The Effect of Covering Corn Silage with Tomato or Apple Pomace on Fermentation Parameters and Feed Quality

Abstract

The current study assessed the effects of covering corn silage with tomato or apple pomace on fermentability and feed quality. The in vitro gas production test was performed using graded 100 mL syringes. Incubation times were 3, 6, 12, 24, 48, 72, and 96 h. In vitro gas generation characteristics were significantly altered by TP (tomato pomace) and AP (apple pomace), both alone and in conjunction with PE (polyethylene) films, regardless of their presence. As a result of the effects found on NH₃-N concentration, aerobic stability, and yeast activity, TP and AP have the potential to become an eco-friendly alternative to PE films. The gas production from the immediately soluble fraction (a) of corn silage was only affected when the corn silage was covered with a combination of AP and PE compared to the CPE group (p < 0.001). The largest cluster includes correlations of the DOM-TDDM (r = 0.90), DOM-AA (r = 0.88), and Ash-TDDM (r = 0.86) correlations. The most substantial negative correlations were identified between DM-CO₂ (r = −82), DM-Yeast (r = −0.79), and CF-DOM (r = −0.79). Nonetheless, the use of pomace as a silage cover presents an inexpensive alternative to plastic films for silage that does not have the environmental problems associated with persistent micro- and nanoplastics.

1. Introduction

Despite all the problems and limitations, the animal husbandry sector is one of the most important and active economic branches [1]. Humanity has a limited amount of protein materials and needs to figure out ways to provide food, which means that we need to take advantage of the smallest efficiency gains possible from waste materials and convert them into food products [2,3]. In anaerobic conditions, ensiling can be used to store and maintain fodder based on natural lactic acid fermentation. To store forage so that ruminants can have access to it throughout the year as a major feed source with high nutritional value is to ensile it [4]. In ensiling, plastic film covers are used to provide anaerobic conditions, which lead to the formation of waste and, what is even worse, the release of persistent micro- and nanoplastics in the ecosystem, which cause environmental pollution [5,6]. In recent years, microplastics have become a significant environmental concern because of their ability to pollute various ecosystems and negatively impact both living organisms and the environment [7,8]. Furthermore, such plastic particles can enter the food chain, end up on our plates, together with plastic additives, and be adsorbed and absorbed into toxins from the environment, which might also have accumulated through bioaccumulation. In order to minimize the impact of micro- and nanoplastics on ecosystems, it is essential to reduce the use of single-use plastics, improve plastics life cycle management, and promote sustainable substitutes and alternatives such as bioplastics. Among that material class, polyhydroxyalkanoates (PHA) can be mentioned, which are naturally occurring polyesters, that are formed as energy storage compounds by several microorganisms and, when extracted, exhibit thermoplastic properties. PHA are biodegradable in all ecosystems, amongst them agricultural soil and seawater. Yet, the market share of such bioplastics is still very low today, and further alternatives are needed. On the one hand, agro-industrial waste can be used as a cover for silage materials, which reduces the costs of supplying plastic film materials, and on the other hand, by maximizing the use of agricultural waste and reducing plastics waste, environmental emissions can be reduced, particularly when the mismanagement of such materials is avoided. Corn silage is a feed source that incorporates various pulps that are produced by industrial processes of transforming raw materials into new products in the food industry. Additionally, as a source of water-rich roughage, these by-products are beneficial to animal husbandry.

Tomato pulp contains cellulose, protein, fat, minerals, phenolic compounds, and carotenoids, some of which are compounds with biological activity, making it a promising alternative feed source. As a feed source for animals, different pulps derived from food industry by-products have been shown to provide added value for animal husbandry as well as reduce the environmental pollution and damage caused by these products [9,10]. According to NRC [11], tomato pulp contains 19–22% crude protein, 11–13% crude fat, and 7–13% ADL (acid detergent lignin). Silage can be made without tomato pulp, but feed material with a high dry matter content is necessary to prevent nutrient losses that may occur due to excessive water drainage during ensiling as well as to increase the silage fermentation quality [12]. Tomato pomace (TP) can be considered as a feed ingredient to valorize it.

