Adsorption Before biochar was investigated and used as a regular feed additive for animals in the early 2010s, charcoal (i.e., biochar made from wood) and activated carbon (i.e., activated biochar when made from biomass; Hagemann et al., 2018) was considered a veterinary drug to tackle indigestion and poisoning. Charcoal was known for many centuries as an emergency treatment for poisoning in animals (Decker & Corby, 1971). Biochar has been and still is used because of its high adsorption capacity for a variety of different toxins like mycotoxins, plant toxins, pesticides as well as toxic metabolites or pathogens. Adsorption therapy, which uses activated biochar as a non-digestible sorbent, is considered one of the most important ways of preventing harmful or fatal effects of orally ingested toxins (McKenzie, 1991; McLennan & Amos, 1989).

From a toxicology perspective, most of the effects of biochar are based on one or several of the following mechanisms: selective adsorption of some toxins like dioxins, co-adsorption of toxin containing feed substances, adsorption followed by a chemical reaction that destroys the toxin and desorption of earlier adsorbed substances in later stages of digestion (Gerlach & Schmidt, 2012). However, classifiable distinctions need to be made to the time-dependent and partly overlapping processes of adsorption, biotransformation, desorption and excretion of the toxic substances throughout the digestive system of animals.

Schirrmann (1984) described the effects of activated carbon on bacteria and their toxins in the gastrointestinal tract as:

1. Adsorption of proteins, amines and amino-acids.
2. Adsorption of digestive tract enzymes, as well as adsorption of bacterial exoenzymes.
3. Binding, via chemotaxis, of mobile germs.
4. The selective colonization of biochar with gram-negative bacteria might result in decreased endotoxin release as these toxins could be directly adsorbed by the colonized biochar when gram-negative bacteria dying-off.

One further major advantage of the use of biochar is its “enteral dialysis” property, that is, already adsorbed lipophilic and hydrophilic toxins can be removed from the blood plasma by the biochar, as the adsorption power of the huge surface area of the biochar interacts with the permeability properties of the intestine (Schirrmann, 1984).

Susan Pond (1986) explained various mechanisms by which biochar can eliminate toxins from the body. First, biochar can interrupt the so-called enterohepatic circulation of toxic substances between the intestine, liver and bile. It prevents compounds such as estrogens and progestagens, digitoxin, organic mercury, arsenic compounds and indomethacin from being taken up in bile. Second, compounds such as digoxin, which are actively secreted into the intestine, can be adsorbed there. Third, compounds such as pethidines can be adsorbed to the biochar, which passively diffuse into the intestine. Fourth, the biochar can take up compounds that diffuse along a concentration gradient between intestinal blood and primary urine.

Redox activity of biochar-based feed additives Although the adsorption capacity is the most prominent function of biochar to explain its positive impacts when fed to animals, adsorption alone cannot explain all phenomena that are observed in biochar feeding experiments. Another pivotal, but still widely overlooked function of biochar is its redox activity. Biochars act as so called geobatteries and geoconductors that can accept, store and mediate electrons from and for biochemical reactions (Sun et al., 2017). Low temperature biochars (HTT of 400–450 °C) function as geobatteries mainly due to their phenol and quinone surface groups. High temperature biochars (HTT >600°), on the other hand, are good electrical conductors (Mochidzuki et al., 2003; Yu et al., 2015). Due to both of these qualities, both, high and low temperature biochars, can act in biotic and abiotic redox-reactions as electron mediators (Van Der Zee & Cervantes, 2009; Husson, 2012; Liu et al., 2012; Kappler et al., 2014; Kluepfel et al., 2014; Joseph et al., 2015a; Yu et al., 2015; Sun et al., 2017). Biochar can accept and donate electrons as, for example, in microbial fuel cells where activated biochar can be used as an anode and as a cathode (Gregory, Bond & Lovley, 2004; Nevin et al., 2010; Konsolakis et al., 2015). The electrical conductivity of biochar is, however, not based on continuous electron flow, like in a copper wire, but on discontinuous electron hopping (Kastening et al., 1997), which is of essential importance for biochar’s function as a (microbial) electron mediator or so-called electron shuttle, facilitating even inter-species electron transfer (Chen et al., 2015). Due to the comparably large size of biochar particles, the electron transfer capacity of biochar’s carbon matrices may lead to a relatively long-distance electron exchange that provides a spatially more extensive accessibility to alternative electron acceptors such as minerals for anoxic microbial respiration (Sun et al., 2017).

