Biomass and nutrient contents in Panicum maximum cultivars irrigated with fish farming effluent

Biomassa e teor de nutrientes em cultivares de Panicum maximum irrigadas com efluente da piscicultura

ABSTRACT

Wastewater reuse is an alternative for irrigated agriculture in semi-arid regions, due to water support and nutritional supply. Thus, the objective was to evaluate biomass production and nutrient contents in three cultivars of Panicum maximum irrigated with fish farming effluent. The experiment was carried out in a greenhouse, with a complete randomized block design in a split-splitplot scheme. The plot consisted of three types of irrigation management (public-supply water (control), control + conventional fertilization, and irrigation with fish farming effluent). The subplot consisted of three cultivars of P. maximum (Tanzania, Mombasa, and Massai). The sub-subplot consisted of four cutting times (45, 90, 135, and 180 days after sowing). At each cutting time, biomass production and nitrogen, phosphorus, potassium, calcium, magnesium, sodium, iron, copper, manganese, and zinc contents were determined. Irrigation with fish farming effluent increases sodium content in all P. maximum cultivars and causes reduction in biomass production. The salinity of fish farming effluent increased calcium, magnesium, iron, manganese, and zinc contents in the tissues.

Keywords
Fertilization; Massai; Mombasa; Water reuse; Tanzania

RESUMO

O reuso de águas residuárias é uma alternativa para a agricultura irrigada em regiões semiáridas, devido ao suporte hídrico e aporte nutricional. Assim, objetivou-se avaliar a produção de biomassa e a concentração de nutrientes em três cultivares de Panicum maximum irrigadas com efluente da piscicultura. A pesquisa foi conduzida em casa de vegetação, com delineamento em blocos casualizados completos em esquema de parcelas subsubdividida no tempo. A parcela foi constituída por três manejos de irrigação (água de abastecimento (testemunha), testemunha + adubação convencional e irrigação com efluente da piscicultura). A sub-parcela foi constituída por três cultivares de P. maximum (Tanzânia, Mombaça e Massai). A sub-subparcela foi composta por quatro tempos de corte (45, 90, 135 e 180 dias após a semeadura). Em cada tempo de corte foram determinadas a produção de biomassa e as concentrações de nitrogênio, fósforo, potássio, cálcio, magnésio, sódio, ferro, cobre, manganês e zinco. A irrigação com efluente da piscicultura aumenta o teor de sódio de todas as cultivares de P. maximum, o que acarretou redução na produção de biomassa. A salinidade do efluente da piscicultura aumentou os teores de cálcio, magnésio, ferro, manganês e zinco nos tecidos.

Palavras-chave
Adubação; Massai; Mombaça; Reuso de água; Tanzânia

INTRODUCTION

Management of water resources is one of the biggest problems faced by humanity, mainly due to the process of urbanization, industrial growth and irrigated agriculture (VÖRÖSMARTY et al., 2015). However, the random nature of these resources, which undergo changes caused by both climate change and human activities, makes them even more precious (PEDRO-MONZONÍS et al., 2015).

In some areas of the Brazilian semi-arid region, the low availability of good-quality water for irrigation has stimulated the use of lower-quality water, especially saline water (SILVA et al., 2014). In most cases, both surface and subsurface waters have high concentrations of salts (PRAXEDES et al., 2022). The great challenge is to develop proper management of these waters with high salt concentrations because, when managed incorrectly, they cause problems of soil salinization and desertification of areas (SÁNCHEZ; NOGUEIRA; KALID, 2015).

Among the alternatives of irrigation with saline water, the reuse of fish farming effluent is technically feasible for the cultivation of plants that are moderately tolerant to salinity, such as forage grasses, maize and bean (SIMÕES et al., 2016; SILVA et al., 2018a). Some studies have shown that drip irrigation with fish farming effluent in a differentiated management promotes segregation in the generating sources, minimizes environmental effects and maximizes social and economic benefits for rural communities (SOUZA et al., 2022).

Among the main advantages related to the reuse of fish farming effluent, the most prominent are the high organic content and saving in plant water demand, with probable reduction in soil chemical fertilization, which may result in increments in biomass production and yield of crops, enabling low-cost agricultural production, especially in regions affected by water scarcity problems (SALGADO et al., 2018).

The reuse of lower-quality water can meet the water requirement of plants, increasing nutritional supply and reducing the environmental impact caused by the release of this nutrient-rich effluent into the groundwater (MOURA; LOPES; HENRY-SILVA, 2014). However, water salinity greater than 2.0 dS m^-1 significantly reduces the biomass production of P. maximum, mainly due to specific ions such as Na⁺ and Cl^- (DIAS et al., 2019; PRAXEDES et al., 2019; SILVA et al., 2020).

Given the importance of developing technologies that allow the reuse of saline water in agriculture and a production system that can benefit small farmers, the objective of this study was to evaluate biomass production and nutrient contents in three cultivars of P. maximum irrigated with fish farming effluent.

