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Crop Science 42:1966-1973 (2002)
© 2002 Crop Science Society of America

CROP ECOLOGY, MANAGEMENT & QUALITY

Nitrogen Application and Critical Shoot Nitrogen Concentration for Optimum Grain and Seed Protein Yield of Pearl Millet

Charles Kennedy*,a, Paul Bellb, David Caldwellc, Bob Habetzd, Jim Rabbe and M. A. Alisonf

a Dep. of Agronomy, LSU Ag Center, Baton Rouge, LA 70803
b Dep. of Agronomy, LSU Ag Center, Baton Rouge, LA 70803
c Red River Research Station, LSU Ag Center, Bossier City, LA 71113
d Rice Research Station, LSU Ag Center, Crowley, LA 70527
e Red River Research Station, LSU Ag Center, Bossier City, LA 71113
f Macon Ridge Branch, Northeast Research Station, LSU Ag Center, Winnsboro, LA 71295

* Corresponding author (ckennedy{at}agctr.lsu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Pearl millet [Pennisetum glaucum (R) Br.] is a promising alternative grain crop for the Southeast and mid-South regions of the USA, but additional N fertility information is needed to optimize its production. Identification of a critical tissue N level at early growth stages is necessary to determine the need for subsequent applications of N. Field experiments were conducted to determine the response of hybrid ‘HGM 100’ grain yield, protein yield, and shoot N concentration to N application rate and to determine the relationship of grain yield and protein yield with shoot N concentration at different growth stages. Grain yield response to N fertilizer applications from 0 to 168 kg ha-1, in 34-kg increments, varied among five field environments ranging from sandy loam to silty clay loam soils and resulted in a moderately low correlation (r2 = 0.36). Relative total protein yield was more consistent (r2 = 0.67). Tissue N concentration of shoots harvested 44 ± 4 d after seeding [early boot(EB)] was related to relative grain yield (r2 = 0.65). A critical N concentration of 31 g kg-1 dry matter (dm), determined by regression and "old" Cate–Nelson procedures, resulted in optimum grain yield. On a per hectare basis, total relative protein yield also correlated moderately high with shoot tissue N concentration at EB (r2 = 0.70). The use of Cate–Nelson procedures determined a critical N concentration in shoot tissue at EB of 32 g kg-1 dm. Determination of a critical N level at an earlier growth stage resulted in low correlations with yield parameters. On the basis of critical values at the EB stage, N amendments could possibly be applied in time to enhance some yield components, although the effective application window is narrow.

Abbreviations: PI, panicle initiation • EB, early boot • dm, dry matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
PEARL MILLET is a promising alternative grain crop for the Southeast and mid-South regions of the USA. It has excellent nutritional composition (Hill and Hana, 1990), making it competitive with corn (Zea mays L.) and sorghum [Sorghum bicolor (L.) Moench]. Moreover, pearl millet is considered to be more drought and acid-soil tolerant than other grain crops (Ahlrich et al., 1991). Pearl millet is also a short-season crop, maturing in about 100 d or less, which makes it an excellent candidate for multiple cropping systems.

To use pearl millet effectively in production systems, area-specific information on the N fertilizer requirement for the crop needs to be acquired. Work in the southeastern USA by Gascho et al. (1995) indicated a linear yield response to N fertilizer rate by pearl millet up to the maximum application of 90 kg ha-1. In the same region, Menezes et al. (1999) suggested 112 kg N ha-1 as the optimum fertilizer rate in soils deficient in mineral N, but cautioned that N fertilizer application could decrease grain yield under conditions of drought, late planting, or increased levels of mineral N in soil. Protein yield, however, could continue to increase regardless of grain yield response. Recommendations have been slow to develop because of the lack of consistent response by pearl millet to N fertilizer applications. The inconsistency has been attributed to an efficient redistribution within the plant (Smith et al., 1990), and variable residual soil N (Maranville and Sirifi, 1995) coupled with the exceptional ability of the plant to extract subsoil nitrate (Menezes et al., 1997). Previous research in this region of the mid-South has shown no consistency in the relationship between available soil N and crop response to N. The dynamic nature of soil N cycling in highly weathered soils in humid, warm-temperate conditions has limited the ability to obtain consistent and reliable correlations with mineral or organic N and yield of crops (Ebelhar, 1990; Pettiet, 1990).

