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Crop Science 42:1556-1563 (2002)
© 2002 Crop Science Society of America

CROP ECOLOGY, MANAGEMENT & QUALITY

Nitrogen Effects on Grain Yield and Yield Components of Leafy and Nonleafy Maize Genotypes

Carlos Costaa, Lianne M. Dwyerc, Doug W. Stewartc and Donald L. Smith*,b

a Univ. of Passo Fundo, Passo Fundo, RS 99001-970, Brazil
b Dep. of Plant Science, McGill Univ., Macdonald Campus, 21,111 Lakeshore, Ste-Anne-de-Bellevue, QC, H9X 3V9 Canada
c Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre, Ottawa, ON, K1A OC6 Canada

* Corresponding author (dsmith{at}macdonald.mcgill.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Effects of N fertilization have been extensively studied for conventional maize (Zea mays L.) hybrids but not for genotypes bearing the leafy and reduced-stature traits which differ significantly in canopy and root morphology. We tested the hypothesis that genotypes carrying the leafy trait (taller plants with more leaves, greater leaf area development, and greater rooting systems) would show differing responses to N availability in terms of grain yield and yield components from those of conventional maize hybrids. The experimental design was a split-plot in a randomized complete block design with four blocks repeated across two growing seasons at each of two field sites. The treatments were N fertilization rates (0, 85, 170, and 255 kg N ha-1) as the main plot factors and genotypes as the subplot factors. The genotypes were leafy reduced stature (LRS), nonleafy normal stature (NLNS), leafy normal stature (LNS), nonleafy reduced stature (NLRS), and conventional hybrid checks of early (P3979) and late maturity (P3905). The latter consistently yielded best and the NLRS hybrid worst; however, the genotypic grain yield ranking varied between sites. Overall, the LRS outyielded its conventional counterpart (P3979) by 12% at one site and by 26% at the other. No significant N x hybrid interactions were detected for grain yield. We inferred that using leafy genotypes in maize production would not require additional N fertilization compared with their conventional maize hybrid counterparts.

Abbreviations: ED, ear diameter • EL, ear length • GY, grain yield • HI, harvest index • KNR, kernel number per row • KRN, kernel row number • LNS, leafy normal stature • LRS, leafy reduced stature • NLNS, nonleafy normal stature • NLRS, nonleafy reduced-stature • P3905, Pioneer 3905 • P3979, Pioneer 3979 • RUE, radiation use efficiency • SDM, stover dry mass


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NEW MAIZE GENOTYPES adapted to short-season areas such as the St. Lawrence lowlands of eastern Canada, where limited heat units and photosynthetically active radiation are available during the growing season, are needed. Nonadapted hybrids develop shorter plants with fewer and smaller leaves, resulting in lower yields than they would have in longer season areas (Chase and Nanda, 1967). Low radiation availability during the grain filling period is particularly limiting to yield. Genetically altering maize to increase the leaf number per plant could increase grain yield under short season conditions. Alterations resulting in leaf arrangements that maximize light interception and optimize radiation use efficiency (RUE) could further improve yield.

Genotypes carrying the leafy trait showed greater numbers of above-ear leaves (Shaver, 1983) and on average, two more leaves, in total, than the conventional genotypes (Begna et al., 2001). Consequently, they have higher leaf area indices (Stewart and Dwyer, 1993; Modarres et al., 1997b; Begna et al., 1999; Dijak et al., 1999) than their conventional counterparts. A shorter vegetative period, longer grain-filling period, and higher yields were noted for leafy genotypes than conventional genotypes (Begna et al., 1997a; Modarres et al. 1997a), making the leafy genotypes particularly well suited for short season areas. The reduced-stature trait leads to plants that develop and mature quickly. Leafy reduced stature genotypes matured faster, yielded more, and were more tolerant to high planting densities than their conventional counterparts when grown under field conditions in Québec (Begna et al., 1997a; Modarres et al., 1998). As RUE increases with leaf N content (Muchow and Sinclair, 1994) and leaf N content increases with N fertilizer rate (Touchton et al., 1979), an understanding of N fertilization-hybrid interactions is important.

