Sources and dosages of Nitrogen applied with urea coated with polymers in Marandu Palisade Grass

Nitrogen is one of the most important nutrients for the production of forage grasses, and the most expensive one. The scope of this research was to evaluate the application of four sources of urea (conventional and coated with polymers) under different dosages, in mass production and nitrogen content and absorption from two cuts of Brachiaria brizantha cv. Marandu. The experiment was conducted at a greenhouse in a randomized block design. The fertilizers used were conventional urea, urea covered with a polymer layer, urea covered with sulfur and urea covered with a boron and copper compound. The dosages used were 30, 60, 90 and 120 kg ha-1 of nitrogen. Fresh and dry mass, root weight and nitrogen content from first and second harvest were evaluated. On the first harvest, there was no significant difference between the treatments for the variables analyzed. On the other hand, second harvest presented difference in fresh and dry mass and nitrogen content. Regression analyses showed a linear increase with all fertilizers used, but this growth was more accentuated for the polymerized fonts. All polymerized ureas allowed higher mass production for Marandu palisadegrass, at tillering and second cut. Foliar nitrogen availability of polymerized ureas did not differ from conventional urea, being recommended to reach pastures with higher quality and productions and to decrease nitrogen losses on the system.


Introduction
Forage grasses represent plants of economic interest.Species from the gender Brachiaria are used in tropical countries as an option to pastures formation, because of the adaptability to different environmental conditions and facility in pasture's management (EUCLIDES et al., 2014).Marandu palisadegrass (Brachiaria brizantha cv.Marandu) is a perennial forage grass with cespitose growth habit, forming clumps up to one meter in diameter and tillers with 1.5 meter of height.It has horizontal, short, tough and curved rhizomes.It has deep roots, which help them to survive along dry periods (FONTES et al., 2014).
Nitrogen is one of the most important nutrients for the production of forage grasses, and the most expensive one.With the correct application, it can increase the amount of protein and the quality of the graze (PINHEIRO et al., 2014).The lack of nitrogen in plant provokes the yellowing of older leaves, decreasing photosynthesis's rate.Therefore, low productivity rates can be the result of an inappropriate nitrogen fertilization (GÄRDENÄSA et al., 2011).
Methodologies optimizing the application and efficiency of nitrogenous fertilizers have increased in the last years.These techniques can increase efficiency according to two groups: slow release fertilizers (covered, encapsulated, insoluble or slowly soluble in water) and stabilized fertilizers (nitrification inhibitor or urease inhibitors).Slow release fertilizes can also be divided in two more groups: condensation compounds of urea (low solubility and slow release) and encapsulated or coated products (controlled release) (CANTARELLA, 2007).
In general, the choice of a nitrogenous fertilizer is based on the cost of nitrogen by unit and by the availability and efficiency from the source (KERING et al., 2011).The most sold fertilizers in the market are urea, ammonium nitrate and ammonium sulphate.Urea [CO(NH₂)₂] is a solid granulated fertilizer with around 45% of nitrogen in the amide form.The advantages of urea are high concentration of nitrogen and low costs of fabrication, transportation, storage and application.However, it has disadvantages like higher hygroscopicity and volatility compared to other sources (HALVORSON; BARTOLO, 2014).
Technological efforts have been made to help the losses of ammonia from volatility, like coating urea with sulfur, new polymers and urease inhibitors (RUZEKA et al., 2014).Polymers are long chains of repeated structural units called monomers.Each polymer behaves differently to encapsulation, varying its release, which may be controlled by humidity or temperature (THAPA et al., 2015).The use of coated urea with polymers is a strategy that has been widely tested because encapsulated sources have a slow release compared to conventional soluble sources (FIGUEIREDO et al., 2012).
Considering the importance of nitrogen fertilization, as well as their topdressing for a quality pasture formation, this study had the aim of evaluate the effect from application of four sources of urea (conventional and coated with polymers) under different dosages, in mass production, root weight and nitrogen content at two cuts of B. brizantha cv.Marandu.