As another byproduct, apple pomace (AP) can be used as an alternative feed source for livestock, too, due to its large quantities that stem from, e.g., apple juice production. In addition, apple pulp can be used as a carbohydrate source in the ensiling of low-carbohydrate feeds. It was concluded by Gadulrab et al. [13] that dry apple pomace (150 g/kg DM, with DM being dry matter or dry mass) was effective in reducing methane emissions, improving nutrient digestibility, and enhancing volatile fatty acid (VFA) concentrations in dairy cows. A study conducted by Bartel et al. [14] found that apple pomace as a feed additive influences lysosomal degradation processes as well as rumen fluid oxidation-reduction potential. Furthermore, using waste to cover the vessel reduces microplastics being released into the environment as would be the case with classic films on one hand, but also reduces production costs and increases waste productivity on the other. Hence, we hypothesized that pomace or tomato covers would affect feed quality and fermentation parameters. Thus, this study investigated how cover crops such as tomato or apple pomace affect feed quality and fermentation parameters.

2. Materials and Methods

2.1. Ethical Standards

There was no animal participation, experimentation, or use of animal tissue in this study. It was therefore not necessary to obtain ethics committee approval.

2.2. Preparation of Silage with Tomato or Apple Pomace

Tomato and apple pomace (TP, AP) were obtained from the Tat factory in the Torbali/Izmir region, Türkiye (38°9′6″ North 27°21′44″ East). Subsequently, they were sealed in plastic bags to preserve moisture and shape and stored in a refrigerator at +4 °C for no more than 48 h before the silage-making process. Corn harvested in the milk stage from a local farm (Pioneer variety 32K6) was cut into theoretical length of 1.5–2.0 cm using a harvesting machine. Then, chopped second crop whole corn was randomly subdivided into 5 experimental treatments: (i) corn silage + cover of polyethylene (C+PE); (ii) corn silage with tomato pomace covering (CTP); (iii) corn silage with tomato pomace covering + cover of polyethylene (CTP+PE); (iv) corn silage with apple pomace covering (CAP); (v) corn silage with apple pomace covering + cover of polyethylene (CAP+PE). Five vessels for each experimental group were prepared. Vessels were cylindrical plastic containers (5 L volume; about 16 cm diameter × 25 cm height). Each vessel was filled with manually pressed samples. C+PE vessels were filled with only corn and covered with polyethylene, CTP vessels were filled with 75% corn and 25% (top covering) tomato pomace, CTP+PE vessels were filled with 75% corn and 25% (top covering) of tomato pomace and covered with polyethylene, CAP vessels were filled with 75% corn and 25% (top covering) of apple pomace, CAP+PE vessels were filled with 75% corn and 25% (top covering) of apple pomace and covered with polyethylene. All the vessels were equipped with a gas release valve on the lid. The silage density was determined as 758.32 ± 12.12 kg/m³ for C+PE, 721.44 ± 14.78 kg/m³ for CTP, 724.31 ± 13.66 kg/m³ for CTP+PE, 729.77 ± 16.21 kg/m³ for CAP, 732.08 ± 13.61 kg/m³ for CAP+PE (values expressed as mean and SD, standard deviation). The vessels were opened after 30 days. The trial was replicated for 5 times, once per week, for a total of 25 vessels for each experimental group (5 vessels × 5 replicates).

2.3. Chemical Analyses

All samples were analyzed in triplicate, considering each of the 5 replicates as the experimental unit and each vessel as the technical replicate. Dry matter (DM) was determined using standard procedures [15] (method 930.15). Ash was determined by standard procedures [15] (method 942.05) using a muffle furnace at 550 °C for 16 h. Fat was determined using the Soxhlet extraction procedure [15] (Method 991.36), and crude protein (CP), and Ammonia-N (NH₃-N) were determined by Kjeldahl N × 6.25 procedure [15] (Method 968.06). Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined with the ANKOM fiber analyzer according to Van Soest et al. [16], and the values were corrected for residual acid-insoluble ash. For NDF determination, sodium sulfite was added to the solution as described in De Bellis et al. [17].