During the microbial decomposition of organic substances in the gastrointestinal tract and particularly in the anaerobic rumen, digestive microbes require a terminal electron acceptor to get rid of surplus electrons that accumulate during the degradation of organic molecules. As electrons do not exist in a free state under ambient environmental conditions and cannot be stored in large enough quantities by cells, organisms always depend on the availability of both an electron donor (e.g., the metabolized organic matter) and an acceptor to which surcharge electrons can be transferred. This usually occurs in so-called redox reactions where molecules or atoms that donate an electron are coupled through electro-chemical reactions with molecules or atoms that accept an electron. To allow this electron transfer, these chemical or biochemical redox-reactions usually have to take place in very close (molecular) proximity.

The coupling of electron donating and electron accepting reactions can, however, be bridged by so-called electron mediators or electron shuttles. Those electron meditators can take up an electron from a chemical reacting molecule, solid interphase or microorganism and provide it to another molecule, atom, solid interphase or microorganism. Well known and investigated electron mediating compounds include thionine, tannins, methyl blue or quinone, showing comparable capacities to humic substances and biochar (Van Der Zee et al., 2003; Liu et al., 2012; Bhatta et al., 2012; Kluepfel et al., 2014).

A well-balanced animal feed regime should contain multiple electron mediating substances. In the high-energetic diets used in intensive livestock farming, the supply with electron-shuttling substances is, however, often insufficient (Sophal et al., 2013). When inert or other non-toxic electron mediators like biochar or humic substances are added to high-energy feed, several redox reactions may take place more efficiently, which could in turn increase the feed intake efficiency (Liu et al., 2012; Leng, Inthapanya & Preston, 2013). Biochar, specifically, can act as both a sole electron mediator or a synergistic electron mediator that increases the efficiency of other mediators (Kappler et al., 2014).

Inside the gastro-intestinal tract, nearly all feed-degrading reactions are facilitated by microorganisms (mostly bacteria, archaea and ciliates). Within those reactions, bacterial cells may transfer electrons to biofilms or via biofilms to other terminal electron acceptors (Richter et al., 2009; Kracke, Vassilev & Krömer, 2015). However, biofilms are rather poor electric conductors and the electron-accepting capacity is low. Hence, microbial redox reactions can be optimized by electron shuttles, such as humic acids or activated biochar whose electrical conductivity is 100–1,000 times higher than that of biofilms (Aeschbacher et al., 2011; Liu et al., 2012; Saquing, Yu & Chiu, 2016). Although the conductivity of non-activated biochar is lower compared to activated biochar, it has been shown that it can efficiently transfer electrons between bacterial cells (Chen et al., 2015; Sun et al., 2017). Bacteria were shown to donate an electron to a biochar particle while other bacteria of different species took up (accepted) an electron at another site of the same biochar particle. The biochar acts here like a “battery” (or electron buffer) that can be charged and discharged, depending on the need of biochemical (microbial) reactions (Liu et al., 2012). Moreover, as biochar can be temporarily oxidized or reduced by microbes (i.e., biochar is depleted or enriched in electrons), it can buffer situations with a (temporary) lack of electron donors or terminal electron acceptors (redox buffering effect) (Saquing, Yu & Chiu, 2016). A principal aim of feeding biochar to animals could thus be to overcome metabolic redox limitations by enhancing electron exchange between microbes, and between microbes and terminal electron acceptors.

