Accumulation and partition of macronutrients in Pennisetum purpureum cv . Roxo managed under different growth ages and seasons Acúmulo e partição de macronutrientes em “ Pennisetum purpureum ” cv . Roxo manejado em diferentes idades e épocas de crescimento MARANHÃO

Universidade Federal do Ceará, Centro de Ciências Agrárias, Programa de Pós-Graduação em Zootecnia, Fortaleza, Ceará, Brasil. Universidade Federal do Ceará, Centro de Ciências Agrárias, Departamento de Zootecnia, Fortaleza, Ceará, Brasil. Universidade Federal do Ceará, Centro de Ciências Agrárias, Departamento de Ciências do Solo, Fortaleza, Ceará, Brasil. Embrapa Caprinos e Ovinos, Sobral, Ceará, Brasil. Universidade Federal do Ceará, Centro de Ciências Agrárias, Departamento de Zootecnia, Fortaleza, Ceará, Brasil. *Endereço para correspondência: theysonduarte@gmail.com


INTRODUCTION
An efficient management of fertilization requires knowledge of nutrient accumulation (ECHER et al., 2009).The knowledge of the nutritional requirements in each phase of the growth cycle would make it possible to improve the synchrony between availability and nutrient requirements of the plant, making it possible to increase the agronomic and economic efficiency of the system.The accumulation and exportation of nutrients by the plant, on the other hand, varies according to the age of the plant, plant's organ, species, cultivar, type of nutrient, management, and edaphoclimatic conditions in which the canopy develops (COSTA et al., 2010;SILVA et al., 2015).Diffused in tropical and subtropical regions, elephant grass (Pennisetum purpureum) has been distinguished by relevant and appreciable agronomic and bromatological characteristics for the production of ruminants.It is quite responsive to management; however, very demanding in soil fertility for maintenance of its productive potential, which may vary according to the edaphoclimatic conditions and cultivar adopted.Most studies on nutrient accumulation are carried out in intensive irrigation systems, making it difficult to extend the results to plants managed on dry lands.It is worth mentioning the scarcity of studies on the partition of nutrients in forage plants.Thus, the demand for research on nutrient accumulation in tropical regions is explicit, considering the variation of rainfall, in particular regarding the requirements of macronutrients by elephant grass cv.Roxo.It is also worth noting that studies of this nature make it possible to generate predictive models of macronutrient accumulation by elephant grass under dry conditions.Thus, the objective was to evaluate the macronutrient accumulations in the leaf, stem, and shoot of elephant grass cv.Roxo managed at different ages and seasons of growth under waterless conditions.

MATERIAL AND METHODS
The experiment was carried out at the Núcleo de Ensino e Estudos em Forragicultura of the Departamento de Zootecnia of the Centro de Ciências Agrárias of the Universidade Federal do Ceará-NEEF/DZ/CCA/UFC, located at latitude 03° 44' 32' S' and longitude 38° 34' 40''W in Fortaleza-CE, with a rainy tropical Aw' climate classification according to Köppen (1936).The study was carried out from May to November of 2012, in an area of elephant grass (Pennisetum purpureum) cv.Roxo established five years prior to the study.The soil of the area is classified as yellow Argisol, with a sandy texture.Three successive cycles of growth in distinct seasons (rainy, transition, and dry) were evaluated according to rainfall during the experimental timeframe.There was accumulated rainfall of 373.30, 17.30, and 9.60 mm in the rainy, transition, and dry seasons, respectively.While the potential evapotranspiration of the crop presented values of 39.43, 46.11, and 52.58 mm day -1 for rainy, transition, and dry season, respectively.The climatological variables were obtained at the Meteorological Station of the Universidade Federal do Ceará, Campus do Pici (Figure 1).The accumulation of macronutrients was quantified at seven growth ages (9, 18, 27,36, 45, 54, and 63 days after cutting close to the soil) during three successive growing cycles.The design was completely randomized in a split-plot time arrangement, where the ages were the plots and the seasons (rainy, transition, and dry) the subplots, with three replications.The area of each experimental unit was 3.0 x 3.5m.Prior to the experimental period, two standardization cuts were performed (January and April of 2012) aiming to homogenize the canopy.Due to the standardization cuts, phosphate fertilization was applied in January based on the soil analysis at the time, which justifies the high phosphorous contents at the later soil analysis, making cover phosphate fertilization not necessary.

