Cashew nutshell pellets as an agro-industrial by-product for ruminant feeding: an in vitro evaluation of digestibility, fermentationn parameters and gas production

Mardiansyah¹, Mukh. Arifin, Agung Purnomoadi, Endang Purbowati and Vita Restitrisnani

Department of Animal Science, Faculty of Animal and Agricultural Sciences, Diponegoro University, Semarang, Central Java, Indonesia (50275)
mardiansyahakhtar@gmail.com
¹ Department of Animal Science, Faculty of Animal Science, University of Mataram, Mataram, West Nusa Tenggara, Indonesia (83115)

Abstract

Cashew nutshell is an abundant agro-industrial by-product in Indonesia with potential application as an alternative feed resource for ruminants. This study aimed to evaluate the effects of graded inclusion levels of cashew nutshell pellets on in vitro digestibility, rumen fermentation characteristics and gas production. Five complete pelleted diets were formulated containing cashew nutshell at 0, 5, 10, 15 and 20% of dry matter and incubated in vitro using rumen fluid collected from adult sheep. The experiment was arranged in a completely randomized design with four replicates per treatment. Parameters measured included total gas production, in vitro dry matter digestibility (IVDMD), in vitro organic matter digestibility (IVOMD), ruminal pH, ammonia (NH₃), total volatile fatty acids (VFA) and methane (CH₄) production. Increasing levels of cashew nutshell significantly increased gas production, ruminal pH and total VFA concentration, while NH₃ and CH₄ production decreased (p< 0.05). However, IVDMD and IVOMD declined at higher inclusion levels, particularly at 15 and 20%, indicating a negative effect on nutrient digestibility when excessive amounts were used. These results suggest that cashew nutshell pellets can modulate rumen fermentation and reduce methane production, although high inclusion levels may impair digestibility. In conclusion, cashew nutshell pellets show promise as a sustainable agro-industrial by-product feed for ruminants, with moderate inclusion levels offering the most favorable balance between fermentation efficiency and nutrient utilization.

Keywords: cashew nutshell, digestibility, gas production, methane, rumen fermentation

Introduction

The Indonesian agri-food industry generates substantial amounts of by-products with considerable potential for use as alternative ruminant feed resources. One such by-product is cashew nutshell (Anacardium occidentale), derived from cashew nut processing activities. Agricultural and agro-industrial residues have increasingly attracted attention as feed ingredients due to their availability, lower cost and potential contribution to sustainable livestock production systems (Purbowati et al 2025). Cashew nutshells are characterized by high fiber content and the presence of bioactive compounds, particularly anacardic acid, which have been reported to influence ruminal fermentation patterns and mitigate methane emissions (Kang et al 2018; Oh et al 2017).

Previous studies have demonstrated that the utilization of agro-industrial by-products in pelleted ruminant diets can improve feed handling characteristics and support acceptable levels of nutrient digestibility and ruminal fermentation. (Ali et al 2023) reported that pelleted feeds formulated from oil palm fronds and Indigofera zollingeriana silage produced favorable in vitro digestibility and gas production profiles, highlighting the potential of pelleting technology to enhance the utilization of fibrous feed resources. Similar findings were reported by Wulandari et al (2020), who observed that substitution of conventional feed ingredients with protected mixture feedstuffs altered ruminal fermentation characteristics without adversely affecting digestibility.

Studies specifically examining cashew nutshell supplementation have indicated its potential to improve organic matter digestibility and reduce methane production during rumen fermentation (Compton et al 2023; Goetz et al 2023). Additional evidence suggests that inclusion of cashew nutshell in ruminant diets can modulate rumen microbial populations and improve feed utilization efficiency (Compton et al 2023; Jayanegara et al 2015; Mota and Silva, 2011; Narabe et al 2021) Changes in methanogenic microbial communities associated with cashew nutshell supplementation have also been linked to reductions in methane emissions (Koike et al 2020)

Despite these promising findings, comprehensive evaluation of graded inclusion levels of cashew nutshell in pelleted ruminant diets remains limited, particularly with respect to dry matter digestibility, organic matter digestibility, ruminal fermentation characteristics and gas production. In vitro techniques provide a controlled approach to assess fermentation kinetics, gas production, ruminal pH, ammonia concentration and volatile fatty acid production, which are critical indicators of rumen fermentation efficiency and nutrient utilization.

