Grain protein as a post-harvest index of nitrogen status for winter wheat in the northern Great Plains

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1 Grain protein as a post-harvest index of nitrogen status for winter wheat in the northern Great Plains Richard E. Engel 1, Dan S. Long 2, and Gregg R. Carlson 3 1 Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717, USA; 2 USDA-ARS Columbia Plateau Conservation Research Center, Pendleton, OR 97801, USA; 3 Northern Agricultural Research Center, Montana State University, Havre, MT 59501, USA. Received 15 November 2005, accepted 13 December Engel, R. E., Long, D. S. and Carlson, G. R Grain protein as a post-harvest index of nitrogen status for winter wheat in the northern Great Plains. Can. J. Plant Sci. 86: The use of grain protein as a post-harvest index of N fertility status has been promoted for spring wheat (Triticum aestivium L.) through the establishment of critical levels for segregating wheat into N deficient vs. N sufficient classes. The objectives of this study were to evaluate this concept for winter wheat in the northern Great Plains; and to estimate the added N requirements necessary to achieve maximum yield when protein concentrations fall below the critical level. A field study consisting of three water regimes, four cultivars, and five fertilizer N levels was conducted near Havre, MT. A consistent relationship between relative yield and grain protein was found and a critical protein concentration of 121 mg g 1 was defined using Cate-Nelson R 2 statistics. Protein concentrations below the critical level were associated with yield losses from N deficiency (79% frequency), while protein concentrations the critical level were associated with N sufficiency (93% frequency). Under conditions of moderate N deficiency (68 99% of maximum), protein concentration could be used to estimate the amount of additional N that would have been needed to achieve maximum yields. This is accomplished by first calculating the difference between the critical and actual protein concentration (expressed in mg g 1 protein). This protein deficit is then multplied by a fertilizer N equivalent that varied from 20 to 38 kg N ha 1 (according to the precipitation environment) for each 10 mg g 1 rise in protein desired. Key words: N sufficiency, N deficiency, critical protein concentration, plant available N Engel, R., Long, D. S. et Carlson, G. R Évaluation du bilan azoté du blé d hiver après récolte dans les grandes plaines du nord selon la teneur du grain en protéines. Can. J. Plant Sci. 86: D aucuns préconisent qu on utilise la teneur en protéines du grain comme indice post-messianique pour déterminer la fécondité du blé de printemps (Triticum aestivum L.) d après la quantité d azote, bref qu on répartisse les grains de blé en diverses classes carencées ou pas en N selon certains seuils critiques. Les auteurs ont tenté d évaluer l utilité d un tel concept pour le blé d hiver cultivé dans le nord des grandes plaines et d estimer l apport de N nécessaire pour obtenir un rendement maximal quand la concentration de protéines tombe sous le seuil critique. À cette fin, ils ont entrepris une étude sur le terrain portant sur trois régimes hydriques, quatre cultivars et cinq taux d engrais N, près de Havre, au Montana. Ils ont découvert l existence d une relation stable entre le rendement relatif et la teneur en protéines du grain puis fixé la concentration critique de protéines à 121 mg par gramme en recourant aux valeurs R 2 de Cate-Nelson. Une teneur en protéines inférieure à cette concentration donne lieu à des pertes de rendement attribuables à une carence en N (fréquence de 79 %) alors qu une teneur en protéines égale ou supérieure à la concentration critique se traduit par une quantité suffisante de N (fréquence de 93 %). Quand il y a carence modérée de N (de 68 à 99 % du maximum), on pourrait se servir de la teneur en protéines pour estimer la quantité de N supplémentaire dont on pourrait avoir besoin pour parvenir au rendement optimal. Pour cela, on calcule d abord l écart entre la concentration critique et la concentration réelle de protéines (exprimées en mg de protéines par gramme); ensuite, on multiplie le déficit de protéines par un équivalent en engrais N allant de 20 à 38 kg de N par hectare (selon l importance des précipitations) pour chaque tranche supplémentaire de 10 mg de protéines par gramme souhaitée. Mots clés: Suffisance en N, carence en N, concentration critique de protéines, N disponible pour la plante Nitrogen is the nutrient most frequently affecting