2.4. OMD, ME and NEL Contents

Metabolizable energy (ME) and organic matter digestibility (OMD) of silages were determined by the following equations reported by Menke and Steingass [18]. The net energy lactation (NEL) calculation was made based on the equality reported by Palangi [19]:

OMD [%] = 15.38 + 0.8453 × GP + 0.0595 × CP + 0.0675 × ash

(1)

ME, [MJ/kg DM] = 2.20 + 0.1357 × GP + 0.057 × CP + 0.002859 × EE²

(2)

NEL [MJ/kg DM] = 0.101 × GP + 0.051 × CP + 0.112 × EE

(3)

(GP: 200 mg gas production of 24 h, CP: crude protein (%), and EE: ether extract (%)).

2.5. In Vitro Gas Production

All rumen fluid sampling procedures were carried out immediately after the animals were slaughtered at the abattoir. Rumen fluid was collected from a total of four 15-month-old Charolais steers raised on the same farm. All animals were fed ad libitum with the same feed ratio (84.2% of dry matter, 14.9% of crude protein, 10.1% of crude fiber, 27.2% of NDF, 10.4% of ADF, and 2.2% of ADL). Ruminal content samples, filtered through eight layers of gauze cloth, were gathered in thermos flasks that had been pre-filled with distilled water at 39 °C to prevent thermal shock to the rumen fluid as described by De Bellis et al. [17]. CO₂ was introduced into the headspace to maintain an anaerobic environment, and the samples were transported to the laboratory within 30 min. Following transportation, the upper layer of ruminal contents was removed, and the rest was thoroughly mixed and homogenized under a CO₂ atmosphere for 1 min to eliminate any remaining particles or attached organisms. The mixture of fluid and contents was then filtered through 6 layers of cheesecloth to create the inoculum for the in vitro fermentation process [20].

The artificial rumen fluid consisted of (added in order) 500 mL H₂O, 0.1 mL solution A, 200 mL solution B, 200 mL solution C, 1 mL resazurin (0.1%, w/v) solution D, and 40 mL reduction solution E. This mixture was then kept under CO₂ in a 39 °C water bath and stirred using a magnetic stirrer. Solution A consisted of 13.2 g CuCl2·2H2O, 10.0 g MnCl₂·4H₂O, 1.0 g CoCl₂6·H₂O, 8.0 g FeCl₂*6H₂O and was made up to 100 mL with water. Solution B consisted of 35 g NaHCO₃ and 4 g NH₄HCO₃ added up to 1000 mL with water. Solution C consisted of 5.7 g Na₂HPO₄, 6.2 g KH₂PO₄, 0.6 g MgSO₄*7H₂O added up to 1000 mL with water. The solution D consisted of 0.5 g resazurin added up to 100 mL with water. The solution E is the reduction solution, it consisted of 95 mL H₂O, 4 mL 1 N-NaOH, 625 mg Na₂S*9H₂O. Approximately 0.200 g dry weight of the sample was weighed into calibrated glass syringes (100 mL) as detailed in Palangi and Macit [21]. The total gas volume of silages was recorded at 3, 6, 12, 24, 48, 72, and 96 h and corrected with blank bottles and standard alfalfa hay as proposed by Menke et al. [22]. Gas chromatography (GC, Agilent 6890N) was used in the chemical analysis of the volatile fatty acids. The cumulative gas production of silage samples was fitted to an exponential model, using the Solver function in Excel 2016.

$$Y={a+b(1-e}^{-ct})$$

(4)

In the above equation, Y is the volume of gas produced at time t (ml); a is the volume of gas produced from the immediately soluble fraction of the samples (ml); b is the volume of gas produced from the insoluble fraction of the samples; c is the gas production rate constant (ml/h); and t is the incubation time (h).