MATERIAL AND METHODS

The study was conducted in a greenhouse, located in the Department of Agronomic and Forestry Sciences of the Federal Rural University of the Semi-Arid Region (UFERSA), East campus, in Mossoró, RN, Brazil, from July 2019 to January 2020. The greenhouse had metal structure, transparent plastic material cover and 50% shade net walls, with area of 126 m² and ceiling height of 4.0 m. The values of air temperature and relative humidity, monitored inside the greenhouse with a digital thermo-hygrometer (MINIPA©) during the experimental period, are presented in Figure 1.

Figure 1
Temperature and relative humidity recorded inside the greenhouse during the experimental period.

The experiment was conducted in a complete randomized block design, with eight replicates, in a scheme of split-split plots in time. The plot consisted of three types of irrigation management (IM), which were: public-supply water (IM1-control), control + conventional fertilization (IM2) and irrigation with fish farming effluent (IM3). The subplot consisted of three cultivars of P. maximum, which were: Tanzania (C1), Mombasa (C2) and Massai (C3). The subsubplot consisted of four cutting times (cycles), at 45, 90, 135 and 180 days after sowing.

Panicum maximum grass was cultivated in a Latossolo Vermelho Amarelo distrófico argissólico (Oxisol) (SANTOS et al., 2018), with sandy loam texture, collected in the 0.0-30.0 cm layer at the Rafael Fernandes Experimental Farm, belonging to UFERSA, located in the Lagoinha district, rural area of the municipality of Mossoró, RN, and its physicalchemical characteristics (TEIXEIRA et al., 2017) are presented in Table 1.

Table 1
Chemical and physical attributes of the soil used in the experiment.

pH	OM	P	K +	Na+	Ca2+	Mg2+	Al3+	H+Al SB	t	T	V	ESP
	(%)	(mg dm-3)			(cmolc dm-3)						%
5.30	1.67	2.1	54.2	21.6	2.70	0.90	0.05	1.82 3.83	3.88	5.65	68	2.0
BD		ECse		Sand				Silt			Clay
(kg dm-3)		(dS m-1)	(g kg-1)
1.60		0.11		820				30			150

²⁺²⁺⁺⁺₄³⁺⁺³⁺

Soil preparation consisted of air drying and sifting, to remove impurities such as roots and gravel. Before planting the crop, liming was performed by applying calcium hydroxide (CaOH₂), according to the base saturation obtained in the soil analysis, with 54% calcium. The soil was corrected to increase base saturation to 90%. After 15 days of acidity correction, the soil under IM2 was fertilized according to the recommendations of Holanda et al. (2017), applying 600 mg of P₂O₅^-, 600 mg of K₂O, and 600 mg of N per pot, through fertigation, using monoammonium phosphate, potassium chloride and urea, respectively. After the first cutting, a topdressing application was made in the IM2 treatment (public-supply water + fertilization), with the dose of 40 and 20 kg ha^-1 of N and K₂O, respectively.

The plants were grown in lysimeters (plastic pots) with 20 dm³ of soil, perforated at the base, which received a 3-cmthick layer of crushed stone no. 1 and 2-mm-mesh nylon screen. The experimental unit consisted of one lysimeter, and the spacing was 0.50 m between pots and 1.0 m between rows. Panicum cultivars were manually sown on July 15, 2019, by placing approximately 20 seeds per pot at a depth of 1.0 cm. The seeds showed a germination power of 90% after 7 days and, when they reached 5.0 cm in height, thinning was performed, leaving only one plant per pot.

During the experimental period, daily irrigations were carried out, in the morning, based on the drainage lysimetry method using a drip irrigation system, formed by a Metalcorte/Eberle self-venting circulation motor-pump unit, actuated by a single-phase motor, with 210 V voltage and 60 Hz frequency, installed in a 60-L reservoir, with spaced emitters of 16-mm-diameter lines and flow rate of 2.5 L h^-1. Two types of water were used for irrigation, one from the supply network (PSW) of UFERSA’s Central Campus, from the Water and Sewage Company of Rio Grande do Norte (CAERN), and the other consisting of fish farming effluent (FFE), collected in the Aquaculture Sector of UFERSA’s East campus. Public-supply water was stored in a 1000-L water tank and the fish farming effluent was stored in two 2000-L water tanks. The chemical characterization of the water used in the experiment is presented in Table 2.

Table 2
Chemical characterization of the waters used in the experiment.

Public-supply water
Period	K+	Na+	Mg2+	Ca2+	Cl-	CO32-	HCO3-	SAR
(Days)	mmolc L-1							(mmolc L-1)-0.5
1-45	0.28	4.56	0.80	0.70	2.40	0.40	2.90	5.20
46-90	0.31	6.54	0.30	1.10	3.00	0.50	3.00	7.80
91-135	0.18	6.05	0.40	0.90	2.80	0.20	2.80	7.50
136-180	0.20	7.62	0.30	0.70	2.60	0.30	2.6 0	6.37
Period	pH	EC	N	P	Cu	Fe	Mn	Zn
(Days)		dSm-1	1 mg L-1			-1 mg L-1
1-45	7.82	0.50	0.10	0.10	0.02	0.05	0.03	0.08
46-90	8.29	0.55	0.10	0.10	0.03	0.06	0.05	0.05
91-135	8.00	0.60	0.15	0.10	0.03	0.04	0.03	0.07
136-180	7.93	0.65	0.10	0.10	0.02	0.07	0.03	0.04
Fish farming effluent
Period	K+	Na+	Mg2+	Ca2+	Cl-	CO32-	HCO3-	SAR
(Days)	mmolc L-1							(mmolc L-1)-0.5
1-103	0.65	17.02	8.90	11.90	21.5	1.30	3.00	5.22
104-180	0.67	16.10	8.10	10.10	18.40	1.60	3.30	5.30
Period	pH	EC	N	P	Cu	Fe	Mn	Zn
(Days)		dSm-1	-1 mg L		-1 mg L
1-103	8.15	3.65	2.30	0.79	0.03	0.65	0.08	0.10
104-180	8.45	3.00	2.35	0.72	0.04	0.50	0.05	0.15

