Tannin Supplementation in Animal Feeding; Mitigation Strategies to Overcome the Toxic Effects of Tannins on Animal Health: A Review

Optimum animal production depends on the type of feedstuffs available for the whole year. Plants such as trees and shrubs have significance in the animal production due to high nutritious values of tannins which reduce the rumen protein degradability and improve the feed digestibility as well as amino acid absorption in the small intestine. But, some of these plant materials are usually rich in anti-nutritional factors, known as tannins. Tannins are a complex group of plant secondary compounds widely distributed in the plant kingdom. Tannins have both deleterious and advantageous effects depending on the types, doses, composition and concentration of tannins. Rural farmers face fodder scarcity during the winter season. Therefore, tannins containing browse species may help farmers to use tannins as supplementary feed for livestock. But higher concentrations of tannins cause metabolic disorder which in results reduces the animal productivity. Therefore, it is need to investigate the effects of tannin supplementation in animal feeding; mitigation strategies to overcome the toxic effects of tannins on animal health. In this review paper, we have discussed safety and hazard associated with tannins in animal feeding.


Introduction
Tannins are group of plant polyphenols present in all vascular plants including leguminous plants, forages, flowers, fruits, grasses and leguminous trees (Blackmon et al., 2016). First time, the word tannins was used by Seguim to explain the compound present in various plants extracts that have feature of converting animal hides to leather (Elgailani & Ishak, 2016). Tannins are generally classified into two groups: hydrolysable tannins (HT) and condensed tannins (CT) ( Table 1). Tannins have both harmful and beneficial effects depending on their nature, concentration, animal species, animal health and feed composition (Figures 1a and 1b). Several studies have reported that tannin based forages have anti-bloat and anthelmintic characteristics which improve the quality of meat and milk products, fatty acids composition and ruminant antioxidant status (Blackmon et al., 2016). In addition, these tannins containing plants also contribute to inhibit the enteric methane emission (Anantasook et al., 2016;Gunun et al., 2017).    (Ngwa et al., 2002). Less total feed intake and less growth rates were also observed in animals consuming tannins based A. sieberiana pods and leaves of A. cyanophylla (Acacia saligna). The adverse effect of tannins (dose dependent) on growth rate was reported due to decrease feed intake and protein digestibility (Blackmon et al., 2016).
Tannins reduce cell wall digestibility by binding bacterial enzymes or forming indigestible complexes with carbohydrates. Another study stated that sheep fed CT containing A. cyanophylla diet had reduced digestibility of organic matter and fiber fractions. Tannins higher levels (about 5-9%) reduce the digestibility of the fiber in the rumen by inhibiting the activity of bacteria or anaerobic fungi . High levels of tannins more than 5% badly also reduce the feed intake and above 9% tannins cause mortality in the animal . The quantity of carbohydrate digestion depends on the type of the tannins feed degradability, fermentation and the amount of feed intake and excretion. If protein resists degradation or feed deficient in protein, (reduce microbial growth in the rumen) that can hinder carbohydrate metabolism (Shewangzaw, 2016). If ruminants consume high tannin diet, it can induce a deficiency of rumen-degradable N that indirectly reduces the function of structural carbohydrates (Shewangzaw, 2016). CT not only attaches to protein but also inhibit the carbohydrates metabolism in the rumen by making carbohydrates complexes resulting undegradation of carbohydrates . Thus, there is need to focus tannin plants contents and tannins plant digestibility. A similar study also reported that it is important to differentiate between high phenolic plants (toxic), tannins containing nutritious plant (safe to use) and tannins based plants comprising more fibrous diet (Chafton, 2006).