2.6. Silage Microbiological Analyses

A 10 g silage sample and 90 mL sterile 0.1% peptone water were transferred into sterile stomacher bags for 2 min of homogenization, followed by serial dilutions for microbiological analyses.

Total Bacteria Count: Plate Count Agar (Oxoid, Hampshire, UK) medium was used to count total mesophilic aerophilic microorganisms. Colonies formed after the plates were incubated at 30 ± 1 °C for 72 h were counted.

Yeast and Mold Counting: Yeast Extract Chloramphenicol Agar, the pH of which was adjusted to 6.8, was used for yeast and mold counting. It was incubated at 25 °C for 3 days for yeasts and 5 days for molds, and the colonies counted after incubation were given as total yeast-mold CFU/g [23], with CFU being colony-forming units.

Lactobacillus/Lactococcus spp. Enumeration: Identification of possible Lactobacilli and Lactococci was performed using MRS (de Man, Rogosa, Sharpe) (Merck, Darmstad, Germany) and M17 agar (Merck, Darmstad, Germany), respectively. Incubation of the Petri dishes was carried out for 48–72 h under anaerobic conditions at 42 °C and 48–72 h under aerobic conditions at 37 °C, respectively. In MRS Agar Petri dishes, the anaerobic environment was provided by the AnaeroGen agent (Oxoid, Hampshire, UK) placed in anaerobic jars (Oxoid, Hampshire, UK) [24,25].

E. coli and Coliform Group Bacteria Counting: Chromocult TBX Agar with pH adjusted to 7.2 was used for E. coli and coliform counting. After incubation at 37 °C for 24–28 h, bright green colonies emerged as E. coli, and purplish colonies were counted as coliform bacteria [26].

Clostridium spp. Count: In the research, 9 mL of Reinforced Clostridial medium prepared in Durham glass tubes was used. In the analysis performed with the EMS technique, serial dilutions are used to seed the tubes, kept in a water bath at 75 °C for 10 min, covered with 2 cm of melted solid paraffin, and incubated for 7 days at 37 °C. A tube in which gas formation is detected at the end of incubation is considered positive. The results were interpreted according to the EMS table [27].

2.7. Determination of Aerobic Stability

One of the best-known tools for evaluating aerobic exposure is the bottle system, which relies on trapping CO₂ gases in a KOH solution, as previously described by Ashbell et al. [28]. In cases where a data logger is not available, this approach, which is used as a reference method, was also used in this study. In the 5-day aerobic stability test, 1.5 L gas-impermeable and corrosion-resistant polyethylene bottles were deployed and CO₂ production was used as an indicator Ashbell et al. [28].

$${CO}_2(\mathrm{g}/\mathrm{k}\mathrm{g})={\displaystyle \frac{0.044\times V_t\times V_{KOH}}{V_a\times W_f\times DM(\mathrm{g}/\mathrm{k}\mathrm{g})}}$$

(5)

In the above equation, $V_t$ is the volume of 1 N HCl required to titrate the sample (mL); $V_{KOH}$ is the total volume of 20% KOH used in the analysis (mL); $V_a$ is the volume of KOH added in the lower part of the bottle (mL); $W_f$ is the fresh weight of silage weighted in (kg).

2.8. Statistical Analysis

All data were expressed as mean ± standard error of mean (SEM). The significance of each experimental group with respect to the control group, traditionally preserved corn silages (CPE), was assessed by means of ANOVA followed by Dunnett’s multiple comparison test. To assess the strength and direction of association between fermentation characteristics, chemical and microbial compositions, and CO₂ production during aerobic stability values, a correlogram was created for visualization in the “corrplot” package as previously described by wei et al. [29]. In addition, the biplot order was shown using the R packages “ggplo” plus “factoextra” and “FactoMineR”, which are based on principal component analysis (PCA), following a modified version of Türkgeldi et al. [30].

