1. Introduction

Various feed ingredients for livestock can be sourced from plantation yields, food crops, and horticultural products (Munadi et al., 2023), including their byproducts. The potential use of agricultural byproducts such as untreated or corn stover for livestock feed ingredients rests on the fact that they are available all year round, in large quantities, and relatively inexpensive (Brunetti et al., 2022). Unfortunately, the utilization of these feed ingredients has not yet been fully implemented due to the limited number of efficient and low-cost processing technologies (Pomar et al., 2021). When an efficient and low-cost processing technology is available, farmers often lack sufficient knowledge and technical capacity to use it (Vastolo et al., 2024).

Corn stover is one of the most underutilized corn byproducts in developing countries (Kitaw et al., 2024). It contains sufficient nutrients as a feed ingredient, which can be enhanced by implementing commonly available feed processing technologies. For example, the corn stover could be processed into silage using feed fermentation technology with better nutrients and digestibility (Pérez et al., 2020). Another instance is the potential use of the largely discarded Ceara rubber tree leaves as livestock feed ingredients. Currently, untreated Ceara rubber tree leaves are used, in limited amounts, to feed large ruminant livestock owing to their high levels of phosphorus and potassium (Deng et al., 2023; Julianto et al., 2022). Making forage silage that combines corn stover, rubber tree leaves and corn flour as a fermentation starter with the level of each material: 60%, 30% and 10% can increase the value of crude protein from 8.47% to 16.04%) with a fairly ideal fermentation pH of pH 4.47, dry matter content (35.80%) and Fleigh value (97.6) which is quite good (Kurniawan et al., 2019). Based on the results of the study, it is evident that the implementation of fermentation technology on high fiber and low crude protein feed ingredients combined with ingredients containing nitrogen sources such as Ceara rubber tree can improve the nutrient content of silage from agricultural by-products. It also indicates that feed fermentation using Ceara rubber tree leaves could be converted into more digestible feed ingredients for livestock (Galla et al., 2020; Koakoski et al., 2024). The challenge of using Ceara rubber tree leaves as a feed ingredient is the presence of unwanted chemical compounds within the leaves, primarily cyanide and tannin, which are toxic to livestock (Sun et al., 2023). The ensilage process could efficiently reduce the levels of HCN (cyanide) and tannins in Ceara rubber tree leaves to a safe level for livestock to feed on.

In this study, we selected Etawa Crossbred goat to represent the smaller size livestock in determining the fermentation characteristics and in vitro nutrient digestibility of diets based on corn stover (Zea mays L., 1973) and Ceara rubber tree leaves (Manihot glaziovii Mull.Arg., 1874) silages. Etawa Crossbred goat is a popular livestock breed due to its significant contribution to meat and milk supply for human consumption in many countries (Suranindyah et al., 2020), including Indonesia (Arief et al., 2020). The goat breed was selected as the research subject of this study because it exhibits sensitivity to certain types of feed and possesses unique characteristics in its rumen digestion system. Hence, understanding the fermentation characteristics and nutrient digestibility of both silages could fill the knowledge gaps in developing more efficient, environmentally friendly, and sustainable ration formulations for Etawa Crossbred goats and other livestock. Because experiments evaluating feed digestibility in vivo require a large number of experimental animals which of course require high costs and long time, the use of in vitro fermentation methods has been widely developed and is quite effective in research in the field of animal nutrition. Through in vitro fermentation research, it is possible to evaluate the relationship between microorganisms and their host regarding the digestion process of feed consumed by domestic livestock (Wang, 2023). This can be understood because each type of feed has different complexity and structure of nutrient compounds so that it has different patterns and levels of fermentation by microorganisms in the digestive system of ruminants (Palmonari et al., 2021).

