SciELO - Scientific Electronic Library Online

 
vol.44 número5Parásitos gastrointestinales del guajolote silvestre de Gould (Meleagris gallopavo mexicana): abundancia, distribución, prevalencia y diversidadComportamiento productivo de cultivares de zanahoria y achicoria con un sistema de intercultivo en surcos y cultivos únicos índice de autoresíndice de assuntospesquisa de artigos
Home Pagelista alfabética de periódicos  

Serviços Personalizados

Journal

Artigo

Indicadores

Links relacionados

  • Não possue artigos similaresSimilares em SciELO

Compartilhar


Agrociencia

versão On-line ISSN 2521-9766versão impressa ISSN 1405-3195

Agrociencia vol.44 no.5 México Jul./Ago. 2010

 

Fitociencia

 

Uptake and nitrogen efficiency in forage maize: effects of nitrogen and plant density

 

Absorción y eficiencia del nitrógeno en maíz forrajero: efectos del nitrógeno y la densidad de población

 

S. Fallah* , A. Tadayyon

 

Department of Agronomy and Plant Breeding. Faculty of Agriculture. Shahrekord University, P. O. Box 115. Shahrekord, Iran. *Author for correspondence: (falah1357@yahoo.com).

 

Received: December, 2009.
Approved: January, 2010.

 

ABSTRACT

Efficient nitrogen (N) fertilizer management is critical for the economic production of maize (Zea mays L.) and the long–term protection of environmental quality. To study the effects of N doses and plant density on the N uptake and efficiency on forage maize cv. SC 704, an experiment was conducted in the Shahrekord region (32° 21' N, 50° 49' E; altitude 2050 m), Iran, at the Agricultural Research Station of Shahrekord University, during the 2007 growing season. The experiment was arranged with four plant densities (92 600, 104 200, 119 000 and 138 900 plants ha–1) and four nitrogen doses (200, 240, 280 and 320 kg ha–1) in a randomized complete block design with four replications. Increases of the plant density led to a significant N uptake in stalk, grain and whole plant. Also, N use efficiency, N uptake efficiency and N harvest index were significantly higher with increasing plant population. However, plant population showed no significant effect on leaf N uptake and N consumption efficiency. Increasing the N doses resulted in a significant higher leaf, stalk, grain and whole plant N uptake. However, N use, uptake and utilization efficiency were significantly decreased. The N harvest index showed different response to the N doses and there was no significant effect. It may be concluded that the optimum plant density for silage maize production can be beyond 138 900 plants ha–1 and utilization of a N dose higher than 240 kg ha–1 should be avoided, in order to minimize N losses.

Key words: Zea mays L., density, efficiency, fertilization, uptake.

 

RESUMEN

Un manejo eficiente de los fertilizantes nitrogenados es crítico para la producción económica del maíz (Zea mays L.) y la protección a largo plazo de la calidad ambiental. Para estudiar los efectos de las dosis de nitrógeno (N) y la densidad de población en la absorción y la eficiencia del N en el maíz forrajero cv. SC 704, se realizó un experimento en la región Shahrekord (32° 21' N, 50° 49' E; altitud de 1050 m), Irán, en la Estación de Investigación Agrícola de la Universidad de Shahrekord, durante la temporada de cultivo de 2007. El experimento tuvo cuatro densidades de población (92 600, 104 200, 119 000 y 138 900 plantas ha–1) y cuatro dosis de nitrógeno (200, 240, 280 y 320 kg ha–1), en un diseño en bloques completos al azar con cuatro réplicas. Un aumento de la densidad de población produjo un aumento significativo en la absorción de N medida en el tallo, el grano y la planta completa. Además, la eficiencia de uso del N, la eficiencia de absorción y el índice de captación de N también aumentaron significativamente con mayores densidades. Sin embargo, las hojas no mostraron un efecto significativo en la absorción de N ni eficiencia de consumo del N. Un aumento de la dosis de N causó un aumento significativo en absorción de N en tallo, grano y planta completa. Sin embargo, la eficiencia de uso, absorción y utilización de N disminuyó significativamente. El índice de captación de N mostró una respuesta distinta a la dosis de N y sin un efecto significativo. Se puede concluir que la densidad de población óptima para la producción de maíz para ensilaje puede ser más de 138 900 plantas ha–1 y que el uso de una dosis de N mayor a 240 kg ha–1 debe evitarse, para minimizar su pérdida.