The use of tissue N concentration to establish the optimum fertilizer rate has been used for several crops including rice (Oryza sativa L.) (Sheehy et al., 1998) and wheat (Triticum aestivum L.) (Vaughn et al., 1990; Baethgen and Alley, 1989). Analyzing the N concentration of tissue prior to or at early reproductive development will help assess the need for supplemental fertilizer application of pearl millet. Work by Maman et al. (1999) and Coaldrake and Pearson (1985) determined the tissue N concentration required for optimum growth rate in pearl millet but not for grain yield. Menezes et al. (1999) suggested a critical tissue N concentration for grain yield, but it was based primarily on results from nonfertilized control plots. Our objective was to (i) determine plant response to applied N fertilizer in multiple environments and (ii) determine prebloom critical N concentrations of shoot tissue of pearl millet for grain yield and kernel protein yield.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
The pearl millet hybrid HGM 100 was grown in five edaphic and climatic environments encompassing three locations and two years. It was grown at Crowley, LA (30° 15' N, 92° 21' W) in a Crowley silt loam (fine, smectitic, superactive, hyperthermic, Typic Albaqualf) in 1998 and 1999. At Bossier City, LA (32° 25' N, 93° 38' W) the soils were a Caplis very fine sandy loam (coarse-silty over clayey, mixed, superactive, calcareous Thermic Oxyaquic Udifluvent) in 1998 and a Moreland silty clay loam (very-fine, smectitic, thermic Oxyaquic Hapludert) in 1999. The millet was also seeded into a Gigger silt loam (fine-silty, mixed, thermic Typic Fragiudalf) at Winnsboro, LA (32° 15' N, 92° 21' W) in 1999. Temperature and rainfall data were collected by recording weather stations located within 10 km of the field sites. Previous crops were cotton (Gossypium hirsutum L.) at Winnsboro and Bossier City and mixed pasture at Crowley. Amendments of 0 to 90 kg P2O5 ha-1 and 0 to 100 kg K2O ha-1 were applied depending on soil test recommendations for each soil type for sorghum. Row widths were 0.81 m at Crowley, 0.96 m at Bossier City, and 1.02 m at Winnsboro. All plots were four rows wide and ranged from 10 to 15 m in length depending on location. Seeding dates were in late April to early May with the exception of Crowley in 1999, which was seeded in early June. Seeding rates were 3.4 kg ha-1 at Bossier City and Winnsboro and 4.5 kg ha-1 at Crowley. Nitrogen was broadcast-applied as ammonium nitrate (34-0-0) about 1 wk post emergence at rates of 0, 34, 67, 101, 134, and 168 kg N ha-1.

Ten shoots per plot were harvested at ground level at 31 ± 5 d after seeding and again at 44 ± 4 d after seeding. The interval between harvests was 10 to 14 d depending on environment. Shoots were checked for presence of an initiated panicle within the culm. Plants in the first harvest did not consistently have an initiated panicle as determined with a dissecting microscope but were considered at or near the panicle initiation stage (PI) based on the work of Teare et al. (1995). Plants in the second harvest had a differentiated panicle and were considered to be in early boot stage (EB). The shoots were rinsed to remove soil contaminants, dried at 65°C for 48h, and ground in a Wiley mill with a 0.5-mm sieve.

