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Tannins are a heterogeneous group of polyphenolic polymers distinguishable into three main groups: hydrolyzable tannins (HTs), condensed tannins (CTs), and flavotannins (FTs). HT and CT compounds are found in plants and represent the classes of natural substances that research has focused on due to their effects on ruminal fermentation parameters, microbiota, biohydrogenation, protein precipitation, and milk production and composition.

Tannins occur naturally as secondary metabolites of plants and are present in many forages consumed by ruminants. Plants act as a reservoir of specialized metabolites derived from secondary metabolism, making them a renewable source of bioactive compounds, which can be employed differently for animal health and welfare.

Plant secondary metabolites are biologically active molecules not involved in primary biochemical processes, such as plant growth, development, and reproduction. Most of these compounds have biological activity on microorganisms, affecting their growth rates, and also impact animal metabolic processes, as is the case with tannins.

According to Figure 1, hydrolyzable tannins are polyphenolic compounds that can be hydrolyzed in the presence of water, producing phenolic acids and sugars. They are primarily formed from gallic acid or ellagic acid and sugars. Condensed tannins, also known as proanthocyanidins, are formed by the condensation of flavonoids, specifically catechins and epicatechins, and are not hydrolyzed in the presence of water.

Studies on plant extracts and secondary metabolites (tannins, essential oils, saponins, flavonoids) in the diets of ruminants highlight the need to utilize compounds to control specific microbial populations in order to modulate ruminal fermentation. Thus, these are natural alternatives with significant potential aimed at increasing livestock productivity and reducing environmental pollutants such as methane (CH4), CO2, phosphorus, and nitrogen in manure.

Therefore, the focus of research on the use of tannins is to elucidate the systemic effects of supplementation in ruminants, as the literature presents a variety of results depending on the type, amount consumed, structure of the compound, molecular weight, and physiological state of the species consuming it. Among the variables that directly influence results on animal performance and the possibilities of using tannins—such as method of delivery, doses, and form of the product (liquid, microencapsulated, molecular structure)—this article provides some considerations presented in the scientific literature on the use of tannins in ruminant feeding.

Tannins and the Modulation of Rumen Microbiota

To understand the action of tannins in the rumen, it is first necessary to recall the microorganisms that make up the bacterial community of a ruminant. This composition directly depends on the species, the type, and the chemical composition and/or frequency of the diet. In all ruminants, bacteria belong to the phyla Firmicutes, Bacteroidetes, Proteobacteria, Fibrobacteria, and, to a lesser extent, Tenericutes and Actinobacteria. Their classification usually considers relative dietary substrates, allowing for the distinction between cellulolytic, amylolytic, proteolytic, lipolytic, methanogenic, saccharolytic, tanninolytic, pectinolytic, ureolytic, acetogenic, and acidogenic bacteria.

The three main cellulolytic bacteria found in cows and other ruminants are Fibrobacter succinogenes (Gram-), Ruminococcus flavefaciens (Gram+), and Ruminococcus albus (Gram variable), whose fermentation end products primarily consist of acetate, butyrate, propionate, and CO2. The most important pectinolytic species in bovine metabolism are Lachnospira multiparus, capable of reducing pectin to oligogalacturonides, providing a large amount of acetate, as well as Prevotella ruminicola and Butyrivibrio fibrisolvens. Proteolytic bacteria in the rumen, such as Clostridium, Bacilli, and Proteobacteria, break down proteins into smaller peptides, while lipolytic representatives include Anaerovibrio lipolytica and Butyrivibrio fibrisolvens. Other microorganisms in the bacterial community include protozoa and anaerobic fungi, which are important because they are involved in the degradation of lignocellulosic components.

Methanogenic archaea constitute the majority of the methanogen community in most ruminants, with research focusing on understanding how dietary interventions and relationships with other microorganisms can modify CH4 emissions. Examples of methane-producing genera include Methanobacterium, Methanobrevibacter, Methanosphaera, and Methanothermobacter.

