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Published online 4 April 2007
Published in Agron J 99:599-606 (2007)
DOI: 10.2134/agronj2005.0290
© 2007 American Society of Agronomy
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Crop Rotations

Nodulating and Non-Nodulating Soybean Rotation Influence on Soil Nitrate-Nitrogen and Water, and Sorghum Yield

Nanga Mady Kayea, Stephen C. Masonb,*, Tomie D. Galushac and Martha Mamod

a 2901 18th St., NW., Apt. 408, Washington, DC 20009
b Dep. of Agronomy, 229 Keim Hall, Univ. of Nebraska, Lincoln, NE 68583-0915
c 202 KCR, Univ. of Nebraska, Lincoln, NE 685830-0915
d 242 Keim Hall, Univ. of Nebraska, Lincoln, NE 68583-0915

* Corresponding author (smason1{at}unl.edu)

Received for publication October 19, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Soybean [Glycine max (L.) Merr.] rotation has been shown to enhance grain sorghum [Sorghum bicolor (L.) Moench] growth and yield due in part to N contribution. Sorghum grain and stover yield, yield components, soil water and soil NO3–N were measured in a long-term rotation study in 2003 and 2004 on a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll). The objectives were to separate biologically fixed N from other rotation effects on sorghum grain and stover yields, and to relate yield to yield components, soil NO3–N and water contents. The cropping sequences were continuous grain sorghum, and sorghum rotated with non-nodulating or nodulating soybean. Soil amendment treatments consisted of control (zero), manure (17–25 Mg dry matter ha–1 yr–1), and N (41 kg ha–1 for soybean and 84 kg ha–1 yr–1 for sorghum). Cropping sequence x soil amendment interaction effects were found for most parameters measured. High soil NO3–N following soybean rotation and from amendment application promoted plant growth leading to low soil water content at anthesis, and increased kernel weight, grain and stover yield. Rotation with non-nodulating soybean without soil amendment increased grain yield by 2.6 to 3.0 Mg ha–1 and stover yield by 1.5 to 1.8 Mg ha–1 over continuous sorghum without soil amendment. Rotation with nodulating soybean further increased grain yield by 1.7 to 1.8 Mg ha–1 and stover yield by 0.6 to 0.9 Mg ha–1. Biologically fixed N effects accounted for only 35 to 41% of enhanced sorghum yield due to crop rotation with soybean. Soil NO3–N during vegetative growth, plant height, soil water content at anthesis and kernel weight were the most important parameters related to sorghum grain yield across cropping sequences and soil amendments.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
GRAIN SORGHUM is an important grain crop in the central Great Plains due to its drought tolerance (Sander and Frank, 1980), high nutrient use efficiency (Maranville et al., 1980), and use as livestock feed. During the past 50 yr, sorghum grain yields have increased by 139% largely due to improved hybrids and soil water management (Unger and Baumhardt, 1999). The major challenge to further increase dryland sorghum grain yields is synchronization of plant N needs with plant-available water (Khosla et al., 2000).

Rotating soybean with sorghum has been shown to increase grain yield (Gakale and Clegg, 1987; Roder et al., 1989, Peterson and Varvel, 1989), increase stover yield (Franzluebbers et al., 1995), alter soil stored water (Roder et al., 1989), increase soil N (Bagayoko et al., 1992), reduce N fertilizer requirement (Hanson et al., 1988), reduce plant nematodes and diseases (Bagayoko et al., 2000), improve soil physical properties (Gakale and Clegg, 1987), alter microbial biomass (Roder et al., 1988), enhance vesicular arbuscular mycorrhizae (Ellis et al., 1992), and reduce yield variability (Varvel, 2000).

Biological N fixation by soybean has been commonly thought to account for the "N credit" to succeeding cereal crops through residue, root and nodule decomposition. However, N balance studies have shown that soybean removes more N than it fixes (Zapata et al., 1987) and amount removed varies across environments (Bundy et al., 1993). It appears that the soybean "N credit" results largely from differences in immobilization and mineralization caused by different residue types (Smith and Sharpley, 1990; Green and Blackmer, 1995), amount of residue (Green and Blackmer, 1995), soil type (Bundy et al., 1993) and residue incorporation (Smith and Sharpley, 1990; Torbert et al., 1998).