The redox-active carbonaceous backbone of the biochar as well as minerals it contains, such as iron (Fe(II) and/or Fe(III)) and manganese (Mn(III) or Mn(IV) minerals), can electrically support microbial growth in at least four different ways: (1) as an electron sink for heterotrophy-based respiration, (2) as an electron sources for autotrophic growth, (3) by enabling cell-to-cell transfer of electrons and (4) as an electron storage material (Shi et al., 2016). It can be hypothesized that enabling of extracellular electron transfer contributes to a more energy efficient digestion resulting in higher feed efficiency when activated or non-activated biochar is administered. Moreover, the electrochemical effects need to be considered as a major factor for explaining possible shifts in the functional diversity of the microbial community in the digestive system (Prasai et al., 2016). Leng, Inthapanya & Preston (2012) also suggested that electron transfer between biochar and microorganisms could be one of the reasons why feeding biochar to cows led to reduced methane emissions in their studies (see chapter 6).

It is further very likely that biochar has the function of a redox wheel in the digestive tract, comparable to FeIII–FeII-redox wheels. It could act jointly as an electron acceptor and donator coupling directly various biotic and abiotic redox-reactions comparable to mixed valent iron minerals (Davidson, Chorover & Dail, 2003; Li et al., 2012; Joseph et al., 2015a; Quin et al., 2015). Beside its polyaromatic backbone, biochar contain, depending on the production process, a multitude of volatile organic carbons (VOC) (Spokas et al., 2011). Some of the pyrolytic VOCs are strong electron acceptors and may act, like a redox wheel similar to how quinone works (Van Der Zee et al., 2003). Some of these pyrolytic VOCs that often undergo oxidative modifications during the aging of biochar (Cheng & Lehmann, 2009) are so-called redox-active moieties (RAMs) that have been shown to contribute to the biodegradation of certain contaminants (Yu et al., 2015). It can be surmised that in the digestive tract, a multitude of RAMs, adsorbed on the surfaces of biochar particles, can act as redox-wheels with various microorganisms. It can be further hypothesized that when biochar buffers electrons in the vicinity of redox active surface groups, it may provide stabile micro-habitats with different redox-pH-milieus for different species of microorganisms (Yu et al., 2015). Moreover, biochar adsorbs certain feed and metabolic substances like tannins, phenols or thionin, which are also electron acceptors and which might further increase the electron buffering of biochar particles during its passage through the digestive tract (Kracke, Vassilev & Krömer, 2015).

Biochar, wood vinegar (i.e., aqueous solutions of condensed pyrolytic gases) and humic substances can act as redox buffering substances (Husson, 2012; Kluepfel et al., 2014) which may explain why the feeding of biochar, pyrolytic vinegar and humic substances often show similar effects; and why the blending of biochar with wood vinegar or humic substances seems to reinforce the effects (Watarai, Tana & Koiwa, 2008; Gerlach et al., 2014). However, unlike both dissolved organic substances, biochar provides a highly porous framework with high specific surface area, where humic-like substances or pyrolytic vinegar can be adsorbed and unfurl three-dimensionally as a coating of the inner-porous aromatic carbon surfaces of biochar. Due to the redox buffering effect of biochar blended with humic substances or wood vinegar, variations of the redox potential may be minimized in the proximity of biochar particles, which could support those species of microorganisms that find their optimum at these redox potentials (Kalachniuk et al., 1978; Cord-Ruwisch, Seitz & Conrad, 1988). Biochar particles may thus provide selective hotspots of microbial activity. It can be assumed that the buffering of the redox potential as well as the effect of electron shuttling between microbial species can have a selective, microbial milieu forming effect, which facilitates and accelerates the formation of functional microbial consortia (Kalachniuk et al., 1978; Khodadad et al., 2011; Sun et al., 2017).

The mechanistic understanding of biochar used as feed additive, especially with regard to its impact on microbial mediated redox reactions, is clearly in its infancy (Gregory, Bond & Lovley, 2004; Nevin et al., 2010; Konsolakis et al., 2015). However, we hypothesize with some confidence that biochar has a direct electro-chemical influence on digestive reactions, and that this is one, if not the main, reason for the extremely varying effects of different biochars. Electrical conductivity, redox potential, electron buffering (poising) and electron transfer capacity (shuttling) of a given biochar depend highly on the type of pyrolyzed feedstock, pyrolytic conditions (Kluepfel et al., 2014; Yu et al., 2015) and especially on pyrolysis temperature (Sun et al., 2017). The higher the temperature above 600 °C, the better is the electron transfer rate and electrical conductivity (Sun et al., 2017). However, the higher the VOC content of, for example, lower-temperature biochars and higher abundance of surface functional groups on lower temperature biochars (400–600 °C), the more important the mediated electron transfer onto/from the biochar may become (Joseph et al., 2015a; Yu et al., 2015; Sun et al., 2017). In addition, the mineral content of biochars should be taken into account as well, since it does not only influence biochar’s electro-chemical behavior, but it may also catalyze various biotic and abiotic reactions (Kastner et al., 2012; Anca-Couce et al., 2014).