Figure 1. Climatic data of the experimental period
Before the beginning of the experiment, soil chemical analysis of the experimental area of the layer of 0 to 20 cm was carried out, presenting the following results: pH in water 6.0; 20.89 g kg -1 of O.M.; 397 and 3.91 mg dm -3 of P and K, respectively; and 2.8, 1.5, 0.15, and 0.04 cmolc dm -3 of Ca ++ , Mg ++ , Al 3+ , and Na + , respectively.Maintenance fertilization was carried out with doses equivalent to 200 kg ha -1 of K 2 O (potassium chloride), 600 kg ha - 1 year -1 of nitrogen (urea), and 50 kg ha - 1 of FTE BR-12.The recommendations of potassium and FTE BR-12 were based on the Fertility Commission of the State of Minas Gerais (CFSEMG, 1999).Fertilization was fractionated into two doses, the first one (100% of FTE BR-12, 50% of nitrogen, and 50% of potassium) was administered at day 18 and the second one was administered at day 27.It should be noted that the fertilization was carried out exclusively during the rainy season, due to the high temporal variation of rainfall characteristic of the region, so that it was decided to administer the nutrients at the time of highest rainfall probability.At the end of each growing season, a soil chemical analysis (layer 0-20 cm) was performed to characterize soil fertility (Table 1).At each growing age in the three cultivation periods, a sample of total forage biomass (TFB) was collected, using a frame of known area (1.0 x 1.0 m).TFB was fractionated into green leaf blade biomass, green stem biomass, and dead forage biomass (data not shown).
All the fractions of the plant were predried in a forced ventilation oven at 65 °C until reaching constant weight.The predried biomass of the plant fractions were summed and used to estimate the TFB productivity of elephant grass cv.Roxo managed under cutting at ground level.The pre-dried green leaf blade and stem biomasses were ground and submitted to nitroperchloric digestion to determine the nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) contents according the methodology described by Silva (2009).The accumulated amount of each nutrient in leaves and stems of elephant grass cv.Roxo was determined by multiplication of the estimated biomass of each fraction (kg ha -1 ) versus the content of the respective nutrients.The accumulation of nutrients in the shoot was obtained by the sum of the accumulated amounts in the leaf and the stem, in each growth age.Data were submitted to analysis of variance, mean comparison test, and regression analysis.The interaction age x season was deployed when significant (p<0.05) by F test for nutrient accumulation in leaf, stem, and shoot fractions.Only the main effect of the seasons was presented for nutrient content.The seasons (rainy, transition, and dry) were compared by the Scott Knott test (p<0.05).The ages were evaluated by regression analysis (T test, p<0.05).The models were selected from the significance of linear and quadratic coefficients and the coefficient of determination.As a tool for statistical analysis, the computational software System for Statistical and Genetic Analysis (SAEG) version 9.1 (UFV, 2007) was used.