One tool to attempt to separate biologically fixed N from other rotation effects is the inclusion of nodulating and non-nodulating soybean isolines into a crop rotation. Maloney et al. (1999) used this method for a maize (Zea mays L.)–soybean rotation study and found that N fixation by soybean was not responsible for the enhanced yield from rotation, while Gentry et al. (2001) and Bergerou et al. (2004) found biologically fixed N to be a minor contributor in similar studies. Since biological N fixation by soybean has a large energy requirement (Heytler and Hardy, 1984) one would expect the non-nodulating isolines to produce higher yields in the presence of adequate N. However, studies including nodulating and non-nodulating soybean isolines with high N application rates up to 600 kg ha–1 have shown similar yields (Weber, 1966; Cooper and Jeffers, 1984; Jeppson et al., 1978; Vasilas and Ham, 1984).

Sorghum yield, in combination with yield component differences, reflect presence and timing of stress conditions or differences in production practices (Maman et al., 2004). When early season stress on tillering (i.e., panicles per square meter) is small, the number of kernels per panicle (Limon-Ortega et al., 1998; Larson and Vanderlip, 1994, M'Khaitir and Vanderlip, 1992; Norwood, 1992) and kernel weight (Saeed et al., 1986; Rajewski et al., 1991; Norwood, 1992) have both been found to be the most important yield components. The number of panicles per square meter has been found to increase with N application (Kamoshita et al., 1998; Rajewski et al., 1991). In a recent study by Maman et al. (2004), kernel weight was found to be the most important yield component of sorghum across different environments and water regimes in the central Great Plains.

The objective of this study was to evaluate nodulating and non-nodulating soybean isolines and soil amendment application to separate the biologically fixed N and other rotational effects on sorghum grain and stover yields and yield components, and relate these to soil NO3–N and water content differences. An improved understanding of crop rotation benefits could equip producers with management tools to further enhance sorghum grain yield.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Research was conducted at the University of Nebraska Agricultural Research Development Center near Mead, NE, on a Sharpsburg silty clay loam in 2003 and 2004. The long-term rotation study was established in 1980, and modified in 1991 to incorporate nodulating and non-nodulating isolines of the soybean variety ‘Clark’. The experimental factors were cropping sequence and soil amendment. The cropping sequences were continuous sorghum, sorghum rotated with nodulating soybean, and sorghum rotated with non-nodulating soybean. The three soil amendments were zero fertilizer, fresh beef feedlot manure at 17.3 Mg ha–1 on dry matter basis in 2003 and 25.9 Mg ha–1 in 2004 (Table 1) applied in spring and incorporated by disking, and N fertilizer as NH4NO3 at 84 kg N ha–1 for sorghum and 41 kg N ha–1 for the soybean crop. Manure and N fertilizer had been applied to the same plots since 1980. The goal was to apply 15 to 20 Mg ha–1 manure on dry matter basis each year, but differences in manure application rate varied due to differences in water content of the fresh manure, as was the case in 2003 and 2004. In addition, N content of manure varied due to weather and length of time in storage. The greater manure application and higher N concentration of manure resulted in more total N applied in 2004 than in 2003 (Table 1). The N supplied by manure in the 1st year was estimated (DeLoughery and Wortmann, 2003) to be approximately 46 kg ha–1 in 2003 and 128 kg ha–1 in 2004. Nitrogen fertilizer was hand-applied at the V6 growth stage following soil and plant sampling, which corresponded to the stage of rapid sorghum growth and N uptake (Vanderlip, 1993).


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  		Table 1. Analysis of beef feedlot manure applied to plots in 2003 and 2004.

