Characterization and Energy Potential of Broiler Manure Reared under Different Flooring Materials

Abstract

Broiler deep litter is composed of cellulose, manure, feathers, and feed, and after reuse through several flocks, may be used as a fertilizer. Alternative litter materials such as plastic flooring have been researched but little is known about the characteristics of the waste produced. We analyzed the properties of broiler manure from different flooring systems and assessed its potential for biodigestion. Broiler chicks (1500 one-day-old) were reared using five flooring materials: wood shavings (WS); plastic floor (PP); 50% shavings + 50% plastic floor (PP + WS); plastic floor with antimicrobial additive (PPA); and 50% shavings + 50% plastic floor with an antimicrobial additive (PPA + WS). Waste collection was done at 14, 28, and 42 days of growth. The total residue (kg) produced was quantified on day 42. The total solids (TS), volatile solids (VS), neutral detergent fiber (NDF), acid detergent fiber (ADF), nitrogen (N), and pH were analyzed at 14, 28, and 42 days of growth, using the residue coefficient (RC) on the 42nd day. The contents of ST, SV, NDF, ADF, C, and N were determined for anaerobic digestion. The concentrations of O₂, CO₂, and CH₄ produced by anaerobic digestion were analyzed. The flooring material did not affect the volatile solids’ contents. The lowest NDF and ADF were found in plastic floor residues. The pH of the manure linearly increased over time as the birds grew. The presence of wood shavings in the manure was decisive for the production of biogas. The treatments with the plastic floor without shavings obtained the highest N content with less residue. Plastic flooring with wood shaving is not recommended as it increases waste generation. The use of plastic flooring reduced the amounts of waste generated and promote a greater yield of biogas with the anaerobic digestion of excreta.

1. Introduction

Brazil’s broiler production is the world’s largest exporter of chicken meat and the third largest producer [1]. With the increase in flocks, there is also the need to decide the correct destination for waste from the poultry farming system. Broiler deep litter consists of a substrate composed of cellulose (usually wood shavings and rice husks), excreta, feathers, and feed, reused throughout several flocks. It is used as a fertilizer but can damage the environment if not treated adequately [2]. In some areas of Brazil, poultry litter is an alternative to an energy matrix through its biogas production. However, it has limitations due to the variation in the materials used as deep litter [3].

The anaerobic digestion process in batch biodigesters is the most suitable treatment for fibrous residues and those with contaminants, and when the dry type (minimum of 30% dry matter) depends on the inoculum used to facilitate and optimize the process, resulting in a constant production of biogas [4]. The batch type fits this production characteristic as poultry waste is not dealt with daily but per flock. For anaerobic digestion to be efficient, factors such as the substrate and co-substrate composition and quality, temperature, pH, the amount of organic load, and the microbial dynamics must be optimized to obtain better process results [5].

Biomass of animal origin does not have its energy potential used in the practice of waste application as fertilizers. The harm caused by this practice has been the subject of research due to possible pathogens and medication residues, such as antimicrobials, in soils [6,7]. Poultry manure has a high potential, with a biogas production of 130–270 N L/kg of substrate, a methane production of 70–140 N L/kg of substrate, and a methane yield of 200–360 N L/kg of dry organic matter, and is always superior when compared to the performance of cattle and pig manure [8].

However, the materials commonly used as bedding for poultry have lignin in their composition and are less susceptible to microbiological degradation [9], thus interfering with biogas production. Despite their complex structure and challenging degradation, wood shavings help with methane production. Due to the high content of ammonia contained in the excreta of the birds, there is an inhibition of the micro-organisms desirable for biodigestion, unbalancing the C/N ratio, and meaning that a stable and efficient process is not obtained since each phase of the anaerobic digestion process has a specific need according to the degradation stage [5,10].

The total and volatile solid contents of the substrates are essential to determine the biogas and methane yields during anaerobic biodegradation. While the total solids represent the substrate dilution and its consequent organic load, the volatile solids are related to the total organic matter available to be converted into biogas. Some studies relate the interaction between these two factors and the improvement in methane generation, as studied by [11], who demonstrated a positive effect between the reduction of TS and VS concentrations in substrates being digested, and a higher efficiency in methane production.

In this sense, raising chickens on alternative bedding materials such as plastic flooring may change the characteristics of the manure compared to those present when wood shavings are used as the litter material. Nanotechnology can be added next to the plastic flooring, which aims to produce materials with specific characteristics through the arrangement of particles on a nanometric scale [12]. Besides being a substitute for shavings, this type of plastic floor also brings antibacterial properties to chicken litter in its composition, favoring the control of micro-organisms within poultry facilities [13].