A direct relationship between N fertilization rate and maize plant growth and grain yield has been widely demonstrated (Zhang et al., 1993; Jokela and Randall, 1989; McCullough et al., 1994). However, studies with conventional maize hybrids (Chevalier and Schrader, 1977; Pérez Leroux and Long, 1994) have shown that maize genotypes vary in their response to N availability, reflecting variations in their relative abilities to absorb native or fertilizer N from the soil (N uptake efficiency), and in their relative efficiencies in using acquired N to produce yield components (N use efficiency) (Chevalier and Schrader, 1977; Moll et al., 1982). In addition, there is a growing public awareness of nonpoint source pollution of agroecosystem origin. This has led to N being targeted for study both as a plant nutrient (Rice et al., 1995) and as a pollutant (Gaines and Gaines, 1994; Patni et al., 1996). Minimizing drainage water NO-3–N while maintaining or improving maize yield is an ongoing challenge. Therefore, N fertilization effects on newly available genotypes, such as leafy, need to be studied to ascertain whether they will produce maximum return with minimal environmental hazards when integrated into a maize production system.

The objective of this study was therefore to compare the effects of N fertilization rates on the yield and yield components of maize genotypes containing the leafy (Lfy) and reduced-stature (rd1) traits, singly or in combination, with conventional maize genotypes.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sites and Experimental Design
The study was carried out during the 1996 and 1997 growing seasons at two experimental sites. One site was located at the Lods Agronomy Research Centre, Macdonald Campus, McGill University, Ste-Anne-de-Bellevue, QC, Canada (45°28' N, 75°45' W), where the soil was a Chateauguay clay loam (fine, mixed, nonacid, frigid, typic Humaquepts). The other site was located at the Central Experimental Farm of Agriculture and Agri-Food Canada in Ottawa, ON, Canada (45°23' N, 75°43' W), where the soil was a well-drained sandy loam of the Uplands Association (coarse loamy, mesic, mixed, Haplorthods). At both sites, the experiments were conducted in the same area of the field for two consecutive growing seasons. Both sites had been planted to cereals (wheat and barley) for at least 2 yr prior to this study.

A split-plot design with four blocks was used for each field experiment. The main plot treatments consisted of four N fertilization rates (0, 85, 170, and 255 kg N ha-1). The subplot treatments were six maize genotypes. The subplots were 4.56 by 6 m and consisted of six rows with a 0.76-m spacing between rows. The four rates of N fertilizer were each applied as a single application of NH4NO3 in the spring of each year, just before seeding. The seeding dates in 1996 were 22 May at the Ottawa site and 20 May at the Macdonald site. In 1997, seeding dates were 19 May at the Ottawa site and 23 May at the Macdonald site. Sufficient P and K (1996) and P (1997) to support a yield goal of 9 Mg ha-1 were applied at seeding, according to soil test recommendations for each site. In both years at the Macdonald site and in 1996 at the Ottawa site, weeds were controlled by preemergence application of Primextra Lite [Dual (Metalochlor, 2-Chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide plus atrazine] applied at a rate of 7 L ha-1 and incorporated into the soil by harrowing (Kongskilde Triple K). At the Ottawa site in 1997, weeds were controlled with the herbicide combination Fieldstar (Flumetsulam, N-(2,6-difluorophenyl)-5-methyl-(1,2,4) triazolo(1,5-a)pyrimidine-2-sulfonamide plus Dual 960E (Clopyralid; 3,6-Dichloro-2-pyridinecarboxylic acid) incorporated into the soil at a rate of 5 L ha-1.