Despite being abundantly available, corn stover and Ceara rubber tree leaves are rarely used as livestock feed (Khan et al., 2023; Tresia et al., 2023). In addition, specific literature addressing the fermentation characteristics and nutrient digestibility of diets based on corn and Ceara rubber tree leaves silage are few and far between. Therefore, this research aimed to determine the fermentation characteristics of and nutrient availability in corn silage and Ceara rubber tree leaves silages via in vitro method in the rumen fluid of Etawa Crossbred goats.

The findings from this research could potentially contribute to addressing food security challenges and foster the rapid utilization of locally sourced feed ingredients, particularly from agricultural byproducts, to support the livestock industry. The findings are also expected to guide farmers in designing optimal rations for Etawa Crossbred goats based on these two silage ingredients. Similarly, the results of the study, where through the intervention process of fermentation technology treatment makes corn cobs more potential to be used in formulating rations in the ruminant industry (cattle, goats and sheep) either as basal rations or as concentrates. Through the process of physical treatment (grinding) and fermentation combined with high protein rubber tree leaves, corn cob waste with abundant biomass is easier to store, preferred and consumed by livestock. In addition, through the fermentation process also causes better palatability of the by-product feed ingredients, the nutrient content can be improved because the crude fiber decreases and the crude protein content can be increased (Irwansyah and Junaedi, 2019). If this can be produced in this study, the processed product of fermented corn cobs combined with Ceara rubber tree leaves is very promising in providing cheap, highly nutritious feed in the ruminant fattening industry.

2. Material and Methods

2.1. Research materials

The fodder ingredients used for silage making were corn straw and ground white meat, Ceara rubber tree leaves. Concentrate feed was formulated using corn stover, rice bran, pollard, coconut meal, tapioca, molasses, and urea. Materials used for in vitro measurement of feed digestibility according to Tilley and Terry (1963) consists of: rumen fluid as a source of microbial inoculum from adult male PE goats taken using vacuum pumps and stomach tubes, saturated HgCl₂ solution, saturated NaCO₃ solution, 0.005 N H₂SO₄ solution, boric acid indicator, 0.5 N HCl solution, 15% H₂SO₄ solution, 0.5N NaOH solution, PP indicator solution (Phenol Phtaline 0.1%), Whatman No.4 paper, distilled water and McDoughall solution.

The equipment used consists of: scales (analytical scales, feed scales and livestock scales), stomach tube, vacuum pump, hot plate stirrer, shaker water bath, digital pH meter (Hanna type Hi 98107), eppendorf tubes, 60 ^oC oven and 105 ^oC 105°C oven (Memmert 40050-IP20), furnace (Memmert 100-800), conway dish, burette, syringe, centrifuge, hot water flask, cool box, fermenter tube, erlenmeyer flask, oven, tweezers, rubber cap, micro volumetric pipette size 0. 1 ml, bulp, measuring cups of various sizes, horn spatula, glass and metal stirrers, CO₂ gas cylinder, schott bottle, freezer, VFA distillation apparatus, crucible cup, rubber tube, measuring glass, Erlenmeyer flask, desiccator, glass beaker, spatula, gauze cloth and muffle furnace (Furnace PA214).

2.2. Research procedure

1. Silage Feed Preparation

The making of silage based on corn straw and Ceara rubber tree leaves was carried out using an optimized method from Campbell (2014). Corn straw and Ceara rubber tree leaves are chopped to a size of ± 2-3 cm and weathered for ± 24 hours so that the moisture content is up to 60% (Umiyasih and Wina, 2008). Corn straw and Ceara rubber tree leaves are mixed with ground corn as a starter according to the treatment until homogeneous, The material that has been mixed is put into a jar (silo) and compacted, the silo is closed and tightened with duct tape to make it airtight (anaerobic) and stored in a storage room that is protected from direct sunlight for 21 days. After 21 days, the jar (silo) was opened and the product silage mixture of corn straw and Ceara rubber tree leaves was removed and weighed and observed the physical quality of silage ranging from pH, odor/aroma, color and texture. Furthermore, the samples were aerated and dried in a 60⁰C oven and could be mixed with the research concentrate feed.