growth and quality of spring and winter wheat (Triticum aestivum L.) in the Northern Great Plains. Since the soils of this region were first cultivated. instances of N deficiency have increased over time due to a loss of indigenous soil N. In addition, cultivar development efforts by plant breeders and improved cultural practices to conserve water (no-till) and control weeds have led to higher yield potentials and plant N requirements. Traditionally, soil testing has been used in this region to predict potential N deficiency problems and characterize N status of farm fields. More recently, investigators in North Dakota (Goos 1984) and Montana (Engel et al. 1999) have promoted the concept of 425 using grain protein at harvest as a biological index of soil N sufficiency or deficiency in spring wheat. This approach is based on the premise that a consistent relationship between grain yield (expressed in relative terms) and protein concentration exists in wheat such that a critical protein level for N sufficiency can be defined. Grain protein levels that fall below a critical level are associated with wheat yields that have been depressed by inadequate N, while protein levels greater than, or equal to, the critical level are associated with environments where N did not limit yields. Critical grain protein concentrations for spring wheat in North Dakota (Goos 1984) and Montana (Engel et al. 1999)

2 426 CANADIAN JOURNAL OF PLANT SCIENCE have been reported to be 140 and 135 mg g 1, respectively. In the Canadian Prairies the critical protein level approach for establishing N sufficiency and deficiency in wheat has been met with mixed results (Selles and Zenter 2001), or viewed less enthusiastically (Fowler 2003). Selles and Zenter (2001) found it to be a useful guide only where spring wheat was not severely water stressed, i.e., following summerfallow. Under these conditions, they found a critical grain protein concentration of 128 mg g 1 marked the separation between N deficient and N sufficient wheat for the realization of full yield potential in spring wheat. Fowler (2003) found wheat grain yield-protein relationships, defined using nonlinear regression procedures, to differ significantly with growing season environment and cultivar selection. This made it difficult to define a single grain protein level associated with deficiency vs. sufficiency. In the northern Great Plains, most investigations of the critical grain protein level approach for evaluating N status are limited to hard red spring wheat. Though this concept was first tested for hard red winter wheat in the central Great Plains (Goos et al. 1982), only a few reports are available for winter wheat in this region. This study summarizes the results from a 2-yr study that included four winter wheat cultivars grown under a gradient of water and N fertility. The objectives were: (i) to define a critical grain protein level for segregating wheat into N deficient vs. N sufficient classes provided a consistent relationship could be found between grain yield and protein; and (ii) to estimate the added N requirements necessary to reach maximum yield for wheat falling below the critical level of N sufficiency. MATERIALS AND METHODS Field Methods (Site Description, Experimental Design, and Cultural Practices) A 2-yr experiment was conducted in a 4-ha field on the Telstad-Joplin (fine loamy, mixed, superactive, frigid Aridic Argiustolls) soil association at the Montana State Universisty Northern Agric. Res. Center (Havre, Montana, USA, N, W) in 2000 and The study was moved to a different location within the field before the beginning of each season. Wheat was seeded into standing barley stubble in both seasons. Soil inorganic NO 3 -N levels in 0-60 cm depth revealed <20 and 28 kg N ha 1 (Table 1) in September 1999 and 2000, respectively. Yield responses to fertilizer N were anticipated at this site based on the soil N test levels, particularly where rainfall was augmented with irrigation to improve soil moisture. For each year of the study, the field site was divided into three distinct water environments or irrigation blocks (15.2 m 73 m) referred to as a low, moderate, and high precipitation regime. A solid-set irrigation system with impact sprinklers (6.1 m 15.2 m nozzle spacing) was used to create each water environment according to the following scheduling scheme. In the low-precipitation regime, wheat was grown under dryland conditions, except for a single application of water equivalent to 6.3 cm in 2000 and 3.8 cm in 2001 at tillering (Zadok stage 26 30). In the