The present study aimed to characterize and quantify energy production potential through the biodigestion of broiler manure (pure and with wood shavings) using different arrays of flooring and litter.

2. Materials and Methods

This study was conducted in experimental housing in Dourados, Mato Grosso do Sul, Brazil (latitude 22°13′18″ south, longitude 54°48′23″ west, and 448 m altitude). The house had 4 m² boxes equipped with pendular drinkers, tubular feeders, double curtains, and evaporative plates and nebulizers to control the internal temperature. In the first three days of the experiment, shredded paper was used to facilitate the access of the chicks to the feed, and was removed after this period. A protection circle was used, reducing the area for the birds for up to ten days, and infrared lamps (250 W) were used for heating in the initial rearing phase (from 1 to 15 days old). The animal-rearing phase lasted 42 days, and the experiment was carried out during the winter.

A total of 1500 one-day-old male broiler chicks of the Ross 408^® strain were used, distributed in a completely randomized design across the five treatments: wood shavings (WS); plastic floor (PP); 50% plastic floor + 50% wood shavings (PP + WS); plastic floor with a nanotechnology antimicrobial additive (MpZn_1300, TSNano, Florianopolis, Brazil) (PPA); and 50% plastic floor with a nanotechnology antimicrobial additive (MpZn_1300, TSNano, Florianopolis, Brazil) + 50% wood shavings (PPA + WS), shown in Figure 1. We used six repetitions, totaling 30 boxes, with 50 birds in each. According to the production phase, the experimental diet was provided ad libitum, based on corn and soybean meal, meeting the nutritional requirements proposed by [14]. All birds were reared using the flock density of 12.3 birds/m².

2.1. Bedding Material

The high-density polyethylene (HDPE) non-commercial plastic floor with wavelengths of ultraviolet light (UV) protection was designed for assembly inside the experimental boxes, with insertable plates measuring 100 × 60 cm and perforations measuring 12 × 12 mm for the passage of excreta. The plates were suspended through profiles 20 cm from the floor of the aviary. The antimicrobial product (MpZn_1300, TSNano, Florianopolis, Brazil) was injected during manufacturing in the treatments that used the antimicrobial addition. The total area of the boxes was 2.90 × 1.40 m, and in the 50% of wood shavings +50% plastic floor treatment, the floor corresponded to a longitudinal strip of 2.90 × 0.70 m, and the shavings completed the sides, totaling 2.90 × 0.70 m of the area with wood shavings.

The material was turned over weekly in the treatments with wood shavings (total or partial). During the experimental period, they presented a decrease in height since there was no material replacement.

2.2. Material Characterization, Collection, and Analysis

Waste collections were performed at 14, 28, and 42 days of production. An opening was made in the plastic floor to scrape the excreta below the floor (Figure 2). Three collection points were carried out in the treatments with wood shavings (total or partial), and the sample was homogenized. On the 42nd day, the birds were weighed, and the total residue was collected from the boxes to quantify the residue, including the material that was adhered to the plastic floor (kg).

The analysis of the total solids (TS) and volatile solids’ (VS) contents was done according to the methodology described by [15]. The analyses of the neutral detergent fiber (NDF), the acid detergent fiber (ADF), and the total nitrogen (crude protein) were measured using the Kjeldahl method, described by [16]. The pH analysis was performed following Normative Instruction 17/2007 [17].

The residue coefficient (CR) was calculated using the ratio between the amount of residue generated in each treatment (according to the type of floor or shaving bed) and the live weight (LW) of the chickens produced at the end of the 42 days of rearing, according to adaptations from [18,19] by Santos [20] (Equation (1))

CR = broiler litter (TS, kg)/mass of broiler generated (LW, kg)

(1)

2.3. Biodigestion Analyses

The initial TS content was set at 4% for the formulation of the substrates for biodigesters. Previously, swine manure inoculum was prepared to accelerate the degradation of the substrates. The analyses for the characterization of the beds were considered and used to prepare loads of biodigesters that were diluted with water and homogenized in an industrial blender, determining the contents of TS, VS, NDF, ADF, C, and N.

Batch biodigesters with a capacity of 1.5 L each were used where the substrates were placed for fermentation (1.3 L per experimental unit) at a rate of 15 per litter collection, totaling 45 biodigesters made of PVC, as described by [21]. The gas analysis was performed with a gas analyzer (GA—21 Plus analyzer, Madur Electronics, Vienna, Austria) to determine the concentrations of O₂, CO₂, and CH₄.