1. Preparation of McDougall's Solution

Making 6 liters of McDougall solution is done by putting 5 liters of distilled water into a 6 liter flask. Into the flask containing distilled water, the ingredients of the McDougall solution consisting of: NaHCO₃ (58.8 g), Na₂HPO₄.7H₂O (42 g), KCl (3.42 g), NaCl (2.82 g), MgSO₄.7H₂O (0.72 g) and CaCl₂ (0.24 g). The addition of CaCl₂ was done last after the other ingredients were dissolved completely. The neck of the flask was washed with distilled water until the water level in the flask reached the mark. The solution mixture was shaken slowly using a stirrer with CO₂ gas until the pH of the solution reached 6.8. During the stirring process the pH of the solution is always checked to keep it at a normal pH. The solution that has been made is warmed until the temperature reaches 39°C. The flask containing the McDougall solution was kept shaken (stirred) while being supplied with CO₂ gas to maintain the pH of the solution to remain constant at pH 6.8.

1. Preparation of 0.2% Pepsin Solution.

The preparation of 0.2% pepsin solution was carried out by weighing 2 g of pepsin with a pepsin: distilled water ratio of 1:1000. Pepsin was dissolved in 850 ml of ion-free water, 17.8 ml of concentrated HCl was put into a measuring flask and distilled water was added until the surface of the solution in the flask reached the mark.

1. Preparation of Borate Indicator Solution

   • Preparation of Solution A. Boric acid (H₃BO₃ crystal) was weighed as much as 4 g, dissolved in 70 ml of distilled water and then heated on a water bath until boric acid and water were completely dissolved. The solution was cooled and put into a 100 ml measuring flask.

   • Preparation of Solution B. 66 mg of bromine cresol green (BCG) and 33 mg of methyl red (MR) were weighed and put into a 100 ml volumetric flask. Into the flask containing BCG and MR, 95% alcohol was added little by little until both ingredients were completely dissolved until the solution in the flask reached the mark. The indicator borate solution was prepared by putting 20 ml of solution B into a volumetric flask containing the cooled solution A. Sterile distilled water was added until the solution in the flask reached the mark.

2. Rumen fluid collection

Rumen fluid was collected as a source of microbial inoculums from PE goats kept in individual pens and fed the research ration. The rumen fluid of PE goats was collected after 4 hours of feeding in the morning, through the mouth using a stomach tube connected to a vacuum pump. The collected rumen fluid was put into a warm thermos (39°C) and immediately taken to the laboratory to be used as a source of microbial inoculums or for measuring microbial populations and fermentation characteristics. The rumen fluid collection procedure was approved by Ethical Approval from Animal Care and Use Committee (AUAC) Bogor Agricultural University, No. 08-2015 IPB.

1. In vitro Fermentation

In vitro fermentation was carried out according to the method of Tilley and Terry (1963). Clean fermenter tubes of 100 ml size were prepared with the number as needed to measure the variables of fermentation characteristics consisting of pH levels, N-NH3 concentration, total VFA production, dry matter digestibility and organic matter digestibility and crude protein digestibility. Shaker bath containing water was turned on until the water temperature reached 39^oC. 500 mg of treatment feed sample was put into each fermenter tube and 40 ml of McDougall solution was added. Furthermore, each fermenter tube was added 10 ml of PE goat rumen fluid while stirring slowly (30 seconds) until the feed, McDougall solution and rumen fluid were homogeneously mixed. Each fermenter tube was supplied with CO₂ gas for 30 seconds to achieve the incubation pH (pH 6.5-6.9) and then incubated on a shaker bath at 39⁰C for 4 hours. The fermenter tubes were removed from the shaker bath and centrifuged to obtain the supernatant required for the measurement of fermentation characteristics (pH, N-NH3, total VFA production). Fermenter tubes for measurement of dry matter digestibility (DMD) and organic matter digestibility (OMD) were incubated in a shaker bath for 48 hours at 39 ⁰C.