 
The experiment was conducted in a randomized complete block design with a split-plot treatment arrangement and four replications. The whole plot was cropping sequence while the subplot was soil amendment. The subplots were 9.1 m wide and 7.9 m long. For weed control, Dual-II Magnum [S-metolachlor: (1S)-2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] was sprayed as a pre-emergent herbicide at a rate of 1.1 kg a.i. ha–1, Basagran [bentazon; 3-(1-methylethyl)-1 H-2, 1,3-benzonthiadiazin-4 (3-one 2,2-dioxide)] at a rate of 1.1 kg a.i. ha–1 was applied at emergence, and hand weeding was done as needed. In 2003 during grain-fill, grasshoppers [Melanoplus differentialis (Thomas)] were controlled by aerial application of Lorsban [Chlorpyrifos: 0,0-diethyl 0-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] at 0.11 kg a.i. ha–1.

The drought and lodging tolerant sorghum hybrid DKS42-20, with 105- to 110-d relative maturity and intermediate stay-green rating was planted in both years. Nodulating and non-nodulating isolines of soybean variety Clark with a Group IV maturity classification and indeterminate growth habit were planted. Tillage consisted of disking on 21 May 2003 and 27 May 2004. Crops were planted on 22 May 2003 and 27 May 2004 using a six-row John Deere 7100 maxi-merge planter (John Deere, Moline, IL) in 76-cm row spacing at the rate of 39 285 kernels ha–1 in 2003 and 32 190 kernels ha–1 in 2004.

Soil Nitrate-Nitrogen and Water
Soils were sampled to a depth of 120 cm at planting, V6, anthesis and physiological maturity growth stages for determination of soil NO3–N and soil H2O contents. Two soil cores for each plot were gathered using a Giddings Soil (Windsor, CO) probe, and 30-cm depth increments were composited. Soil samples were analyzed for soil NO3–N using the automated cadmium reduction method analyzed by flow injection analysis (Gelderman and Beegle, 1998) following calcium phosphate extraction (Combs et al., 1998). Soil water was determined as the difference between soil weight at sampling and following oven drying at 45°C until a constant weight was reached. Volumetric water was calculated using soil bulk densities determined as the averages of 30-cm depth increments down to 120-cm depth in the fall of 2003. Soil bulk densities were similar, with an average surface 30-cm bulk density of 1.49 g cm–3 for nonmanured plots and 1.46 g cm–3 for manured plots. In addition, the surface 30-cm increment was sampled at planting in both years and analyzed for Bray-P1, and extractable K, S and Zn, and soluble salts.

Sorghum Grain and Stover Harvest
Grain yields were hand-harvested from an area of 9 m2 in the middle of each 12-row (8 by 9 m) plot, and were corrected to 140 g kg–1 water content. Panicles in the harvest area were counted and threshed at physiological maturity. Sorghum plants were harvested and stems, leaves, and panicles were separated and oven dried for 6 d at 60°C and used to calculate stover yield as the sum of leaf dry and stem dry weight per hectare. During grain fill, heights of four sorghum plants in middle rows of each plot were measured.

Statistical Analysis
Data were analyzed by ANOVA, appropriate orthogonal contrasts and Pearson correlations using SAS proc mixed procedures (Littell et al., 1996). All factors were considered fixed except replication, and P ≤ 0.05 was used to declare significant differences. Stepwise regression was used to identify the parameters most closely associated with grain yield. Years were analyzed separately due to presence of heterogeneity of variances.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Soil and Climate
Soil test results indicated that the 23-yr cropping sequence on the experimental site had no effect on soil nutrient levels and organic matter concentration (Table 2). Although significant, cropping sequence influence on soil pH and electrical conductivity were small. Soil amendment treatments, especially plots with manure application since 1980, affected soil nutrient levels. In both years, manured plots had higher levels of all measured soil parameters. Soil nutrient levels for all plots were greater than the sufficiency level for grain sorghum production, although Zn in nonmanured plots was marginal (Ferguson, 2000).


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  		Table 2. Soil test results for the upper 30 cm of a Sharpsburg silty clay loam soil at Mead, NE, in 2003 and 2004 (averaged across cropping sequences).