3. Results

3.1. Nutritional Composition

The chemical composition of corn silage that uses organic waste as a covering material is presented in Figure 1. A reduction in DM, ash and NDF (p < 0.0001) is evident in corn silages that use a combination of TP and PE (CTP+PE) as cover material, relative to traditionally preserved corn silages (CPE). On the contrary, an increase is observed in EE, CP, CF, ADF (p < 0.0001), and ME (p = 0.0289) in the CTP+PE group. Similarly, corn silages employing AP and PE as covering materials exhibit lower levels of DM (p = 0.0016), ash, CP and ME (p < 0.0001) content compared to CPE. Conversely, the CF and ADF values of the CAP+PE surpass those of the CPE group (p < 0.0001). On the other hand, a reduction in the values of ash (p = 0.0324), EE (p = 0.0203), CF (p = 0.050), NDF, and ME (p < 0.0001) values is discernible in commercial corn silages when TP is used as covering material, relative to silages in the CPE group. Compared to silages in the CPE group, the CTP group had higher levels of DM (p < 0.0001), CP (p = 0.0002), and ADF (p = 0.0011). Similarly, a reduction in ash, NDF, and ME (p < 0.0001) was noted in the group solely employing AP as the covering material, concomitant with an increase in cellulose fiber (CF) and acid detergent fiber (ADF) values (p < 0.0001).

The pH levels in groups utilizing AP either alone (p < 0.0001) or in combination with PE (p < 0.0001) as a covering material were found to be lower than those observed in CPE (Figure 2). There were no noticeable differences between the group using normal corn silage (CPE) and the group using TP as the sole cover material (p > 0.05). However, the combination of TP with PE as a cover material resulted in a lower pH (p = 0.0275). The use of TP and AP, either solely (p < 0.0001) or in combination with PE (p = 0.0179 for CTP+PE and p < 0.0001 for CAP+PE), results in a notable reduction in the content of NH₃-N compared to CPE. Furthermore, it is noteworthy that AP when used as the sole cover material, exhibits the lowest NH₃-N content (1.80 g/kg TN) among all groups (TN = total nitrogen). Furthermore, no significant differences were observed when TP was used as a covering material only in the LA content of corn silage (p > 0.05), whereas the combined application of TP and PE as covering materials resulted in a reduction in the LA (p = 0.0004) content (p = 0.0004) compared to CPE. Conversely, a decrease in the LA content was observed in corn silages where AP was employed either as the sole (CAP, p = 0.0002) covering material or in combination with PE (CAP+PE, p = 0.0006). The AA (acetic acid) content of corn silages exhibited a decrease compared to CPE when TP and AP were used solely or in combination with PE as covering materials (p < 0.0001). In particular, the highest decrease (74.32%) in AA content was observed in the cohort using AP solely as a cover material. Similarly, the utilization of TP and AP, alone or in combination with PE, results in a reduction in BA (butyric acid) content compared to CPE (p < 0.0001). In particular, the highest decrease in BA content is observed in corn silages employing AP as the sole covering material and in silages using TP in combination with PE (87.10%).

3.2. Bacterial and Yeast Count

A comparison with CPE shows that the total bacterial count in corn silages is not affected by covering TP and AP alone or in combination with PE (p > 0.05, Figure 3). However, the enumeration of Lactobacillus spp. in corn silage exhibited an increase when sealed with TP alone (CTP, p = 0.0079) or in combination with PE (CTP+PE, p < 0.0001). On the contrary, the enumeration of Lactococcus spp. decreased in corn silage sealed solely with AP (CAP, p = 0.0150) or a combination of TP and PE (CTP+PE, p = 0.0008). Furthermore, AP in combination with PE was found to inhibit coliform bacteria proliferation in the ensilation process when utilized as a covering material in corn silages (p = 0.0248). Corn silage sealed only with TP showed a noticeable increase in yeast count (CTP, p = 0.0044). However, combining TP and PE substantially reduced yeast proliferation in silages (CTP+PE, p < 0.0001). Furthermore, corn silage sealed with a combination of AP and PE as cover material showed a significant reduction in yeast proliferation (CAP+PE, p < 0.0001).