Palabras clave: Zea mays L., densidad, eficiencia, fertilización, absorción.

 

INTRODUCTION

Enhancement of uptake and utilization efficiency of nutrients in forage maize production are required to achieve activity and uniformity in the process of absorption, translocation, assimilation and distribution of nutrients in plants (Zebarth and Sheard, 1992). In high plant density, ear and grain sterility increases due to intraspesific competition for assimilates starting with the flowering stage (Tollenaar, 1977). Andrade et al. (1999) point out that plant density has a significant effect on dry matter distribution within vegetative and reproductive plant sink and on seed formation response to available source of a single plant. Therefore, a greater than optimum plant density reduces seed number per ear, mean seed weight and ear lengths (Bavec and Bavec, 2002).

Cox and Cherney (2002) show that the density of 116 000 compared to 80 000 maize plants ha–1resulted in 3 g kg–1 lower crude protein, whereas Al–Kaisi and Yin (2003) point out that N uptake was more effective for dry weight than nitrogen concentration per plant. In that experiment, nitrogen uptakes under two low plant densities were significantly lower than in two higher plant densities. Nitrogen fertilizer is a favorite means for enhancement of maize yield (Wienhold et al., 1995; Gehl et al., 2005), but inappropriate management causes environmental pollution (Al–Kaisi et al., 1999). When the nitrogen in soil is low (100 kg N ha–1), utilization of fertilizer increases plant yield (Wienhold et al., 1995). The application of nitrogen above the appropriate levels may cause nitrate accumulation in lower parts of the root expansion and consequently there is a risk for soil nitrogen leaching (Ferguson et al., 1991; Schepers et al., 1991; Sogbedji et al., 2000).

Subedi et al. (2006) point out that forage maize increase exponentially with increasing nitrogen application rate and maximum yield (10.3 Mg ha–1) is obtained at a rate of 225 kg N ha–1. Besides, maize silage yield increase from 150 to 225 kg N ha–1linearly and after that there is a quadratic–plateau response. A study carried out by Ulger et al. (1997) for two years show that with increasing levels of nitrogen fertilizer (from 200 to 250, 300 and 350 kg N ha–1), crude protein content (10.4 %) increased in the first year; however, no effect was obtained in the second year. Ma et al. (1999) point out that with increased N application (from 100 to 200 kg N ha–1) the forage maize (9.01 Mg ha–1) increased linearly.

In normal conditions there is a direct relation between nitrogen application and grain yield (Al–Kaisi and Yin, 2003). Cox and Cherney (2001) point out that the interaction of row space × plant density × dry matter and quality of forage maize is not significant. However, they recommend a N dose of 140 to 250 kg ha–1 for an appropriate yield production (11.44 Mg ha–1) (Al–Kaisi and Yin, 2003).

The increase of dry matter production, optimal use of nitrogen fertilizer and prohibition of nitrogen leaching in environment are very important in forage production systems. So, in this research nitrogen uptake and efficiency under different levels of N and maize density were investigated.

 

MATERIALS Y METODS

A field experiment was conducted in the research field station of Shahrekord University Agricultural College, Iran, located at 32° and 21' N and 50° and 49' E and an altitude of 2050 m. The soil is calcareous developed in limestone with a clay loam texture and low in organic C content and total N content. A representative soil sample was air dried, and sifted through a 2–mm sieve for laboratory analysis. The soil EC–value was measured using saturated paste methods (Janzen, 1993). Soil pH was determined in a 1:2 (w/v) soil/ water suspension using a pH meter with a glass electrode. The total N concentration was measured by the Kjeldahl method (Bremner, 1996), and available P and K were extracted by the Mehlich–1 method; P was determined colorimetrically and K by atomic absorption spectroscopy (Donohue et al., 1983). Total organic carbon in soil was determined only in samples from the 0 to 30 cm depth by wet digestion method (Snyder and Trofymow, 1984). All concentrations were expressed on an oven–dried weight basis. Main variables of the 0–30–cm surface layer are shown in Table 1.