^++2+2+-₃^2-₃^-

At the end of the experiment, soil samples were collected in each experimental plot studied (IM1, IM2, IM3), for the three Panicum maximum cultivars, sent to the laboratory and characterized for soil fertility (Table 3) according to Teixeira et al. (2017). The three cultivars were analyzed in four cuttings, at 45, 90, 135 and 180 days after sowing. The plants were cut at 10 cm height above the soil surface, using scissors. The plant material was placed in paper bags, dried in an air circulation oven at temperature of 65 °C until reaching constant weight, and subsequently weighed to obtain dry biomass production.

Table 3
Chemical attributes of the soil at the end of the experiment.

	OM	N	P	K+	Na+	Ca2+	Mg2+	(H+Al)
Cultivar	g kg-1			mg dm-3		cmolc dm-3
IM1 - irrigation with public-supply water (control)
Tanzania	8.9	0.4	10.9	16.5	31.1	1.0	0.6	0.0
Mombasa	9.9	0.5	10.9	17.1	39.8	1.2	0.4	0.0
Massai	8.4	0.4	11.5	18.4	35.3	1.0	0.6	0.0
IM2 - irrigation with public-supply water + fertilization with NPK
Tanzania	9.8	0.5	25.0	33.6	35.6	1.2	0.4	0.0
Mombasa	10.5	0.5	15.0	34.8	37.5	1.2	0.4	0.0
Massai	9.1	0.5	18.0	33.1	37.8	1.0	0.7	0.0
IM3 - irrigation with fish farming effluent
Tanzania	17.7	0.5	22.9	68.2	33..2	2.6	2.7	0.0
Mombasa	19.3	0.6	21.0	75.5	491.0	2.7	2.7	0.0
Massai	20.5	0.6	24.6	77.8	488.6	3.1	1.8	0.0
	Fe	Mn	Cu	Zn	pH	EC	CEC	ESP
Cultivar	mg dm-3				H2O	dS m-1	cmolc dm-3	%
IM1 - irrigation with public-supply water (control)
Tanzania	8.4	10.1	0.2	2.0	7.6	0.2	1.8	7.6
Mombasa	7.0	9.2	0.3	1.8	7.4	0.2	1.8	9.5
Massai	8.1	8.6	0.3	1.7	7.4	0.2	1.8	8.5
IM2 - irrigation with public-supply water + fertilization with NPK
Tanzania	10.9	9.3	0.2	1.8	7.6	0.3	1.8	8.4
Mombasa	9.5	9.8	0.3	1.6	7.5	0.2	1.9	8.8
Massai	9.7	9.1	0.3	1.9	6.8	0.4	1.9	8.4
IM3 - irrigation with fish farming effluent
Tanzania	8.9	10.8	0.4	1.9	7.2	1.2	6.9	20.8
Mombasa	14.6	9.0	0.3	1.8	7.2	1.2	7.7	27.6
Massai	13.6	10.9	0.3	1.6	7.2	0.9	7.2	29.4

²⁺²⁺⁺⁺₄

Plant dry matter samples were crushed in a Wiley mill and analyzed for contents of nitrogen (N), phosphorus (P), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), copper (Cu), manganese (Mn) and zinc (Zn). In the laboratory, the material was subjected to sulfuric acid wet digestion (H₂SO₄ + H₂O₂) in an open system, to determine the total leaf N contents by the Kjeldahl method (MIYAZAWA et al., 2009). Nitric digestion (HNO₃) was performed in a microwave oven to determine the contents of P, K, Na, Ca, Mg, Fe, Cu, Mn and Zn by means of inductively coupled plasma atomic emission spectrometry (MIYAZAWA et al., 2009).

The data obtained in the experiment were subjected to analysis of variance, F test (p≤0.05); in case of significance, the means were compared by Tukey test at 1% and 5% probability levels, using SISVAR 5.6 software (FERREIRA, 2019).

RESULTS AND DISCUSSION

The interaction between irrigation management and cultivars (p<0.05) and the interaction between irrigation management and cutting times (p<0.01) significantly influenced the biomass production of P. maximum grass.

In the interaction between irrigation management and cultivars for P. maximum grass, it was found that biomass production is independent of the studied variety of P. maximum grass, because higher biomass production is obtained under irrigation with public-supply water and fertilization with NPK (IM2) (Table 4). The biomass production of irrigated and fertilized grass is clearly higher than that found in the other treatments. Canto et al. (2013) and Galindo et al. (2018a) also observed increments in biomass productivity in fertilized pastures.