Grain yield was determined either by machine harvesting the two inside rows of each plot or by means of a panicle covering method similar to that of Menezes et al. (1999) when potential bird depredation was high. In our experiment, 15 panicles were bagged at random post-anthesis. At harvest maturity (post-physiological maturity plus additional dry-down days), the total number of mature panicles in the two inside rows were counted, and the bagged panicles were harvested and threshed. Grain weight and moisture content was determined on these panicles, and plot yield was estimated by the product of panicle number x grain weight per panicle adjusted to 130 g kg-1 dm moisture content. Kernel protein was determined by pulverizing 30 g of harvested grain for 45 s in a Braun KSM2 coffee grinder (Woburn, MA) modified to a grinding volume of 103 cm3. The pulverized and homogenized grain was able to pass through a 0.5-mm sieve. Nitrogen concentration for shoot tissue and grain was determined on a Leco FP-428 N analyzer (St. Joseph, MI). Kernel protein concentration was calculated as the product of g N kg-1 dm grain x 6.25 (Garret, 1974). Total protein yield (ha-1) was the product of kernel protein concentration of grain x grain yield per hectare. A crude estimate for available soil N and excess fertilizer N applied was determined as the difference between applied N fertilizer amount and the amount of N removed in grain harvest.

The experimental design at each location was a completely randomized block with four replications. Analysis of variance and regression analysis were performed with SAS (SAS, 1985). Data comparisons across environments were accomplished on a relative basis by dividing treatment means by the largest mean value per each environment and multiplying by 100. Relative optimum grain and protein yield across environments were determined on the basis of lowest yields that were NS from maximum yield. This was 88 and 87% of maximum for grain and protein yield, respectively. We used these values as the benchmark criteria to establish a critical shoot N concentration for grain and protein yield. Critical N concentrations were determined by Cate–Nelson procedures (Cate and Nelson, 1965, 1971) as well as normal regression methods. The r2 values determined in this study are for regression analyses of treatment means (Gomez and Gomez, 1984).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
Weather and Soil Conditions
Weather conditions in 1998 were less optimal for crop growth (i.e., drier and/or warmer than normal), especially at Bossier City where, from May through July, maximum daily temperatures averaged 35°C and were over 37°C 39% of the time, mean daily temperatures averaged 28°C, and total rainfall during that time was only 78 mm (Fig. 1) . Maximum daily temperatures at Crowley during that time averaged about 32°C with mean daily temperature averaging 28°C and 206 mm of rainfall distributed more towards the end of that period. In 1999, temperatures at Crowley from June through August were similar to the crop growing period in 1998 and rainfall during the growing period was 275 mm, which was more evenly distributed than in 1998. Maximum daily temperature during May through July in 1999 at Bossier City averaged 32°C, mean daily temperature averaged 26°C, and rainfall totaled 327 mm. Temperatures at Winnsboro in 1999 were similar to those at Bossier City and rainfall amounts totaled 372 mm. Differences primarily in weather conditions coupled to some extent with differences in soil type could explain most of the yield differences among environments.



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  		Fig. 1. Weather variables before, during, and after the pearl millet growing period for five environments.

 
Although available soil N was not directly determined for this study, there was native soil N available for crop growth as evidenced by the negative N balance between N removed in grain and 0 fertilizer N applied, which ranged from about 16 to 50 kg N ha-1 (Fig. 2) . Weather conditions predominately affected yield potential and therefore the amount of available soil N removed, but soil type also had some influence. The Moreland silty clay loam (BC99) had less soil N removed by the 0 N fertilizer application than silt loam soils (C99, W99) with similar weather conditions (Fig. 1) indicating the higher clay content contributed less available N.



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  		Fig. 2. The presumed balance of N remaining after harvest of HGM 100 pearl millet as determined by the difference in N fertilizer applied and N harvested in grain among five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. Negative values at 0 applied N represent nonfertilizer N removed from soil. Error bars represent ±1 SE. All regression equations were significant to P < 0.05 and coefficients of determination were >0.90.