The study consisted of four nitrogen doses (200, 240, 280 and 320 kg N ha–1) as urea and four plant densities (92 600, 104 200, 119 000 and 138 900 plants ha–1), as a factorial experiment in a randomized complete block design with four replications. Each plot consisted of six rows of maize spaced 0.60 m with 6 m length. Phosphorus and K fertilizers were applied preplanting according to recomendations derived from the soil test.

Under favorable climate condition, soil was plowed once by moldboard plow and twice by disk harrow in two vertical directions. Before utilization of disk harrow, the soil was sprayed with 2 L of EPTC herbicide for weed control. After complementary land preparation, one third of nitrogen fertilizer as urea, 100 kg ha–1 P as super–phosphate triple and 100 kg ha–1K as potassium solphate (based on soil test) were added to soil and thereafter furrows were prepared. Corn seeds were planted at 5 cm depth on top furrows in form of multi–seeds per plant point on May 1, 2007; average day and night temperature was 12 °C. In each point of planting 3 seeds were placed and, in order to gain appropriate densities in 3–4 leaf stage, extra seedlings were hand thinned while soil was in field capacity condition. The corn seeds for planting were cultivar of hybrid 704 single cross. Irrigation was applied 5 to 7 d per week based on environmental conditions and hand weeding was performed.

At the 50 % kernel milk–line stage, 20 plants from row 5 were harvested and dry weights of the forage biomass were measured after drying the samples at 80 °C until a constant weight. The nonfermented silage yield was expressed as DM silage yield (kg ha–1) based on the number of plants at harvest in each treatment.

After chopping (4–5 cm pieces) the leaves, stalk and ear of plants, sub samples were taken and oven–dried at 80 °C until a constant weight to determine the DM. To determine N content of leaf, stalk and grain at the 50 % kernel milk–line stage (whole plant moisture ≈65 %), 10 plants from row 3 were harvested and total N concentrations in the plant (whole plant, leaf, stalk and grain) samples were determined by Kjeldahl (Bremner and Breitenbeck, 1983) after drying the samples at 80 °C until a constant weight. Nitrogen uptake was calculated by multiplying dry weight by nitrogen concentration in leaves, stalk and grain.

The following variables were calculated for each treatment: N use efficiency (NUE; kg kg–1) as the ratio of dry matter to N supply, where N supply is the sum of soil at planting, mineralized N, and N fertilizer; N uptake efficiency (NUpE;kg kg ~ 1) as the ratio of total plant N uptake to N supply; N utilization efficiency (NUtE; kg kg–1) as the ratio of dry matter to total plant N uptake; and N harvest index (NHI; kg kg–1) as the ratio of N in grain to total plant N uptake. Nitrogen efficiency terminology follows Pederson et al. (2002).

Analysis of variance were performed using SAS (SAS Institute Inc., 2001). Plant density and nitrogen doses means were analyzed using F–protected Least Significant Difference (LSD; p<0.05).

 

RESULTS Y DISCUSSION

The plant density by nitrogen rate interaction was not significantly different for all variables except stalk nitrogen uptake.

Forage biomass

Total DM was significantly increased (p<0.01) with increasing plant density. Maximum total DM was obtained at the 138 900 plants ha–1 (Table 2), whereas Andrade et al. (2002) report a higher plant density and Al–Kaisi and Yin (2003) point out a lower value.

