1. Introduction

Livestock production is one of the major activities for human beings to generate varieties of products including milk, egg, and meat used for human food. It is also essential to the nutritional worth and economy of the world [1]. Therefore, improving animal feed must be considered to satisfy the economic and nutritional aspects of human beings [2]. People of the world should take into account both quantity and quality of feed to fulfill animal nutritional requirements and to increase animal production and productivity [3, 4].

Animal production and productivity are decreased due to metal toxicity and bioaccumulation by the food chain, which could result in harmful outcomes in animals, plants, and their consumers [5, 6]. Nickel (Ni), mercury, and cadmium (Cd) are among those toxic and highly water-soluble metals from contaminated soil, agricultural waste, grazing pastures, and vegetation [7–9]. Animals that are poisoned by mercury cannot produce liver, meat, or kidneys suitable for human consumption. Because cell toxicity and dysfunction can be caused by heavy metals through binding to protein sites and displacing the original metals from their native binding sites [9].

Therefore, inclusion of biochar is the best solution to meet the animal nutrient requirements and modulation of heavy metals. It can improve adsorption of organic compound nutrients like, fatty acids, urea, and amino acids. Urea is adsorbed by biochar particles when utilized as bedding material. This preserves urea molecules from volatilizing and ammonifying that improves nutrient requirement of animals through retention and slow release of nitrogen, phosphate, and potassium [7, 10]. Similarly, biochar can immobilize heavy metals from contaminated feed and soil through surface adsorption (create binding sites with heavy metals) and ion exchange (reduces the mobility of heavy metals) [11, 12].

The history of biochar used for soil activities began about 7000 years ago [13]. It can be prepared from organic materials like wood, straw, manure, crop leftovers, and leaves heated from 350 to 1000°C in anaerobic conditions to partially pyrolyze [14, 15]. In the past 10 years, using and manufacturing biochar have been common and have become popular as a feed additive for animals and soil amendment for crop production [3].

Among the most common problems in livestock production industries include health issues, dietary nutrient deficiency-related issues (grass tetany, milk fever, hemorrhage, anemia, Keshan’s disease, and goiter), and metabolic disorders such as carbohydrate metabolism (hypoglycemia, ruminal acidosis, and ruminal bloat), abnormal lipid metabolism (ketosis and milk fat depression in dairy cows, abnormal amino acid metabolism (edma, hyperhomocysteinemia, gout hyperammonemia, anemia) are the major one [16]. It has been reported that feeding biochar as a feed additive served as one of the best solutions to treat digestive disorders and metabolic abnormality [3, 10, 17]. Because biochar had the characteristics of toxin adsorption (bind toxins like dioxins, glyphosate, mycotoxins, and pesticides), improve nutrient intake and utilization, gut microbiota modulation through encouraging useful microbial populations and inhibits the growth of harmful bacteria, reduced methane production by preventing the activity of methanogen bacteria, and efficiency of nutrient retention leads to improve overall animal health and digestive disorder treatment [18, 19].

In the current scenarios, the absence of enough feed additives for livestock production, the increase in greenhouse gas (GHG) emission from ruminant animals, and the digestive or metabolic disorders due to malnutrition system for livestock production need alternative measurements. Of those avilable alternative measurements, previous studies have recommended that the inclusion of biochar in a feed has paramount importance. It can increase animal growth performance and health status [17, 20] and reduction of enteric methane production [21, 22]. However, besides the above importance, long-term feeding of biochar may have a risk. Because biochar had a potential to adsorb drug, essential feed compounds and shifts the microbiota dynamics in the digestive system of animals [23].

Therefore the objective of this review is to summarize existing studies on the role of biochar as a feed additive in animal performance, its importance for immunization of the digestive system, micro-biota dynamics, and reduction of enteric methane production. The review will summarize and indicate the maximum inclusion level of biochar as a feed additive for different group of animals.

2. Methods of Review

The topic was proposed based on its current issues and the importance of biochar for livestock production and methane emission mitigation. An understanding of the effect of biochar on animal performance, digestibility, micro-biota dynamics, and reduction of enteric methane production is very important. This review followed a systematic review method. Research articles on the use of biochar as feed additive were collected via using common search engines including Google Scholar, PubMed, and Web of Science with search terms such as biochar, feed additives, methane emission reduction, the effect of biochar on livestock performance, and methane emission (Figure 1). Then, after a screening of related and appropriate articles from different journal sites, articles were downloaded and stored in the EndNote library. Further screening was conducted to remove similar articles or duplicated articles.