Table 4
Dry biomass production (g pot^-1) of P. maximum grass for a split-split-plot scheme as a function of irrigation management (IM), cultivars (C) and cutting times (CT), respectively.

Cultivar		Irrigation Management (IM)
	IM1	IM2	IM3
Tanzania	41.79 b	85.54 a	43.70 b
Mombasa	47.84 b	82.89 a	43.04 b
Massai	48.31 b	84.32 a	41.63 c
Cutting Times (CT)	IM1	IM2	IM3
CT1	21.18 b	33.08 a	20.58 b
CT2	50.93 b	99.21 a	50.22 b
CT3	48.20 b	94.97 a	43.82 b
CT4	63.60 b	109.73 a	56.55 c

p

The results for grass irrigated with public-supply water (IM1) and grass irrigated with fish farming effluent (IM3) were similar among cultivars, except for the cv. Massai, which obtained lower biomass production when irrigated with fish farming effluent (41.63 g pot^-1) compared to the production obtained under irrigation with public-supply water (48.31 g pot^-1) (Table 4). The salinity of the effluent (3.0-3.65 dS m^-1) associated with the restriction of the container used in the experiment intensifies the soil salinization process, causing delay in the expected biomass production. The literature shows that waters with electrical conductivity greater than 2.0 dS m^-1 significantly reduce Panicum maximum biomass production, mainly due to specific ions such as Na⁺ and Cl^- (DIAS et al., 2019; PRAXEDES et al., 2019; SILVA et al., 2020).

In the interaction between irrigation management and cutting times, the best biomass production was observed in grass irrigated with public-supply water and fertilized with NPK (IM2). The values of biomass production for grass irrigated with public-supply water (IM1) and grass irrigated with fish farming effluent (IM3) were similar in all cuttings, except for the fourth, when the biomass production of grass irrigated with public-supply water (63.60 g pot^-1) was higher than that obtained with irrigation with fish farming effluent (56.55 g pot^-1) (Table 4). However, the literature has reported reductions in biomass production since the first cut (DIAS et al., 2019; PRAXEDES et al., 2019; SILVA et al., 2020). In this context, it is understood that prolonged irrigation with IM3 reduces the biomass production of P. maximum cultivars. Table 3 shows that the soil irrigated with fish farming effluent obtained higher nutrient accumulation and higher percentage of exchangeable sodium (sodium that is bound to negative charges of the soil). The lower biomass production of P. maximum grass irrigated with fish farming effluent occurred due to the increase in soil salinity and sodicity.

The interaction between irrigation management (IM), cultivars (C) and cutting times (CT) was significant for nitrogen and calcium contents (p≤0.05) and for phosphorus, potassium, magnesium and sodium contents (p≤0.01) in P. maximum grass. In the first cut, the N contents of Mombasa and Massai cultivars were lower when plants were irrigated with publicsupply water and fertilized with NPK (IM2) (10.77 and 10.01 g kg^-1, respectively), compared to plants irrigated with fish farming effluent (IM3) (12.80 and 11.77 g kg^-1, respectively) (Table 5). In the second cut, there was no difference caused by irrigation management for these two cultivars. However, in the third cut the highest N contents for the three cultivars were obtained when plants were irrigated with public-supply water and fertilized with NPK (IM2). In the fourth cut, the N contents of the P. maximum cultivars were similar under all types of irrigation managements.

Table 5
Nitrogen (N), phosphorus (P) and potassium (K) contents in P. maximum grass plants in split-split-plot scheme as a function of irrigation management (IM), cultivars (C) and cutting times (CT), respectively.

Cultivar (C)	Irrigation Management (IM)		Cutting Times (CT)
		CT1	CT2	CT3	CT4
		N (g kg-1)
Tanzania	IM1 IM2	8.37 bB 11.43 aA	5.25 cC 9.83 aA	3.28 bC 6.07 aA	0.77 aA 1.86 aA
	IM3	11.27 aB	8.06 bB	3.99 bA	0.82 aA
Mombasa	IM1 IM2	12.36 aA 10.77 bAB	8.64 aA 9.52 aAB	4.76 abA 5.09 aA	0.68 aA 0.88 aA
	IM3	12.80 aA	9.95 aA	3.61 bA	1.48 aA
Massai	IM1 IM2	11.16 abA 10.01 bB	7.10 aB 8.26 aB	4.27 bAB 6.02 aA	0.82 aA 1.09 aA
	IM3	11.77 aAB	7.26 aB	3.66 bA	0.88 aA
		P (g kg-1)
Tanzania	IM1 IM2	2.12 bA 2.99 aB	2.03 bA 3.55 aA	2.17 bA 4.10 aA	1.74 bA 1.52 bA
	IM3	2.76 abA	2.53 bA	2.46 bA	2.56 aA
Mombasa	IM1 IM2	2.17 bA 3.09 aB	1.80 bA 3.70 aA	1.53 bA 4.10 aA	1.26 aAB 1.83 aA
	IM3	1.83 bB	2.41 bA	2.12 bA	1.77 aB
Massai	IM1 IM2	2.58 bA 4.09 aA	1.70 aA 2.00 aB	1.86 bA 3.58 aA	0.76 bB 1.45 aA
	IM3	1.76 cB	2.19 aA	2.47 bA	1.60 aB
		K (g kg-1)
Tanzania	IM1 IM2	7.42 cA 9.06 bA	5.80 bB 7.45 aA	8.06 aA 8.32 aA	6.91 bA 6.27 bB
	IM3	10.40 aA	8.35 aA	8.44 aA	9.31 aB
Mombasa	IM1 IM2	7.50 bA 8.57 aA	5.91 bB 7.02 aA	6.75 bB 8.12 aA	6.29 cA 7.37 bA
	IM3	7.50 bB	7.70 aA	8.73 aA	9.18 aB
Massai	IM1 IM2	7.94 cA 9.46 bA	7.32 abA 7.08 bA	8.08 aA 8.21 aA	6.95 bA 7.32 bA
	IM3	10.81 aA	8.27 aA	8.95 aA	10.42 aA