Methane arises from the fermentation of food in the rumen (87–90%) and in the large intestine (10–13%) of ruminants through the action of methanogenic archaea. Methanogens reduce CO2 to CH4 using hydrogen during the final phase of microbial fermentation in the rumen. From the perspective of reducing methane production, tannins can act directly or indirectly in the rumen. The indirect action is due to their effects in reducing the degradability of plant material. The direct effect occurs on methanogenic microorganisms, inhibiting their activity. Additionally, tannins can reduce the accessibility of methanogens to H₂ by promoting an increase in propionate, a gluconeogenic precursor, whose fermentation pathway, unlike that of other VFAs (volatile fatty acids), does not lead to the release of hydrogen. Therefore, the reduction of methane production through the use of plant extracts rich in tannins can be justified by a direct reduction or inhibition in the population of protozoa and/or methanogens, inhibition of fibrinolytic enzyme activity, digestibility of feed, and alterations in the profiles of VFAs, which are involved in different metabolic pathways and can lead to macroscopic effects in the animal.

Moreover, tannins have an affinity for binding to dietary proteins, thereby reducing excessive protein degradation in the rumen and increasing availability for absorption in the intestines of ruminants. Excess protein in the diets of ruminants, consumed through grazing or unbalanced diets, is degraded into ammonia in the rumen and, at least in part, is not utilized by microorganisms. Ammonia needs to be metabolized in the liver and is excreted largely as volatile urea through urine. Thus, excess protein in the diets of cattle is a burden on metabolism and the environment.

Therefore, dietary supplements that provide the desired effect on reducing proteolysis in the rumen and increasing the availability of this protein in the abomasum are viable and promising alternatives, such as chestnut extract (Castanea sativa), which is mainly composed of hydrolyzable tannins. In an in vitro trial, it was observed that chestnut extract reduces ruminal protein degradation without influencing microbial protein synthesis. In vivo, researchers found that supplementation with chestnut extract reduced nitrogen losses and methane emissions from the manure of dairy cows. Concurrently, milk production was not negatively affected, and no toxic effects or decreases in feed intake were reported in dairy cows.

One way to assess the effect of tannins is through the urinary purine derivatives, which allow estimation of the amount of microbial protein produced in the rumen and digested in the duodenum, as they primarily originate from microbial nucleic acids in the rumen and their derivatives. The concentrations and excretion of urinary nitrogen in ruminants depend partly on the amount of ammonia formed in the rumen, which increases with excess protein available in the diet relative to energy supply. With the reduction of protein degradation in the rumen, other effects may be observed, including reduced nitrogen excretion into the environment, suppression of intestinal parasites, improvements in immune responses, reproductive efficiency, and a slight increase in milk production.

In the literature, the biological response of the interaction of tannins with the microbiota depends on the considered dose level. Hydroalcoholic extracts from chestnut (Castanea spp.), sumac (Rhus typhina), mimosa (Mimosa tenuiflora), and quebracho (Schinopsis balansae), all supplemented at a dosage of 1 mg, inhibited methanogens, while at different doses, they exhibited varying behaviors against Fibrobacter succinogenes, Ruminococcus flavefaciens, and anaerobic fungi.

Tannins: Palatability and Concentration in the Diet

Tannins have been present in the diet of cattle for thousands of years, since plants developed this defense mechanism against predators and microorganisms. An interesting fact is that, despite their long presence in the cattle diet, their effects on dairy cattle are not fully understood, and there is significant variability in the results of experiments testing different levels of tannin inclusion in diets.

Most research reports that tannins in their pure form can cause the development of conditioned food aversion due to their astringent taste. Therefore, an important point to consider when offering tannins to animals is how they will be presented. This aversion to tannin-rich foods is related to the production of proline-rich proteins (PRP) in saliva, which can bind to dietary tannins to inactivate them. It is the binding of these proteins to tannins that produces the astringent taste and subsequent development of food aversion. Cattle and sheep lack PRP; therefore, the decrease in dry matter intake due to the astringent taste mechanism associated with tannins may not occur in sheep and cattle. However, other proteins are present in the saliva of cattle fed tannin-rich diets, which have a high affinity for tannins but are not rich in proline; these salivary proteins tend to form tannin-protein complexes.