2.3. Ration treatment and research design

1. Ration Treatment

The nutrient content of the treatment diets are presented in Table 1, Table 2 and Table 3.

Ceara rubber tree leaves, and 7.5% ground corn) + 40% concentrate), T3 (ration containing 60% forage (corn stover silage made with 70% corn stover + 25% Ceara rubber tree leaves + 5% ground corn) + 40% concentrate), T4 (ration containing 60% forage (corn stover silage made with 75% corn stover + 22.5% Ceara rubber tree leaves + 2.5% ground corn) + 40% concentrate).

1. Research Design

This research designed using of Randomized Complete Block Design (Ibrahim and Abdullahi, 2023) with four treatments formed based on the periods of rumen fluid collection from Etawa Crossbred goats. Each treatment was repeated four times. The ration treatments (T) applied in this research include were:

• T1: Silage with a composition of 60% corn stover, 30% Ceara rubber tree leaves, and 10% ground corn, supplemented with 40% concentrate).

• T2: Silage with a composition of 65% corn stover, 27.5% Ceara rubber tree leaves, and 7.5% ground corn, supplemented with 40% concentrate,

• T3: Silage with a composition of 70% corn stover, 25% Ceara rubber tree leaves, and 5% ground corn, supplemented with 40% concentrate,

• T4: Silage with a composition of 75% corn stover, 22.5% Ceara rubber tree leaves, and 2.5% ground corn, supplemented with 40% concentrate.

2.4. Data analysis

All data from the observed parameters were statistically analyzed using analysis of variance (ANOVA) of randomized block design using SPSS (Statistical Package for the Social Sciences) (IBM®SPSS® version 21.0, 2012 Armonk, New York, USA). The Normal distribution of the data was tested by the Shapiro Wilk normality test. A P>0.05 was considered indicative of a normal distribution. Data area reported as mean ± standard deviation or median and range (minimum-maximum values) depending on normal or normal distribution, respectively. The mean value data of the replicates were measured based on the periodization of PE goat rumen fluid collection as a source of microbial inoculums (Ibrahim and Abdullahi, 2023). Normally distributed variables were analyzed by analysis of variance (ANOVA) of randomized block design using SPSS (Statistical Package for the Social Sciences) (IBM®SPSS® version 21.0, 2012 Armonk, New York, USA). If significant differences were found, a subsequent post hoc test was done using the Duncan Multiple Range Test (DMRT) to determine the difference between the treatments. The significance level was set at P<0.05.

3. Results

3.1. Characteristics of in vitro fermentation

The in vitro fermentation characteristics of the rations based on corn stover silage (Zea mays) and Ceara rubber tree leaves (Manihot glaziovii) in Etawa crossbred goats are presented in Table 4.

3.1.1. Acidity (pH)

The average rumen pH (incubation) exposed to rations based on corn stover silage (Zea mays) and Ceara rubber tree leaves (Manihot glaziovii) in Etawa crossbred goats is presented in Figure 1.

The analysis of variance indicated that the rations based on corn stover silage and Ceara rubber tree leaves did not significantly affect rumen pH values (P>0.05). The best fermentation pH was achieved by P1 treatment followed by P3 and P4 treatments. Nevertheless, the pH values of all treatments were within the pH ideal values to support an optimal process of in vitro fermentation (McDonald et al., 2010).

3.1.2. N-Ammonia (N-NH₃) concentration

We found through analysis of variance that the rations based on corn stover silage and Ceara rubber tree leaves significantly affect (P<0.05) the concentration of N-Ammonia (N-NH₃). The concentration of N-Ammonia (N-NH₃) resulting from the treatment involving the addition of fermented Ceara rubber tree leaves using rumen fluid from Etawa Crossbred goats is presented in Figure 2.