pp

In the first cut, N contents ranged from 8.37 to 12.80 g kg^-1, but in the fourth cycle the values were on the order of 0.68 to 1.86 g kg^-1 of N. From the third cut, there was a sharp decrease in N content. These results indicate that in the third cycle a supplementation with N would be required. However, nutrient contents may vary depending on plant age and stage of development because, with the maturity stage, forage species tend to decrease their N, P and Mg contents and increase their Ca content (MALAVOLTA et al., 1974). The decrease in N contents in this study did not mean decrease in biomass production (Table 4). However, the biomass production of fertilized grass (IM2) was up to two times higher than that of unfertilized grass (IM1 and IM3).

In the cultivars, the N contents of Tanzania grass plants irrigated with fish farming effluent (IM3) were higher than those of plants irrigated with public-supply water without fertilization (IM1). And higher N contents were observed for the cv. Tanzania, with IM2, in the first, second and third cuts (Table 5). The cv. Tanzania was more responsive to N fertilization in the first two cuts than the cultivars Mombasa and Massai. When it did not receive N (IM1), the cv. Tanzania obtained the lowest leaf N contents, indicating that this cultivar is more sensitive to N availability in the soil. The cv. Mombasa obtained the highest N contents in the first and second cut, when compared to the other cultivars, being the most efficient in extracting N from the soil. Thus, the cv. Mombasa has high response to N fertilization, showing rapid growth of the aerial part (GALINDO et al., 2018b). The available N in the fourth cut was not enough for a good performance of this cultivar. In the cv. Massai in IM2, N contents were lower than those in the cultivars Mombasa and Tanzania in the first and second cuts. Silva et al. (2018b) found that Massai grass has low utilization of N from N fertilization due to volatilization.

The highest P contents of P. maximum cultivars from the first to the third cut occurred in IM2, except for cv. Massai in the second cut, which did not differ for irrigation management. In the fourth cut, the cv. Tanzania obtained higher P content when irrigated with fish farming effluent. However, there was no difference in P contents as a function of irrigation management for the cv. Mombasa. However, the cv. Massai in the fourth cut obtained higher P contents under IM2 and IM3. The cv. Tanzania, when irrigated with fish farming effluent, obtained the highest P contents in the first and fourth cuts compared to the cultivars Mombasa and Massai. Over the cutting times, the P content in the IM1 treatment decreased, but the P values remained stable in IM2 and IM3 until the third cut. In the fourth cut, there was a considerable decrease in the P contents of the cultivars, mainly in IM1 and IM2, and in the cultivars Mombasa and Massai under IM3. It is worth pointing out that the cv. Tanzania, when irrigated with IM3, obtained stable P contents throughout the cycles, which ranged from 2.76 to 2.46 g kg^-1 (Table 5).

Regarding the behavior of the cultivars throughout the cycles, the cv. Tanzania in the fourth cut also obtained the highest P content when irrigated with IM3, indicating that this cultivar is more efficient in extracting P from the soil than the others, even under conditions of higher soil salinity (Table 5). However, under conditions of high soil salinity and sodicity, as observed in IM3 (Table 3), even with higher soil P levels than in IM2, the cultivars Mombasa and Massai had difficulty absorbing P in the saline soil. Conversely, the cv. Massai, compared to the others, showed lower P content when irrigated with IM3, which indicates that this variety is more sensitive to low P availability in the soil than the others, especially under conditions of high salinity (Tables 3 and 5). Low P contents may compromise the structural development of Panicum grass, so the addition of this nutrient is fundamental for the development of this crop (FLORENTINO et al., 2022). In this context, the ability of P. maximum grass to absorb P under conditions of high salinity, as observed in the cv. Tanzania, may be an indication of salinity tolerance, since the cv. Massai, which had lower P contents in the fourth cut, obtained the lowest biomass production when irrigated with fish farming effluent (Table 4).

In the analysis of IM within the C x CT interaction, the cultivars Tanzania and Massai in the first cut had the highest K contents with IM3. For the cv. Mombasa, the highest K content occurred in IM2 (Table 5). In the second cut, the K contents obtained under IM2 and IM3 were similar in the cultivars Tanzania and Mombasa, being higher than those found under IM1. For the cv. Massai, the highest K content, in the second cut, occurred in IM3. In the third cut, there was no difference between the types of irrigation management for the cultivars Tanzania and Massai. In turn, in the cv. Mombasa the K contents found under IM2 and IM3 were higher than those found under IM1 (control). In the fourth cut, the K contents of the three cultivars of P. maximum irrigated with fish farming effluent (IM3) were higher than those found with IM1 and IM2.