Total biomass was significantly increased as the N dose increased up to 320 kg ha–1. However, there were no significant differences in forage dry matter between 240, 280 and 320 kg N ha–1 (Table 2). Results reported by Al–Kaisi and Yin (2003) and Subedi et al. (2006) support our findings for reducing N fertilizer doses.

Leaf nitrogen uptake

There was no significant difference in leaf N uptake among plant density treatments, but there was a significant response to nitrogen fertilizer (Table 2). The nitrogen uptake by leaves increased as nitrogen consumption increased, but no significant differences were observed between 280 and 320 kg N ha–1.

As approximately 50 % of leaf nitrogen affect photosynthetic plant system, the development of a photosynthetic system parallel to leaf expansion by promotion of N consumption may increase proportion of N uptake by leaves. Ulger et al. (1997) report that the highest N content of leaves was 3.46 % with a 350 kg ha–1 N fertilizer supply, whereas the lowest content, 2.58 %, was recorded with the 200 kg N ha–1 dose.

Stalk nitrogen uptake

Plant density and nitrogen dose affected stalk N uptake, and a plant density × N dose interaction was found. Thus, for 104 200 plants ha–1, the responses of stalk N uptake to different doses of N varied with other plant densities (Figure 1).

It looks like that for low density (104 200 plants ha–1) it is not likely to be found an effect of low com1petition similar to a density of 92 600 plants ha–1, and stalk biomass would be similar for densities of 119 000 and 138 900 plants ha–1. Thus, there are significant interaction effects between plant density and nitrogen uptake.

Grain nitrogen uptake

The effect of plant density and N dose on grain nitrogen uptake was significantly different (p<0.01) (Table 2). The increase of grain N accumulation due to an increase in plant density was gradual, where density of 119 000 plants ha–1 between continuous doses of nitrogen consumption was significantly different. But no significant differences were observed densities between 138 900 and 119 000 plants ha–1. Maximum production of biomass was found only for greater density and no differences were observed for grain N concentration, except for the lowest density. The increase in nitrogen uptake by grain was for a density of 138 900 plants ha–1. Our results confirm a similar finding reported by Al–Kaisi and Yin (2003).

As nitrogen dose increased from 200 to 320 kg ha–1 there was an increment in grain N uptake (Table 2), which shows that the highest N rate promoted a larger whole plant nitrogen uptake and, consequently, a better vegetative growth. Subedi et al. (2006) indicate that grain yield was exponentially increased with N rate and maximum yield is obtained with 225 kg N ha–1. And with increased nitrogen consumption, the response of grain nitrogen is due to yield promotion (Ulger et al., 1997).

Whole plant nitrogen uptake

Nitrogen uptake per hectare was affected by plant density and nitrogen dose (Table 2). Minimum rate of plant nitrogen uptake was observed for 104 200 plants ha–1 and 92 600 plants ha–1, whereas with a density of 139 800 plants ha–1 more biomass was produced due to the availability of nitrogen (Uhart and Andrade, 1995). As a result, a maximum rate of nitrogen was absorbed (Al–Kaisi and Yin, 2003). It seems that lower competition for a density of 92 600 plants ha–1 causes an increase in N concentration in plant organs, especially in stalk, and in spite of receiving favorable light condition and probably high assimilation speed, stalk nitrogen was not reduced. However, at the density of 104 200 plants ha–1 there was apparently more competition and an increase in remobilization of stalk nitrogen reduced nitrogen uptake per hectare (Anderson et al., 1984).

Uptake of nitrogen was directly related to N dose (kg ha–1), but no significant difference was observed between 280 and 320 kg ha–1 (Table 2). Usually, nitrogen availability increases nitrogen uptake, followed by an increase in nitrogen concentration in plant organs (Ma et al., 1999). Regarding the role of N in photosynthetic system, the rate of dry matter production also increased. In our experiment, nitrogen concentration and dry matter production in the 320 kg N ha–1 treatment was higher, which may have promoted nitrogen uptake per hectare. Halvorson et al. (2002) point out that 1production of forage maize removed 213 kg N ha–1 from soil, which was approximately 35 % more than grain N uptake, whereas Torbert et al. (2001) reported that approximately 144 kg N ha–1 were absorbed for a production of 14 355 kg dry matter ha–1.