We found through the Duncan test showed that the highest average value of N-NH₃ during fermentation was measured in T3 treatment and followed by T4 treatment. The average N-NH₃ value of T3 treatment was significantly different (P<0.05) compared to that of T1 but not significantly different from that of the T2 and T4 treatments. Similar to pH values, the values of N-NH₃ in all treatments were within the ideal values (14-21 mM) (McDonald et al., 2010) to provide sufficient required by fermentative organisms to grow during the in vitro incubation.

3.1.3. Production of total volatile fatty acid

The production of total volatile fatty acids (VFA) resulting from the treatment with the addition of fermented Ceara rubber tree leaves using rumen fluid from Etawa crossbred goats is presented in Figure 3.

Based on the normality test using Saphro-Wilk, the total VFA production data has a significance value greater than the 5% alpha significance level (P<0.05) so that the total VFA production data is concluded to be normally distributed. Meanwhile, the results of the homogeneity test show that based on the mean value, the variance is relatively homogeneous, was Sig. 0.431 (P>0.05), then it is concluded that the total VFA production data has a relatively homogeneous variance.

We found through analysis of variance that the rations based on corn stover silage and Ceara rubber tree leaves did not have a significant effect (P>0.05) on the total VFA production. The averages of total VFA production obtained in this study showed the concentration of total VFA in T2 treatment was quantitatively higher compared to the other treatments, where T1 treatment showed the lowest value while T3 and T4 were relatively the same. The total VFA production dynamics in all treatments varied, which did not correspond to a higher or lower amount of used corn stover, Ceara rubber tree leaves and ground corn. Nevertheless, the values of total VFA produced in all treatments were considered within the standard value to provide sufficient carbon compound (Sun et al., 2021) in the fermentation process, indicated by a stable microorganism population and normal nutrient digestibility.

3.2. Nutrient digestibility

Dry and organic matter digestibility provides insights into how efficiently animals can digest and utilize nutrients from the consumed diet. The higher the values of DMD and OMD, the better animals can utilize the available nutrients in the diet for growth, production, and other bodily functions. Factors such as the type of food, composition of the diet, and the health condition of the animals can influence the values of DMD and OMD. The nutrient digestibility parameters, including DMD and OMD of rations based on corn (Zea mays) stover silage and rubber leaves (Manihot glaziovii) in Etawa crossbred goats, are presented in Table 5.

3.2.1. Dry matter digestibility

Based on the normality test using Saphro-Wilk, DMD data has a significance value greater than the 5% alpha significance level (P<0.05) so that the DMD data is concluded to be normally distributed. Similarly, the homogeneity test results show a relatively homogeneous mean with Sig. 0.431 (P>0.05), then it is concluded that the DMD data has a relatively homogeneous variance.

We found through analysis of variance that the use of corn stover and Ceara rubber tree leaves-based rations did not have a significant effect (P>0.05) on the percentage of DMD of the ration.

The DMD averages obtained in this study show relatively varied values among the treatments. Ration T2 containing 60% forage (corn stover silage made with 65% corn stover, 27.5% Ceara rubber tree leaves, and 7.5% ground corn) had the highest DMD percentage compared to the other treatments (T2, T3 and T4) (Figure 4).

3.2.2. Organic matter digestibility

Based on the normality test using Saphro-Wilk, OMD data has a significance value greater than the 5% alpha significance level (P<0.05) so that the OMD data is concluded to be normally distributed. Similarly, the homogeneity test results show a relatively homogeneous mean with Sig. 0.431 (P>0.05), then it is concluded that the OMD data has a relatively homogeneous variance.

We found through analysis of variance that the rations based on corn stover silage and Ceara rubber tree leaves had no significant effect (P>0.05) on the percentage of OMD of the ration. The histogram of the average values of organic matter digestibility for rations based on corn stover (Zea mays) silage and Ceara rubber tree leaves (Manihot glaziovii) is presented in Figure 5.