Genotypes
The six genotypes were selected on the basis of their contrasting canopy architectures, as well as preliminary observations suggesting differences in rooting pattern (Modarres, 1996, personal communication). The genotypes (Fig. 1) represented four genotypic groups: LRS, NLNS, LNS, and NLRS. In addition, conventional commercial hybrids, Pioneer 3905, (P3905, requiring 2800 heat units, °C) as the check hybrid for late maturity, and Pioneer 3979 (P3979, requiring 2550 heat units, °C) as the check for early maturity (Fig. 1) were grown. Maize genotypes bearing the leafy trait have more leaves above the ear, a lower ear placement, a higher yield potential, and are generally earlier maturing (Shaver, 1983). The general range of plant heights for the genotypes used in this study were previously recorded: 0.80 to 1.20 m for LRS, 2.20 to 2.85 m for LNS, 0.60 to 1.10 m for NLRS, 2.00 to 2.67 m for P3905 and P3979 (Modarres et al., 1997c; Begna et al., 1999), and 2.18 m for NLNS (Dijak et al., 1999). Each genotype was seeded at its optimum population density (plants ha-1): 100000 for LRS, 150000 for NLRS (Begna et al., 1999), 55000 for NLNS and LNS (Modarres and Smith, 1996, personal communication), and 75000 for P3905 and P3979 (Russel, 1985; Tollenaar, 1991). Plant populations from 65000 to 75000 plants ha-1 are generally recommended for conventional maize genotypes in Québec and Ontario.



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  		Fig. 1. Examples of plant canopy architectures of field-grown maize genotypes at silking stage.

 
Yield Component Assessment
At harvest, for each genotype, plants from 2 m of the two central rows were hand-cut at the soil surface. Maize ears were removed, shelled, and both cob and grain total fresh mass determined. The whole stalks (stem plus leaf sheaths) were bulked, chopped, and weighed. Subsamples of cobs, stalks, and grain were weighed and dried to a constant weight at 80°C in a forced hot-air dryer and reweighed. Final grain yield was expressed in Mg ha-1, adjusted to a 155 kg water Mg-1 basis.

The variables recorded were: (i) stover, cob, and total biomass yields, (ii) ear length (EL) and diameter (ED), (iii) number of kernel rows per cob, (iv) number of kernels per cob row, (v) harvest index (HI), and (vi) grain yield.

Data Analysis
Statistical analyses were performed using SAS for Windows Release 6.12 (SAS Institute, 1997). The SAS procedures used for the ANOVA and normality tests were GLM (general linear model) and UNIVARIATE, respectively. Protected ANOVA LSD tests were used to assess the differences between treatment means (Steel and Torrie, 1980). Regression models were fitted to the data to describe the relationship between N rates and stover dry matter and N rates and grain yield and were carried out with the SAS procedure REG (SAS Institute, 1997).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Environmental Conditions
Temperatures in both site–years were similar to the 30-yr averages (Fig. 2) . While average temperatures for both sites in 1996 and 1997 were fairly similar (17.5°C in 1996 and 16.8°C in 1997), there was 175 mm more rainfall in 1996 than in 1997 at the Ottawa site, with below normal precipitation in June, July, and August 1997, but July 1996 precipitation was 1.7 times normal. Total rainfall for the Macdonald site in 1997 was greater than the 30-yr average for May, June, July, and August (Fig. 2). In contrast, August 1996 precipitation at Macdonald was only 25% of normal.



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  		Fig. 2. Monthly mean temperature and total rainfall during the 1996 and 1997 growing seasons and 30-yr mean at the Macdonald and the Ottawa experimental sites.

 
Stover Plus Husk Dry Mass
Nitrogen fertilization rates had no effect on any of the variables studied for the Macdonald site in 1996 (Table 1) . For this site, 1997, there was a positive effect of increasing N-fertilization rate on stover dry mass and total biomass (stover plus husks, cobs and grain, on a dry mass basis). For the Ottawa site, there were positive N fertilization effects on cob and total dry mass in 1996, and for stover dry mass, and total biomass in 1997 (Table 2) . Such an increase in maize dry mass with increasing N fertilization has also been shown elsewhere (Jokela and Randall, 1989; Piekielek and Fox, 1992; Guillard et al., 1995). Because there was no N x genotype interaction for any site–year, the response of stover dry mass (SDM in Mg ha-1) to applied N (in kg ha-1) was regressed using pooled data across the genotypes for each year, and within site data. Data from the Macdonald site were best fit by a linear model (SDM = 4.37N + 4699.22; r2 = 0.90, P < 0.05) while that of the Ottawa site were best fit by a quadratic model (SDM = -0.049N2 + 17.86N + 6752.58; r2 = 0.99, P < 0.01). While dry mass was greater for the highest N fertilizer rate in both years at the Macdonald site, at the Ottawa site, the highest dry mass occurred at the 170 kg N ha-1 rate. The N fertilizer vs. dry mass yield relationships were consistent within sites among years.