Nitrogen use efficiency (NUE)

Nitrogen use efficiency was significantly affected by plant density and by N dose (Table 3). Maximum and minimum N use efficiency was due to densities of 138 900 and 92 600 plants ha–1 (Figure 2). At a density of 138 900 plants ha–1, a larger number of plants per unit area showed an increased nitrogen consumption and then increased biomass production. Thus, at this density, probably the nitrogen losses were minimum and we can conclude indirectly that a side effect of environment is minimum in such density. This finding agrees with the conclusion of Barbieri et al. (2008), who report that a high plant density increased NUE by 12 and 15 % expressed as DM or grain yield per unit of available nitrogen.

 

Nitrogen use efficiency showed a reciprocal relation with nitrogen utilization. The consumption of 200 kg N ha–1 was most efficient but an increased consumption reduced this efficiency (Figure 3). It seems that a fraction of relatively high doses of nitrogen was not absorbed by the plants and probably were lost by plants as leaching. Therefore, efficiency rate of 240, 280 and 320 kg N ha–1 treatments were reduced. Regarding maize N use efficiency, Guillard et al. (1995) and Halvorson et al. (2005) report similar results.

Nitrogen utilization efficiency (NUtE)

There was no significant effect of plant density on N utilization efficiency, but the effect of nitrogen consumption was significantly different for this variable (Table 3). An increase of nitrogen consumption caused significant a reduction in NUtE (Figure 3). This suggests that the treatment of 200 kg N ha–1 caused the least nitrogen leaching in the environment and probably part of the nitrogen may be removed by leaching before absorption by roots, since the plants were not able to use extra elements through the root zone. Therefore, an excess of nitrogen may be followed by water pollution (Al–Kaisi et al, 1999).

Nitrogen uptake efficiency (NUpE)

The NUpE was significantly affected by plant density and by N dose (Table 3). High plant density (138 900 plants ha–1), caused maximum NUpE (Figure 2). High density show a maximum leaf area which leads to maximum use of environmental resources, increasing absorption, translocation, assimilation and distribution of elements in the plant (Zebarth and Sheard, 1992). Thus, there is a balanced distribution of dry matter between vegetative and reproductive plant components (Andrade et al., 1999) and nitrogen consumption is efficient with no loss. However, at low plant density a portion of nitrogen is not absorbed by the roots, there is a delay in canopy formation and finally yield is reduced (Andrade et al., 1999), causing reduction of nitrogen uptake efficiency.

Maximum nitrogen uptake efficiency corresponded to the treatment of 240 hg N ha–1 (Table 3) and it was significantly different with 320 kg N ha–1 (Figure 3). This result suggests that plants used effectively 240 kg N ha–1 and that there was a balance between plant requirement and soil nitrogen availability; a higher N dose will increases its discharge. Doses of 225 kg N ha–1 (Subedi et al., 2006) and 140 to 250 kg N ha–1 (Al–Kaisi and Yin, 2003) support the statement that an optimum uptake efficiency is achieved with 240 kg N ha–1.

Nitrogen harvest index (NHI)

There was a direct relationship between NHI and plant density (Figure 2), and maximum NHI corresponded to a density of 138 900 plants ha–1 (Table 3). Upper leaf area index observed in a density of 138 900 plants ha–1 prepared a desirable photosynthetic potential and apparently more assimilates were rapidly allocated toward grain (Zebarth and Sheard, 1992). Therefore, the production of more grain in this treatment caused an increase of nitrogen harvest index.

 

CONCLUSIONS

The density of 138 900 plants ha–1 elicited a significant absorption of nitrogen fertilizer. As a result, nitrogen was used in photosynthesis, the biomass production showed maximum consumption and uptake efficiency and, finally, the nitrogen harvest index was the highest.