Material and methods
The study was conducted in a greenhouse of Universidade Federal de Uberlândia, at 18.885º S and 48.259º W. The experiment design was a randomized block design (RBD) in a 4 x 4 + 1 factorial (fertilizers x dosages) and three blocks.The fertilizers used were conventional urea, urea covered with a polymer layer, urea covered with sulfur, and urea covered with a boron and copper compound.All the fertilizers had the same amount of nitrogen (45% of N).The dosages used were 30; 60; 90 and 120 kg ha -1 of nitrogen at planting and coverage.An additional control without any application of nitrogen was added to the experiment.
The soil used was an Oxisol, medium texture, compound by 423 g kg -1 of gravel, 205 g kg -1 of fine sand, 36 g kg -1 of silt and 336 g kg -1 of clay.The pH in water was 5.8; content of available phosphorus of 1.7 mg dm -3 ; available potassium of 21 mg dm -3 ; sulfur of 9 mg dm -3 ; organic matter of 21.0 mg dm -3 and saturation of bases of 75%.It was performed a correction with single superphosphate in soil, equivalent to 120 kg ha -1 before sowing.
Sowing was conducted with 25 seeds of B. brizantha cv.Marandu per pot.Each pot contained 0.3 kg dm -3 of soil and was drilled in the bottom to remove water excess.The thinning was made twenty days after sowing, letting 4 plants per pot.A cut of 10 cm height was held in 30 days after sowing.Topdressing was superficial without incorporation, after the first cut.Irrigation was carried out in an interval of two days, throughout the experiment with the help of tensiometers.The second cut was held at 60 days after sowing.
After cut, the leaf blades and other components were weighed in a precision balance to calculate the fresh mass production.Then, they were dried in a kiln at 65 ºC and removed when it got constant mass.After drying, the samples were weighed for dry mass production calculation (SILVA, 2009).A leaf analysis was also conducted in order to determine the nitrogen content with two newly expanded leaves of the plant, according to Kuhinara, Maeda e Alvarez (2005) methodology.At the end of the experiment (second cut), the plants were removed, separating vegetative portion from the roots, and then they were weighted separately.The leaves and roots were also dried in a kiln at 65 ºC for dry mass calculation.
For normality of residuals and homogeneity of variances Shapiro Wilk and Levene's tests were performed, respectively (p-value=0.05).Analysis of variance (ANOVA) was performed with 0.01 of significance.In case of significant difference, Tukey's test was used to compare the means at 0.05 of significance.Dunnett's test was performed to compare treatments with the additional control.Regression analysis was performed for the dosages.R software was used to run the statistics.

Results and discussion
On first cut, there was no significant difference between the treatments for the variables analyzed (Table 1).The fonts of urea did not differ from the control either, just for nitrogen content.For initial development of B. brizantha, there was no interference from the polymers or the dosage of urea.However, the treatments differed from the control in nitrogen content, showing the importance of a correct adubation for the plants.Alexandrino et al. (2004) demonstrated how important nitrogen is to leaf appearance and elongation rate in B. brizantha.On the other hand, second cut presented difference in fresh and dry mass.Dunnett's test showed difference between all treatments compared to control, which was expected (Table 2).Nitrogen fertilization is a strategy that allows increase of volume density of forage, and especially leaves production, due more leaf emergence and elongation, elevating dry mass production (FRAME et al., 2013).Despite increase of mass with the treatments, no difference was detected in root weight.Radicular system of Brachiaria is more affected by potassium and phosphorus than nitrogen (COSTA et al., 2009).Independently of the type of polymerized urea, Tukey's test revealed increase on plant mass when compared to conventional urea, showing that when urea is revested, the chances of losses for ambience are decreased, confirming the remarks of Ruzeka et al. (2014).Regression analyses (Figure 1) adjusted a linear increase for all fertilizers used; however, this rise was more accentuated for the polymerized fonts.Other authors found the same effects with the rise of urea dosages in Marandu palisadegrass (PRIMAVESI et al., 2006;COSTA et al., 2009).The content of mass in coated ureas duplicated from the smallest to the highest dosage used, and the high coefficients of determination from the equations confirm how responsive Marandu palisadegrass was to the rise of nitrogen availability.Even if conventional urea had raised fresh mass content too, it was not observed difference for dry mass content.
For the first cut, the content of nitrogen in all treatments were statistically equal (17.15 g kg -1 ) differing only from the control (9.37 g kg -1 ).All coated ureas had the same behavior than conventional urea for nitrogen foliar assimilation.That information proved that these fonts are well absorbed by the plant, and their slowly liberation did not affect nitrogen availability for plant on current planting.The most worrying scenario for coated ureas is when their slow liberation do not provide the correct nitrogen quantity (SILVA et al., 2012).Besides that, all coated ureas provided more mass production, essential for a good pasture.Farruggia et al. (2014) revealed nitrogen content and mass production are the most important attributes for quality pasture.

Conclusion
All polymerized ureas allowed higher mass production for Marandu palisadegrass at tillering and second cut.Foliar nitrogen availability of polymerized ureas did not differ from conventional urea, being recommended to reach pastures with higher quality and productions and to decrease nitrogen losses on the system.Future studies need to be performed to evaluate the effects on a Marandu palisadegrass pasture all year long.

Figure 1 .
Figure 1.Fresh and dry mass of B. brizantha cv.Marandu submitted to different rates four types of urea ( conventional urea;urea covered with a polymer layer; urea covered with sulfur; urea covered with a boron and copper compound).

Figure 2 .
Figure 2. Foliar nitrogen content from second cut of B. brizantha cv.Marandu submitted to dosages of urea fertilizers.

Table 1 .
Analyses of variance for fresh and dry mass, nitrogen content and root weight of two cuts of B. brizantha submitted to dosages of nitrogen fertilizers.

Table 2 .
Fresh and dry mass from second cut of B. brizantha cv.Marandu under application of different polymerized fertilizers.
Means, followed by the same small letters in columns do not differ significantly by Tukey's test (p-value ≤0.05).*Controldiffers from treatments by Dunnett's test (p-value ≤0.05).Source: Elaborated by the authors (2017).