Specific toxin adsorption

Adsorption of mycotoxins The contamination of animal feed with mycotoxins is a worldwide problem that affects up to 25% of the world’s feed production (Mézes, Balogh & Tóth, 2010). Mycotoxins are mainly derived from mold fungi, whose growth on fresh and stored animal feed is difficult to prevent, especially in humid climates. Mycotoxin-contaminated feed can result in serious diseases of farm animals. To protect the animals, adsorbents are usually added to the feed to bind the mycotoxins before ingestion. In addition to the frequently used aluminosilicates, activated carbon and special polymers are increasingly being used (Huwig et al., 2001).

One of the most common mycotoxins is aflatoxin (Alshannaq & Yu, 2017), which has, therefore, been used in numerous studies as a model substance to investigate the adsorption behavior of biochar and how it reduces the uptake of the toxin in the digestive tract and hence in the animal blood and in milk (Galvano et al., 1996a). Galvano et al. (1996b) were able to reduce the extractable aflatoxin concentration in animal feed by up to 74% and the concentration in milk by up to 45%, by adding 2% activated biochar to pelleted aflatoxin-spiked feed for dairy cows. The non-systematic comparison of different activated biochars, however, showed that there are large differences in the adsorption efficiency between different types of (activated) biochar.

Diaz et al. (2002) showed in an in vitro sorption batch study that four different activated carbons adsorbed 99% of the aflatoxin B from a 0.5% aflatoxin B-spiked solution when activated biochars were dosed at 1.11 g on 100 ml. However, when Diaz administered 0.25% activated carbon to aflatoxin-B contaminated feed for dairy cows a year later (Diaz et al., 2004), they were unable to demonstrate any significant reduction in aflatoxin B levels in the milk. Here, it has to be considered that in the in vivo test, an insufficiently characterized (activated) biochar was fed at a low concentration of 0.25% of the feed fresh weight, whereas in the in vitro studies, the biochar was added at 1% to the aqueous solution, that is, four times higher, and in the absence of a feed matrix.

Galvano et al. (1996a) also investigated the adsorption capacity of 19 different activated carbons for two mycotoxins, ochratoxin A and deoxynivalenol, and found that the activated biochar adsorbed 0.80–99.86% of the ochratoxin A and up to 98.93% of the deoxynivalenol, depending on the type of activated biochar. The large range of results clearly confirms the importance of a systematic characterization and classification of biochar properties. However, Galvano et al. concluded that neither the iodine number used for activated biochar characterization, nor the Brunauer–Emmet–Teller specific surface area derived from N2 gas-adsorption isotherms allowed straightforward predictions of the adsorption capacity for these mycotoxins.

Di Natale, Gallo & Nigro (2009) compared various natural and synthetic adsorbent feed additives for dairy cows to reduce the aflatoxin content in milk. Activated biochar showed the highest toxin reduction capacity (>90% aflatoxin reduction in milk with 0.5 g aflatoxin per kg diet). Analytical studies of the milk quality also showed slight positive effects on the milk composition with regard to organic acids, lactose, chlorides, protein content and pH. The authors explained the high adsorption capacity with the high specific surface area in combination with a favorable micropore size distribution of the biochar, and the high affinity of aflatoxin for the polyaromatic surface of the biochar in general (Di Natale, Gallo & Nigro, 2009).

Bueno et al. (2005) investigated the adsorption capacity of various doses of activated biochar (0.1%, 0.25%, 0.5%, 1%) for zearalenone, a dangerous estrogenic metabolite of the fungus species Fusarium, for which so far no treatment agents had been found. In vitro, all zearalenone could be bound at each of the four biochar doses. However, in vivo, where a wide variety of mycotoxins and numerous other organic molecules compete with the free adsorption surfaces of biochar, hardly any specific adsorption could be achieved.