moderateprecipitation regime, wheat received irrigation at tillering Table 1. Irrigation amounts applied and preplant soil NO 3 -N levels (0 60 cm depth) for the three precipitation regimes, and rainfall for growing season (Apr. 01 to physiologic maturity). Havre, Montana Irrigation applied Soil NO 3 -N levels Precipitation regime (cm) (kg ha 1 ) Low Moderate High Rainfall z z Rainfall totaled 6.6 cm for the high regime in 2001 due to a late-season event that occurred after the low and moderate regimes had reached maturity, while the high water regime wheat remained green. (Zadok stage 26 30), flag-boot (Zadok stage 39 45), and late heading-anthesis initiation (Zadok stage 57 65). In the high-precipitation regime, wheat received irrigation as in the moderate-precipitation regime, plus an additional irrigation during early grain-fill (Zadok stage 75 80). Each precipitation regime consisted of a randomized complete block design, split-plot arrangement of four winter wheat cultivar (CDC Kestrel, McGuire, Rampart, Erhardt) main-plots, and five N fertilizer level sub-plots (or 20 cultivar N level treatment combinations). The treatment combinations were replicated four times. Fertilizer N levels were adjusted for each water regime. Fertilizer N rates were equivalent to 0, 22, 45, 90, and 134 kg N ha 1 in the low-precipitation regime; and 0, 28, 56, 112, and 168 kg N ha 1 in the moderate regime. In the high-precipitation regime, fertilizer N rates were equivalent to 0, 34, 67, 134, and 202 kg N ha 1 in 2000 and 0, 39, 78, 156, and 234 kg N ha 1 in Individual plots were approximately 1.8 m wide and 6.1 m long. Cultural Practices and Lab Analyses Seeding occurred on 1999 Oct. 01 and 2000 Sep. 27. Seeding and fertilizing were accomplished in a single-pass by means of a no-till small-plot grain drill equipped with separate cones and spinner-dividers for fertilizer and seed (Engel et al. 2003). Row spacing and seeding rates were 30 cm and 215 pure live seeds m 2, respectively. Fertilizer N was applied as granular urea (subsurface band) and ammonium nitrate (surface broadcast) in 2000 and 2001, respectively. Sufficient triple superphosphate was applied with the seed to ensure adequate P nutrition according to soil test recommendations. Broadleaf weeds were controlled by application of herbicides. Irrigation amounts were measured with catch-cans placed inside each precipitation regime, and rainfall was recorded with a gauge placed next to the study area. Water was applied at a rate of approximately 20 mm h 1. Grain harvest of the field plots was accomplished by trimming approximately 0.5 m off the ends of each plot, then combining the four center rows with a small-plot combine. Subsamples of the grain were collected for protein determinations. Grain subsamples (25 g) were ground in a Udy Mill prior to N determination using an automated dry combustion instrument, or Dumas procedure (Leco Corporation, St.

3 ENGEL ET AL. N STATUS OF WINTERWHEAT 427 Table 2. Analysis of variance z of cultivar (C) and fertilizer N (FN) main effects and interaction on grain yield and protein concentration of winter wheat over three precipitation regimes (low, moderate, and high) and two growing seasons. Havre, Montana Precipitation Grain yield Grain protein Year regime C FN C FN C FN C FN Probability > F 2000 Low NS y < < NS Moderate NS < < High < < < NS 2001 Low NS NS NS NS < NS Moderate NS < < High NS < < < NS z ANOVA was conducted using PROC MIXED in SAS version 9.0 (SAS Institute Inc. 2002). y NS = not significant at, or below, 0.10 level. Joseph, MI). Protein concentrations were calculated by multiplying Leco N 5.7. Protein values were corrected to 135 g kg 1 moisture basis on gravimetric moisture determinations of all subsamples. Soil samples were dried in a lowtemperature oven (45 C) prior to extraction with 1 M KCl (5:1 extract to soil ratio) for NO 3 -N determinations. Aliquots of the extracts were injected into an autoanalyzer for NO 3 -N determination using Cd reduction to NO 2 and a modified Griess-Ilosvay method (Mulvaney 1996). Statistical Analyses Analyses of variance of grain yield and grain protein concentration were performed using PROC MIXED in SAS version 9.00 (SAS Institute, Inc. 2002). Block and block cultivar terms were treated as random effects. Regression analyses of grain yield vs. available N were determined on treatment means using PROC GLM. Effects were declared significant at P < 0.10 level unless otherwise specified. Wheat yields were expressed in relative terms, or as a percentage of the plateau yield (100%) using an