth (r2 = 0.74).
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Table 6. Linear relationships between preplant soil NO3–N contents at several depth increments and the 60- to 90-cm depth from 21 site-years.
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Our results reflect those from an earlier Wisconsin study conducted on over 2000 samples by Ehrhardt and Bundy (1995). They concluded that a 60-cm soil sampling depth can be reliably used in a preplant soil NO3 test for corn when NO3–N content from the 60- to 90-cm depth is estimated with a predictive model. Economic gains using N rate recommendations based on measured (0 to 60 cm) plus predicted (60 to 90 cm) preplant soil NO3 contents (r2 = 0.97) were nearly identical to the measured values to a 90-cm depth (Fig. 5).
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CONCLUSIONS
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Preplant soil NO3–N content had the strongest relationship with the EONR for wheat compared with other soil sampling times and diagnostic tests. The combination of soil NO3–N content at GS 25 plus total plant N uptake at GS 30 was also related to the EONR but would be more labor intensive and costly. Excessive N availability from N fertilizer additions at sites with high soil NO3 contents resulted in significant yield reductions due to increased lodging and lower grain test weights. Nitrogen rate recommendations adjusted for preplant soil NO3–N resulted in average economic gains of $29 ha–1 (median $17) and increased linearly as soil NO3–N values increased from 56 to 355 kg ha–1. Gains were as high as $40 ha–1 due to lower N fertilizer costs where soil NO3–N contents were 56 to 200 kg ha–1 and as high as $110 ha–1 where soil NO3–N contents were >200 kg ha–1 due to the prevention of yield losses associated with excessive N availability. A 60-cm soil sampling depth can be reliably used for a preplant soil NO3 test for wheat when NO3–N content from the 60- to 90-cm depth is estimated with a predictive model based on NO3 contents in the 30- to 60-cm depth.
A preplant soil NO3 test for winter wheat has greater profit potential compared with corn production systems since economic gains can result from avoiding yield losses due to excessive N availability in addition to reduced N fertilizer cost. In states with livestock-based farming systems, such as Wisconsin, significant levels of soil NO3 can carry over to the following crop due to previous N additions including manure, legumes, and fertilizer N. The potential for high economic gains coupled with the reduced risk of NO3 loss to the environment due to excessive N additions make the preplant soil NO3 test a practical N management tool for winter wheat production systems.
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ACKNOWLEDGMENTS
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The authors are grateful to Julie Studnicka and Peter Wakeman from the Department of Soil Science for field and laboratory work; Mark Martinka from the Agronomy Department for planting, harvesting, and lodging evaluations at the Chilton and Racine sites; and the staff at the Lancaster and Arlington Research Stations.
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NOTES
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Research supported by the Wisconsin Fertilizer Research Fund, the University of Wisconsin Nonpoint Pollution and Demonstration Project, and the College of Agricultural and Life Sciences, University of Wisconsin–Madison.
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