The substrates were prepared with water, inoculum, and fresh residue. After being made, the digesters were completed with this mixture, and the biogas productions were followed throughout the time that the substrates continued producing biogas. Gas analysis was performed from the digesters’ first biogas production, which occurred at different times according to the experimental treatment. The biogas productions were followed throughout the time that the substrates continued producing biogas, which was somewhere between 120 and 150 days.

The volume of biogas produced daily was determined by measuring the vertical displacement of the gasometers and multiplying the recorded data by the area of its internal cross section (0.00785 m²). Temperature, biogas volumes, and environmental measurements were recorded throughout the process. The volume of biogas produced was corrected for 1 atm and 0 °C conditions using a combination of Boyle’s and Gay-Lussac’s laws, recorded in liters (LN), as described by [22]. The reductions of TS, VS, and fibrous constituents were calculated considering the amounts of these constituents at the fermentation process’s beginning (influent) and end (effluent).

2.4. Statistical Analysis

Data were evaluated for the normality of residuals using the Shapiro–Wilk test, and for the homogeneity of variances using the Levene test. Thus, an analysis of variance was performed using the PROC MIXED from SAS [23]. The statistical models included the type of floor as a fixed factor, and the animal effect, nested within each treatment, was included as a random factor in all models (command RANDOM). When significant, Tukey’s test was performed to compare the means of the treatments, adopting a 95% significance.

The data measured over the weeks in which the birds were reared were submitted to an analysis of variance through the MIXED procedure of SAS (SAS 9.3) [23] with the use of the REPEATED command in which the evaluation days were considered repeated measures in time. The statistical models included the type of floor as a fixed factor, and the animal effect, nested within each treatment, was included as a random factor in all models (command RANDOM). When the effects of the interactions between the types of floor and the time were significant, these were unfolded and evaluated through regressions using orthogonal polynomials. The significance level for all analyses performed was 5%.

3. Results and Discussion

As the birds’ rearing time increased, there was an increase in pH in all evaluated treatments, with an increasing linear effect (p < 0.0001) (

Table 1. Litter characterization of different broiler litter materials: wood shavings (WS); plastic floor (PP); 50% plastic floor +50% wood shavings (PP + WS); plastic floor with a nanotechnology antimicrobial additive (PPA); 50% plastic floor with nanotechnology antimicrobial additive +50% wood shavings (PPA + WS), and the effect and regression equations found.

Variables	Day of Growth	Treatments						EPM	p-Value
		WS	PP	PP + WS	PPA	PPA + WS	Ave		Litter	Rearing Time	LxT
pH	14	6.80	6.84	6.50	7.08	6.52	6.75	0.111	0.2666	<0.0001	0.1139
	28	7.93	6.99	7.60	7.19	7.21	7.38
	42	8.42	8.41	8.13	8.10	8.32	8.28
	Ave	7.71	7.41	7.41	7.45	7.35	7.46
TS (%)	14	68.21	63.67	58.33	64.25	61.87	63.26	1.540	0.0029	0.0057	0.0125
	28	56.74	42.44	63.85	39.47	67.61	54.02
	42	63.79	57.22	65.88	52.16	62.33	60.27
	Ave	62.91 a	54.44 ab	62.68 a	51.96 b	63.93 a	59.18
VS (% of TS)	14	93.90	87.16	91.69	86.30	91.12	90.05	0.458	<0.0001	0.0033	0.0475
	28	90.73	86.97	93.21	86.54	93.44	90.18
	42	89.46	85.54	88.99	87.50	88.82	88.06
	Ave	91.36 a	86.55 b	91.30 a	86.80 b	91.12 a	89.42
NDF (% of TS)	14	55.42	22.12	51.78	25.99	45.70	40.2	2.012	<0.0001	0.1461	0.1109
	28	58.91	24.51	52.19	25.74	49.79	42.25
	42	44.15	25.17	47.53	29.26	46.79	38.582
	Ave	52.82 a	23.93 b	50.50 a	27.00 b	47.47 a	40.34
ADF (%of TS)	14	39.32	6.10	30.58	7.01	30.58	22.72	1.996	<0.0001	0.0726	0.1390
	28	37.66	8.27	38.09	7.79	31.95	24.75
	42	27.23	6.37	28.40	7.33	25.95	19.06
	Ave	34.74 a	6.91 b	32.36 a	7.38 b	29.49 a	22.17
N (%of TS)	14	2.13	4.42	2.98	5.46	2.59	3.52	0.154	<0.0001	<0.0001	<0.0001
	28	2.59	4.34	4.08	4.40	4.50	3.98
	42	2.42	3.47	2.21	3.33	2.63	2.81
	Ave	2.38	4.07	3.09	4.40	3.24	3.43
Variable	Effect	p-value	Equation							R2
pH	lin	<0.0001	y = 0.05464x + 6.14533							0.7078
ST (PP)	quad	0.0028	y = 0.09186x2 − 5.37440x + 120.90667							0.8131
SV (PPA + WS)	quad	0.0035	y = −0.01769x2 + 0.90810x + 81.880							0.8273
SV (WS)	lin	0.0003	y = −0.15833x + 95.79889							0.8567
N (PPA)	lin	0.0004	y = −0.07583x + 6.33889							0.8940
N (PPA + WS)	quad	0.0003	y = −0.00966x2 + 0.54214x − 3.28333							0.9083
N (PP)	quad	0.0213	y = −0.00202x2 + 0.07893x + 3.52667							0.8948
N (PP + WS)	quad	<0.0001	y = −0.00759x2 + 0.39750x − 1.27333							0.9504