approach similar to that of Goos et al. (1982). The intent of the approach used by Goos et al. (1982) was to normalize yields across diverse environments and growing conditions by expressing yield as a function (or a percentage) of the yield potential, or yield plateau, where N was not limiting. In this study, each season precipitation cultivar treatment combination was interpreted to represent a unique environment due to the obvious effects of water, season, and cultivar selection on yield. Plateau yields were calculated as the mean yield of N rate(s) not significantly different from the highest yielding N treatment for each season precipitation regime cultivar treatment combination. Single degree of freedom contrasts using the Estimate statement in SAS were conducted to perform the comparison tests (and establish statistical significance) among season precipitation regime cultivar treatment means. The statistical procedure described by Cate and Nelson (1971) was used to separate the data set into low (N deficient) and high (N sufficient) classes for calculation of R 2 values and determination of the critical grain protein concentration. The Cate and Nelson procedure consisted of ordering the data into an array (X,Y or grain protein concentration, relative grain yield) based upon a ranking of the X values. The (X,Y) pairs were maintained in this order throughout the analysis. Starting with a protein cutoff concentration that placed two or more points in the low class, the data set was divided into two classes. Protein cutoff values were then increased sequentially. At each cutoff value an R 2 value was calculated from the equation: 1 (CSS l + CSS h )/CSS t where CSS l and CSS h is the corrected sum of squares of grain yield for low and high protein classes, respectively, and CSS t is the corrected sum of square of grain yield for the entire data set. The critical protein concentration was defined where R 2 was maximized. Cate and Nelson R 2 values (Cate and Nelson, 1971) were calculated using a custom designed software program (Selles, F personal communication Semiarid Prairie Agricultural Research Centre, Swift Current, SK., Canada). RESULTS AND DISCUSSION Critical Grain Protein Concentration Absolute grain yield and grain protein responded to N fertilizer in 2000 and 2001 in all precipitation regimes, except for the low regime in 2001 (Table 2). The results were not surprising given the low indigenous soil N levels observed at this site. The response to N fertilizer, plus the approach of augmenting rainfall with irrigation was effective in creating a wide range of yield potentials and grain protein concentrations (Table 3). Grain yield levels in the low-precipitation regime were extremely small in 2000 due to the absence of appreciable rainfall during the growing season. Yield was affected by cultivar fertilizer N interactions in several instances. In general, McGuire and CDC Kestrel were the most and least responsive cultivars to N fertilization, respectively, as evidenced by the difference between the maximum-yielding N fertilized treatments and the unfertilized treatment (0 N). Grain protein concentrations protein levels were affected by cultivar selection in all precipitation regimes, except the low regime in CDC Kestrel typically had a significant lower (P < 0.05 level) grain protein level than the other cultivars tested, reflecting the low protein characteristic of this genotype. Expressing yield in relative terms, or as a percentage of the maximum plateau yield, provided a method for normal-

4 428 CANADIAN JOURNAL OF PLANT SCIENCE Table 3. Gain yield (kg ha 1 ) and protein (mg g 1 ) response to N fertilizer by four winter wheat cultivars under varying precipitation regimes for two growing seasons ( ) at Havre, Montana Parameter Maximum Precipitation Y=yield, Fertilizer N level z plateau regime Cultivar (P protein) N 1 N 2 N 3 N 4 N 5 yield y 2000 Low Erhardt Y 2009c 2313bc 2478b 2930a 3092a 3011 P CDC Kestrel Y 2222b 2439b 3171a 3119a 2814a 3035 P McGuire Y 1101c 2459b 2603b 3092a 3064a 3078 P Rampart Y 1992c 2612b 2670ab 2865ab 3011a 2849 P Moderate Erhardt Y 1857c 2888b 3589a 3838a 3787a 3738 P CDC Kestrel Y 2812c 3017c 3700b 4128a 3598b 4128 P McGuire Y 1772d 2629c 3559b 4047a 4163a 4105 P Rampart Y 2034d 2791c 3316b 4064a 3893a 3961 P High Erhardt Y 1474d 2771c 3265b 4389a 4552a 4471 P CDC Kestrel Y 2407d 3254c 4415b 4796b 5231a 5231 P McGuire Y 1312d 2435c 3587b 4925a 5061a 4993 P Rampart Y 1846e 2958d 4159c 5144b 5570a 5570 P Low Erhardt Y 550a 494a 492a 681a 704a 585 P CDC Kestrel Y 529a 883a 576a 