An increase in nitrogen consumption caused an increase in nitrogen uptake by different plant organs. However, nitrogen efficiency for use, utilization and uptake was significantly reduced. Therefore, in order to prevent nitrogen loss to environment whithout reducing biomass production, in forage maize it is better to use 138 900 plants ha–1 or higher, and avoid doses above 240 kg N ha–1.

 

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from Shahrekord University.

 

LITERATURE CITED

Al–Kaisi, M. M., and X. Yin. 2003. Effects of N rate, irrigation rate, and plant population on corn yield and water use efficiency. Agron. J. 95: 1475–1482.         [ Links ]

Al–Kaisi, M. M., A. F. Berrada, and M. W. Stack. 1999. Dry bean yield response to different irrigation rates in southwestern Colorado. J. Prod. Agric. 12: 422–427.         [ Links ]

Anderson, E. L., E. J. Kamprath, and R. H. Moll. 1984. Nitrogen fertility effects on accumulation, remobilization, and partitioning of N and dry matter in corn genotypes differing in prolificacy. Agron. J. 76: 397–404.         [ Links ]

Andrade, F. H., P. Calvino, A. Cirilo, and P. Barbieri. 2002. Yield response to narrow rows depends on increased radiation interception. Agron. J. 94: 975–980.         [ Links ]

Andrade, F. H., C. Vega, S. Uhart, A. Cirilo, M. Cantarero, and O. Valentinuz. 1999. Kernel number determination in maize. Crop Sci. 39: 453–459.         [ Links ]

Barbieri, P. A., H. E. Echeverría, H. R. Saínz Rozas, and F. H. Andrade. 2008. Nitrogen use efficiency in maize as affected by nitrogen availability and row spacing. Agron. J. 100:1094–1100.         [ Links ]

Bavec, F., and M. Bavec. 2002. Effect of plant population on leaf area index, cob characteristics and grain yield of early maturing maize cultivar (FAO–100–400). Eur. J. Agron. 16: 151–159.         [ Links ]

Bremner, J. M., and G. A. Breitenbeck. 1983. A simple method for determination of ammonium in semimicro–Kjeldahl analysis of soils and plant materials using a block digester. Commun. Soil Sci. Plan. Anal. 14: 905–913.         [ Links ]

Bremner, J. M. 1996. Total nitrogen. In: Sparks, D. L., A. L. Page, P. A. Helmke, and R. H. Loeppert (eds). Methods of Soil Soil Analysis. Part 3. SSSA Book Ser. 5. SSSA and ASA, Madison, WI. pp: 1085–1122.         [ Links ]

Cox, W. J., and D. J. R. Cherney. 2001. Row spacing, plant density, and nitrogen effects on corn silage. Agron. J. 93: 597–602.         [ Links ]

Cox, W. J., and D. J. R. Cherney. 2002. Evaluation of narrow–row corn forage in field–scale studies. Agron. J. 94: 321–325.         [ Links ]

Donohue, S. J., R. H. Brupbacher, R. A. Isaac, J. D. Landcaster, A. Mehlich, and D. D. Scott. 1983. Reference soil test methods for the southern region of the United States. Southern Coop. Ser. Bull. 289. Univ. of Georgia, Athens.         [ Links ]

Ferguson, R. B., C. A. Shapiro, G. W. Hergert, W. L. Kranz, N. L. Klocke, and D. H. Krull. 1991. Nitrogen and irrigation management practices to minimize nitrate leaching from irrigated corn. J. Prod. Agric. 4: 186–192.         [ Links ]

Gehl, R. J., J. P. Schmidt, L. D. Maddux, and W. B. Gordon. 2005. Corn yield response to nitrogen rate and timing in sandy irrigated soils. Agron. J. 97: 1230–1238.         [ Links ]

Guillard, K., G. F. Griffin, D. W. Allinson, M. M. Rafey, W. R. Yamartino, and S. W. Pietrzyk. 1995. Nitrogen utilization of selected cropping systems in the U. S. Northeast: I. Dry matter yield, N uptake, apparent N recover, and N use efficiency. Agron. J. 87: 193–199.         [ Links ]