A study with Holstein dairy cows investigated to what extent the negative effects of fungal-contaminated feed silage can be reduced by co-feeding activated biochar at 0, 20 or 40 g daily (Erickson, Whitehouse & Dunn, 2011). Cows fed the biochar amendment and the contaminated silage had higher feed intake and improved digestibility of neutral detergent fiber, hemicellulose and crude protein and had higher milk fat content compared to the control without biochar. When the same daily amounts of biochar were administered to uncontaminated quality silage, no changes in digestion behavior, milk quality or any other effect on the dairy cows could be detected. However, the authors showed in a second experiment that cows, when given the choice, clearly preferred good quality silage to contaminated silage either with or without biochar. They concluded that farmers should focus on providing high quality feed rather than mitigating negative effects of contaminated silage with biochar.

While Piva et al. (2005) found no protection against the injurious effects of fumonisin, a highly toxic mycotoxin, following a 1% addition of biochar to the feed of piglets, Nageswara Rao & Chopra (2001) showed that the addition of biochar to aflatoxin B1 contaminated feed of goats reduced the transfer of the toxin (100 ppb) to the milk by 76%. In the latter trial, the efficiency of activated biochar was significantly higher than that of bentonite (65.2%). Both adsorbents did not affect the composition of goat’s milk nor the average level of milk production.

In vitro studies with porcine digestive fluids showed high rates of adsorption of Fusarium toxins such as deoxynivalenol (67%), zeralenone (100%) and nivalenol (21%) through activated biochar (Avantaggiato, Solfrizzo & Visconti, 2005; Döll et al., 2007). On the other hand, Jarczyk, Bancewicz & Jedryczko (2008) found no significant effect when they added 0.3% activated biochar to the diet of pigs. Neither in the blood serum nor in the kidneys, the liver or in the muscle tissue could the ochratoxin concentrations be reduced by this small amount of supplement with uncharacterized industrial biochar (Jarczyk, Bancewicz & Jedryczko, 2008). However, no adverse effect was noted either.

Mycotoxins often cause serious liver damage in poultry. Biochar administered at daily rates of 0.02% of the body weight significantly increased the activity of key liver enzymes (Ademoyero & Dalvi, 1983; Dalvi & Ademoyero, 1984). While aflatoxin (10 ppm) reduced feed intake and weight gain of broiler chickens, the addition of 0.1% biochar to the feed (w/w) reversed the negative trend (Dalvi & McGowan, 1984).

Comparing the effect of activated biochar with a conventionally used alumina product (hydrated sodium calcium aluminosilicate), it was found that the alumina product resulted in considerable liver and blood levels of aflatoxin B when administered at 0, 40, 80 μg AFB1 per kg diet, but not when combined with a 0.25% and 0.5% biochar treatment (Kubena et al., 1990; Denli & Okan, 2007). In another study, activated biochar reduced the concentration of aflatoxin B in the feces of chickens for fattening, but only if the biochar was administered separately from the feed (Edrington et al., 1996). However, Kim et al. (2017) showed with an in vivo pig feeding trial that the aflatoxin absorption capacity was reduced by 100%, 10% and 20%, respectively, for three different biochars supplemented at 0.5% to the same basal diet, again demonstrating the importance of considering specific biochar properties. The importance of dosage was confirmed in another recent poultry trial where 0.25% or 0.5% activated biochar was added to an aflatoxin B1 contaminated diet, decreasing aflatoxin B1 residues in the liver of the birds by 16–72%, depending on the aflatoxin B1 and biochar dosages (Bhatti et al., 2018).