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  		Table 1. Stover (SDM) and cob dry mass (CDM), total biomass yield (TB), harvest index (HI) and grain yield (GY) of maize genotypes for the Macdonald site in 1996 and 1997.

 

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  		Table 2. Stover (SDM) and cob dry mass (CDM), total biomass yield (TB), harvest index (HI) and grain yield (GY) of maize genotypes for the Ottawa site in 1996 and 1997.

 
Genotypic effects were observed for all traits as was expected for this wide range of maize architecture types (Tables 1 and 2). Nonleafy normal stature and P3905 generally had the largest stover, cob, and total dry masses for all site–years while NLRS had the lowest. These results confirm those reported by Modarres et al. (1997a) for normal-stature inbred lines. One might have expected that the early maturing LRS, which was 0.46 m shorter than its early maturing conventional counterpart P3979, would have generated less stover dry mass. However, both genotypes had essentially equal stover dry mass across sites and years. The leafy genotypes, LRS and LNS, had more total leaves and above-ear leaves than the taller conventional genotypes (the early maturing P3979 and late maturing P3905). While leaves made up 26.9%, stem 52.2%, and ear 20.9% of LRS total dry mass, these values were 19.3, 49.7, and 31.0% for P3979 (data not shown). It is important to note that NLRS (average height of 1.38 m at maturity) generally yielded less aboveground dry mass. Higher levels of kernel breakage occurred during shelling of the late maturing NLNS (higher cob moisture content than its counterparts) than did with other hybrids. These breakage losses were minimized by delaying harvest as long as was possible. The high cob moisture content found for the NLNS hybrid was consistent with those reported in other studies (Modarres et al., 1998b). Lower cob moisture content is generally associated with ease of shelling, as genotypes with high cob moisture contents exhibit lower shelling ratios (Cross et al., 1987). Higher cob moisture content may cause kernel damage, resulting in losses in grain yield (McGhee, 1971). Differences among maize hybrids in the rate at which cob moisture is lost during maturation have been previously reported for conventional maize hybrids (Cross and Kabir, 1989). There is, however, no clear indication from this study that maize genotypes bearing the leafy trait would show lower stover and grain moisture contents at harvest than their conventional hybrid counterparts. The lateness and generally high grain moisture content at harvest make the NLNS less suitable to Canada's short-season areas where earlier maturity is important for higher probability of complete harvest as well as lower drying costs.

Harvest Index
At the Macdonald site, HI ranged from 0.47 to 0.53 in 1996 and from 0.52 to 0.62 in 1997 (Table 1), while at the Ottawa site it varied from 0.42 to 0.58 in 1996 and from 0.43 to 0.66 in 1997 (Table 2). In 1997, applied N rates influenced HI at both sites: HI increasing with N rate at the Macdonald site, but peaked at 85 kg N ha-1 at the Ottawa site. Soil type (sandy loam) and low July and August rainfall may have been responsible for the decline in HI > 85 kg N ha-1. Ample early season moisture levels, conducive to good canopy development, could result in more leaf area (potential transpiratation) and thus more rapid depletion of soil water and development of moisture stress during a period of low rainfall. This would mean greater stress levels for the high N plants, leading to a lower HI and yield. A N x genotype interaction was observed only for the data at the Ottawa site in 1997. In particular, greater discrimination in HI occurred among hybrids at the low N rates (i.e., 0 and 85 kg N ha-1) than at the intermediate or the highest N rates (i.e., 170 and 255 kg N ha-1). In 1997, at the Ottawa site and across N rates, NLNS showed a lower HI than other genotypes, which did not differ in HI amongst themselves (Table 2). Except for the 1996 Ottawa data, HI values were all within the range reported by Begna et al. (1997a). Generally, NLRS showed high HI values and NLNS low values. The greater HI reported for LRS genotypes by Begna et al. (2000)(1997b) was not observed. This suggests that some genetic variation may exist among LRS genotypes in terms of HI.