Total solids (TS); volatile solids (VS); neutral detergent fiber (NDF); acid detergent fiber (ADF); and nitrogen (N). Bird standard error (EPM); LxT interaction effect (Litter type × Time). Mean values in the same row followed by different lowercase letters differ significantly (p < 0.05).

).

The progressive increase in pH is a common occurrence during animal rearing. It has a determining effect on the higher emission of NH₃ from chicken litter or excreta [24]. If associated with an increased humidity and temperature, it may compromise the loss of N from the deep litter and the broiler-rearing environment. The critical humidity for ammonia volatilization from poultry litter was reported between 38.3 and 51.1% by [25], in the temperature range between 18.3 and 40.6 °C.

We found an interaction for the TS between time and bedding materials (p = 0.0125), showing a positive quadratic behavior in the PP treatment (p = 0.0028). However, the R² in the other treatments was not significant. An interaction was found between the treatments and the time of raising the birds for the variable VS (p = 0.0475) with a negative quadratic effect in the treatment PPA + WS (p = 0.0035) and a linear negative in the treatment WS (p = 0.003). In the remaining treatments, the equations were not significant. This condition of decreasing VS deposition in the composition of the mass of waste generated is possibly associated with a higher proportion of excreta concerning the mass of wood shavings with the increase in the age of the broilers. Although the SV contents are higher in the shavings than in the excreta, the components of this fraction present a high degree of complexity due to the presence of lignin and fibrous constituents, as reported by [9]. Thus, the NDF and ADF contents were influenced only by litter materials (p < 0.0001),

The NDF and ADF results had a significant difference only for the bedding materials (p < 0.0001), with the presence of wood shavings (total or partial) determining the results, thus showing higher levels of NDF and ADF. There was an interaction between the time of rearing the birds and the litter materials used for nitrogen content, with a decreasing linear effect for PPA (p = 0.0004) and a negative quadratic effect for PPA + WS, PP, and PP + WS, not obtaining a significant equation for the treatment WS.

The bedding materials did not affect the volatile solids’ contents (p = 0.2773), significantly affecting the other evaluated variables (

Table 2. Contents of total solids (TS), volatile solids (VS), neutral detergent fiber (NDF), acid detergent fiber (ADF), and total nitrogen (N) (crude protein) of residues of different broiler litter materials—wood shavings (WS); plastic floor (PP); 50% plastic floor + 50% wood shavings (PP + WS); plastic floor with a nanotechnology antimicrobial additive (PPA); and 50% plastic floor with a nanotechnology antimicrobial additive +50% wood shavings (PPA + WS)—on the 42nd day.

Variable	Treatment					EPM	p-Value
	WS	PP	PP + WS	PPA	PPA + WS
TS production from litter (kg)	63.27 b	37.68 c	78.07 a	39.79 c	91.02 a	4.959	<0.0001
Live weight (kg)/box (kg)	104.25 a	53.37 b	105.59 a	60.54 b	109.22 a	5.479	<0.0001
Residue coefficient (RC) (litter TS (kg)/live weight (kg)	0.61 c	0.70 bc	0.73 ab	0.65 bc	0.83 a	0.019	0.0002
VS (% of TS)	87.32	91.31	90.87	89.62	92.07	0.728	0.2773
NDF (% of TS)	51.19 b	30.90 c	63.56 a	30.55 c	60.10 ab	3.349	<0.0001
ADF (% of TS)	33.10 b	13.53 c	46.50 a	12.56 c	45.35 a	3.467	<0.0001
N (% of TS)	2.10 b	3.79 a	2.08 b	3.34 a	2.28 b	0.184	<0.0001
N (kg) produced in the box	1.32 b	1.42 ab	1.63 ab	1.33 b	2.06 a	0.088	0.0187

Bird standard error (EPM); Mean values in the same row followed by different lowercase letters differ significantly (p < 0.05).