The average values of organic matter digestibility in all treatments were measured >97%, indicating a very high and efficient nutrient digestibility. T1 treatment produced the best organic matter digestibility, followed by T2, T3 and T4.

4. Discussion

The parameters of fermentation characteristics (pH, N-NH₃, and total and individual VFA) are the primary indicators to explain good nutrient digestibility in vitro and in vivo (McDonald et al., 2010). These parameters indicate that anaerobic bacteria responsible for the rumen fermentation process can grow and facilitate the fermentation process. The typical pH values in the rumen range between 6 to 7, which is ideal for supporting the rumen's microbial growth and fermentation processes (McDonald et al., 2010). The achievement of normal pH levels in the in vitro process is an important indication that the fermentation process carried out is successful in reducing of cyanogenic glycocides content and other anti-nutrients such as anti-trypsin, oxalate, phytic acid and tannin compounds in Ceara rubber tree leaves. Cyanogenic glycocides are one of the secondary components that can affect the rumen ecosystem because they can inhibit the fermentation activity of rumen microbes on feed substrates to produce VFA and N-NH3 (Suharti et al., 2021) during the in vitro process. The low VFA production and N-NH₃ concentration produced in the fermentation process will result in negatively impacts the overall condition of the rumen ecosystem and digestive performance and feed utilizing. Obviously, if this is not addressed, it reduces the utilization or crop products as feed ingredients and the productivity performance of ruminants (Makkar, 1993). The effectiveness of the ration fermentation process that can reduce the levels of anti-nutrient cyanogenic glycocides content. It is that supports the creation of normal pH values during the in vitro process has a positive impact on the optimization of the rat enzymes produced by the rumen microbial community function properly in breaking down the nutrient compounds in feed. Therefore, maintaining rumen pH is always crucial to support the survival and stabilization of microbial growth. The fiber content in the feed plays a role in recycling buffers in saliva due to the extended time spent on feeding and rumination processes. A rumen pH below 6 can affect microbes' digestion of feed materials due to a decrease in microbial population (Perez et al., 2024). The high diversity of data obtained on the pH meters and total VFA production obtained in the study may be due to several factors including; (i) the number of replicates (rumen fluid collection period) is not much (only 4 replicates). So that in the next research, more replicates are needed to minimize the diversity of data in this study. (ii) Consistency in following the procedure for implementing the in vitro fermentation process which is not yet optimal according to the method by Tilley and Terry (1963) such as the pH calibration process, equipment validity and accuracy in measuring total VFA production and the accuracy of conducting other studies. (iii) Equipment specifications for measuring total VFA production are still conventional. Therefore, in future research, it is necessary to procure equipment for measuring total VFA and partial VFA production with better accuracy.

We also found in the study, the growth and activity of rumen microorganisms also have to be supported by a sufficient amount of nitrogen in the form of N-NH₃ during the in vitro incubation. The ammonia concentration in the rumen represents a balance between the amount of nitrogen produced, utilized by microorganisms, and absorbed by the rumen. A normal range of N-Amonia concentration to ensure a working rumen ecosystem is between 6 to 21 mM of rumen volume (McDonald et al., 2010). In Figure 2, all the N-NH₃ concentrations were within the optimum values, with the best value recorded in T3 treatment (20.35 mM). The high concentration of N-NH₃ in treatment T3 (20.35 mM) could be attributed to the relatively high content of nitrogen precursors in the feed ration. This high nitrogen availability supports the robust growth of rumen microbes, enabling them to degrade the feed effectively. The slow decline in pH facilitates the growth of Clostridium or enterobacteria, thereby enhancing protein degradation. The degradation of protein results in deamination in the rumen, producing NH₃ and CO₂ (Holik et al., 2019)