3. Year-Wise Publication, Trend Analysis, and Area of Research Focus in Biochar Application

As presented in Table 1, the data collected from the Web of Science revealed 22,620, 176,534, 63,620, and 30,835 for feed additive, improvement of rumen microbiota, animal feed digestibility, and enteric methane reduction, respectively. The overall published data were 293,609 on biochar applications for the livestock sector. This data were higher than the 6005 published data reported by Kumar et al. [29] in the application of biochar for the management of waste, reduction of climate change, soil quality improvement, energy production, and remediation of contaminants from soil, water, and air. This difference might be the increase in research interest, data collection methods, or its wider importance in livestock sector.

The exponential trend analysis for year-wise publication for biochar application was presented in Figures 2 and 3. Based on this result, the application of biochar as a feed additive, improvement of rumen microbiota, animal feed digestibility, and enteric methane reduction in the livestock sector was increased exponentially. From these research topics, the improvement of rumen microbiota was the first followed by animal feed digestibility.

4. Use of Biochar as a Feed Supplement

While using biochar as an animal feed supplement, considering the regulation and feed certification under the European Food Safety Authority (EFSA) is mandatory. The mandate concerns feed safety, rules for organic livestock feeds, sustainability of organic production, labeling of organic products, and repealing restrictions [30]. The commission has checked and approved the biochar made from important plant parts, such as roots, leaves, wood, and feedstock free from any painting and organic and nonorganic contaminants, such as plastic [3].

5. Effects of Biochar Inclusion to Feed on Animal Performance

In livestock farming, supplementing feed with feed additives is a widespread method to improve animal immunity, protein intake, growth efficiency, and livestock production [3]. Feeding a regular dosage of charcoal is widely used to improve animal health and growth performance [3]. Different studies have revealed that adding biochar to feed can help animals grow weight and improve appetite to provide more food [31, 32].

5.2. Impact of Biochar on Small Ruminate Performance

The inclusion of biochar in the feed for small ruminate has a significant impact on growth performance. When compared to control groups, goats’ weight gain was significantly affected by diets supplemented with 1.1% of biochar [35]. As presented in Figure 4, there was an increase in body weight gain by up to 1.5% inclusion of biochar in the sheep experiment [36]. However, when the inclusion level of biochar is greater than 1.5%, there was a reduction of body weight gain in sheep.

5.5. Impact of Biochar on Fish Production

A different study revealed that adding biochar to fish feed has a prominent result [39]. According to Khaki [40], the largest brown trout (Salmo trutta) received biochar-supplemented meals 0.2 and 0.3 g per kg diet was higher weight gain. Growth rates were increased when 36% of biochar was fed to the striped catfish (Pangasius hypophthalmus) that biochar made from 1% of rice husks [41]. The application of active charcoal supplementation in the experiment of Nile tilapia, Oreochromis revealed it can improve fish growth performance and feed intake and the optimum active charcoal level was added to be 7.0 g/kg diet [42]. Generally, Figure 5 revealed that feeding biochar can increase animal growth performance per livestock species.

As Summerside here in Table 1, the maximum inclusion level (0.8%) for cattle can improve from 237.1 to 240.2 kg of body weight gain. Similarly, body weight was increased from 22.41 to 33.81 kg for fish under maximum inclusion level of 4% biochar and for shoat from 25.35 to 33.04 kg in maximum inclusion level (1.8%) of biochar in the feed. However, there was slight similarity in average result of poultry. In conclusion, our review found that biochar had a positive effect on animal body weight gain to the amount indicated by the maximum inclusion levels below.

6. Biochar for Enhancement of Digestive Microbiota

Animal stomachs are more intricate ecosystems with a variety of microbiota. This microbiota exists in the rumen-reticulum, gastrointestinal tract, and cecum in a symbiotic relationship with the host [46]. They degrade any feeds consumed by the host, produce various biochemical substances, and play various roles in the production of volatile fatty acids (VFA), which are used as a source of energy in glycolysis [47]. Creating a solid surface region for microorganisms might effectively transfer substrate, increase the efficiency of adenosine triphosphate (ATP) synthesis, and biochar might promote the formation of specific rumen microbial populations [41].