RESULTS AND DISCUSSION
From the proposed equations, yields of 11,869.26, 5,550.23, and 2,692.68kg ha -1 were estimated at age 63 days after cutting (DAC) in the rainy, transition, and dry seasons, respectively.The positive linear increment for total forage biomass of the elephant grass as a function of the growth ages (Figure 2) helps explain the linear response for the accumulation of most of the studied nutrients.It is worth noting that the forage was managed only up to 63 days of age in order to match the productivity and quality of the biomass produced, since harvest at later ages leads to qualitative reduction of the forage due to a lower leaf/stem ratio, an increase in the rumen undegradable fiber fraction, and a reduction in the crude protein of the forage (BHERING et al., 2008;LOPES et al., 2013a).The studied factors, growth age and growing season, presented an interaction (p <0.001) for nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) contents in leaf and stem fractions, except for the P and K content in the stem fraction (Tables 2 and 3).It should be noted that the macronutrient contents were not submitted to regression analysis, only to mean comparison test, comparing the contents between the growing seasons at each growth age, which were used as indicators of the nutritional status of elephant grass cv.
Roxo.An interaction (age x season) was also observed for nutrient accumulation in the leaf, stem, and shoot of elephant grass cv.Roxo.According to Werner (1997), the appropriate leaf nitrogen content for elephant grass should be between 15 and 25 g kg -1 .In the present study, the leaf nitrogen content (LNC) of elephant grass cv.Roxo remained within the recommended range from day 36 DAC to harvest in the rainy season.For the transition and dry seasons, LNC remained below the lower limit at all ages (Table 2) due to the fact that nitrogen fertilization took place only in the rainy season (Figure 1), resulting in low residual N availability as it is very dynamic in the soil-plant system.From the age of 27, higher LNC and stem nitrogen content (SNC) were observed for the rainy season, intermediate contents for the transition season, and lower contents for the dry season, probably due to the combined effect of nutrient availability and negative water balance since the contact between N and root occurs predominantly through a mass flow process (SILVA et al., 2011;TEIXEIRA et al., 2011), requiring appropriate matrix potential for this to happen (Table 2).The leaf phosphorus content (LPC) of elephant grass cv.Roxo quantified for all ages and seasons was in accordance with the recommendations by Werner (1997) for elephant grass (1 to 3 g kg -1 ), and was similar to the content observed by Santos et al. (2012) (3.60 g kg -1 for elephant grass cv.Roxo), revealing the tolerance of elephant grass cv.Roxo to extended water deficit regarding the absorption of P (Table 2).and stem (SKC) potassium content; CV: coefficient of variation; means followed by corresponding uppercase letters in the same column did not differ between variables by the Scott Knott test at 5% of probability; means with no letters did not present significant interaction (age x season) but only significant difference between the seasons.
From day 54 DAC there was progressive increase in LPC of elephant grass cv.Roxo between rainy, transition, and dry seasons due to the concentration effect of P on foliar biomass in response to the reduction in plant growth (COSTA et al., 2013) The stem phosphorus content (SPC) was not influenced by the growth age, presenting mean values of 5.25, 6.28, and 6.48 g kg -1 for the rainy, transition, and dry seasons, respectively (Table 2).On average, lower SPC was observed in the rainy season in relation to the other seasons, probably due to the effect of nutrient dilution as a consequence of the greater relative growth of the stem regarding the absorption of phosphorus in the rainy season (CARMO et al., 2011).The lack of significance of SPC between the transition and dry seasons reinforces that the forage was able to absorb and maintain similar P contents during these two cycles of regrowth under low availability of water in the soil.Although water deficit compromises contact and nutrient absorption, absorption may occur depending on the water stress time, as observed by Leão et al. (2011).It was found that the leaf potassium content (LKC) of elephant grass cv.Roxo, at all ages and seasons, remained above the upper limit of the sufficiency range (15-30 g kg -1 ) recommended by Werner (1997) 2).The elephant grass yielded the highest LKC in the dry season, at all growth ages (Table 2).This was probably due to the negative water balance that occurred during the dry season (Figure 1), which induced a strategic accumulation of K in the leaves as an adaptive response to stress by water deficit (MAHMOOD et al., 2009).The stem potassium content (SKC) was not influenced by the growth age, presenting mean values of 95.03, 82.06, and 98.32 g kg -1 for the rainy, transition, and dry seasons, respectively.The mean SKC did not differ (p>0.05) between the rainy and dry seasons, both being higher than the transition season.The highest SKC being in elephant grass cv.Roxo cultivated in the rainy and dry seasons may have occurred due to the effect of excessive absorption "luxury consumption" and concentration, respectively (Table 2).The leaf calcium content (LCaC) of the elephant grass cv.Roxo observed was below the sufficiency values (3 to 8 g kg -