Based on the results of the study, the addition of 30% Ceara rubber tree leaves in T1 treatment minimally affects the concentration of N-NH₃ (16.80 mM). The lower concentration of N-NH₃ in T1 treatment may be associated with its higher pH value (7.17) compared to the other treatments. A combination of low N-NH₃ and high pH values will decrease the microbial population responsible for the digestion of feed dry matter (Figure 4). The relatively non-optimal values of N-NH₃ and pH in T1 treatment could be caused by the high concentration of cyanide acid (HCN) due to the inclusion of Ceara rubber tree leaves (30%) in the feed. A high concentration of HCN in feed without being treated with the supplementation of sulfur minerals will inhibit the growth of bacteria by limiting its cytochrome respiratory chain and the electron transport chain (Sombuddee et al., 2022). This process will eventually lead to suppressed nutrient digestibility in livestock (McMahon et al., 1995). In addition, the higher pH value in T1 treatment might have reduced the population of rumen microbes responsible for protein degradation, resulting in a decrease in N-NH₃ concentration during fermentation.

The presence of anti-nutritional compounds in the feed, such as tannins, could also contribute to the difficulty of the rumen microbes in digesting protein in treatment T1. Tannins can protect proteins from rumen microbial degradation due to their ability to bind with proteins, forming complex compounds that reduce protein degradation (Besharati et al., 2022) and subsequently decreasing N-NH₃ concentration. The inhibited growth of microorganisms can change the rumen's microbial population, affecting the rumen microbes' ability to degrade nutrients in the feed. These changes in microbial capability are reflected in the metabolite products, such as the production of N-NH₃ and VFA (Tanuwiria and Hidayat, 2019).

The total VFA production values in this study are within the range considered normal, which ranges between 70 mM and 150 mM (McDonald et al., 2010). The treatment involving a combination of forage and concentrate with the addition of calcium soap of soybean oil resulted in a total VFA production ranging from 49.08 mM to 142.72 mM (Bain et al., 2017). VFA is a product of bacterial activity during fermentation in the rumen. Therefore, increased bacterial population and activity will lead to a higher production of VFA (Bain et al., 2018). A high total VFA production value indicates an adequate energy supply for livestock, as the total VFA is the end product of carbohydrate fermentation and serves as the primary energy source for ruminants from the rumen. A decrease or increase in total VFA production can indicate the ease or difficulty of nutrients in the feed, especially carbohydrates and proteins, to be broken down by rumen microbes, thus serving as a measure of feed fermentability (Wole et al., 2018).

Another factor that can influence the high or low production of total VFA is the level of OMD in the feed ration. Organic matter comprises crude fiber, crude protein, crude fat, and nitrogen-free extract (Bain et al., 2018). The higher the digestibility of organic matter in the feed ration, the greater the total VFA production, and vice versa. Rations for ruminant livestock typically contain structural carbohydrates in the form of roughage (cellulose and hemicellulose) (Izzatullah et al., 2018). In addition, they also contain simple carbohydrates that are easily fermented, such as sugars and starch, which will undergo fermentation to produce Volatile Fatty Acids (VFA), CH, and CO₂.

The lack of significant differences (P>0.05) in the DMD percentages might stems from the relatively equal influence of corn stover, Ceara rubber tree leaves, and concentrate when mixed. Despite the indifferences, the results show that values of dry matter digestibility in all treatments (56.51% to 59.91%) were all within the normal range. Feed digestibility is considered high or low when reaching between 65% to 70% or below 50% (Fredriksz and Joris, 2020), respectively. A wider range of values was observed in a study by Yanuarianto et al. (2020), where corn stover silage mixed with Leucaena leaves and molasses resulted in dry matter digestibility ranging from 47.39% to 61.67%. A relatively good average value of dry matter digestibility in this study likely relates to the positive effects of the silage process which successfully reduce the amount of crude fiber and HCN concentration in the feed. Tanuwiria and Hidayat (2019) suggested that silage process could effectively reduce crude fiber and HCN concentration in feed ingredients containing HCN such as cassava, sorghum cultivar, maize, alfalfa and para tubber seeds. Beside the silage process, rumen microbial system has also evolved to convert cyanide to a less hazardous thiocyanate or directly break down cyanide to-cyanoalanine (intermediate), formic acid, and ammonia. In the rumen, biological and chemical hydrolysis of the glycoside (cyanogenesis) swiftly converted HCN into thiocyanate, resulting in detoxification (Fredriksz and Joris, 2020).