Silage mixtures containing 8.8% and 16.6% of biochar led to increments in total VFA production, which suggests increased microbial fermentation with an increasing quantity of biochar [48]. This is because biochar can increase the number of lactic acid bacteria, restrict butyric acid production, and limit mycotoxin production.

According to Amean and Shujaa [49], Awassi lambs were fed varying percent of biochar supplements. Average daily gain (ADG), final body weight gain, and FCR were significant (p  < 0.05) for ruminant animals due to an increase in bacterial population [33]. They stated that continuous increasing the percent of biochar content in animal feed, and performance may lower due to decreasing microbial productivity and higher urea or nitrogen absorption in the biochar’s pores, which renders these nutrients inaccessible to microbial protein synthesis.

6.1. Microbiota Dynamics in Normal Cattle (bovine) Rumen

Scholars have documented that a single milliliter of rumen fluid from normal cattle contains ~10¹¹ (99%) bacteria, 10⁹ (0.99%) archaea, 10⁶ (0.00099%) protozoa, and 10⁶ (0.00099%) fungi, respectively. Rumen microbes fall into two main categories: those that digest fibre, preferring a pH above 6.2, and those that digest starch, with a preferred pH range between 5.4 and 7.0. These microorganisms exhibit sensitivity to the rumen environment and undergo fluctuations influenced by dietary changes. The consumption of a high-starch diet can induce a shift in rumen pH, leading to alterations in microbe populations [50]. Based on the average result presented in Figure 6, Bacteroidetes (27%), Prevotellaceae (21%), and Firmicutes (19%) were the most dominant bacterial population in the rumen of cows.

7. Effect of Biochar Inclusion on Microbiota Dynamics

The exploration of microbiota dynamics in the rumen of animals involves investigating the intricate interactions within the distinctive digestive system of ruminants. Ruminant animals, such as cows, sheep, goats, and deer, exhibit remarkable microbial diversity within their rumen. In this intricate ecosystem, diverse microorganisms engage in interactions with one another [54, 55].

Various microorganisms present in the rumen, including Bacteria, Protozoa, Fungi, Archaea, and Viruses, contribute significantly to nutrient processing. Their essential roles involve the conversion of plant fiber into short-chain fatty acids and other metabolites. This transformative process is crucial for the production of meat and milk, directly influencing the quality of feed. Furthermore, rumen microbes play a vital role in detoxification and the metabolism of xenobiotics, such as aromatic compounds and secondary plant metabolites [56, 57].

8. Biochar Supplement for Enhancing Digestibility

Leng, Inthapanya, and Preston [85] illustrated that animals fed with biochar as a feed additive has improved feed digestibility and digestion rates. Similarly, Silivong and Preston [35] and Winders et al. [86] investigated that ruminants can digest feed improved by biochar in their diets [87] also confirmed a positive response in dry matter digestibility (DMD) using pine-sourced biochar. In addition to this [88], reported that biochar addition of the in vivo method improves the nutrition content and digestibility of mycotoxin-containing silages. However, Teoh et al. [26] noted that biochar is entirely inorganic matter and is not digested by the rumen microbiota. There was no effect on DMD with biochar supplementation in the study. However, there is evidence suggesting that biochar does not affect digestibility [89, 90].

9. Biochar Supplement for Enteric Methane Reduction

General information on biochar’s contribution to the reduction of methane emission in different livestock species and from in vitro fermentation systems is presented in Table 8. Due to herbivorous animals’ significant contribution to the production of enteric methane, altering rumen fermentation to reduce this greenhouse gas should be given a high priority in the research field [33]. As fermentation duration and methane output increase, relative biochar addition can reduce methane (CH₄) synthesis [85]. Although there is little study on the use of biochar in animal diets, several sources indicated that a decrease in methane (CH₄) gas production without any decline in digestibility [94].