 
The 2004 growing season had greater precipitation than in 2003, especially in July and September (Table 3). Compared to long-term averages, the 2003 average precipitation represented 62% of the long-term precipitation while 2004 represented 71%, both well below the long-term average. Precipitation in 2004 was distributed more uniformly during the growing season than in 2003. Average growing season temperatures were similar, but it was more than 3°C hotter during July and Aug 2003 than in 2004. In 2004, temperatures were higher in May and September, which is important given sorghum's base temperature of 15°C (Kasalu et al., 1993), early and late-season sorghum sensitivity to cold temperature (Eastin, 1976), and temperature effects on the rate of soil N mineralization (Quemada and Cabrera, 1997). Both years had lower temperatures than the 30-yr average.


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  		Table 3. Growing season monthly precipitation and temperature for Mead, NE, in 2003 and 2004. Data were gathered by University of Nebraska weather station located less than 1 km from the experimental site.

 
Soil Nitrate-Nitrogen and Water
In this study, potential soil NO3–N sources were soil organic matter, residual soil NO3–N from the previous year, crop residues (including roots and nodules on nodulating soybean), manure or fertilizer, and NH4–N in the soil. The C/N ratios of crop residues and soil organic matter influence the rate and amount of soil N mineralization and immobilization (Smith and Sharpley, 1990; Green and Blackmer, 1995). In all treatment combinations, the soil NO3–N content decreased greatly between the V6 and anthesis growth stages (Table 4) when the most rapid sorghum growth and N uptake occurred (Vanderlip, 1993). In all cropping sequences, plots receiving soil amendment (either N fertilizer or manure) had the highest soil NO3–N level while the zero soil amendment had the lowest. Soil NO3–N content was greater for manure than N fertilizer plots within cropping sequence; however, soil amendment interacted with cropping sequence at V6 growth stage in 2003, and planting and anthesis stages in 2004. Contrasts indicated that the difference in soil NO3–N between manure and N varied among cropping sequences in these cases. In general, manured plots had greater soil NO3–N than N plots, consistent with the greater total N application with manure (Table 1) than N fertilizer, and the 1 g kg–1 higher soil organic matter content in manured plots in 2003 and 3 g kg–1 higher content in 2004 than in N plots (Table 2). Manure contributed large amounts of NO3–N to the growing crops, but also left large amounts of soil NO3–N in the soil profile at physiological maturity.


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  		Table 4. Influence of cropping sequence and soil amendment on soil NO3–N in the 120-cm depth soil profile in 2003 and 2004, Mead NE.

 
The rotational cropping sequences had higher soil NO3–N levels than for continuous sorghum (Table 4) similar to those reported by Bagayoko et al. (1992). Interactions with soil amendment were present at V6 stage in 2003, and planting and anthesis stages in 2004. At V6 stage in 2003, soil NO3–N in the sorghum following non-nodulating soybean sequence with N fertilizer was greater than for sorghum following nodulating soybean, and the opposite was true for manured plots. Manured plots for sorghum following non-nodulating soybean had greater soil NO3–N than for sorghum following nodulating soybean. Although not always significant, generally plots with sorghum following non-nodulating soybean had intermediate soil NO3–N levels at planting and V6 growth stage, likely due to mineralization of stored N in the intermediate C/N ratio soybean residues (Bergerou et al., 2004; Gentry et al., 2001). The greater soil NO3–N in plots rotated with soybean was likely due to increased mineralization of N from soil organic matter and the low C/N ratio soybean residues including roots. When differences in soil NO3–N occurred between sorghum following nodulating and non-nodulating soybean, the difference was likely due to lower C/N ratio of nodulating soybean residues and decomposition of nodules (Bergerou et al., 2004). Soil NO3–N concentrations were generally consistent with anticipated N supply from manure (DeLoughery and Wortmann, 2003) and fertilizer (Khosla et al., 2000), and from rotation with soybean (Bagayoko et al., 1992; Gentry et al., 2001).