Materials and methods

Pellet preparation was conducted at the Mini Feedmill of the Nutrition Science and Ruminant Laboratory, Faculty of Animal and Agricultural Sciences (FPP), Diponegoro University. In vitro digestibility, ruminal fermentation characteristics and gas production analyses were performed at the Nutrition Science and Ruminant Laboratory, Faculty of Animal and Agricultural Sciences, Diponegoro University. The experimental diets were formulated as complete pelleted feeds containing 70% concentrate and 30% dried forage on a dry matter basis. Cashew nutshell meal was incorporated into the pelleted diets at different inclusion levels (0, 5, 10, 15 and 20%) to replace the complete feed according to the respective treatments. The experiment was arranged in a completely randomized design with five dietary treatments and four replicates per treatment. Each formulated diet was used as a substrate for in vitro rumen fermentation evaluation.

Cashew nutshell management

Anacardium occidentale (cashew) plant, which thrives in tropical regions particularly in Indonesia has two main parts that are commonly utilized: the fruit and its shell (Figure 1). Both parts play an important role in the processing and utilization of the plant.

In this study, cashew nutshell was collected as an agro-industrial by-product from a local cashew processing unit located in Wonogiri Regency, Central Java. The cashew nutshells were cleaned, sun-dried and ground passed through a 1 mm sieve, followed by additional drying to minimize the presence of anti-nutritional compounds. The processed cashew nutshell meal was then incorporated into the experimental diets and pelleted using a laboratory-scale pellet mill with a 3 mm die, without the addition of binding agents. The resulting pellets were air-dried and stored in airtight containers until further in vitro analysis. A portion of the supporting feed ingredients, including dried water spinach, ground peanut shell, pollard, corn gluten feed (CGF), soybean meal (SBM), molasses and mineral mix, was purchased from Andika Feed Store, Semarang City, Indonesia, to ensure uniformity and consistency of feed quality throughout the experimental period.

a. cashew fruit (cashew apple and nut)	b. cashew nutshell (outer shell of cashew nut)
Figure 1. Cashew (Anacardium occidentale)

Sample preparation and chemical analysis

Feed ingredient samples were dried in a forced-air oven at 55 °C until a stable weight was achieved, after which they were finely ground using a Wiley-type mill and sieved through a 1 mm mesh. Chemical composition analyses were conducted to determine dry matter (DM), ash content, crude protein (CP), ether extract (EE) and crude fiber (CF) in accordance with standard AOAC analytical methods (AOAC, 2005). The resulting proximate composition of the feed ingredients is summarized in the following table 1.

Ruminal fluid preparation

Rumen fluid used for in vitro digestibility and ruminal fermentation measurements was collected from clinically healthy adult male sheep (2 years old) slaughtered at the Semarang municipal abattoir during routine commercial operations. Collection of rumen fluid from the abattoir was carried out to avoid invasive procedures on experimental animals and in accordance with animal welfare considerations and the principles of the 3Rs (Replacement, Reduction and Refinement) (Makkar, 2004). Donor sheep originated from a commercial production system and were fed a basal diet consisting of forage and concentrate, representative of typical feeding practices for fattening sheep in the region. Rumen fluid was collected in the morning before feeding and immediately after slaughter (within 15 minutes) to maintain microbial activity. The rumen contents were sampled from the central portion, filtered through four layers of cheesecloth to remove coarse particles, continuously flushed with CO₂ to maintain anaerobic conditions and stored in pre-warmed insulated containers (39 °C). The rumen fluid was then transported directly to the laboratory and used as an inoculum for in vitro rumen fermentation and digestibility assays, following standard procedures described by (Getachew et al 2004; Menke and Steingass, 1988).