Biochar is already used as a beneficial feed ingredient and has a market, and there is in vivo proof that it can reduce greenhouse gas emissions [17]. The average result for methane production from several researchers under in vitro and in vivo experiments was 73.21 g/day with a maximum inclusion level of 3.1% biochar (Table 8). Similarly, the result concluded by Winders et al. [44] in growing steers and finishing cattle, biochar given at 8 g/kg DM decreased methane (CH₄) generation by 9.5 and 18.4%, respectively. An experiment conducted by Cabeza et al. [95] also revealed that, compared to the control group, adding biochar decreased the production of methane (CH4) and total gas. Sirjani, Zahedifar, and Rouzbehan [28] also reported that there was a significant difference (p  < 0.05) in methane gas production between control (21.6%), Diet + biochar (14.5%), Diet + lactobacilli-biochar (13.4%), diet + yeast-biochar (13.2%), and it was revealed that there was the reduction of methane gas production via feeding of biochar as a feed additive.

However Terler et al. [92] was reported that Supplementing with a 4% maximum inclusion level of biochar on dairy cows’ feed will increase methane production from 322 to 348 grams/day (Table 8). More research on methane (CH₄) and hydrogen gas production in vivo models is needed to justify biochar’s potential as a methane mitigation technique properly.

9.1. How Does the Addition of Biochar Reduce Enteric Methane Production?

The addition of biochar to ruminant diets is considered a potential strategy to reduce enteric methane production. Enteric methane is produced during the digestion process in the stomachs of ruminant animals, such as cattle, sheep, and goats [96]. As presented in Table 9, the addition of biochar at 1.29% decreased methane production from 32.4 to 15.7 ppm [34], at 0.8% decreased from 109 to 92.2 g/day [44], at 3% decrease from 141 to 122 g/day [44], and at a 1% decrease from 225.1 to 215.5 g/day[23 for goat, growing and finisher Steers and Beef Heifers, respectively. Methane is a powerful greenhouse gas, and its reduction is important for mitigating climate change [97].

Table 9. Contribution of biochar to methane reduction

Animals	Feedstock	Biochar inclusion level (% or g) and results in parentheses ()			Blend	Types of trial	p-Value	References
Cattle	Powdered activated carbon	0% (9.14) ppm	—	0.5% (1500) ppm	No	Invivo	0.001	[91]
Goat	Rice husks	0% (982), 0.36% (669) ppm	0.85% (686) ppm	1.29% (709) ppm	No	Invivo	0.001	[34]
Goat	Rice husks	0% (32.4) ppm	0.85% (16.3) ppm 0.36% (18.2) ppm	1.29% (15.7) ppm	No	Invivo	0.001	[34]
Growing Steers	Whole pine trees	0% (109) g/d	0.8% (92.2) g/d)	3% (100) g/d	No	Invivo	0.68	[44]
Finishing Steers	Whole pine trees	0% (141) g/d	0.8% (128) g/d	3% (122) g/d	No	Invivo	0.39	[44]
Beef Heifers	Yellow pine	0% (225.1) g/d	0.5% (216.5) g/d	1% (215.5) g/d	No	Invivo	0.43	[23]
Yellow Cattle	Gasifier stove	0% (85.7)	0.6% (76.2)	—	No	Invivo	0.066	[33]
Dairy Cows	Pure ash wood	0 (322) g/day	200 (348) g/day	200 BC + 90 U (371) g/day	Urea	Invivo	0.210	[92]
RUSITEC	Stem wood chips	0 g (59.9) mg/day	20 g/DM + H2SO4 (64.5) mg/day	20 g/DM + HCl (67.6) mg/day 20 g /DM + ZnCl2 (53.8) mg/day	H2SO4, HCl ZnCl2	Invitro	0.23	[27]
RUSITEC	Hardwoods	0% (35.8a) mg/day	3.6% (39.6b) mg/day	7.2% (35.5c) mg/day	No	Invitro	0.10	[26]
In Vitro Rumen	Pine wood chips and corn stover	0 g/kg (26.5) mL/g	81 g/kg CS (27.7) mL/g 81 g/kg pine (25.3) mL/g	186 g/kg CS (27.4) mL/g 186 g/kg pine (27.8) mL/g	No	Invitro	0.619	[93]

• Note: Methane in % of total gas at (Control = 21.6^a), (Diet + biochar = 14.5^b), (Diet + lactobacilli-biochar = 13.4^c), (Diet + yeast-biochar = 13.2^c) p-value = 0.018 [28].
• Abbreviations: BC + U, biochar + urea; CS, corn stover; g, gram; Mcal, megacalorie; mL, milliliter; ppm, parts per million; RUSITEC, rumen simulation technique.