Yield Components
While EL and ED were affected by N rate in both years at the Ottawa site, at the Macdonald site the effect occurred only in 1997 (Tables 3 and 4) . This is evident in Fig. 3 , which shows an increase of both EL and ED from 0 to 85 kg N ha-1 and no effect above this rate. An increase in N fertilization from 0 to 85 kg N ha-1 increased EL by 0.9 and 11% for the Macdonald site, and 8 and 17% for the Ottawa site in 1996 and 1997, respectively. There was an N x hybrid interaction for ED at both sites in 1996. Cob diameter was generally larger at 85 and 170 kg N ha-1 than at the highest N fertilization rates. Across N rates, the NLNS and P3905 hybrids had greater ELs and EDs (Tables 3 and 4) than the other genotypes tested. Hybrids showed differing EDs at each of the N rates. Visual differences can be seen among typical examples of ears of the six maize genotypes collected at the Macdonald site in 1997 (Fig. 4) . There was, in general, a positive relationship between EL and ED. In general, poorer coefficients of correlation of EL vs. ED were found for NLRS as compared with other genotypes (r = 0.35, P > 0.5). The relative contribution of kernel and cob to ED is shown in Fig. 5 .


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  		Table 3. Mean of ear length (EL), ear diameter (ED), kernel row number (KRN), and kernel number per row (KNR) of maize genotypes for the Macdonald site in 1996 and 1997.

 

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  		Table 4. Mean of ear length (EL), ear diameter (ED), kernel row number (KRN), and kernel number per row (KNR) of maize genotypes for the Ottawa site in 1996 and 1997.

 


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  		Fig. 3. Effect of N fertilization rate on ear diameter and on ear length for data pooled across six maize genotypes grown at the Macdonald and Ottawa sites in 1996 and 1997.

 


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  		Fig. 4. Examples of ears of six contrasting maize genotypes grown at the Macdonald site in 1997.

 


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  		Fig. 5. Relative contribution of top and bottom kernels and cob to the total diameter of the ear of maize hybrids grown at the Macdonald site in 1997.

 
There was no N effect on kernel row number (KRN) per ear and kernel number per row (KNR). Each year, within sites, differences were noted between maize genotypes for both KRN and KNR (Tables 3 and 4). The NLNS genotype was generally among those with the greater KRN and KNR, and this was consistent across site–years. The KRN values ranged from 12.5 to 16.0 and KNR from 21.7 to 32.4 at the Ottawa site in 1997 (Table 4). At the Macdonald site, KRN values ranged from 11.7 to 15.3 and KNR from 28.2 to 36.8 (Table 3). These results are consistent with those reported by Begna et al. (1997a), who measured values ranging from 12.0 to 14.7 and 18 to 30 for KRN and KNR, respectively, in a field study conducted under similar experimental conditions. There were no genotype x N interactions for KRN and KNR for any of the site–years.

Grain Yield
Across both sites, genotype in 1996 and both N and genotype in 1997 were the only factors to affect yield (Tables 1 and 2). There was no interaction between the applied N fertilization rates and maize genotype, indicating that N rate effects on grain yield were constant from one maize genotype to another, suggesting that the introduction of leafy genotypes into maize production would not require additional N fertilization compared with their conventional maize counterparts.