The high percentage of DMD in T2 treatment could be attributed to the fact that the feed treatment had balanced protein and energy contents to support the growth and activity of the fermentative bacteria during the in vitro incubation. In contrast, the lower percentages of DMD in other treatments were due to suppressed microbial growth. This lower bacterial growth resulting from lower energy content in the feed was due to the high content of roughage (lignin and cellulose) (Khan et al., 2023). Despite these variations, the DMD values in all treatments still support the growth of rumen microbes and the growth performance of the livestock.

The high percentage of DMD observed in T2 is presumed to be caused by the relatively higher total VFA concentration compared to the other three treatments, specifically at 182.10 mM. The total VFA concentration is related to the chemical composition of the feed, such as fiber and crude protein, pH, and N-NH_3, which can influence the growth of rumen microbes (Shilvia et al., 2023). Optimal microbial growth will significantly increase fermentative activity in breaking down nutrients in in vitro incubation.

As described previously, the presence of HCN in Ceara rubber tree leaves is suspected to improve the digestibility of the ration (Kyawt et al., 2019). The process of reducing cyanide residues or increasing cyanide removal efficiency during acidic fermentation conditions releases cyanogenic glycosides (HCN emission). Within this ensiling process, microbial activity can utilize cyanogenic glycosides as a source of C and N, improving nutrient digestibility (Prachumchai et al., 2021). High digestibility percentages in this study reflect the substantial contribution of specific nutrients to livestock. In contrast, feed with low digestibility indicates a lower capability to supply nutrients for basic livestock sustenance and production purposes. The high organic matter content aligns with the high percentage of OMD in the ration, especially protein. The crude protein content positively influences the value of OMD because protein is one of the nutrients easily degraded by rumen microbes (Makmur et al., 2022). The average values of ODM reaching ≥ 97% are an indication that fermentation characteristics achieved by this study are considered optimal (pH 6.87 to pH 7.17; N-NH₃ 16.80 mM to 20.35mM; and total VFA 101.35 mM to 182.10 mM). These optimal fermentation characteristics imply that the rumen microorganism grew well and could optimally break down the feed nutrient microorganism. Although the in vitro OMD averages were relatively similar between treatments, our data indicate that the fermentation treatment on the combination of maize straw and Ceara rubber tree leaves can produce high effective nutrient digestibility of DMD and OMD, which is quite potential to improve the efficiency of using agricultural by-products in an effort to improve the production performance of Etawa Crossbreed goats both as meat producers and milk production.

5. Conclusions

Based on these primary findings, this study concludes that different combinations of silage based on corn stover and Ceara rubber tree leaves create a normal in vitro fermentation ecosystem to support microbial growth and activity in breaking down feed nutrients effectively. Feed containing 60% forage silage with 65% corn stover, 27.5% Ceara rubber tree leaves, and 7.5% ground corn + 40% concentrate is the best ingredient combination to achieve an optimal fermentation ecosystem in degrading feed nutrients in vitro. The combination of ration feed ingredients that can create a normal fermentation ecosystem with relatively good nutrient digestibility in this study is an important indicator for applying the cultivation of PE goats as milk-producing livestock.

Acknowledgements

We want to extend our gratitude to the Rector of Halu Oleo University and the students for their valuable contributions to the successful execution of this research. Additionally, the author expresses appreciation to the administrators of the Laboratorium Produksi and Satwa Harapan, Laboratorium Analisis Pakan Ternak, at the Faculty of Animal Science, Halu Oleo University.

References

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