In their review article, Toth & Dou (2016) document further conflicting studies in which biochar feeding may or may not mitigate the effects of mycotoxin intoxication. The results of most studies on sorption in aqueous solution (in vitro) did not correlate with the results in corresponding in vivo test results (Huwig et al., 2001). Thus, in vitro studies have to be interpreted with care, because matrix effects can dramatically impact mycotoxin sorption, for example, Jaynes, Zartman & Hudnall (2007) found that an activated carbon (Norit®, Boston, MA, USA) could sorb up to 200 g/kg aflatoxin, but only in clear solution. In a corn meal suspension, sorption capacity was 100 times lower due to matrix effects. Matrix effects in the digestive tract can be expected to be even more complex due to varying pH and redox conditions. Still, based on our review, we conclude that negative effects of certain mycotoxins such as deoxynivalenol (Devreese et al., 2012, 2014; Usman et al., 2015) and zearalenone (Avantaggiato, Havenaar & Visconti, 2004) can be effectively suppressed with rather low dosages of activated biochar amended to feed, while no benefit was found for aflatoxin. It can be hypothesized that (activated) biochar is only able to suppress negative effects of mycotoxins that are rather hydrophobic (Avantaggiato, Havenaar & Visconti, 2004).

However, most of these studies have in common that only commercial activated carbons and biochars were used without proper characterization, that is, systematic trials with biochar of different feedstock (e.g., wood vs. herbaceous feedstock) and production conditions (e.g., temperature) are barely available. Thus, systematization of the results remains difficult.

Adsorption of bacteriological pathogens and their metabolites The use of activated and non-activated charcoals to improve animal health was recommended and studied by German veterinarians as far back as the beginning of the 20th century. In 1914, the adsorbing effect of charcoal for various toxins in the digestive tract was described by Skutetzky & Starkenstein (1914). First experiments with bacterial toxins of Clostridium tetani and Clostridium botulinum as well as with diphtheria toxin were performed as early as 1919 (Jacoby, 1919). In particular, Wiechowski pointed out how important the quality of the charcoal is, and how different the effect of different charcoals on the toxin adsorption can be (Wiechowski, 1914). Ernst Mangold described in 1936 the effect of charcoal in animal feeding comprehensively and concluded: “The prophylactic and therapeutic effect of charcoal on infectious or feeding-related diarrhea is clear, and based on this observation, the co-feeding of charcoal to juvenile animals appears as an appropriate prevention” (Mangold, 1936). At about the same time, Albert Volkmann published his findings about efficient reduction of oocyst excretion resulting from coccidiosis and coccidial infections when charcoal was fed to domestic animals (Volkmann, 1935).

Gerlach et al. (2014) demonstrated that daily supplement of 400 g of a high-temperature wood-based biochar (i.e., HTT 700 °C) significantly reduced the concentration of antibodies against the Botox-producing pathogen Clostridium botulinum in the blood of cattle indicating the suppression of the pathogen. They concluded that the neurotoxin concentration was reduced by the biochar in the gastrointestinal tract of the animals. The feeding of only 200 g of biochar per day did not show the same efficiency. However, when this lower dosage was mixed with 500 ml of lactobacilli-rich sauerkraut juice, a similar significant reduction of Clostridium botulinum antibodies in the blood could be measured.

Knutson et al. (2006) fed sheep infected with Escherichia coli and Salmonella typhimurium 77 g of activated biochar per animal per day. Although Naka et al. (2001) had shown earlier by in vitro trials that E. coli O157: H7 (EHEC) cell counts were reduced from 5.33 × 106 by five mg/ml activated biochar to below 800, the in vivo test by Knutson et al. with the same activated biochar (DARCO-KB; Norit®) revealed no biochar-related binding of either E. coli or S. typhimurium in the gastrointestinal tract of sheep. The authors hypothesized that either the biochar binding sites were occupied by competing substances or other digestive bacteria or that the time between infection with the pathogen and administration of the biochar was too long.

Schirrmann (1984) indicated that biochar has a particularly strong adsorption or suppression capacity for gram-negative bacteria (e.g., E. coli) with high metabolic activity (see more below in section “Administration of Biochar Feed and Biochar Quality Control”: Side effects of biochar). Fecal E. coli counts in manure after feeding 0.25% activated biochar or 0.50% coconut tree biochar were significantly lower than those of the control without biochar in 10 days finishing pig trial, while the number of beneficial bacteria Lactobacillus in feces increased in both biochar treatments (Kim et al., 2017).