For the Macdonald site, grain yield increments from one N-fertilization rate to the next higher one were generally the same for both low and high N rates, whereas at the Ottawa site the greatest yield increments occurred at low N rates (Fig. 6) . Grain yield was 1.43-fold higher with 85 kg N ha-1 than without N application at the Ottawa site, but this difference was just 1.07-fold at the Macdonald site. Increasing N-fertilization from 170 kg N ha-1 to 255 kg ha-1 resulted in no increase in yield for the Ottawa site, but an increase for the Macdonald site (Fig. 6). Maize genotypes were grown in a clay loam soil at the Macdonald site and in a sand loam soil at the Ottawa site. Thus, these results were not surprising as crop response to applied N fertilizer is affected by a number of factors including soil type (Oberle and Keeney, 1990; Lory et al., 1995), crop sequence and supply of residual and mineralized N (Lory et al., 1995). While a linear regression model relating grain yield (GY, Mg ha-1) and N fertilization rate (N, kg ha-1) (GY = 8.62N + 7960.32; r2 = 0.99, P < 0.01) better fitted the Macdonald site data, a quadratic model (GY = -0.143N2 + 44.49N + 8654.03; r2 = 0.85, P < 0.01) best fitted the Ottawa site data (Fig. 6). Both linear (Oberle and Keeney, 1990) and quadratic (Oberle and Keeney, 1990; Stecker et al., 1995) models have been reported for N fertilization-yield relationships for conventional maize hybrids. On the basis of the regression model for the Ottawa site, maximum grain yield was obtained with 156 kg N ha-1, a rate well within the locally recommended range of 120 to 170 kg N ha-1 (Conseil des Production Végetales du Québec, 1996).



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  		Fig. 6. Regression relationships between applied N rates and mean grain yield from data pooled across genotypes and years within sites. Macdonald: GY = 8.62N + 7960.32; r2 = 0.99, P < 0.01. Ottawa: GY = -0.143N2 + 44.49N + 8654.03; r2 = 0.85, P < 0.01.

 
The ranges of grain yields obtained for the Macdonald site (7.0 to 12 Mg ha-1; Table 1) and for the Ottawa site (7 to 15 Mg ha-1; Table 2) in 1996 and 1997 were similar to those reported for other studies under similar environmental conditions (Modarres et al., 1997c; Begna et al., 1997a). The mean grain yield was 9.0 Mg ha-1 at the Macdonald site (Table 1) and 10.7 Mg ha-1 for the Ottawa site (Table 2). At both sites, consistently higher yields were obtained with the late maturing conventional P3905 (Tables 1 and 2). However, the genotypic yield ranking varied within site–year. Overall, while the LRS outyielded its conventional counterpart, P3979, by 12% at the Macdonald site, P3979 outyielded LRS by 26% at the Ottawa site. The grain yield performances of LRS and LNS were greatly affected by bird damage at the seedling stage at the Ottawa site in 1997, which probably affected their overall performance at this site. Thus, the results obtained at the Macdonald site tend to better agree with the observation that the early LRS generally outyields its conventional early maturing counterparts (Modarres et al., 1998).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study constitutes the first attempt to investigate the responses of maize genotypes bearing the leafy trait, reduced-stature trait, or a combination of the two traits and conventional maize genotypes to N fertilizer rates in terms of grain yield and yield components.

At both sites, consistently high and low yields were found for the late maturing conventional P3905 and NLRS genotypes, respectively. The reported higher HI for LRS than conventional genotypes was not observed in this work, suggesting that genetic variation for this trait exists among LRS genotypes. Moreover, the larger canopies and root systems for hybrids bearing the leafy trait did not translate into greater yield. Identification of the physiological mechanisms limiting the translation of the anatomical superiorities of leafy hybrids into greater grain yields, along with how to best fix the expression of the trait, will be important avenues for further research.


   ACKNOWLEDGMENTS
 
We are grateful to Lynne Evenson, Teshome Melkamu, Peter Neave, Doug Balchin, and Dave Meredith for their assistance in field data collection. This research was supported partially by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Collaborative Research Grant. The senior author is grateful to the Brazilian Post-Graduate Federal Agency (CAPES) for financial support.

Received for publication June 23, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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