). The production of live weight (kg)/box was higher in the treatments that had wood shavings (total or partial) in their composition. The residue coefficient (litter/live weight) was higher in the PPA + WS treatment when compared to the WS, PP, and PPA treatments.

The behavior of the residues was similar concerning the NDF and ADF, with the lowest levels found in the plastic floor residues with and without antimicrobials. The highest nitrogen contents were found in the plastic floor residues with and without antimicrobials. When we observe the amount of N produced per treatment, a different behavior occurs, with the PPA + WS treatment presenting higher values than the WS and PPA treatments.

Regarding the production of ST, the plastic floor effectively reduced the waste generated while raising the birds (PP conditions) when used in isolation. However, when associated with wood shavings, the waste generated was higher (floor with WS). When we compared the residue coefficient, the best values occurred with the shaving bed (isolated) or PP (also isolated), and the combination of these two was not beneficial for the generation of the RC (residual coefficient). Based on the residue data, we can recommend PP (without antimicrobials) since it appears to have the lowest RC, as reducing residue generation is a desired condition in poultry farming. At the same time, when subjected to anaerobic digestion, this residue presents the highest values of biogas production (energy yield).

For reducing organic constituents and biogas production, there was an interaction between the rearing time and bedding materials in all variables, with different behaviors (equations in

Table 3. Reduction of biodigestion (Red) of different broiler litter materials over time: wood shavings (WS); plastic floor (PP); 50% plastic floor + 50% wood shavings (PP + WS); plastic floor with a nanotechnology antimicrobial additive (PPA); 50% plastic floor with a nanotechnology antimicrobial additive +50% wood shavings (PPA + WS), and the effect and regression equations found.

Variables		Day of Growth	Treatments					Ave	EPM	p-Value
			WS	PP	PP + WS	PPA	PPA + WS			Litter	Rearing Time	LxT
Red	TS (%)	14	33.39	33.35	38.95	21.87	52.91	36.09	1.703	<0.0001	<0.0001	<0.0001
		28	40.59	56.44	51.19	27.92	40.76	43.38
		42	47.47	62.36	54.96	34.13	50.57	49.89
		Ave	40.48	50.71	48.36	27.97	48.08	43.12
	VS (%)	14	35.56	37.75	41.29	27.51	58.66	40.15	1.758	<0.0001	<0.0001	<0.0001
		28	46.37	61.76	54.08	31.82	43.70	47.54
		42	52.23	65.47	61.67	36.91	55.47	54.35
		Ave	44.72	54.99	52.34	32.08	52.61	47.35
	NDF (%)	14	23.91	28.42	33.11	29.55	19.83	26.96	1.057	<0.0001	<0.0001	<0.0001
		28	33.61	42.71	39.37	28.77	35.03	35.89
		42	35.83	45.42	42.33	31.52	39.24	38.87
		Ave	31.27	38.85	38.27	29.94	31.37	33.94
	ADF (%)	14	15.36	22.03	26.74	22.40	11.86	19.68	1.128	<0.0001	<0.0001	<0.0001
		28	22.56	38.19	31.08	23.01	23.45	27.66
		42	33.31	40.13	32.61	26.31	30.3	31.93
		Ave	22.74	33.45	30.14	23.91	21.87	26.42
	N (%)	14	0.77	2.68	1.23	3.31	1.72	1.94	0.122	<0.0001	<0.0001	<0.0001
		28	2.54	3.18	2.54	2.61	2.66	2.70
		42	3.32	3.3	1.48	2.42	1.73	2.45
		Ave	2.21	3.05	1.75	2.78	2.03	2.36
Variables			Effect	p-value		Equation			R2
Red TS.		(PPA)	lin	<0.0001		y = 0.43786x + 15.71556			0.9579
		(PPA + WS)	quad	<0.0001		y = 0.05602x2 − 3.22071x + 87.02			0.9809
		(PP)	quad	<0.0001		y = −0.043378x2 + 3.48774x − 6.89			0.9967
		(PP + WS)	quad	<0.0001		y = −0.0216x2 + 1.78119x + 18.25			0.9893
		(WS)	lin	<0.0001		y = 0.50274x + 26.41222			0.9937
Red VS		(PPA.)	lin	0.0001		y = 0.33571x + 22.68333			0.9239
		(PPA + WS)	quad	<0.0001		y = 0.06816x2 − 3.93095x + 100.33333			0.9910
		(PP)	quad	<0.0001		y = −0.05175x2 + 3.8881x − 6.6.533333			0.9952
		(PP + WS)	quad	0.0097		y = −0.01327x2 + 1.47095x + 23.29333			0.9909
		(WS)	quad	0.0114		y = −0.01263x2 + 1.3025x + 19.80333			0.9870
Red NDF.		(PPA)	quad	0.0207		y = 0.00902x2 − 0.43464x + 33.86667			0.7581
		(PPA + WS)	quad	0.0002		y = −0.02804x2 + 2.26345x − 6.36333			0.9911
		(PP)	quad	0.0003		y = −0.02953x2 + 2.26107x + 2.55333			0.9861
		(PP + WS)	lin	0.0002		y = 0.32964x + 29.040			0.8848
		(WS)	quad	0.0088		y = −0.01908x2 + 1.49405x + 6.470			0.9544
Red ADF		(PPA)	lin	0.0017		y = 0.13988x + 20.45222			0.7342
		(PPA + WS)	quad	0.0274		y = −0.01208x2 + 1.33512x − 3.99667			0.9849
		(PP)	quad	0.0004		y = −0.03629x2 + 2.67857x − 7.890			0.9792
		(PP + WS)	lin	0.0040		y = 0.20976x + 24.73333			0.7311
		(WS)	lin	<0.0001		y = 0.53417x + 8.25111			0.9562
N		(PPA)	lin	0.0039		y = −0.0304x + 3.61949			0.7446
		(PPA + WS)	quad	<0.0001		y = −0.00478x2 + 0.2681x − 1.09667			0.9797
		(PP)	lin	0.0046		y = 0.02121x + 2.47154			0.7212
		(PP + WS)	quad	0.0002		y = −0.00604x2 + 0.3469x − 2.44			0.9161
		(WS)	quad	0.0018		y = −0.00254x2 + 0.23345x − 2.00			0.9898