In the analysis of C within the IM x CT interaction, the cultivars Tanzania and Massai showed higher K contents under IM3, whereas the cv. Mombasa, in general, showed similar results, with the highest K contents under IM2 and IM3 (Table 5). Irrigation with fish farming effluent increased the K content in the shoots of the P. maximum cultivars, mainly Massai. Fish farming effluent, in addition to sodium, contains high concentration of potassium, which favored the absorption of this nutrient when compared to grass irrigated with public-supply water without and with fertilization with NPK.

According to the calcium (Ca) contents (Table 6), it was found that, in the first cut, the contents of this nutrient were not influenced by IM. In the second cut, it was observed that the cv. Massai had the lowest Ca content in the tillers when irrigated with IM1. For the third and fourth cuts, the highest Ca contents occurred when the cultivars were subjected to IM3 and the lowest Ca contents were obtained under IM1 for Mombasa and Tanzania, respectively.

Table 6
Calcium (Ca), magnesium (Mg) and sodium (Na) contents of P. maximum grass in a split-split-plot scheme as a function of irrigation management (IM), cultivars (C), and cutting times (CT), respectively.

Cultivar (C)			Cutting Times (CT)
	Irrigation Management (IM) CT1		CT2	CT3	CT4
		Ca (g kg-1)
Tanzania	IM1 IM2	3.60 aA 3.77 aA	3.80 aAB 4.11 aA	4.27 bA 4.49 bA	3.71 cB 4.80 cA
	IM3	3.70 aA	4.54 aA	6.04 aA	6.53 aA
Mombasa	IM1 IM2	3.83 aA 3.31 aA	4.10 aA 4.16 aA	3.43 cB 4.61 bA	4.02 bAB 4.41 bA
	IM3	3.79 aA	4.68 aA	5.95 aA	6.66 aA
Massai	IM1 IM2	3.65 aA 3.77 aA	3.17 bB 4.48 aA	4.05 bAB 4.57 bA	4.48 bA 4.24 bA
	IM3	3.63 aA	4.40 aA	5.86 aA	6.60 aA
		Mg (g kg-1)
Tanzania	IM1 IM2	5.74 aA 6.08 aA	4.09 bAB 4.31 bA	4.55 bAB 4.44 bA	2.79 cA 4.49 bA
	IM3	4.34 bC	6.26 aB	8.09 aB	9.87 aA
Mombasa	IM1 IM2	4.53 bB 4.94 bB	4.29 bA 4.52 bA	4.19 bB 4.51 bA	3.21 bA 3.94 bA
	IM3	5.92 aB	7.70 aA	9.35 aA	10.62 aA
Massai	IM1 IM2	5.16 bAB 5.28 bAB	3.28 bB 3.88 bA	5.18 bA 4.27 bA	3.28 cA 4.79 bA
	IM3	6.97 aA	7.58 aA	8.92 aAB	9.74 aA
		Na (g kg-1)
Tanzania	IM1 IM2	2.08 bA 2.14 bA	2.62 bA 2.66 bB	4.00 cA 7.77 bC	5.71 cB 9.73 bB
	IM3	3.11 aAB	5.74 aB	13.11 aA	16.51 aB
Mombasa	IM1 IM2	1.92 bA 2.01 bA	2.95 bA 2.91 bB	4.43 cA 9.44 bB	6.31 cAB 12.93 bA
	IM3	3.75 aA	5.28 aB	13.65 aA	18.80 aA
Massai	IM1 IM2	2.05 aA 2.37 aA	2.76 cA 4.57 bA	4.72 cA 10.66 bA	6.88 cA 12.86 bA
	IM3	2.66 aB	12.86 aA	13.16 aA	15.41 aC

pp

Ca was better absorbed by the Panicum cultivars analyzed when they were subjected to fish farming effluent (IM3), with contents ranging from 3.63 to 6.60 g kg-1, followed by NPK fertilization (IM2), with contents ranging from 3.31 to 4.80 g kg-1 and then by IM1, with contents ranging from 3.83 to 4.48 g kg-1 (Table 6). These results can be attributed to the residual effect of nutrients present in fish farming effluent (IM3), which promoted improvement in plant nutritional status. According to Malavolta, Liem and Primavesi (1986), the adequate Ca contents for Panicum grass range between 1.5 and 6.0 g kg-1. The values observed in the present study are within or very close to the range described Malavolta, Liem and Primavesi (1986).

In the analysis of IM within the C x CT interaction, in the first cut for Mg content (Table 6), it was observed that the cv. Tanzania had the highest Mg contents under IM1 and IM2. For the cultivars Mombasa and Massai, the highest Mg contents were observed when they were irrigated with IM3. In the second, third and fourth cuts, the highest Mg contents for all cultivars analyzed were observed in IM3. The cultivars of P. maximum accumulated Mg in their tissues when irrigated with fish farming effluent (IM3).

