LEAF DIAGNOSIS AND PRODUCTIVITY OF FORAGE SORGHUM FERTIGATED WITH NITROGEN DOSES IN TWO HARVESTS

The aim of the present study was to evaluate the degrees of macronutrients in the diagnostic leaf and the productivity of forage sorghum IPA 467, submitted to fertigation with nitrogen doses, during two harvests (summer and winter), both without regrowth. From January 20 to May 18 and from June 17 and October 14, 2016, two experiments were conducted in Canindé de São Francisco, State of Sergipe, in the Brazilian semi-arid. The experimental design was randomized complete blocks, with four replications. The treatments consisted of four doses of N (0; 80; 160 and 240 kg ha-1). Joint analysis of variance was carried out, considering each different harvest as a new factor. The characteristics evaluated were the content of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) in the leaves, height of the plant, diameter of the stem,green mass productivity, dry mass productivity and percentage of dry mass. The macronutrient content in the leaves was incremented with increasing doses of N, except Ca which presented no influence. The winter crop promoted the highest content of K, while the summer crop presented an elevated absorption of Mg. The varying dosage of N or the harvests season did not influence dry mass productivity (average of 17,386 ton/ha-1). The content of N (23.16-26.68 g kg-1), P (3.16-4.11 g kg-1), K (10.98-33.36 g kg-1), Ca (3.61 g kg-1) and Mg (4.78-9.64 g kg-1) can be considered enough for the full development of the plant.

Sorghum (Sorghum bicolor L.) stands out as a specie of great importance for animal nutrition in the semiarid regions of Brazil. As a plant with immense fodder performance, regrowth capacity and adaptability in dry climates, making sorghum capable of surviving and becoming productive under limited supply conditions (Elias et al., 2016). Because of these characteristics, it is used in numerous products, for grain silage, green cut, animal grazing, and grain, in animal feeds and for human consumption (Buso et al., 2011).
In the Brazilian semiarid, this species has been cultivated both in rainfed condition and under irrigation. Despite its higher tolerance for water deficits when compared to corn, sorghum, does present a negatively affected productivity whenever it is cultivated in semiarid conditions without irrigation due to erratic pluviometric fluctuations (Coelho et al., 2018). However, in irrigated areas the proper hydric regiment of the crop can be maintained throughout the cycle, enabling the use of fertilization techniques through the irrigation water (fertigation), which stands out as a way of applying fertilizers that mostly resembles the plants natural rate of water and nutrient absorption. Fertigated systems make for directly applying fertilizers in the densest root regions, allow for the fractioning of dosages and an increase in the efficiency of fertilization in order to increase production (Souza et al., 2020).
During its development, sorghum is a N demanding plant, this being a determining factor for the plants overall nutrition, its capacity of protein formation, assisting in photosynthesis, and augmenting its nutritional value and forage quality (Macedo et al., 2012). Due to this factor, N fertilization is an option for farmers that desire to increase its productivity. Nirmal et al. (2016) achieved linear increments of forage sorghum yields, up to doses of 310 kg ha -1 of N reaching productions of 60.6 thousand kg ha -1 .
Furthermore, nitrogen fertilization for the cultivation of sorghum is a tried practice that can not only increase the absorption of this N, but of other nutrients, like potassium (K), phosphorus (P), calcium (Ca) and magnesium (Mg) as well (Fonseca et al., 2008;Serrão et al., 2012).
Accordingly, to know the macronutrient contents in sorghum leaves is essential for the management of nutrients, its absorption efficiency and for providing productivity gains and cost saving (Santos et al., 2014).

Material and Methods
The experiment was conducted, at  (Sousa et al., 2010). The average meteorological data during the experimental period was measured from an automatic meteorological station installed six kilometers away from the experiment's area ( Figure 1).

The area's soil was classified as a Chromic
Luvisol, with undulated topography and clayey -granulometric values of 478.20 g kg -1 of sand, 98.00 g kg -1 of silt and 423.80 g kg -1 of clay, with its chemical characteristics (Silva, 2009) Table 1 -Chemical soil analysis of experimental areas of forage sorghum (depth of 0 to 20 cm), in summer and winter harvests.  and 179 mm in summer and winter harvests, respectively. The crop management adopted during the experiment followed those used as standard by local producers in the region.
The weed control was performed using the herbicide atrazina, applied in pre-emergence.
At 79 (summer) and 90 days after planting (winter), due to the booting of the grain, the second mature upper leaf was collected randomly in 10 plants of the usable area of each plot (Silva et al., 2008). Then, the dry mass of these leaves was measured after drying in a forced air circulation oven, with a set temperature of 65ºC, until it reached constant mass.
The leaves were ground in a Willey stainless steel cutting mill (Tecnal -TE-650), using a 2.0 mm mesh sieve. In order to determine the content of N (g kg -1 ), the methodology described by Tedesco et al. (1995), was used. Measuring 200 mg of vegetal sample, and then transferring it to the digester tube, where 1 mL of 30% hydrogen peroxide and 3 mL of sulfuric acid were added.
Subsequently, the mixture was introduced in a block digester at a temperature of 180 to 190 ºC for two hours, followed by another hour at 350 ºC.

Results and Discussion
The joint analysis of variance of the forage sorghum diagnostic leaf, macronutrient contents are presented in Table 2 Table   2).
The excess of N could caused a reduced absorption of N by the crop, due to a probable Leaf diagnosis and productivity of forage sorghum...

high volatilization of ammonia as a result
of the high dose of urea in a soil with a pH value higher than 6.5 (Table 1) The P already available in the soil was sufficient for agricultural cultivation ( Table   1), but the content of P in the diagnostic leaf increased when the N was applied ( Figure 3A).
This suggests that the absorption of P by sorghum plants until anthesis, accompanied the increase in induced growth due to a greater availability of N in soil (Serrão et al., 2012). It is important to emphasize that the P content in diagnostic leaf higher than 2 g kg -1 can be considered suitable for sorghum development (Cantarella et al., 1997), which was measured in all of the treatments.
For the content of K, the diagnostic leaves of the winter harvest presented a linear increase with increasing doses of N ( Figure 3B), reaching According to Table 3, it is gathered that for all of the doses of N evaluated, winter harvest provided the conditions for a higher absorption of K when compared to summer. This can be explained by the greater availability of the nutrient measured in the soil during the winter (Table 1), reducing the relation (Ca+Mg)/K and promoting vantages for the absorption of K in relation to Mg in the second harvest (Table 4). The sorghum harvested during summer, obtained a superior mean content of Mg in the diagnostic leaf with 9.64 g kg -1 , while in winter it was 4.78 g kg -1 (Table 4). The contents of Mg in both harvests can be considered high in relation to suitable content ranges proposed by Malavolta et al. (1989), Cantarella et al. (1997 and Martinez et al. (1999). These results may be due to low ratio of Ca/Mg (1.61 in summer and 2.27 in winter), favoring absorption of Mg The average content of Ca verified in the experiments was 3.61 g kg -1 (  1997;Martinez et al., 1999).
According to the joint analysis of the agronomic characteristics of forage sorghum (

Conclusions
The contents of macronutrients in diagnostic leaf of forage sorghum are increased with increasing doses of N by fertigation, except Ca which was not influenced.
The winter harvest presented a higher content of K in the diagnostic leaf of sorghum, while the summer harvest was more beneficial for Mg absorption.
The doses of N and the harvests do not influence the dry mass yield of sorghum IPA 467.

Acknowledgements
Special thanks are due to the Coordenação