Total solids (TS); volatile solids (VS); neutral detergent fiber (NDF); acid detergent fiber (ADF); and nitrogen (N). Bird standard error (EPM). Quadratic effect (quad); linear effect (lin). (Red).

and

Table 4. Biogas production over time from different broiler litter materials: wood shavings (WS); plastic floor (PP); 50% plastic floor + 50% wood shavings (PP + WS); plastic floor with a nanotechnology antimicrobial additive (PPA); and 50% plastic floor with a nanotechnology antimicrobial additive +50% wood shavings (PPA + WS).

Variable		Day of Rearing	Bedding Material					Ave	EPM.	p-Value
			WS	PP	PP + WS	PPA	PPA + WS			Litter	Rearing Time	LxT
Methane (%)		14	72.30	63.53	67.44	54.40	70.04	66.14	0.719	<0.0001	<0.0001	<0.0001
		28	68.24	63.97	69.30	54.33	63.21	63.81
		42	68.48	68.76	69.09	67.32	68.77	68.48
		Ave	69.67	65.42	68.61	59.68	67.34	66.14
Biogas production potential (L/kg TS)		14	150.88	135.49	165.92	85.22	211.75	149.85	14.432	<0.0001	<0.0001	<0.0001
		28	182.25	310.12	281.46	105.57	174.59	210.79
		42	249.14	449.43	315.15	121.62	279.52	282.97
		Ave	194.09	298.34	254.18	104.14	221.95	214.53
Methane production potential (L/kg TS)		14	108.58	85.74	111.50	48.71	147.88	100.25	10.233	<0.0001	<0.0001	<0.0001
		28	123.97	198.07	194.66	57.22	110.04	136.79
		42	170.19	308.69	217.34	81.47	191.83	193.91
		Ave	134.25	197.50	174.50	62.47	149.92	143.65
Biogas production potential (L/kg VS)		14	163.59	162.55	183.25	100.21	229.93	167.91	17.212	<0.0001	<0.0001	<0.0001
		28	201.46	362.45	302.96	124.40	186.47	235.55
		42	283.69	547.76	349.41	144.24	314.27	327.87
		Ave	216.24	357.58	278.54	122.95	243.56	243.77
Methane production potential (L/kg VS)		14	117.62	102.70	123.00	57.03	160.45	112.16	12.160	<0.0001	<0.0001	<0.0001
		28	136.89	231.32	209.40	67.17	117.31	152.42
		42	193.67	376.17	240.84	86.51	215.55	224.55
		Ave	149.39	236.73	191.08	73.57	164.44	163.04
Variable		Effect	p-value	Equation								R2
Methane	(PPA)	quad	<0.0001	y = 0.04095x2 − 1.93881x + 76.51667								0.9759
	(PPA + WS)	quad	<0.0001	y = 0.03161x2 − 1.81536x + 89.26667								0.9717
	(PP)	quad	0.0268	y = 0.01109x2 − 0.43405x + 67.43333								0.8833
	(PP + WS)	lin	0.0403	0.05917x + 66.95667								0.4161
	(WS)	quad	0.0008	y = 0.01098x2 − 0.75131x + 80.67								0.9562
Biogas production potential (L/kg TS)	(PPA)	lin	0.0008	y = 1.29952x + 65.8911								0.8622
	(PPA + WS)	quad	<0.0001	y = 0.36249x2 − 17.87893x + 389.1433								0.9811
	(PP)	quad	0.0328	y = −0.09009x2 + 16.25726x − 76.31333								0.9967
	(PP + WS)	quad	<0.0001	y = −0.20878x2 + 17.02119x − 33.31667								0.9949
	(WS)	quad	0.0315	y = 0.09063x2 − 1.56595x + 153.18								0.9688
Methane production potential (L/kg TS)	(PPA)	lin	0.0002	y = 1.16988x + 28.85333								0.8576