Pellet making process and experimental diets

The pelleted diets were prepared by weighing all feed ingredients according to the formulation of each treatment, including cashew nutshell meal as a substitution ingredient. All dry ingredients were ground to pass through a ±1 mm sieve, thoroughly mixed and water was added (approximately 10–15% of total weight) to achieve suitable moisture content for pelleting. The feed mash was processed using a pellet mill equipped with a ±4 mm die. Pelleting was conducted under moderate temperatures generated by mechanical friction, without excessive pressure or additional heating, to minimize nutrient degradation. The pellets were oven-dried at 60 °C for 24 hours until a stable moisture content was achieved, cooled to room temperature, packed in airtight containers and stored in a dry place until used for in vitro analysis, following standard feed preparation procedures for rumen fermentation studies (Menke and Steingass, 1988). Visual representation of the pelleted diets formulated with different levels of cashew nutshell meal inclusion (0, 5, 10, 15 and 20%) is presented in (Figure 2). The experimental diets were formulated with increasing levels of cashew nutshell as a partial substitute for the complete feed and the resulting nutrient composition of each treatment on a dry matter basis is presented in Table 2.

Figure 2. Research methodology cashew nutshell in ruminant feed

In vitro rumen fermentation

In vitro rumen fermentation was conducted using rumen fluid collected from adult male sheep (approximately 2 years old) slaughtered at the Semarang municipal abattoir. Donor animals originated from a commercial production system and were fed a standard diet consisting of forage and concentrate, representative of typical feeding practices in the region. Rumen fluid was collected in the morning prior to feeding and immediately after slaughter to ensure optimal microbial activity, following standard procedures for in vitro rumen studies (Menke and Steingass, 1988; Thomas et al 1963). The rumen contents were strained through four layers of cheesecloth to remove coarse particles.

The rumen fluid was subsequently mixed with a mineral buffer solution under strictly anaerobic conditions by continuous flushing with CO₂, as described by McDougall (1948) and used as the fermentation inoculum. Approximately 500 mg of pelleted cashew nutshell meal feed sample (on a dry matter basis) was placed into calibrated glass syringes and incubated with 50 mL of the rumen fluid–buffer mixture. Incubation was carried out at 39 °C for 24 hours following the in vitro gas production technique described by Menke and Steingass (1988). Blank syringes containing only rumen fluid and buffer solution were included to correct gas production values.

Digestibility measurements and fermentation characteristics

After incubation, total gas volume in each syringe was recorded directly from the calibrated scale and corrected using blank values, as recommended by Thomas et al (1963). Fermentation was terminated by cooling the incubation syringes, after which the fermented residues were filtered through ash-free filter paper and oven-dried at 105 °C until constant weight to determine in vitro dry matter digestibility (IVDMD), following the procedure described by Van Soest and Robertson (1985). The dried residues were subsequently ashed in a muffle furnace at 550 °C for 4 hours to determine in vitro organic matter digestibility (IVOMD), using the standard methods outlined by (Makkar, 2004; Mertens, 2002).

Rumen pH was measured immediately after incubation using a calibrated digital pH meter, as detailed by Dijkstra et al (2018). Ammonia (NH₃) concentration was determined from the fermentation supernatant using the phenol–hypochlorite method (Chaney and Marbach, 1962), while volatile fatty acids (VFA) were analyzed using steam distillation or gas chromatography, depending on laboratory equipment availability, as described by Harrison (1975). Methane (CH₄) production was estimated based on the composition of fermentation gases analyzed by gas chromatography (Janssen and Kirs, 2008), or indirectly calculated from total gas production and VFA profiles using commonly applied empirical equations in in vitro rumen fermentation studies.

Experimental design and data analysis

The experimental data were analyzed using analysis of variance (ANOVA) according to a completely randomized design, as described by Montgomery (2017). When significant differences among treatments were detected, Duncan’s multiple range test was applied to compare treatment means at a 5% significance level, following the procedure outlined by Duncan, (1955). All statistical analyses were performed using IBM SPSS software version 30.

This flowchart outlines the research methodology for using cashew nutshells in ruminant feed. It highlights key steps including pellet preparation, cashew nutshell management, rumen fluid collection, in vitro fermentation and data analysis, providing a clear and structured approach to evaluating the potential of cashew nutshells as a sustainable feed ingredient.