Soil water content was similar for all treatment combinations at planting, while at V6 stage it varied by cropping sequence and at anthesis it varied by cropping sequence, soil amendment, and their interaction in both years (Table 5). At V6 and anthesis stages, continuous sorghum plots had more soil water than rotated plots in both years. At anthesis, continuous sorghum plots without soil amendment had the greatest soil water content, continuous with soil amendment had intermediate soil water content, and rotated with amendment had the least soil water content. The lack of soil amendment increased soil water content more for continuous than rotated sorghum. Although manured plots would be expected to have the greatest water holding capacity (Barker, 1996), they had similar water content to N plots and less soil water than zero plots. Observations indicated that greater plant growth and leaf area was present in plots with lowest soil water content at anthesis, suggesting that low soil water content was associated with increased crop water use due to greater growth between planting and anthesis (Vanderlip, 1993).


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  		Table 5. Influence of cropping sequence and soil amendment on soil water content in 120-cm soil profile in 2003 and 2004, Mead NE.

 
Grain Yield
Analysis of variance indicated that sorghum grain yield was influenced by the cropping sequence, soil amendment and their interaction effects in both years (Table 6). Without soil amendment, including non-nodulating soybean in the cropping sequence increased sorghum grain yield by 143% in 2003 and 79% in 2004 over continuous sorghum. In both years, including nodulating soybean in the rotational cropping sequence without soil amendment increased yield by 31% over rotation with non-nodulating soybean. These data suggest that biological N fixation effects of soybean accounted for approximately 35% of the grain yield enhancement due to rotation in 2003 and 41% in 2004, thus most of the rotation benefit was due to effects other than biologically fixed N by the preceding soybean crop, such as immobilization/mineralization of soil N (Smith and Sharpley, 1990; Green and Blackmer, 1995), reduced disease infestation (Bagayoko et al., 2000), improved physical properties (Gakale and Clegg, 1987) and altered soil microbial biomass (Roder et al., 1988). This is similar to the findings of Gentry et al. (2001) and Bergerou et al. (2004) for a maize–soybean rotation. The rotation benefit to sorghum yield was greater in this study than reported by Roder et al. (1989), Peterson and Varvel (1989) and Gakale and Clegg (1987), possibly related to the below average seasonal precipitation (Roder et al., 1989) present in both years (Table 3).


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  		Table 6. Influence of cropping sequence and soil amendment on grain yield and components in 2003 and 2004, Mead, NE.

 
In both years, manure and N fertilizer had a similar effect on sorghum grain yield (Table 6). In the drier 2003 yr (Table 3), when N fertilizer or manure was applied, rotation with non-nodulating soybean increased grain yield by 27 to 29% with no further increase with rotation of nodulating soybean (Table 6). In 2004, the yield enhancement due to rotation with non-nodulating soybean with N or manure was ≤9%, and further yield increase of rotation with nodulating soybean of ≤7%. Application of N or manure resulted in a large grain yield increase of 4 to 5 Mg ha–1 for continuous sorghum, an intermediate increase of 2.6 Mg ha–1 in 2003 and 3.3 Mg ha–1 in 2004 for sorghum following non-nodulating soybean, and smaller increase yield of 0.5 Mg ha–1 in 2003 and 1.8 Mg ha–1 in 2004 for sorghum following nodulating soybean. Roder et al. (1989) and Bagayoko et al. (1992) found a similar trend in grain sorghum yield increase due to application of manure and N. Therefore, yield enhancement and reduced soil amendment application rates can be obtained through use of crop rotation.

Grain Yield Components
Cropping sequence x fertilizer interaction effects were found for panicles per square meter and 100-kernel weight (Table 6) in the drier 2003 yr (Table 2). Soil amendment increased the number of panicles per square meter for continuous sorghum and sorghum rotated with non-nodulating soybean, but not for sorghum rotated with nodulating soybean (Table 6). Kernel weights were similar for all cropping sequences and fertilizer treatments except for N application to sorghum following nodulating soybean, which resulted in 0.27 to 0.40 g per 100 kernels lighter than other treatment combinations. Continuous sorghum produced fewer panicles per square meter than in other cropping sequences. In 2004, manure application increased number of panicles per square meter over N and zero plots, while N and manure application increased 100-kernel weight over zero plots.