Additionally, the composition of the diet directly influences the behavior of tannins in the gastrointestinal tract. Various authors have reported that the results or development of food aversion to diets and reduction in dry matter intake depend on the forage or concentrates used. For example, lactating cows consuming Lotus corniculatus (a forage containing tannins) exhibited greater dry matter intake and lower methane excretion per liter of milk compared to cows fed ryegrass silage (PUCHALA et al., 2005). Other authors, such as Woodward et al. (2001) and Carulla et al. (2005), reported an increase in dry matter intake with supplementation levels of 2.59% and 2.50% of quebracho tannins in the diets.

Regarding milk production effects, studies revealed that the effect of protein precipitation from tannin ingestion, which reduces their degradation in the rumen by the microbial population, resulted in increased milk production in cows (WOODWARD et al., 1999), dairy goats (ROUISSI et al., 2006), and ewes (PENNING et al., 1988). Simultaneously, different doses used by the cited authors provided varying results, reinforcing that the use of tannins is influenced by animal species, environment, diet composition, type and doses of tannins, physiological state of the animals, among others, emphasizing the need for constant technical monitoring to achieve the expected results.

A fundamental issue when using plant extracts as feed additives is the dosage. The lack of standardization among analyses and the use of different standards to express tannin concentrations mean that comparisons between experiments can rarely be made with reasonable confidence.

Similarly to how dietary tannin concentrations can affect the volume of milk produced per cycle, the yield of milk components, as well as the concentrations of milk fat, protein, and lactose, were other factors evaluated by authors, who reported that they were not negatively affected by tannin supplementation. According to Aguerre et al. (2010), the yield and composition of milk fat were not affected by quebracho tannin supplementation up to 1.8% in the diets of dairy cows, and the concentration of milk protein increased with the inclusion of 0.45% tannins, while supplementation with 1.8% reduced the concentration of milk protein.

There are few studies on the impact of tannin-rich plants or extracts on the reproductive performance of adult ruminants. However, short periods of improved nutrient supply through diets containing tannin supplementation before and during reproduction have been shown to influence ovulation rates. These periods also increased the size and/or number of follicles, reduced follicular atresia, altered plasma concentrations of gonadotropins, and increased ovarian sensitivity to gonadotropins. These effects may be related to changes in body weight and condition, energy and protein intake, and absorption of protein from the small intestine (related to tannin supplementation), as well as plasma concentrations of essential amino acids and levels of plasma metabolic hormones, especially insulin.

The action of tannins on reducing embryonic losses may be associated with their ability to precipitate proteins and their availability in the rumen, as a large part of dietary protein is hydrolyzed in the rumen to ammonia, some of which is reincorporated into microbial protein. Excess ammonia is absorbed from the rumen and metabolized to urea in the liver, leading to increased plasma concentrations of ammonia and urea, which may increase the number of early embryonic losses. Overall, the provision of tannins at low concentrations has positive effects on the reproduction of dairy cows.

In conclusion, technical support is extremely important for monitoring the use of tannins, as the effects on rumen fermentation modulation, nutrient utilization, and the performance of ruminants are likely due to the great diversity in the structural characteristics and, consequently, the reactivity of these compounds. The correct selection of dosages is another important issue due to the difficulty in choosing concentrations that positively affect a specific parameter without causing a negative response in others. Rely on our team of specialists to find a solution that best fits your productive realities. Contact us to learn more about the solutions involving hydrolyzable tannins available in our portfolio.

Supplementary Feed for Dairy Cows

Farmatan D is a combination of ellagitannins and essential oils that promotes the reduction of protein degradation in the rumen and increases the utilization of dietary proteins, leading to enhanced milk production, improved milk quality, and better liver health with lower protein inclusion in the diet. Farmatan D also reduces the number of Clostridia spp. in the digestive system, positively shaping the bacterial population.

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