Corn Hybrids Response to Nitrogen Rates at Multiple Locations in Brazilian Amazon

Among the nutritional requirements of corn crop, nitrogen (N) is the element required in greater quantity and, directly responsible for increase or decrease in grain production. The aims of study were to evaluate the effects of applied N rates in topdressing in development and production of corn in Brazilian Amazon. The experiments were installed on 20 January 2014 (Capitão Poço city) and 24 January 2014 (Paragominas city). The experimental design was a randomized block design in a 5 × 2 factorial scheme, consisting of five N rates of topdressing applications (0; 40; 80; 120; and 160 kg ha of N) and two corn double hybrids (AL-Avaré; and AL-Bandeirante), with four replicates. The evaluations of vegetative development components (plant height, height of ear insertion, and leaf area index) were carried out at time of male flowering stage, while evaluations of grain yield components (number of grains per row, grain yield, and harvest index) carried out during the harvest period. The corn hybrids, AL-Avaré and AL-Bandeirante, independent of experimental site, showed the highest technical efficiency between the rates of 80 to 120 kg ha of N. Based on the information obtained in regression analysis verified that Paragominas experiment showed greater vegetative development (plant height, LAI, ear height, number of grains per row, ear length) and higher grain yield compared to corn developed in Capitão Poço experiment.


Introduction
Corn is of great importance for world agribusiness because it is considered the main constituent in production of animal feed.In addition, currently, its production is increasing, for use in ethanol manufacture (Hage, 2013).Although versatile, corn production has basically followed the growth of pigs and chickens productions, both in Brazil and in the World (Belusso & Hespanhol, 2010).
Brazil is the 3 rd largest producer of corn, with an approximate production of 85.5 million tons of grains, in 15.8 million hectares of planted area and grain yield of 5.4 t ha -1 , in which Pará state is the main producer in Brazilian Amazon (Conab, 2015).
Among the nutritional requirements of corn crop, nitrogen (N) is the element required in greater quantity and, directly responsible for increase or decrease in grain production.According to Santos, Silva, Carvalho, and Caione (2010), most of Brazilian Amazon soils have insufficient nitrogen contents, requiring fertilizer supply.
The adequate nutritional management for corn crop, although it has been well studied, mainly, nitrogen application (Farinelli & Lemos, 2012;Souza, Buzetti, Tarsitano, & Valderrama, 2012), still needs more studies, since it is one of factors that most affect the corn yield (Melo, Corá, & Cardoso, 2011).According to Goes,

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Experi
The experi S and 47º2 Amazon, S The clima Finlayson, in Figure 2 The exper that Capit Paragomin Sowing fertilization was based on fertilization and liming recommendations for Pará state (Cravo, Viegas, & Brasil, 2010).The mineral fertilizers used for sowing fertilization was NPK (04-20-20), at rate of 333 kg ha -1 , applying the fertilizers in planting furrow, below and beside the seeds.
Each experimental plot consisted of five rows of plants, 6.0 m in length, spaced by 0.9 m.The numbers of plants used were 5 plants m -1 to obtain a planting density of approximately 55,000 plants ha -1 .In all of the experiments, agricultural treatment procedures and plant protection products used were those recommended for corn crops.

Experimental Design
The experimental design was a randomized block design in a 5 × 2 factorial scheme, consisting of five N rates of topdressing applications (0; 40; 80; 120; and 160 kg ha -1 of N) and two corn double hybrids (AL-Avaré; and AL-Bandeirante), with four replicates.
Nitrogen fertilization was carried out manually, applied in topdressing at a distance of approximately 0.08 m from the rows of plants, at V 4 phenological stage (Ritchie, Hanway, & Benson, 1993), using N-urea at rates of 0, 40, 80, 120 and 160 kg ha -1 of N.

Data Collection
The evaluations were performed in components of vegetative development during the period that corn plant was in male flowering stage (V T phenological stage) (Ritchie et al., 1993).The plant heights (PH) was evaluated by measuring stem length (from the soil surface to base of male inflorescence), while the height of ear insertion (measured from the soil level to first ear) was performed manually, using a tape measure.
In determination of leaf area (LA), five plants of each experimental plot were evaluated, measuring the length (L) and width (W) in cm, to obtain the initial leaf area (LA) (Sangoi, Schmit, & Zanin, 2007).Posteriorly, calculated the leaf area using the following equation: The leaf area index was calculated from leaf area measurements using the following equation: In which, e 1 and e 2 refer to plant spacing in the planting line (m) and between the planting lines (m), respectively.
For the evaluations of grain yield components analyzed the variable number of grains per row, in which ten ears were used per plot, by simple counting of grains present in a row of each ear.After the threshing process, grain yield was obtained by grain mass, determining the water content, corrected to 13% moisture.The harvest index (HI) was calculated as ratio of corn grain dry weight to whole plant dry weight.

Statistical Analysis
The experimental data for each of the two experimental site were assessed using the Shapiro-Wilks (p > 0.01) and Levene (p > 0.01) tests to verify the normality and homoscedasticity waste, analysis was performed using SAS statistical software (Sas, 2008).The data were subjected to an individual analysis of variance using the SISVAR statistical software, and relationship between the residual mean square 7:1 was verified.Finally, data were subjected to an analysis of joint variance, in which determined the effects of N rates in topdressing applications were studied by polynomial regression analysis (p < 0.05) using the SISVAR statistical software (Ferreira, 2011).

Growth and Plant Development Characteristics
The leaf area index of AL-Avaré and AL-Bandeirante hybrids showed statistical differences only in experiment conducted in Capitão Poço.Regarding Paragominas experiment, the average leaf area index was 2.53 and 2.47 for AL-Avaré and AL-Bandeirante, respectively (Figure 3).