Plant Height and Stover Yield
Plants were taller and differences among treatments were greater in the wetter 2004 yr than in 2003 (Table 7). With zero amendment, rotation increased plant height in both years over continuous sorghum, and sorghum following nodulated soybean was taller than following non-nodulated soybean. Soil amendment increased plant height the most for continuous sorghum, intermediate for sorghum following non-nodulating soybean and had no influence on height for sorghum following nodulated soybean.


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  		Table 7. Influence of cropping sequence and soil amendment effects on stover yield, stover N and plant height in 2003 and 2004.

 
Soil amendment increased stover yield in both years over zero plots (Table 7), while in wetter 2004 yr with greater manure N application, manured plots also produced more stover than N plots. Rotation with soybean increased stover yield in 2004. The same trend was present in 2003, but was only significant at P = 0.07.

Correlation with Grain Yield
Grain yield was highly correlated with soil NO3–N at planting, V6 and anthesis growth stages (R = 0.42–0.65); panicles per square meter (R = 0.49–0.77); plant height (R = 0.79–0.82); and stover yield (R = 0.59–0.82); and negatively correlated with soil water at anthesis (R = –0.72 to –0.83) in both years. In addition, in the wetter 2004 season with lower plant population, 100-kernel weight (R = 0.69) was also correlated with grain yield, with 100-kernel weight being more highly correlated than panicles per square meter (R = 0.49) as reported by Maman et al. (2004) and Rajewski et al. (1991). We speculate that the greater importance of panicles per square meter in 2003 was due to dry soil conditions combined with the higher plant population, since water stress during vegetative growth (Tables 2 and 5) increases the importance of the number of panicles produced in grain yield determination (Kamoshita et al., 1998).

In both years, stepwise regression determined the same parameters to be associated with grain yield:

Formula 1[1]

Formula 2[2]

High-yielding plots of sorghum following nodulating soybean with soil amendment (Table 6) had the greatest soil NO3–N present at vegetative growth stage (Table 4) which led to the greatest sorghum plant height (Table 7), consequently higher soil water extraction by the crop during vegetative growth, resulting in lower soil water levels at anthesis (Table 5), leading to production of heavier kernels (Table 6). This study has shown that biologically fixed N from soybean directly contributes only 35 to 42% of the enhanced sorghum yield in crop rotation. Clearly rotation with nodulating soybean, and to a lesser extent with non-nodulating soybean, also increased soil mineralization early in the growing season to release more soil NO3–N that promoted sorghum growth, and soil water utilization to produce heavy sorghum kernels.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Rotation with soybean combined with manure or N fertilizer application have been recommended practices for decades to supply N and optimize grain yield. This study extended the understanding of the role of biological N fixation in grain yield enhancement of maize rotated with soybean in the humid eastern maize belt (Maloney et al., 1999; Gentry et al., 2001; Bergerou et al., 2004) to sorghum rotated with soybean in the semiarid Great Plains. This study also documented that increased soil NO3–N from rotation with nodulating soybean (and to a lesser extent with non-nodulating soybean) and soil amendments increased soil NO3–N which stimulated sorghum growth resulting in lower soil water content at anthesis, and increased grain and stover yield at harvest. Only 35 to 41% of the rotational enhancement of sorghum grain yield was due to biologically fixed N, thus the majority of the rotational grain yield enhancement was attributed to other factors as found by Gentry et al. (2001) and Bergerou et al. (2004) in maize–soybean rotations. Future study on the timing and rate of N mineralization from nodulating and non-nodulating soybean, effect of manure and N fertilizer on rate and timing of N mineralization of soybean residues (including roots and nodules) and soil organic matter, and the influence of nodulating and non-nodulating soybean on the soil microbial communities are merited.


   ACKNOWLEDGMENTS
 
The authors wish to acknowledge the assistance of Drs. Kent Eskridge and Erin Blankenship, Department of Statistics, University of Nebraska for their assistance in analyzing and interpreting the data in this manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Contribution of the Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583-0915. Paper 14709 of the Journal Series of the Nebraska Agric. Res. Div. Research supported by USAID Grant DAN 1254-G-0021 through INTSORMIL, the International Sorghum and Millet Collaborative Research Program.


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





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