1
; WERNER, 1997) and those observed by Santos et al. (2012) (16.11 g kg -1 ) for elephant grass cv.Roxo, except for at ages 27 and 36 DAC for the transition and dry seasons, respectively (Table 3).It should be noted that there was no need to correct soil fertility for Ca; however, potassium fertilization possibly compromised Ca uptake, as reported by Vale et al. (2011), since both minerals compete for the same sites of absorption in the root system.The LCaC of elephant grass cv.Roxo varied between growing seasons from the age of 18 DAC to harvest.Higher LCaC was observed in the dry season (p<0.05) from age 45 DAC to harvest.Possibly, this concentration effect in LCaC was due to the lower production of biomass in the dry season (Figure 2).The stem calcium content (SCaC) of the elephant grass cv.Roxo differed between seasons for all evaluated ages (Table 3), this was possibly because of the low translocation of this nutrient in the plant and because the Ca absorption by the root system was conditioned to rainfall, which was quite irregular during the evaluated seasons (Figure 1).The leaf magnesium content (LMgC) of elephant grass cv.Roxo presented values compatible with the sufficiency range of 1.5 to 5.0 g kg -1 (WERNER, 1997) at all ages and growing seasons, and it was similar to the content reported by Santos et al. (2012) (1.60 g kg -1 ) for elephant grass cv.Roxo.Thus, it was observed that, despite the negative water balance, the plant was able to maintain the adequate LMgC (Table 3).The LMgC and stem magnesium content (SMgC) differed between seasons from the ages of 18 and 27 DAC to harvest, respectively.From age 45 DAC to harvest, elephant grass cv.Roxo presented the lowest LMgC and SMgC in the rainy and transition seasons, probably due to the effect of nutrient dilution in the biomass produced (BATISTA & MONTEIRO, 2010).The leaf sulfur content (LSC) was below the lower limit of the sufficiency range (1 to 3 g kg -1 ; WERNER, 1997) and similar to the content (5.12 g kg -1 ) observed by Silva et al. (2011) for elephant grass cv.Roxo, at all ages and growing seasons.Possibly, the amounts of S supplied via phosphate fertilization (single super phosphate, 8% S) and micronutrient sorted complex (FTE BR-12) were not sufficient to meet the demand for S of elephant grass cv.Roxo.
For the transition and dry seasons, some nutritional unbalance between N and S may have occurred (AVALHÃES et al., 2009;HOJO et al., 2012).LSC and stem sulfur content (SSC) presented oscillation between the growth ages for all the growing seasons, showing no perceptible pattern between the seasons.An increasing response (p<0.001) was observed for the accumulation of macronutrients as a function of the growth age in the leaf, stem, and shoot components of the elephant grass cv.Roxo in the three growing seasons (rainy, transition and dry), except for the accumulation of Ca in the leaf and shoot for the transition season (Tables 4 and  5).Variation (p<0.05) was observed in the magnitude of the accumulation of N, P, K, Ca, Mg, and S as a function of the season for all the components analyzed (leaf, stem, and shoot) (Tables 4 and 5).(SCaA), and shoot (SHCaA) calcium accumulation; leaf (LMgA), stem (SMgA), and shoot (SHMgA) magnesium accumulation; leaf (LSA), stem (SSA), and shoot (SHSA) sulfur accumulation; mean followed by corresponding letters in the same column did not differ (p>0.05) by the Scott Knott test.Significant at the levels of 5% (*), 1% (**), 0.1% (***) of probability; R 2 : coefficient of determination; CV = coefficient of variation.
DAC. Daily stem calcium accumulation rates (SCaA) were estimated at 0.049, 0.015, and 0.011 kg ha -1 day -1 for rainy, transition, and dry seasons, respectively.Calcium was the fifth most accumulated nutrient in the shoot of the canopy in the rainy and transition seasons and the fourth in the dry season.Daily shoot calcium accumulation rates (SHCaA) of the canopy were estimated at 0.15 and 0.09 kg ha -1 day -1 for the rainy and dry seasons, respectively, resulting in accumulations of 5.98 and 4.46 kg ha -1 at age 63 DAC for the rainy and dry seasons, respectively.In the transition season, the SHCaA of the elephant grass, adjusted to the quadratic model, estimated a maximum accumulation of 4.64 kg ha -1 at 59.76 DAC.Considering the growing seasons, LCaA, SCaA, and SHCaA varied according to the growth age of the plant (Table 5).The daily leaf magnesium accumulation rates (LMgA) were estimated at 0.17, 0.08, and 0.06 kg ha -1 day -1 for the rainy, transition, and dry seasons, respectively.The daily stem magnesium accumulation rates (SMgA) were estimated at 0.21, 0.04, and 0.02 kg ha -1 day -1 for rainy, transition, and dry seasons, respectively.Magnesium was the fourth most accumulated nutrient in the shoot of the elephant grass cv.Roxo in the rainy and transition seasons, and the fifth in the dry season.Daily shoot magnesium accumulation