 
Grain Yield Response to N Fertilizer Applications
Maximum grain yields ranged from about 1.0 (BC98) to almost 4.5 Mg ha-1 (W99) among environments and were from 13% (C99) to 150% (BC99) above the unfertilized check. In three of the five environments, pearl millet responded to N fertilizer application (P < 0.05) with a strong trend (P < 0.10) in a fourth environment (Table 1) . Environments that had less response to N fertilizer application (W99, C99) also had or used more N already present in the soil than the responsive environments (Fig. 2). The condition of greater available soil N would limit N fertilizer response by this crop (Maranville and Sirifi, 1995).


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  		Table 1. The effect of N fertilizer application rate on grain yield and kernel protein content and yield of the pearl millet hybrid HGM100 grown under five environments.

 
The relationship between N fertilizer rate and grain yield varied among environments with two environments (BC99, C98) having a predominately linear grain yield response to N fertilizer rate (Table 1). These and two more environments (C99, BC98) also had a quadratic response which indicated a plateau or even decline in yield at higher N rates. Based on linear or quadratic responses, 88% of maximum grain yield was achieved with as little as 4 kg N applied ha-1 (C99) to as high as 111 kg N applied ha-1 (BC99). Because the yield response to N rate was highly variable among the five environments, relative grain yield across all environments was only moderately correlated with N fertilizer application rate (Fig. 3) . This indicated that a single, optimum fertilizer rate for all environments could not be determined with a high level of confidence. Moreover, the historically poor correlation of soil test N with yield in this geographic region would necessitate use of additional tissue testing techniques to determine the need for supplemental N fertilizer applications.



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  		Fig. 3. Relative (percentage of maximum at each environment) grain yield response of the pearl millet hybrid HGM 100 to N fertilizer rates across five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. * = equation significant to at least P < 0.05.

 
Response of Shoot N Concentration to N Fertilizer Applications
Shoot N concentration response to N fertilizer rate was generally linear (Fig. 4) with only slight improvement, if any, when the quadratic model was applied. With the exception of the 1999 Crowley environment, N concentration declined in shoot tissue between PI and EB sampling times. The decline was not proportional across N fertilizer rates, as slopes tended to be steeper at the EB stage than at the PI stage. The drop in N concentration from the PI stage to the EB stage averaged 11 g kg-1 dm for N fertilizer rates that had the lowest amount of tissue N. The greatest N concentration at the PI stage, which occurred with 168 kg N applied ha-1, produced an average drop of only 5 g kg-1 dm by the time of EB. This indicated that plant growth was increasing and/or N uptake diminishing, especially at lower soil N levels. Therefore, plants with higher N concentration in shoot tissue prior to the onset of reproductive growth would have a greater potential for assimilation and/or remobilization of N during grain development and grain fill. While shoot N concentration at the PI stage was related with shoot N concentration at the EB stage in most environments (correlation coefficients ranged from 0.75 to 0.98 with r = 0.5 across environments), the shoot N concentration at the EB stage was a better indicator for grain yield than the shoot N concentration at the PI stage.



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  		Fig. 4. The effect of N fertilizer rate, crop stage, and environment on shoot-tissue N concentration of the pearl millet hybrid HGM 100 among five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. Error bars represent ±1 SE. * = equation significant to at least P < 0.05.

 
Determining a Critical Shoot N Concentration for Grain Yield
Shoot N concentration at the PI stage was correlated with absolute grain yield (r2 = 0.44, P < 0.05, data not presented), whereas N concentration at the EB stage had a better fit with relative grain yield (Fig. 5) . Determining a critical N concentration at EB was more accurate than at PI based not only on regression analysis but also by Cate–Nelson procedures (Fig. 6) . Using 88% relative grain yield as the average equivalent to maximum yield, we found a critical shoot N level of 31g kg-1 dm using a second order regression (Fig. 5), 31 g kg-1 dm using the "old" Cate–Nelson method (Cate and Nelson, 1965) (Fig. 6B), and 26 g kg-1 dm using the "new" Cate–Nelson method (Cate and Nelson, 1971) (Fig. 6C). These values were similar to 28 g N kg-1 dm shoot tissue determined by Menezes et al. (1999) and suggested as a critical value. However, shoot tissue in that study was harvested at a time similar to that of our PI harvests. Using our data from the PI harvest, we found a critical value would have been much higher than that obtained by Menezes et al. (1999), and the accuracy would have been questionable (Fig. 5, 6A, 6B). Alternatively, Maman et al. (1999) indicated that a shoot N concentration of 24 to 28 g kg-1 dm at roughly 40 d after seeding was adequate for optimum crop growth of pearl millet while Coaldrake and Pearson (1985) indicated that tissue concentrations of 13 to 15 g kg-1 dm would suffice. Such differences may be due to the different hybrid or variety used, the lack of range in N applications, and field study vs greenhouse pot study. Regardless, neither study extended this relationship to the partitioning of crop growth into grain yield.