Here’s how the addition of biochar may help in this regard:

• 1.

  Adsorption of Methane: Biochar has a porous structure with a large surface area. When mixed in the feed of ruminants, it can adsorb methane gas within its pores. This decreases the volume of methane released into the atmosphere during the digestion process [98, 99]. Biochar also has electron-mediating characteristics in biological redox reactions [100].

• 2.

  Microbial Interactions: Biochar can influence the microbiota dynamics in the digestive system of ruminants. Methanogenic archaea are microorganisms responsible for methane synthesis in the rumen. The inclusion of biochar may affect the microbial community, possibly decreasing the presence or activity of methanogenic archaea, leading to reduced methane release [101, 102].

• 3.

  Improved Digestive Efficiency: Biochar has the benefit of enhancing the overall digestive efficiency of ruminants. By improving nutrient consumption and fermentation processes in the rumen, biochar is used to a reduction in the fermentation of feed leading to, the mitigation of methane production [103, 104].

• 4.

  Changes in Fermentation Pathways: Biochar can adjust the fermentation pathways in the rumen. It may improve the synthesis of acetate and propionate, which are less methane-producing VFA compared to butyrate. This shift in fermentation pathways can result in lower methane emissions [43].

• 5.

  Altering pH Levels: Biochar can influence the pH levels in the rumen. When included in the rumen, biochar affects pH levels due to its buffering capacity. This means it can stabilize pH by absorbing or releasing hydrogen ions (H⁺) as desirable.

Methane synthesis is sensitive to pH, and by keeping optimal pH states, biochar can potentially decrease the activity of methane-producing microorganisms [26, 105].

Generally, as the conceptual framework presented in Figure 7 revealed, biochar is important for livestock production, microbiota dynamics improvement, and enteric methane reduction.

10. Cost-Effectiveness of Using Biochar as a Feed Additive on a Large-Scale Livestock Production System

As presented in Table 10, there is a significant profit gain per animal product (kg of meat) when biochar is used as a feed additive. Adding biochar to a feed is a low-cost effective method of boosting livestock production margins. Biochar enhances feed efficiency and animals’ well-being. It can significantly increase herd longevity and lowering replacement costs [121]. As average body weight gain results extracted from Table 3 revealed that, it is possible to gain about $ 104.29 per all listed animals. Therefore, we can conclude that by using the correct level of inclusion, it is possible to improve the production cost of livestock and gain a significant profit.

11. Long-Term Feeding and Its Impact on Animal Performance, Gut Micro Flora, and Product Quality

In this review, we did not find a serious side effect of biochar used as a feeding additive for farm animals. There are few reports about the negative effect of biochar as a feed additive. Without considering the biochar’s preparation, using it as a feed additive may hazard rather than increasing growth performance and animal products like egg, meat, and milk [3]. According to Schmidt et al., [17] activated biochar may sometimes adsorb useful and healthy bacterial flora. They stated that long-term biochar feeding may shift microbiota dynamics in the digestion system of animals, and there must be more methodological investigation for digestive and harmful microorganisms before reaching an overall conclusion. In addition, a few negative effects were identified regarding the immobilization of liposoluble feed ingredients (e.g., vitamin E or Carotenoids), which may limit long-term biochar feeding [17]. Additionally, Fujita et al. [122] stated that the inclusion of 0.5% biochar in daily fed of hens affects vitamin E content in the eggs by 40%. Animals fed with biochar have shown improvements in meat quality, including better fat composition, reduced cholesterol levels, and enhanced flavor. In addition, biochar supplementation can lead to lower levels of harmful residues in meat, potentially extending shelf life and improving safety. In dairy cows and laying hens, biochar supplementation has been linked to higher nutrient content in milk and eggs, including increased levels of beneficial fatty acids and vitamins [3, 17].

12. Conclusion and Recommendation

Conflicts of Interest

The authors declare no conflicts of interest.

Author Contributions

Conceptualization and original draft: Bernabas Ayeneshet and Tenaw Temesgen

Source Collection, Writing, review AND editing: All authors collected the relevant published article from highly indexed journals, wrote the full review article and contributed to the review article, and approved the submitted version.

Funding

This review paper was not supported by any projects.