Liquid cattle manure often contains E. coli O157: H7 (EHEC), which can contaminate water and soil and enter the human food chain (Diez-Gonzalez et al., 1998). Biochar can both adsorb E. coli and its toxic metabolites already in the digestive tract, as well as reduce the spread of those bacteria in water and soil by adding it to manure. Gurtler et al. (2014) investigated the effect of various biochar on the inactivation of E. coli O157: H7 (EHEC) when applied to soils. All biochars produced by either fast or slow pyrolysis from switchgrass, horse manure or hardwood significantly reduced EHEC concentrations, with fast pyrolysis of barley and oak log feedstock providing the best results in the contaminated soil mix, where EHEC after 4 weeks were untraceable using a cultivation based assessment (Gurtler et al., 2014).

Abit et al. (2012) investigated how E. coli O157: H7 and Salmonella enterica spread in water-saturated soil columns of fine sand or sandy loam, when the soil columns were blended with 2% of different biochars. While chicken manure biochar prepared at 350 °C did not improve the binding of either bacteria, the addition of biochar prepared at 700 °C from pinewood or from chicken manure significantly reduced the spread of both bacteria. In a later study, the authors showed significant differences in immobilization between the two bacterial strains and suggested that the surface properties of the bacteria played a significant role in the binding of these bacteria to the biochar (Abit et al., 2014). The latter may turn out to be an important insight into biochar—bacterial interaction and needs to be investigated systematically.

Since E. coli infections are likely to spread through cattle herds via water troughs, the prophylactic addition of biochar to trough water may be a preventive measure that should be further investigated.

In the study of Watarai & Tana (2005), the mixture of fodder with 1% and 1.5% bamboo biochar and bamboo vinegar, respectively, slightly but significantly reduced the levels of E. coli and Salmonella in chicken excrement. A patented biochar—wood vinegar product, Nekka-Rich (Besnier, 2014), whose composition was not revealed, showed a highly significant reduction of Salmonella in chicken droppings. It was further found that the biochar—wood vinegar product reduced the pathogenic gram-negative Salmonella enterica bacteria in the droppings, but not the intestinal flora of ubiquitous, non-toxic, gram-positive Enterococcus faecium bacteria (Watarai & Tana, 2005).

A 0.3% bamboo biochar feed supplement (on DM base) suppressed the fecal excretion of gram-negative coliform bacteria and gram-negative Salmonella in pigs up to 20- and 1,100-fold, respectively, compared to controls without biochar (Choi et al., 2009). The effect of biochar on the suppression of both bacterial species was of the same order of magnitude as that of antibiotics. Feeding biochar resulted in a 190-fold increase in the number of beneficial intestinal bacteria and a 48-fold higher level of gram-positive Lactobacilli compared to the treatment with antibiotics (Choi et al., 2009).

In vitro studies revealed that biochar, as well as clay, can efficiently immobilize cattle rotavirus and coronaviruses at rates of 79–99.99% (Clark et al., 1998). Since the diameter of the viral particles were larger than the pore diameters of the clay and most pores of the biochar, the authors suspected that binding was mainly due to the viral surface proteins binding to the biochar.

In vitro and in vivo experiments with bovine calves showed that biochar, especially in combination with wood vinegar, was able to control parasitic protozoan Cryptosporidium parvum infection and to stop diarrhea of calves within one day. The number of oocysts in the feces dropped significantly after a single day of feeding biochar; after 5 days no more oocysts could be found in the feces of the calves (Watarai, Tana & Koiwa, 2008). Similar results were reported when a commercial biochar wood acetic acid product (Obionekk®, Obione, Charentay, France) was tested as feed additive in young goats (Paraud et al., 2011). The mixture administered twice or thrice daily reduced the clinical signs of diarrhea already on the first day, and the oocyst shedding in the feces decreased significantly. Over the period of the study, the mortality of the young goats was 20% in the control group and only 6.7% in the treatment group that received Obionekk® three times per day. Biochar feeding in goats may also reduce the incidence of parasites such as cestode tapeworms and coccidia oocysts (Van, Mui & Ledin, 2006).