Halvorson, A. D., F. C. Schweissing, M. E. Bartolo, and C. A. Reule. 2005. Corn response to nitrogen fertilization in a soil with high residual nitrogen. Agron. J. 97: 1222–1229.         [ Links ]

Halvorson, A. D., R. F. Follett, M. E. Bartolo, and F. C. Schweissing. 2002. Nitrogen fertilizer use efficiency of furrow–irrigated onion and corn. Agron. J. 94: 442–449.         [ Links ]

Janzen, H. H. 1993. Soluble salts. In: Carter, M. R. (ed). Soil Sampling and Methods of Analysis. Lewis, Boca Raton, FL. pp: 161–166.         [ Links ]

Ma, B. L., L. M. Dwyer, and E. G. Gregorich. 1999. Soil nitrogen amendment effects on seasonal nitrogen mineralization and nitrogen cycling in maize production. Agron. J. 91: 1003–1009.         [ Links ]

Pederson, G. A., G. E. Brink, and T. E. Fairbrother. 2002. Nutrient uptake in plant parts of sixteen forages fertilized with poultry litter: Nitrogen, phosphorus, potassium, copper, and zinc. Agron. J. 94: 895–904.         [ Links ]

SAS Institute Inc. 2001. SAS User's Guide: Statistics. Version 8.2. SAS Institute Inc., Cary, NC.         [ Links ]

Schepers, J. S., M. G. Moravek, E. E. Alberts, and K. D. Frank. 1991. Maize production impacts on groundwater quality. J. Environ. Qual. 20: 12–16.         [ Links ]

Sogbedji, J. M., H. M. Van Es, C. L. Yang, L. D. Geohring, and F. R. Magdoff. 2000. Nitrate leaching and nitrogen budget as affected by maize nitrogen rate and soil type. J. Environ. Qual. 29: 1813–1820.         [ Links ]

Subedi, K. D., B. L. Ma, and D. L. Smith. 2006. Response of a leafy and non–leafy maize hybrid to population densities and fertilizer nitrogen levels. Crop Sci. 46: 1860–1869.         [ Links ]

Snyder, J. D., and J. A. Trofymow. 1984. Rapid accurate wet oxidation division procedure for determining organic and inorganic carbon in plant and soil samples. Commun. Soil Sci. Plant Anal. 15: 1587–1597.         [ Links ]

Tollenaar, M. 1977. Sink source relationships during reproductive development in maize: A review. Maydica, Bergamo 22: 49–75.         [ Links ]

Torbert, H. A., K. N. Potter, and J. E. Morrison. 2001. Tillage system, fertilizer nitrogen rate, and timing effect on corn yields in the Texas Blackland Prairie. Agron. J. 93: 1119–1124.         [ Links ]

Uhart, S. A., and F. H. Andrade. 1995. Nitrogen deficiency in maize. I. Effects on corn growth, development to dry matter–partitioning, and kernel set. Crop Sci. 35: 1375–1383.         [ Links ]

Ulger, A. C., H. Ibrikci, B. Cakir, and N. Guzel. 1997. Influence of nitrogen rates and row spacing on corn yield, protein content, and other plant parameters. J. Plant Nutr. 20: 1697–1709.         [ Links ]

Wienhold, B. J., T. P. Trooien, and G. A. Reichman. 1995. Yield and nitrogen use efficiency of irrigated corn in the northern Great Plains. Agron. J. 87: 842–846.         [ Links ]

Zebarth, B. J., and R. W. Sheard. 1992. Influence of rate and timing of nitrogen fertilization application on yield and quality of hard red winter wheat. Can. J. Plant Sci. 72: 13–19.         [ Links ]

Creative Commons License Todo o conteúdo deste periódico, exceto onde está identificado, está licenciado sob uma Licença Creative Commons