	(PPA + WS)	quad	<0.0001	y = 0.30517x2 − 15.52x + 304.49								0.9843
	(PP)	lin	<0.0001	y = 7,396,214x − 26.29778								0.9943
	(PP + WS)	quad	0.0002	y = −0.15429x2 + 12.42036x − 33.00								0.9917
	(WS)	quad	0.0113	y = 0.07868x2 − 2.20583x + 123.18333								0.9656
Biogas production potential (L/kg TS)	(PPA)	lin	0.0004	y = 1.5725x + 77.9544								0.8948
	(PPA + WS)	quad	<0.0001	y = 0.43689x2 − 21.45357x + 44,368,667								0.9746
	(PP)	lin	<0.0001	y = 13.7575x − 28.59333								0.9951
	(PP + WS)	quad	0.0005	y = −0.18685x2 + 16.39810x − 10.66333								0.9922
	(WS)	quad	<0.0001	y = 0.11315x2 − 2.0475x + 169.11								0.9575
Methane production potential (L/kg VS)	(PPA)	quad	0.0406	y = 0.04898x2 − 1.33283x + 66.03								0.9391
	(PPA + WS)	quad	<0.0001	y = 0.36066x2 − 18.22929x + 344.90667								0.9792
	(PP)	lin	<0.0001	y = 9.7667x − 36.79889								0.9916
	(PP + WS)	quad	0.0012	y = −0.14022x2 + 12.06095x − 18.43333								0.9878
	(WS)	quad	0.0232	y = 0.09569x2 − 2.64274x + 135.80333								0.9532

Total solids (TS); volatile solids (VS); neutral detergent fiber (NDF); acid detergent fiber (ADF); and nitrogen (N). Bird standard error (EPM). Quadratic effect (quad); linear effect (lin).

). The different types of floor affected the reductions in solid constituents in this study. Although the antibiotic in the plastic floor was not quantified, it is believed that it was incorporated into the excreta and that, even in small amounts, compromised the digestion of the substrates in the biodigesters. Antibiotics cause instability in the anaerobic digestion process. However, if excessive doses do not occur, the micro-organisms involved may adapt and recover [26]. When comparing the reductions in solids in the substrates originating from the PPA with the PPA + WS, which also had the antibiotic (half of the area covered by shavings), greater reductions were found in the litter, as the shavings may have diluted the concentration of antibiotics in the substrate composition.

The most significant reduction in the TS and VS (62.36 and 65.47%, respectively, at 42 days) for the substrates generated with the excreta produced in the PP is an already foreseen behavior since only the excreta were collected in this environment. As the birds age, there is also more outstanding waste production and feed remains that could serve as a substrate for micro-organisms. Higher VS reductions represent a more effective degradation of organic matter, which will result in greater biogas production [27]. The reduction in VS is close to that observed by [28], with a 66.27% reduction when carrying out the biodigestion of laying hen manure.