rates (SHMgA) were quantified as 0.38, 0.13, and 0.08 kg ha - 1 day -1 , for the rainy, transition, and dry seasons, respectively, resulting in accumulations of 20.86, 7.47, and 5.01 kg ha -1 day -1 at 63 DAC for the rainy, transition, and dry seasons, respectively.Considering the growing seasons, it was found that LMgA, SMgA, and SHMgA presented higher values in the rainy season, intermediate values in the transition season, and lower values in the dry season from age 45 DAC to harvest.The daily leaf sulfur accumulation rates (LSA) of elephant grass cv.Roxo were estimated at 0.038, 0.008, and 0.001 kg ha -1 day -1 for the rainy, transition, and dry seasons, respectively.The daily stem sulfur accumulation rates (SSA) were estimated at 0.063, 0.006, and 0.002 kg ha -1 day -1 for the rainy, transition, and dry seasons, respectively.Sulfur was the sixth most accumulated nutrient in the shoot the canopy of elephant grass cv.Roxo in the three growing seasons.The daily shoot sulfur accumulation rates (SHSA) of the canopy were estimated at 0.102, 0.014, and 0.004 kg ha -1 day -1 for the rainy, transition, and dry seasons, respectively, resulting in accumulations of 4.82, 0.914, and 0.272 kg ha -1 at age 63 DAC in the rainy, transition, and dry seasons, respectively.Considering the growing seasons, it was found that the LSA varied according to age, while the SSA and SHSA presented higher values in the rainy season, intermediate values in the transition season, and lower values in the dry season from age 45 DAC to harvest.The elephant grass cv.Roxo partitioned more N to the leaf than to the stem, showing a more pronounced effect in the dry season in relation to the others (Figure 3A).The partition of nutrients in the plant is the product of the increase in biomass by the nutrient content in the plant tissue.Thus, the largest partition of N being observed in the leaf in the present study resulted from the higher LNC in relation to the SNC (Table 2), due to the function of nitrogen in plant metabolism, since it is a constituent of chlorophyll besides directly participating in photosynthesis, which occurs predominantly in the leaf (MARSCHNER, 2012;LOPES et al., 2013b). of S similar to the rainy season was observed, changing only in the magnitude of the response.From age 9 to 36 DAC, a linear decrease of S was observed in the leaf (Figure 3F), which was also observed by Zobiole et al. (2010).In the interval between 45 and 63 DAC, an allocation superior to 60% of S was observed in the stem of the shoot of the canopy.Despite the accumulation rates of N, P, K, Mg, and S by elephant grass cv.Roxo showing positive linear behavior, it was noticed that the accumulation of those nutrients was greatest between the ages 27 and 36 DAC in the rainy season and between the ages 36 and 45 DAC for the transition season.The response in the present study is justified by the maximum population density of tillers being observed at 31 and 39 DAC in the rainy and transition seasons, respectively, as observed by Maranhão et al. (2018).As for the dry season, the accumulation of nutrients was enhanced between the ages of 18 and 27 DAC.Possibly, this pronounced accumulation at the beginning of the growth cycle was due to the mobilization of organic reserves presents in the rhizomes of elephant grass.The macronutrient accumulations and contents in the biomass of the shoot of elephant grass cv.Roxo were modified according to the growing age and season.The accumulation of macronutrients in elephant grass cv.Roxo showed the following decreasing order: K > N > P > Mg > Ca > S for the rainy and transition seasons and K > P > N > Ca > Mg > S for the dry season.The age with the greatest nutrient accumulation varied according to the growing season of elephant grass cv.Roxo.The accumulation pattern of macronutrients by elephant grass cv.Roxo along the growth cycle allows recommendation of maintenance fertilization with the nutrients N, P, K, Ca, Mg, and S at the beginning (18 days after cutting) and in the middle phase of the growth cycle, between the ages 27 and 36 days after cutting during the rainy season.

Table 1 .
Chemical soil attributes at the end of each cultivation season of elephant grass cv.

Table 2 .
Nitrogen, phosphorus, and potassium contents in the leaf and stem fractions of elephant grass cv.Roxo at different growth ages in three growing seasons

Table 3 .
Calcium, magnesium, and sulfur contents in elephant grass cv.Roxo at different growth ages in three growing seasons SSC) sulfur content; CV: coefficient of variation; means followed by corresponding uppercase letters in the same column did not differ (p>0.05) by the Scott Knott test.

Table 4 .
Nitrogen, phosphorus, and potassium accumulation in elephant grass cv.Roxo at different growth ages and seasons

Table 5 .
Calcium, magnesium, and sulfur accumulations in elephant grass cv.Roxo at different growth ages in three growing seasons