Role of Tannins on Animal Production
Tannins have different effects on lactating animals. A study in dairy cattle has investigated that L. corniculatus increased 60% milk production and 10% milk protein quantity than control group (without CT). Min et al. (2003) reported that, ewes feeding CT based L. corniculatus had no effect on milk let-down in the early lactation, but secretion rates of whole milk, protein and lactose were increased by 21,14 and 12%, respectively during mid and late lactation (Min et al., 2003). In dairy cows, CT increased milk quantity but no effects were observed on feed intake. However, various studies described that black wattle tree (Acacia mearnsii) bark containing CT concentration had no effects on dairy cows milk production (Gerlach et al., 2018). Dairy ewes fed quebracho tannins (QT) and acacia species had decreased milk yield. Several other studies suggest that mechanism behind increase milk production was due to protein protection from rumen microbial degradation which results in enhanced milk production in dairy cows, goats and sheep (Attia et al., 2016). These consequences were expected due to chemical composition, properties and chemical reactivity of tannins which in turn deviates the rumen microbial ecosystem and volatile fatty acids concentration (Valentina Vasta et al., 2010;Minieri et al., 2014;Carreno et al., 2015).
Protein plays a key role in the clean wool production consisting high cysteine quantity in the feed but amino acids containing sulphur has harmful effect on wool production. CT forages increase loss rate of cystine from blood plasma, mainly due to reducing the loss of sulphur containing amino acids (SAA) in the rumen. Addition of CT in diet increase fleece growth due to increasing the absorption of SAA (Chanjula et al., 2004;Waghorn, 2008;Pathak et al., 2017). Wool growth depends upon both the type and concentration of CT. Some studies revealed that CT containing Hedysarum coronarium (sulla) and L. pedunculatus above 50 g/kg DM impair wool growth. However, the response of wool growth was variable when CT quantity reduced below 22 g CT/kg DM. According to Priolo et al. (2000), CT extracted from L. corniculatus 35 g/kg DM increased the wool growth in lambs. Therefore, CT based L. corniculatus 22-38 g/kg DM demonstrated better effects for wool production. The results variation was due to difference in chemical composition and concentration of CT, which can affect its biological activity (Pathak et al., 2017). Tannins plants are good source of amino acids which plays an important role in the process of ovulation (Attia et al., 2016). The nutritional effect is possibly connected with hormonal pathway of ovary. It has reported that essential amino acid (EAA) and plasma branched chain amino acid (BCAA) are mutually related to increases the intravenous infusion of BCAA (Puchala et al., 2005;Ying et al., 2011). On other hand, in vitro study reported that the sensitivity of muscle protein to insulin have been enhanced by BCAA. Numerous studies determined that only enzyme BCAA transferase present in the ovaries. These consequences explain that cytosolic enzyme-BCAAT play a role for the progressive decrease of BCAA in skeletal muscles of sheep (Faure, Glomot, & Papet, 2001). The above finding shows that BCAA have a direct positive effect on ovaries. But the phenomenon behind this enhancing effect is unknown.
Rumen microbial fermentation plays an important role in the prevention of tannins toxicity (EFSA, 2014). Therefore, mature animals can tolerate HT concentration in the range 15,000-25,000 mg/kg diets without affecting performance, milk quantity or growth factor Liu et al., 2013). Toxicity seems to be more prone to calves, but the high quantity of tannins diet at > 1500 mg/kg feed produce clinical symptoms as compared to recommended dose for flavoring practices (EFSA, 2014). More specifically, higher tannins concentration causes toxicity than low concentration. Mostly, tannins were derived from commercial products sweet chestnut (Castanea sativa). Tannin toxicity has been investigated for various species of ruminal bacteria for example Streptococcus bovis, Fibrobacter succinogenes, Ruminobacter amylophilis, Butyvibrio fibrosolvens, and Prevotella ruminicola. These tannins plants cause enzyme inactivation, substrate deficiency, damage membranes, ulcers in the rumen, necrosis of the mucosal folds and metal ion deficiency which in turn, affect the rumen digestibility (Pérez et al., 2011).
A study in pigs for fattening traits has found that growth performance was not affected by the addition of 2000 mg TA/kg from sweet chestnut (Prevolnik et al., 2012). A similar study has investigated that average daily gain in piglets were unaffected with the supplementation of 1500 mg of a TA/kg feed, and no negative effects were observed on blood profile during 21 days trials (Bilić-Šobot et al., 2016). On the other hand, another similar study described that piglets fed 125, 250, 500 or 1,000 mg TA/kg diet had increased feed conversion ratio, reduced average daily gain, red blood cell count, and hematocrit respectively, under 28 days experiments (Lee et al., 2010). The blood profile seemed to be more prone at 1,000 mg tannin/kg. However, the effects were not clear at lower concentration. For animal husbandry application, the disadvantageous effects appear at 250 mg tannin/kg diet (Bilić-Šobot et al., 2016). A study on male rabbits in New Zealand has revealed that there was no deleterious effect associated with tannin supplementation as compared to control groups (without tannic acid (TA) from chestnut wood) under 21 days experiments (Liu et al., 2011(Liu et al., , 2012. A study has found that rats given different concentration of tannic acid at 5, 10, 15 and 20 g/kg feed respectively influences the iron absorption (Afsana et al., 2004;Karamac, 2009).