675a 795a 691 P McGuire Y 505a 515a 477a 637a 655a 559 P Rampart Y 715a 555a 582a 552a 756a 632 P Moderate Erhardt Y 2071c 2566b 2980a 3140a 3137a 3086 P CDC Kestrel Y 2412c 2922ab 3127a 3094a 2780b 3048 P McGuire Y 1739d 2582c 2919b 3340a 3154ab 3247 P Rampart Y 2126c 2820b 3081ab 3120a 3116a 3106 P High Erhardt Y 1463d 2896c 3823b 4990a 4955a 4973 P CDC Kestrel Y 2335d 3219c 4016b 4944a 5139a 5042 P McGuire Y 1479d 2941c 3934b 4977a 5090a 5034 P Rampart Y 2317d 3101c 4241b 5047a 5225a 5136 P z N fertilizer levels N1, N2, N3, N4, N5 were 0, 22, 45, 90, and 134 kg ha 1 in the low regime; 0, 28, 56, 112, and 168 kg ha 1 in the moderate regime; 0, 34, 67, 134, and 202 kg ha 1 in the high regime-2000 and 0, 39, 78, 156, and 234 kg ha 1 in the high regime y Plateau yield is the average of yields not significantly different from the maximum for each season precipitation regime cultivar. Statistical significance was conducted using single df contrasts via the Estimate statement in SAS version (SAS Institute Inc. 2002). a d Grain yield values across rows followed by the same letter are not significant different at the P < 0.10 level. izing yield vs. protein relationships across the precipitation regimes. A scatter diagram of relative yield vs. grain protein (Fig. 1) produced similar relationships to those presented earlier by Engel et al. (1999) and Goos et al. (1982). A critical grain protein percentage of 121 mg g 1 (Fig. 2) was defined using the R 2 value statistical approach of Cate and

5 Table 4. Approximate fertilizer N equivalents required to raise grain protein concentration 10 mg g 1 based on interpretation of grain protein vs. available N relationships Year Growing Grain protein vs. Fertilizer N and season available N equivalent to raise water regime precipitation z function slope y protein 10 mg g 1 (cm) (kg N ha 1 ) 2000 low moderate moderate high high z Growing season precipitation = rainfall plus irrigation applied (see Table 2). y Slopes derived from Fig. 3. x Fertilizer N equivalent = inverse of slope 10. Nelson (1971). Protein levels above the critical protein level had a high probability (55 of 59 episodes, or 93%) of being associated with N sufficiency. Protein levels at, or below, the critical were associated with N deficiency at a somewhat lower probability or 79% of the time (48 of 61 episodes). Also, for episodes where grain protein falls below the critical level there is no correlation between relative yield (R 2 < 0.01) and protein. The results from this study indicate that grain protein can serve as a qualitative index of N fertility status within a field. Although this index is derived post-harvest, the information provides a grower or consultant with knowledge that their N fertility program resulted in yields that were not limited by inadequate N in instances where grain protein levels rise to, or above, the critical level. Conversely, where wheat protein levels fall below the critical level the information provided indicates a potential problem (i.e., inadequate N) in the current N fertility program may exist. As with other indices or measures of N fertility status (e.g., soil NO 3 -N testing), application of grain protein values is imperfect and a degree of caution needs to be exercised. Fowler (2003) noted concerns related to the effect of wheat cultivars and classes on grain protein-yield relationships. In this study there were 13 episodes, where protein fell below the critical level, but significant yield losses to N deficiency were not observed. Eight of these 13 episodes were associated with the cultivar CDC Kestrel. This suggests that some biases may exist in applying critical grain protein information across all cultivars. Also, there are examples in the literature where N excess can lead to yield reductions under moisture limited environments (Engel 1991; Fowler et al. 1989; Nielsen and Halvorson 1991). In the past, we have observed this phenomenon to be particularly important during summers of extremely high temperatures (Engel 1991). Therefore, high protein levels are not a guarantee that yields have been maximized, but more an indication that yield losses from inadequate N have not occurred. Also, the relationships in Fig. 1 do not provide any consideration for fertilizer N and grain prices, and protein premiums that would affect a grower s economic return or minimum quality standards that impact marketing of North American grain. ENGEL ET AL. N STATUS OF WINTERWHEAT 429 Fertilizer N Deficit Required to Reach N Sufficiency Level Under conditions of N deficiency, it might be useful