The digestibility values for corn silages in which TP and AP were used as covering materials are presented in Figure 4. Only in silages where TP was the sole covering material did the TDDM value of corn silages not change (p > 0.05). Compared to CPE silages, the lowest TDDM was observed in the group where AP and PE were used together as cover material (652.4 g/kg vs. 698.7 g/kg). Similarly, the DOM of corn silage exhibited a decrease when TP or AP was used as a sole covering material, or in combination with PE (p < 0.0001).

The absence of yeast formation observed in the CTP+PE group over a 180-day ensiling period led to the expectation of minimal CO₂ gas formation in this group during the aerobic stability test. However, after the 7-day aerobic stability test, the group using TP as the sole cover material (CTP) showed the lowest measured amount of CO₂ (1.35 g/kg of DM, p < 0.0001) as shown in Figure 5.

3.3. In Vitro Gas Production

Figure 6 illustrates the in vitro gas production curve derived from 96 h of gas production values, while

Table 1. Estimated degradability parameters of C+PE (ensiled corn covered with polyethylene), CTP [ensiled corn (75%) covered with tomato pomace (25%)], CTP+PE [ensiled corn (75%) covered with tomato pomace (25%) and covered with polyethylene], CAP [ensiled corn (75%) covered with apple pomace (25%)], CAP+PE [ensiled corn (75%) covered with apple pomace (25%) and covered with polyethylene] samples.

	a	b	c	a + b
CPE	0.95 ± 0.17	68.68 ± 0.30	0.086 ± 0.0004	69.63 ± 0.14
CTP	0.41 ± 0.10	67.89 ± 0.15	0.085 ± 0.0002	68.29 ± 0.15 ****
CTP+PE	1.35 ± 0.25	66.01 ± 0.22 ****	0.081 ± 0.0010 ****	67.36 ± 0.12 ****
CAP	0.38 ± 0.33	65.64 ± 0.27 ****	0.082 ± 0.0011 **	66.02 ± 0.18 ****
CAP+PE	−0.41 ± 0.18 ***	65.67 ± 0.13 ****	0.082 ± 0.0007 ***	65.26 ± 0.11 ****

**: p < 0.01; ***: p < 0.001; ****: p < 0.0001. a = rapidly soluble fraction (%); b = slowly degradable fraction (%); c = degradation rate constant (%/h) of fraction ‘b’.

outlines the estimated parameters associated with this curve. The gas production from the immediately soluble fraction (a) of corn silage was only affected when the corn silage was covered with a combination of AP and PE compared to the CPE group (p < 0.001) whereas gas production of the insoluble fraction (b) of the corn silage decreased when the corn silages were sealed with a combination of TP and PE (CTP+PE, p < 0.0001), as well as in cases where AP alone (CAP, p < 0.0001) or in combination with PE (CAP+PE, p < 0.0001) was employed compared to the CPE group. A consistent reduction in the gas production rate was noted across all groups, with the exception of the GTP group, where TP served as the sole covering material (p < 0.0001 for CTP+PE, p < 0.001 for CAP+PE, and p < 0.01 for CAP).

Pearson’s correlation analysis was used to examine the relationship between the utilization of organic waste (TP and AP) as covering materials and various aspects of corn silage, which includes their chemical and microbiological composition, fermentation properties, digestibility levels, and the amount of CO₂ generated during aerobic exposure, and the resultant data are depicted as a correlogram in Figure 7. Following hierarchical clustering, the analyzed parameters were segregated into three clusters. The largest cluster includes correlations of the DOM-TDDM (r = 0.90), DOM-AA (r = 0.88), and Ash-TDDM (r = 0.86) correlations. In the second cluster, a positive correlation is also evident between CF-EE (r = 0.81) and CF-ADF (r = 071). In the third group, although the NDF-CO₂ correlation is significant, the correlation coefficient is relatively low (r = 0.39). On the contrary, the most substantial negative correlations were identified between DM-CO₂ (r = −82), DM-Yeast (r = −0.79), and CF-DOM (r = −0.79).