In the analysis of C within the IM x CT interaction, in the first cut for Mg content (Table 6), it was observed that the cv. Mombasa under the three types of irrigation management obtained the lowest Mg contents compared to the others. In the second and third cuts, the cv. Mombasa obtained the highest Mg contents for all types of irrigation water, except for the third cut of the cv. Mombasa irrigated with IM1, when Mg contents were higher than those found in the other cultivars under this management condition. It is worth pointing out that the cultivar Tanzania absorbs the lowest Mg contents when irrigated with fish farming effluent in the first, second and third cuts, compared to the others, with contents of 4.34, 6.26 and 8.09 g Mg kg^-1 of dry matter, respectively. However, the values obtained for Tanzania grass are higher than those found in the literature, which range from 1.8 to 2.5 g (Mg) kg^-1 of dry matter (SILVEIRA; MONTEIRO, 2010). The high Mg contents observed in P. maximum cultivars can be explained by the high concentrations of Mg in irrigation water, especially in the fish farming effluent (Table 2).

For the Na content in the analysis of IM within the C x CT interaction, it was observed that, for all cultivars at all cutting times, the highest Na content was obtained when plants were irrigated with fish farming effluent, except in the first cut of the cv. Massai, when there was no difference between the types of irrigation management (Table 6). The high Na contents in P. maximum cultivars irrigated with fish farming effluent are due to the high Na concentrations present in the effluent, ranging from 16.1 to 17.02 mmol_c L^-1 (Table 2), thus influencing the biomass of the cultivars, especially Massai (Table 4). Thus, a careful management of drip irrigation is needed for using fish farming effluent in the cultivation of P. maximum cultivars, mainly with the use of a drainage system, since it poses risk of toxicity to plants, due to the high concentrations of Na and Cl ions, which may reduce biomass accumulation, as verified by other authors (DIAS et al., 2019; PRAXEDES et al., 2019; SILVA et al., 2020).

For the Na contents of the P. maximum cultivars within the IM x CT interaction, it was found that the cultivar Mombasa under IM2 and IM3 had the highest Na contents in the fourth cut, followed by the cultivars Tanzania and Massai, respectively (Table 6). The biomass production of the cv. Massai was reduced when it was irrigated with IM3 compared to IM1 (Table 4), but the cv. Massai had the lowest Na content in the tissues, in the fourth cut when compared to the others, indicating that this cultivar is more sensitive to Na accumulation in tissues and expends more energy in the process of Na exclusion in the roots than the other cultivars studied, which compromises its biomass accumulation. Conversely, the cultivars Mombasa and Tanzania can better cope with high Na contents in tissues, since they had similar biomass production between IM1 and IM3 (Table 4). The highest Na contents were verified in the fourth cut, coinciding with the period in which there was a difference in biomass production between IM1 and IM3 (Table 4). Therefore, it can be inferred that Na contents above 13.11, 13.65 and 13.16 g hg^-1 of dry matter (verified in the third cut), in the leaves of the cultivars Tanzania, Mombasa and Massai, respectively, can cause toxicity and biomass loss (Tables 4 and 6).

The IM x CT x C interaction was significant for the iron content (p<0.05) and for the zinc and copper contents (p<0.01) of P. maximum grass. The manganese (Mn) content of P. maximum grass was significantly affected (p<0.01) only by cutting time.

For Fe content in each cut (Table 7), it was found that, in the first, second and third cuts, IM3 promoted higher Fe contents in all cultivars. In the fourth cut, only the cultivars Tanzania and Massai showed higher contents of the micronutrient when irrigated with IM3, while Mombasa showed higher Fe contents when subjected to IM1 and IM2. It was observed that Fe contents increased with shoot maturity. However, in the fourth cycle, the values decreased. This was probably a consequence of the competitive inhibition between Fe and Mn, resulting in the negative effect of Fe absorption. The range of the micronutrient Fe found in the study was 37.94-223.58 mg kg^-1. For Werner et al. (1997), the range considered suitable for iron is 50-200 mg kg^-1 contained in the shoot dry matter. Therefore, only in the first cut the plants were Fe deficient, although they showed no visual symptoms, and this nutrient was supplied by irrigation.

Table 7
Iron (Fe), zinc (Zn) and copper (Cu) contents of P. maximum grass in a split-split-plot scheme as a function of irrigation management (IM), cultivars (C) and cutting times (CT), respectively.