to know the N deficit, or amount of additional fertilizer N that would have been needed to reach the critical level. A grower could use this information to assist in the refinement of future N fertilizer programs. Estimates of this available N deficit can be made with some limitations from grain protein-available N relationships. Scatter diagrams of protein vs. available N illustrate a J-shaped relationship for most environments (Fig. 3). The one exception occurred in the low-precipitation regime in 2001 where yield was severely impacted by drought (< 890 kg ha 1 ) and protein levels were extremely high (153 to 192 mg g 1 ). Under most environments, the first increment of N fertilization decreased the protein concentration, or did not affect protein concentration while increasing yield. This Steenbjerg-type response (Steenbjerg 1951) to N was particularly apparent in the high-precipitation regimes where yield potentials exceeded 5000 kg ha 1. Regression analyses reveal that linear-type functions provide a good fit to the data if the unfertilized (0 N) wheat is not considered in this analysis (Fig. 3). Soil N levels were extremely low at this field, hence wheat not receiving N exhibit symptoms of N deficiency, i.e., chlorosis, by the tillering stage. In 2000, wheat not receiving N produced yields that were 61.8, 54.4 and 26.8% (mean of four cultivars) of the maximum plateau yield for the low, moderate, and high precipitation regimes, respectively. In 2001, wheat not receiving N produced yields that were 67.5 and 37.5% of the maximum plateau yield (mean of four cultivars) for the moderate, and high precipitation regimes, respectively. Although, yield losses of this magnitude from inadequate N can occur in the field, they are not common in most present day crop production systems where growers apply even modest amounts of fertilizer N. To predict or estimate fertilizer or available N deficits under these instances of moderate N deficiency one first calculates the inverse of the slopes for the linear functions in Fig. 3. This expresses the N equivalent, or amount of N required to change protein by 1.0 unit (i.e., mg g 1 ) on a perhectare basis. This information is then applied to protein values that fall below the critical level. For example, the inverse (1/x) of the slope for the low precipitation regime in 2000 is 1/ This equates to approximately 2 kg ha 1 available N equivalent, to change protein concentration 1 mg g 1. Therefore, winter wheat with a protein content of 111 mg g 1 would have required approximately 20 kg additional N (2 10 = 20) on a per-hectare basis to raise protein 10 mg g 1 to the 121 mg g 1 critical level. In this study the predicted N equivalent, or fertilizer N equivalent, to change protein was fairly stable from 15 to 21 cm of growing season precipitation. This range in precipitation encompasses that frequently observed during many seasons in northern Montana (location of this study) and adjacent areas of southern Alberta and Saskatchewan. As growing season precipitation increased above 21 cm the fertilizer N equivalent rose. This result was not unexpected given that yield potentials (or the sink size) increases with

6 430 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 1. Relative grain yield vs. protein relationships for winter wheat. Dark circles indicate episodes where yield was significantly (0.10) below the maximum. Light circles indicate episodes where yield was not significantly different from the maximum. Fig. 2. Cate-Nelson R 2 analysis of winter wheat relative yield vs. grain protein relationships. Critical protein concentration is defined where R 2 values are maximized. growing season precipitation. Under yield potentials in the kg ha 1 range (high regimes for 2000 and 2001) the fertilizer N equivalent to raise protein 1 mg g 1 was approximately 3.8 kg N on a per-hectare basis. Although cultivar selection affected protein concentration, regression analyses (results not shown) revealed the slope terms for the linear models in Fig. 3 were not significantly affected by cultivar selection (i.e., cultivar N rate interaction term was not significant). Hence, the application of a fertilizer N equivalent value for protein correction may not be greatly impacted by cultivar selection. Future Applications The potential application of protein information at harvest for N deficiency or sufficiency diagnosis may take on added significance as optical sensors for combines, enabling onthe-go analysis of grain protein, become commercially available (Long et al. 2005; Long and Rosenthal 2005).