Corn silage characteristics are influenced to various degrees by organic waste use as covering materials (Figure 8). According to PCA (principal component analysis), the first two PCs (principal components) account for 58.40% of the total variability. Specifically, PC1 explains 36.64% of the total variability, while PC2 accounts for the remaining 21.76%. Additionally, upon closer examination of this figure, corn silages in which TP and PE were used jointly as covering materials exhibit differences in silage characteristics compared to other groups. Traditionally preserved corn silages (CPE) have been determined to share similarities with corn silages in which only TP (CTP) is used as the covering material but differ from other silage categories. Furthermore, it is evident that corn silages in which AP is used alone (CAP) or in combination with PE (CAP+PE) as a covering material possess similar properties but diverge from other groups.

4. Discussion

Several studies have clearly demonstrated that forage crops, which provide essential nutrients to ruminant animals, can be stored as silage for extended periods of time with minimal to no loss [31,32,33]. These studies also show that the amount of anaerobiosis achieved in the completed vessel and its sustained maintenance throughout the feed-out period is the most important factor affecting feed preservation efficiency during ensiling [34]. Prior research has established that forage crops can be preserved as silage in bags, piles, tower vessels, and horizontal vessels. Out of these, horizontal vessels are the favored method owing to their substantial storage capacity, low construction expenses, and comparatively minimal long-term maintenance demands [35]. Furthermore, it is widely acknowledged that the most commonly used sealing strategy is to cover horizontal vessels with PE films [36]. However, due to the high O₂ permeability of PE films, the quality of the silage in the upper and side layers of horizontal vessels can deteriorate, which then threatens the health of animals fed silage with low nutritional value and compromises feed intake and digestibility [36,37].

However, since the early 1990s, the fact that PE films are non-biodegradable and difficult to recycle has prompted researchers to look for alternative covering materials to utilize in silage production, which plays an important role in the nutrition of ruminants [38,39]. That drive has been intensified by more recent findings about the ubiquitous micro- and nanoplastics pollution that is intrinsically linked to the use of fossil, non-degradable plastics such as polyethylene. Instead of using PE, it was proposed [40] that using biofilms and organic covers, which offer effective protection against air leakage and can be broken down when combined with silage during feeding, can offer a more sustainable and low-cost alternative. In recent years, oxygen barrier (OB) films made of polyamides (PA) and ethylene-vinyl alcohol (EVOH) [40], as well as edible films based on casein and soybean meal [39], have been used for this purpose, and additional potential lies in films made from biobased and biodegradable plastics (bioplastics) such as polyhydroxyalkanoates (PHA), or PBAT (polybutylene-adipate-co-terephtalate). In contrast, research on organic waste as a covering material is scarce. As a result, TP and AP were used as cover materials in this investigation and compared to traditionally preserved corn silage.

It is commonly recognized that feed quality is significantly impacted by sealing techniques to prevent air entry during storage. Regarding the physical properties of the organic waste used, Figure 1 indicates that after 180 days of storage, TP exhibits greater air permeability compared to AP and PE, and hence the elevated DM content observed in the CTP group is closely associated with the prevalent evaporation in this particular group. On the contrary, the use of AP as a cover material did not result in any discernible difference in the dry matter content compared to conventionally preserved corn silage (CPE). According to research using OB films, silage had accelerated aerobic degradation due to an increase in the rate of transmission of water vapor, leading to an increase in porosity in conjunction with moisture loss [41]. These losses are well known to be caused by yeast metabolism, which consumes a large portion of soluble carbohydrates and lactates while producing CO₂, water, ethanol, and energy (heat) [42]. Therefore, it was expected that the group that used TP as cover material (CTP) during the aerobic exposure period would produce more CO₂ than the control group (CPE), considering the increase in the yeast population under normal circumstances. However, the results show that TP inhibits yeast activity and reduces CO₂ production due to its antioxidant properties of lycopene [43] and the presence of phenolic acids, especially caffeic, procatchoic, vanillic, catechin, and gallic acids [44]. Another point is that AP-covered silages (CAP) are not as permeable to air as PE-covered silages (CPE) because they have the same or less DM. However, it should be noted that in silages where TP and AP are used as cover materials, a drop in NDF and an increase in ADF are significant signals of oxidation. The reason behind this is that hemicellulose is typically consumed after non-fiber carbohydrates (such as sugar, starch, or pectin) when there are significant oxidation losses [40].