Cultivar (C)			Cutting Times (CT)
	Irrigation Management (IM) CT1		CT2	CT3	CT4
		Fe (mg kg-1)
Tanzania	IM1 IM2	39.13 bA 38.19 bA	94.31 bA 88.16 bA	71.56 bA 76.56 bA	66.88 aAB 56.69 aB
	IM3	54.67 aA	199.57 aB	212.80 aAB	69.31 aAB
Mombasa	IM1 IM2	37.94 bA 41.13 bA	90.00 bA 88.94 bA	70.44 bA 66.71 bA	72.25 aA 73.63 aA
	IM3	60.38 aA	194.63 aB	201.50 aB	60.25 aB
Massai	IM1 IM2	48.38 bA 43.56 bA	89.86 bA 88.67 bA	76.56 bA 70.75 bA	56.14 bB 69.67 abAB
	IM3	67.50 aA	223.58 aA	218.38 aA	77.30 aA
		Zn (mg kg-1)
Tanzania	IM1 IM2	42.63 aA 53.19 aA	35.31 bA 42.50 bA	35.44 bB 42.13 bA	25.19 bA 24l81 bA
	IM3	43.88 aA	57.00 aB	58.56 aA	43.75 aA
Mombasa	IM1 IM2	41.44 bA 43.44 abA	34.31 bA 35.69 bA	42.38 bA 50.29 bA	23.69 bA 26.06 bA
	IM3	55.13 aA	63.03 aB	65.13 aA	45.75 aA
Massai	IM1 IM2	41.56 bA 50.44 abA	34.13 bA 40.75 bA	49.94 aA 52.38 aA	26.00 bA 26.38 bA
	IM3	54.64 aA	85.90 aA	61.13 aA	42.63 aA
		Cu (mg kg-1)
Tanzania	IM1 IM2	5.50 aA 4.19 bA	6.44 aA 6.75 aA	3.75 bA 3.69 bA	3.69 aA 2.00 bA
	IM3	3.44 bB	6.81 aB	5.44 aA	3.56 aA
Mombasa	IM1 IM2	5.19 aA 4.57 aA	6.94 abA 5.94 bA	3.44 bA 3.56 bA	3.00 aA 2.50 aA
	IM3	4.57 aA	7.29 aB	5.88 aA	3.44 aA
Massai	IM1 IM2	5.88 aA 5.06 aA	6.31 bA 2.19 cB	3.69 bA 3.56 bA	3.69 aA 2.19 bA
	IM3	5.07 aA	8.57 aA	5.81 aA	3.50 aA

pp

Regarding the Zn content of the P. maximum cultivars (Table 7), it was observed in the first cut that the cultivars differed as a function of IM, with higher Zn contents in the cv. Tanzania when subjected to IM2, and in the cultivars Mombasa and Massai when subjected to IM3. For the second, third and fourth cuts, it was observed that the highest Zn contents were found when the cultivars were subjected to IM3.

The IM1 treatment promoted lower Zn supply, and IM3 promoted greater Zn supply. There was a decrease in Zn contents from the fourth cut, when plants were irrigated with IM1 and IM2. With IM3, there was a decrease, but not so significant. In the four cuts evaluated, the Zn contents were within the range indicated by Werner et al. (1997), who reported adequate values between 20 and 300 mg kg^-1 for forage species in ruminant feeding, and those obtained in this study are between 23.69 and 85.90 mg kg^-1.

In relation to the Cu contents of the P. maximum cultivars for the first cut, the cultivars Mombasa and Massai were similar under all irrigation managements (Table 7). The cv. Tanzania, during this period, had higher Cu contents in IM1. In the second and third cut, IM3 favored Cu absorption by plants, which was possibly due to the presence of Cu in the effluent. In the fourth cut, there was no influence of IM on Cu absorption by plants, and this decrease is explained by the low availability of Cu in the soil. In general, when comparing the Cu contents obtained in this study (Table 7) with the levels suggested by Guimarães, Ferreira and Carvalho (1980), it can be affirmed that the contents of this element in the plants can be considered adequate in the first and second cuts and low in the third and fourth cuts, since the authors classified them as low up to 4 mg kg^-1 and as adequate between 5 and 15 mg kg^-1.

Regarding the Mn content of the P. maximum cultivars (Table 8), it was observed that there was no difference with the use of IM, that is, the cultivars were similar in the absorption of this micronutrient. However, maturity influenced the availability of Mn in the cultivars, with a decrease along the cuts.

Table 8
Manganese (Mn) content of P. maximum grass as a function of cutting times (CT).

Cutting Time	Mn (mg kg-1)
CT1	69.42 a
CT2	37.06 c
CT3	40.82 b
CT4	34.97 c

p

The Panicum cultivars evaluated had low tolerance to Mn, which was possibly due to competition with Fe, because these elements are antagonistic, so a high content of Fe (Table 7) inhibited the absorption of Mn in the cultivars (GONÇALVES JR et al., 2015). When comparing the Mn contents of the Panicum cultivars with the levels suggested by Werner et al. (1997), it can be affirmed that the contents of this element in the first and third cuts can be considered adequate and low in the second and fourth cuts, since the range considered adequate for forage grasses according to these authors is 40-250 mg kg^-1.

CONCLUSIONS

The highest biomass production and the best nutritional status are obtained in irrigated and fertilized grass, but fish farming effluent can be used in the irrigation of P. maximum, assuming production losses.

Irrigation with fish farming effluent increases sodium content in P. maximum plants, which led to a reduction in the biomass of all cultivars, requiring monitoring of salinity in the P. maximum cultivars.

The salinity of fish farming effluent increased Ca, Mg, Fe, Mn and Zn contents in the tissues, resulting in nutritionally acceptable values for the P. maximum cultivars.

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Publication Dates

• Publication in this collection
  30 Oct 2023
• Date of issue
  Oct-Dec 2023

History

• Received
  02 Aug 2022
• Accepted
  02 Mar 2022