7 ENGEL ET AL. N STATUS OF WINTERWHEAT 431 Fig. 3. Grain protein available N relationships for winter wheat grown under three precipitation regimes. Mean of four cultivars (Erhardt, CDC Kestrel, McGuire, Rampart). Once this technology becomes available, agronomists and land managers will be able to use the concepts presented here to identify areas of fields where N fertility programs are adequate (or inadequate) on a site-specific basis. This information could be very helpful in directing soilssampling efforts across large field landscapes, and developing updated soil fertilization programs. Interested readers may wish to consult Long et al. (1999) and Long et al. (2000) for details on using a geographic information system and maps of grain yield and grain protein to compute site-specific N requirements for spring wheat. As with other nutrient diagnosis and recommendations programs, utilization of grain protein as a diagnostic tool for evaluating N status has its limitations. However, as grain growers typically have good recollections and interest in the protein concentration of their wheat we believe this can be an important tool in nutrient management. Cate, R. C., Jr. and Nelson, L. A A simple statistical procedure for partitioning soil test correlation data into two classes. Soil Sci. Soc. Am. Proc. 35: Engel, R.E Simulated growing season precipitation and nitrogen effects on winter wheat yield. Agron. J. 83: Engel, R. E., Long, D. S., Carlson, G. R. and Meier, C Method for precision nitrogen management in spring wheat: I. Fundamental relationships. Prec. Agric. 1: Engel, R. E., Fischer, T., Miller, J. and Jackson. G., A small-plot seeder and fertilizer applicator. Agron. J. 95: Fowler, D. B Crop nitrogen demand and grain protein concentration of spring and winter wheat. Agron. J. 95: Fowler, D. B., Brydon, J. and Baker, R. J Nitrogen fertilization of no-till winter wheat and rye. I. Yield and agronomic responses. Agron. J. 81: Goos, R. J., Westfall, D. G., Ludwick, A. E. and Goris, J. E Grain protein content as an indicator of N sufficiency for winter wheat. Agron. J. 74: Goos, R. J Post-harvest evaluation of nitrogen management a new approach for selling soil testing to wheat farmers. J. Agron. Educ. 13: Long, D., Engel, R. and Carpenter, F On-combine sensing and mapping of wheat protein concentration. Crop management. [Online] Available: element/cmsum2.asp?id=4841. Long, D. and Rosenthal, T Evaluation of an on-combine wheat protein analyzer on Montana hard red spring wheat. Pages in J. Stafford, ed. Precision Agriculuture 05. Wageningen Academic Publishers, Wageningen, the Netherlands. Long, D., Engel, R. and Carlson, G Method for precision nitrogen management in spring wheat: II. Implementation. Prec. Agric. 2: Long, D., Engel, R. and Reep, P Grain protein sensing to identify nitrogen management zones in spring wheat. Site-specific management guidelines. SSMG-24. Potash and Phosphate Institute, Norcross, GA. [Online] Available: Mulvaney, R. L Nitrogen-inorganic forms. Pages in D. L. Sparks et al. eds. Method of soil analysis. Part 3. Chemical methods. SSSA, Inc., Madison, WI. Nielsen, D. C. and Halvorson, A. D Nitrogen fertlilty influence on water stress and yield of winter wheat. Agron. J. 83: SAS Institute, Inc SAS for Windows. Release 9.0. SAS Institute, Inc., Cary, NC. Selles, F. and Zentner, R. P Grain protein as a post-harvest index of N sufficiency for hard red spring wheat in the semiarid prairies. Can. J. Plant Sci. 81: Steenbjerg, F Yield curves and chemical plant analysis. Plant Soil 3:

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