As is well known, protein degradation is directly related to the DM content and only indirectly to the soluble sugar content, and the lower NH₃-N content in TP (CTP) or AP-covered silage (CAP) compared to PE-covered silage (CPE) suggests less water infiltration in organic waste-covered silage. Denoncourt et al. [39] found similar outcomes when they applied a biodegradable coating material to corn silage. These authors made their films from soy and casein. The presence of less than 10 g/kg of NH₃-N also suggests that corn silage was harvested within the normal range and at similar levels of DM [45]. McDonald et al. [46] observed that mold activity boosted protein breakdown and consequent NH₃-N formation in silage. The lack of visible mold and relatively low NH₃-N readings in our research, on the other hand, might be interpreted as a sign of the absence of aerobic spoilage. However, after 180 days of ensiling, the groups that covered TP alone (CTP) or TP in combination with PE (CTP+PE) showed an increase in CP. This could mean that the plant tissues have started to deteriorate, and the NH₃-N content is getting higher. Similar results were also reported by Supel et al. [47].

Previous research has shown that yeast populations exceeding 5 log cfu/g (on a fresh matter basis) are more vulnerable to aerobic deterioration [35]. The decrease in yeast count in this study might be attributed to the TP characteristics mentioned above, as well as the high number of Lactobacillus spp., which is known to drop pH more quickly. Lactobacillus spp., naturally found in the epiphytic flora of TP and AP, may have contributed to the establishment of a homofermentation pattern in the vessel by increasing its population and influencing the LA:AA ratio [48] (LA = lactic acid, AA = acetic acid). However, if AP is used as a cover material, co-inoculation with heterofermentative bacteria such as Lentilolactobacillus buchneri may be required because homofermentative fermentation is well known to result in loss of DM [49]. Furthermore, it has been observed that silages treated with AP (CAP and CAP+PE) exhibit a lower pH level compared to corn silages coated with PE (CPE). Furthermore, it is possible that the citric and malic acids found in AP also contributed significantly to the reduction of this pH [50]. This could be because AP contains more water-soluble carbohydrates than TP [51] and therefore provides a better environment for lactic acid bacteria to develop.

Research has shown that the kinetic parameters of ruminal degradation, as well as the mineral, carbohydrate, and protein fractions, are essential [52]. By synchronizing carbohydrate and nitrogen breakdown in contemporary ruminant feed formulations, energy and nitrogen losses due to ruminal fermentation can be mitigated. Also, the efficiency of microbial synthesis can be improved [53]. In the current study, both the overall gas production rate and the total gas production rate were considerably lowered by the use of TP and AP as cover materials. This is most likely owing to the low pH of silages containing TP and AP as cover materials, which reduces the action of cellulolytic bacteria in the rumen fluid. These findings are consistent with those obtained in the studies by Pirmohammadi et al. [54] and Dai et al. [50].

PCA analysis clearly showed that corn silages with TP as the cover material and silages with PE as the cover material had similar qualities. As a result, replacing PE film with organic waste provides for less environmental impact. Furthermore, it was determined that DM losses can be prevented by co-inoculating AP with heterofermentative bacteria.

5. Conclusions

In conclusion, the use of TP and AP as alternative covering materials for corn silage has significant impacts on the chemical composition of the silage, as well as its fermentation characteristics, microbial composition, and aerobic stability. Results show that TP and AP significantly alter DM content, nutritional composition, pH levels, microbial population, and in vitro gas generation characteristics whether used alone or in conjunction with PE films. Furthermore, the effects on NH₃-N concentration, aerobic stability, and yeast activity demonstrate the potential of TP and AP as eco-friendly substitutes for PE films. More studies are needed to optimize their use and investigate possible synergies with microbial inoculants.