Regarding the NDF and ADF reductions, lower values were observed at 14 days for the different floors, mainly in the WS and the PPA + WS, since at the beginning of the broilers, the presence of wood shavings was greater in proportion to the excreta than in the final period of poultry production, in addition to the complexity of the constitution of the shavings, which limits its degradation. The shavings contain around 30% lignin [29], the cell wall component with the slowest degradation, which hinders the action of hydrolytic enzymes secreted by micro-organisms to break down materials, even hemicellulose and cellulose, which could reduce their efficiency at converting organic matter into biogas [22].

The maximum potential for biogas production (Table 4) at 14 days occurred in the substrates formed with the PPA + WS residues (211.75 and 229.93 L/biogas/kg of TS and VS, respectively). At 28 and 42 days, the behavior of the potential biogas production was different, with the residues from the PP being the ones that produced the most biogas, with the production at 24 days being 310.12 and 449.43 L/kg of TS and SV, respectively. At 42 days, the production reached 362.45 and 547.76 L/kg of ST and SV, respectively. During the entire biodigestion period, the lowest biogas production occurred in the substrates formed with the PPA residues, where its maximum production potential was 121.62 and 144.76 L/biogas/kg of TS and VS, respectively.

The waste from the PPA + WS may have shown the greatest potential for biogas production at 14 days compared to 28 days, a different behavior from the other substrates due to the proportion of broiler-rearing waste in the affluent composition. It is believed that the combination of fibrous material, represented by wood shavings and excreta, may have resulted in a better adjustment of the C:N ratio, thus favoring biogas production.

Possibly due to the presence of wood shavings, the antimicrobials on the floor using nanotechnology affected the PPA + WS in a less impactful way than the PPA alone. When only the PPA excreta was used, the biogas production potentials were reduced, reinforcing the beneficial effect of shavings and excreta for biogas generation.

Miah et al. [30] conducted an anaerobic co-digestion of poultry litter with bovine manure. The authors reported that the inclusion of 75% of chicken litter made of wood shavings together with 25% of bovine manure reached the maximum potential for biogas production (480 L biogas/kg) concerning the isolated digestion of bovine manure (400 L biogas/kg), demonstrating that chicken litter has a good potential for biogas production, and that in co-digestion it can favor production, generating higher values than with waste digested alone. The substrates formed only with the PP excreta at 14 days possibly had a lower biogas production potential than the PPA + WS substrates due to the high concentration of N (Table 1) present in the excreta. Nitrogen in the residue is beneficial for microbial growth during digestion. However, the high concentration of this constituent releases large quantities of ammonia, and the surplus of ammonia inside the biodigester decreases biogas production, interrupts methanogenic activity, and inhibits digestion on account of the excessive production of volatile fatty acids [31].

When substrates from the PP excreta collected at 28 and 42 days were used, the biogas production had an inversion of behavior compared to the substrates formed with the 14-day residues since, in these two periods, the biogas production potentials were higher in the PP residues concerning the PPA+ WS residues. This behavior can be explained by the reduction in the nitrogen content of the residues collected in these two periods (Table 1), possibly associated with N volatilization during poultry rearing. In this way, ammonia generation in the biodigester may have been reduced, allowing for better substrate fermentation conditions. The low potential for biogas production in the substrates formed with PPA excreta during the three collection periods probably has a direct relationship with the antimicrobials on the floor, as previously mentioned, since antimicrobials inhibit microbial activity [32].

The proportions of methane in the biogas were influenced by the bed type, mainly in the material collected at 14 days. This resulted in higher methane production potentials for the substrates prepared with waste collected from the floor with the WS and the PPA + WS, and the lowest concentrations in the PPA antimicrobials in the excreta from the PPA; the high concentration of nitrogen in the material collected at 14 days interfered with the microbial action, as previously discussed. With regards to the co-digestion of fresh poultry excreta and agricultural waste, Abouelenien et al. [10] observed an increase in methane production (502 mL g⁻¹ SV) compared to the isolated digestion of fresh excreta (493 mL g⁻¹ SV), as the accumulation of ammonia can cause a slight inhibition of methane production. However, methanogenic bacteria can adapt to the ammonia concentration and return to methane production.

4. Conclusions

The plastic floor proved to be a viable alternative for the production of broiler chickens, allowing for lower waste coefficients if used alone. In contrast, its use in combination with wood shavings cannot be recommended because it increases waste generation in poultry production. The plastic floor with antimicrobials interfered negatively with the biogas and methane production potentials. However, the residues from the plastic floor were the ones that presented the best energy yield.

The production of broiler chickens on plastic flooring is recommended to reduce the amount of waste generated and promote a greater yield of biogas and methane with the anaerobic digestion of excreta, consequently reducing the impact of chicken production on the environment.