Result and discussion

Gas production and digestibility

The increasing levels of cashew nutshell resulted in different responses on fermentation characteristics and digestibility parameters. As presented in (Table 3), gas production increased consistently from CN0 to CNS20, indicating enhanced overall fermentation activity. This trend is consistent with Goetz et al (2023) and Oh et al (2017) who reported that the active compounds in cashew nutshell liquid can modulate rumen microbial activity and stimulate fermentation, leading to higher gas production. However, although gas production increased, the values of IVDMD and IVOMD tended to decrease at higher inclusion levels, particularly at CNS15 and CNS20. This pattern suggests that excessive levels of cashew nutshell may suppress cellulolytic microbes and consequently reduce dry matter and organic matter digestibility. Similar observations were reported by Adetunji et al (2020) and Compton et al (2023), who found inhibitory effects of cashew nutshell supplementation when applied at high concentrations. Other studies in ruminant nutrition also confirm that phytogenic compounds may improve fermentation efficiency but can depress digestibility at non-optimal doses (Ali et al 2023; Kang et al 2018; Wulandari et al 2020). Therefore, these findings indicate that moderate inclusion levels are likely more beneficial, while excessive supplementation may increase gas production but negatively affect nutrient utilization.

Ruminal fermentation profile

The inclusion of cashew nutshell showed a clear modulation of ruminal fermentation characteristics. Ruminal pH increased progressively with higher CNSL levels, ranging from 6.61 at CN0 to 7.04 at CNS20, indicating a shift toward a slightly less acidic environment. This response agrees with Oh et al (2017) and Compton et al (2023) who reported that bioactive compounds in CNSL can modify rumen microbial activity, leading to improved buffering capacity and stabilization of fermentation conditions. Conversely, NH₃ concentrations decreased significantly (p< 0.05) with increasing CNSL supplementation, suggesting reduced protein degradation or improved microbial nitrogen utilization. Similar reductions in ammonia nitrogen with CNSL supplementation have been documented by Adetunji et al (2020) and Goetz et al (2023), who associated this trend with inhibition of hyper-ammonia producing bacteria.

Total VFA increased progressively from CN0 to CNS20, indicating an enhancement of overall fermentation efficiency despite the increase in ruminal pH. This is consistent with Kang et al 2018) and Narabe et al (2021) who reported that CNSL stimulates rumen fermentation by selectively modifying microbial communities involved in carbohydrate fermentation. Interestingly, methane production decreased significantly with increasing CNSL levels, with the lowest value observed at CNS20 (20%). This finding supports previous studies demonstrating the strong antimethanogenic potential of CNSL through disruption of methanogenic archaea and hydrogen-utilizing microbial pathways (Chung et al 2024; Cuervo et al 2024; Dijkstra et al 2018; Goetz et al 2023; Koike et al 2020). Overall, these results indicate that cashew nutshell supplementation can improve rumen fermentation efficiency, reduce nitrogen losses and effectively lower methane production, although the optimal inclusion level should consider its balance with nutrient digestibility responses discussed earlier. The detailed results are presented in the following table 4.

Overall, the results of this study indicate that cashew nutshell pellets have the potential to be used as an agro-industrial by-product feed source for ruminants. The in vitro evaluation demonstrated that their inclusion influenced rumen fermentation characteristics, nutrient digestibility and gas production, suggesting a possible role in modulating rumen microbial activity. Although variations in digestibility and fermentation responses were observed across different inclusion levels, the general trend supports the feasibility of cashew nutshell pellets as a complementary feed resource.

Conclusion

The results of this in vitro study indicate that cashew nutshell pellets have the potential to be used as an agro-industrial by-product in ruminant feeding systems. The inclusion of cashew nutshell pellets influenced gas production, digestibility and rumen fermentation characteristics, with responses varying depending on the level of incorporation. These findings suggest that cashew nutshell pellets may serve as an alternative feed resource to support more sustainable feeding strategies. However, there are limitations to the use of cashew nutshell pellets, particularly regarding their optimal inclusion levels, potential toxicity and the variability in animal responses. Further in vivo studies are needed to confirm their effects on rumen metabolism, animal performance and overall safety, particularly concerning the impact on long-term health and productivity, before practical application in livestock production.

Acknowledgments

The authors gratefully acknowledge the financial support from the Indonesian Education Scholarship (Beasiswa Pendidikan Indonesia). Appreciation is also extended to the Faculty of Animal and Agricultural Sciences, Diponegoro University, for the academic support, research facilities and guidance throughout the study.

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