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  		Fig. 5. The relationship between shoot-tissue N concentration and relative (percentage of maximum at each environment) grain yield at different crop stages for the pearl millet hybrid HGM 100 across five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. * = equation significant to at least P < 0.05.

 


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  		Fig. 6. The use of old (A and B) and new (C) Cate-Nelson procedures to determine the critical N concentration of shoot tissue at different crop stages for optimum relative grain yield of the pearl millet hybrid HGM 100. Arrows in A and B would represent accurate diagnoses of deficient or adequate levels of N. Intersection of the vertical line with the y axis represents the critical N value in A and B, whereas the highest coefficient of determination (r2) value produced by a shoot N concentration represents the critical value in C.

 
We are uncertain if N fertilizer supplements applied after the EB stage would be effective enough to maximize yields to that of earlier applications. On the basis of plant development rates of pearl millet (Teare et al., 1995) and N uptake at different growth stages of the species (Spitalniak et al., 1995), less than 25 d would be available for N application and uptake if critical N concentration was determined at EB. This is the average time from EB to 50% stigma emerged, when N uptake by the plant diminishes. However, results from the work of Coaldrake et al. (1987) with P. americanum L. indicate switching from low soil N status to higher N status by mid panicle differentiation can increase kernel weight and N concentration. Seed numbers in that study were unaffected, as they are established earlier in ontogeny. It would appear that there is a narrow window for N application after EB in which yield components and N concentration could be positively affected. Therefore, rapid turn-around of tissue analysis results and sufficient soil moisture would be critical for success. Additionally, we do not have a model to predict the amount of fertilizer N to apply to achieve optimum yield on the basis of N content of shoot tissue at the EB stage.

Protein Yield Response to N Fertilizer Applications
In contrast to grain yield, kernel protein concentration response to N fertilizer rate was linear with little improvement as a second order response (Table 1). The maximum increase in protein in the grain over the control plots averaged 54 g kg-1 dm across environments. However, the total amount of grain protein yield (protein ha-1) was linked with total grain yield, and a quadratic model improved the response prediction to N fertilizer application in four of the five environments. In all but one environment (BC99), N fertilizer rates of 34 to 67 kg ha-1 were sufficient for optimum grain protein yield. On a relative basis, protein yield response to N fertilizer application rate combined across environments was more linear than that of relative grain yield and had a better fit (Fig. 7) . This was probably due to the direct relationship between N application rate and kernel protein concentration (Table 1), which was also a component of grain protein yield. With optimum protein yield averaging 87% of maximum (Table 1), an application of about 100 kg N ha-1 would achieve the average optimum according to the quadratic equation (Fig. 7). However, the upper 95% confidence interval could justifiably bring the application rate down to about 65 kg N ha-1 with the caveat that fine-textured soils would require higher rates of N fertilizer. This rate of N was similar to that estimated for relative optimum grain yield with a low r2 (Fig. 3). This recommendation was somewhat lower than what was suggested by Menezes et al. (1999), possibly because of soil type or weather differences.



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  		Fig. 7. Relative (percentage of maximum at each environment) protein yield (ha-1) response of the pearl millet hybrid HGM 100 to N fertilizer rates across five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. * = equation significant to at least P < 0.05.