Mitigation Strategies to Overcome Tannin Toxicity
Various studies provide mitigation strategies to control the injurious effects of tannins feeding (Figure 4). There are a number of effective techniques to extract tannins from plants. Local feedstuffs containing tannins can be effectively used for livestock if a simple and economically viable detannification process is developed. Wood ash is a good source of alkali which has practiced at an industrial scale (Brown et al., 2016). Specifically, it has been described that soaking feed with alkaline or water solutions separate tannins toxic compounds from the most nutritive parts and thus enhance the palatability of feed (Kyarisiima & Svihus, 2004). A study has investigated that magadi soda containing alkalies (sodium carbonate, sodium bicarbonate and sodium sesquicarbonate) has minimized 40 to 50% tannins toxic effect in sorghum (Ben Salem et al., 2005). Wood ash solutions have also been utilized for treatment of tannins containing millet and sorghum for human intake. Therefore, application of wood ash has potential for detannification of tannins compounds.  Vol. 12, No. 4; protect the feed protein from rumen degradation. The effect of PEG depends on the proteins level in the forages. The forages containing higher proteins reduce the effect of PEG (Medugu et al., 2012). Therefore, the above findings suggest that addition of PEG in tannins containing diets may beneficial for ruminants (Petek & Dikmen, 2006).
Various studies have found that extraction with aqueous organic solvents such as ethanol (40%), acetone (30%), and methanol (50%) decreased 70% tannins from oak leaves (Vitti et al., 2005). The benefit of organic solvents is that it can be reused and tannins can be utilized for tanning of leather. Plants treated with alkaline solution reduced 70 to 90% tannins concentration. The reduction in tannins was due to oxidation of tannins by oxygen at high pH level. Application of oxidizing agents and alkalis has the potential to inactivate tannins in oak leaves (Vitti et al., 2005). Both chemical and protein precipitation techniques showed that tannins storage at different temperature and pH values decreased tannin levels (Medugu et al., 2012). The stirring of plants containing tannins increased the rate of inactivation due to increase oxygen exposure. These findings suggested that the decrease in tannins was due to tannins inactivation. In addition, several industries can use hydrogen peroxide (oxidizing agent) to detanninified oak leaves as well as local feedstuffs for example Acacia karroo, A. nilotica, Mangifera indica and Tamarindus indica (Brown et al., 2016). It reduces 99% tannin contents in these plants.
These detannification techniques can be applied to remove tannin from the agro-industrial by-products which may help to control the problem forages toxicity in future (Medugu et al., 2012).

Safety and Risk Associated With Tannin Feeding
Tannic acid as a feed additive is safe (dose dependent) for all animals and no environmental hazardous effects have been documented. However, in vitro trials in rats found genotoxicity, but in vivo studies did not report genotoxicity and oral exposure related carcinogenicity (EFSA, 2014). Panel on Additives and Products used in Animal Feed (FEEDAP) also did not report tannins based reproductive toxicity. In addition, the above panel has declared that tannins caused hazardous effects to workers via inhalation or direct exposure with mucous membrane, eye and skin (EFSA, 2014). Therefore, it is recommended that caution should be taken before use of tannic acid in animal ration (Table 2).

Conclusion and Future Recommendation
Tannic based plants have both advantageous and injurious effects on animal health depending on the amount of feed intake, concentration, composition and time duration. These plants have potential to control gastrointestinal parasites and reduce the ruminal tympany. These plants (trees, shrubs and bushes etc.) can be used for livestock to fulfill the requirement of rural farmer during fodder scarcity in the winter season. But preventive measures should be taken before application of tannins in the animal ration. Tannin concentration more than 5% badly affects voluntary feed intake and cause mortality. Thus, mitigation strategies may help to remove the toxic effects of tannins containing feedstuffs which might favorable for animal production. Therefore, optimum amount of tannin containing plants should be used in the animal feeding to enhance ruminant production. Very fewer studies have focused the effects of HT on animal health. Hence, further research is required to understand the potential role of HT in the animal feeding. It is necessary to explore the better utilization of tannin-containing plant species and evaluate HT associated toxicity in animals. Taken together, it has been suggested that the 15 mg TA/kg diet is safer for all animals.