 
Determining a Critical Shoot N Concentration for Protein Yield
The use of tissue N concentration at EB as a criterion for applying additional N for achieving a relative optimum grain protein yield was slightly different than for relative optimum grain yield. By regression analysis, shoot tissue N at EB was about 36 g kg-1 dm for optimum grain protein yield (Fig. 8) compared with 31 g kg-1 dm for optimum grain yield (Fig. 6). A shoot tissue N concentration of 36 g kg-1 dm at EB stage would require, on average, about 50% more N fertilizer to reach that level compared with 31 g N kg-1 dm (Fig. 4). The use of Cate–Nelson procedures resulted in a critical shoot N concentration of 32 g N kg-1 dm at EB (Fig. 9B, 9C) . This resulted in better agreement between grain protein yield and grain yield with shoot N concentration at EB and suggested that 32 g N kg-1 dm may be the upper limit for the critical N level at EB for both yield parameters. Moreover, the balance between N applied and N removed in grain averaged about 30 kg ha-1 more using the higher criterion of 36 g N kg-1 dm than the lesser value (Fig. 10) . The application of 60 to 65 kg N ha-1 resulted in 50 kg N ha-1 or less remaining after harvest (Fig. 2), which was similar to that found when the criterion of 32 g N kg-1 dm at EB was used (Fig. 10). Although the data shown in Fig. 2 and 10 are crude estimates of applied N remaining in the environment, they do indicate the potential for adding excess N to certain environments with over application of fertilizer N. As with relative grain yield, relative protein yield did not have a high correlation with N concentration of shoot tissue at the PI stage (Fig. 8, 9A, 9C).



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  		Fig. 8. The relationship between shoot tissue N concentration and relative (percentage of maximum at each environment) protein yield (ha-1) at different crop stages for the pearl millet hybrid HGM 100 across five different environments; Crowley (C), Bossier City (BC), and Winnsboro (W) in 1998 and/or 1999. * = equation significant to at least P < 0.05.

 


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  		Fig. 9. The use of old (A and B) and new (C) Cate–Nelson procedures to determine the critical N content of shoot tissue at different crop stages for optimum relative protein yield (ha-1) of the pearl millet hybrid HGM 100. Arrows in A and B represent accurate diagnoses of deficient or adequate levels of N. Intersection of the vertical line with the y axis represents the critical N value in A and B, whereas the highest coefficient of determination (r2) value produced by a shoot N concentration represents the critical value in C.

 


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  		Fig. 10. Amount of applied nitrogen presumed remaining after grain harvest of the pearl millet hybrid HGM 100 in five environments as it relates to shoot N content at early boot stage. * = equation significant to at least P < 0.05.

 

   CONCLUSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
No consistent response among five environments was found between relative optimum grain yield and N fertilizer rate. This resulted in a prediction equation across environments that had a moderately low fit (r2 = 0.36*). Weather conditions and soil type contributed to this variability. Grain protein yield on a relative basis had a more consistent response to N application (r2 = 0.67*) than grain yield with 65 kg N ha-1 usually providing optimum grain protein yield in four of five environments. A shoot tissue N concentration >= 31 g kg-1 dm at early boot was highly correlated to optimum relative grain yield (r2 = 0.65*) and was considered the critical N concentration at that growth stage. Critical shoot tissue N for relative grain protein yield at EB varied depending on the analysis method used, but 32 g N kg-1 dm was considered to be the best choice, as it would require less N applied and would have less excess N remaining after harvest. The results of this study contribute to the paradox that additional N amendments would be most effective if applied early in crop ontogeny, but the need for amendment can only be accurately assessed later in crop ontogeny. Thus, the window of opportunity for effective amendment is relatively narrow.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Approved for publication as manuscript No. 01-09-0535 by the Director of the La. Agric. Exp. Sta., LSU Ag Center.

Received for publication June 8, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 





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HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
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