**6. Plant metabolites**

*Livestock Health and Farming*

this basis are:

used are listed as follows:

ruminants [45].

which do not compete with human feeding. Therefore, diverse advantages can be observed when utilizing alternative forages such as (a) a considerable reduction in the feeding source costs; (b) exploitation of nutrients contained in agriculture by-products which otherwise would not be used (these by-products could be high in rapidly digestible carbohydrates or in fiber, both suitable for ruminants); and (c) an increase in the by-product cost which eventually will create economic benefits for producers and the productivity chain supply. Moreover, some agricultural by-products do not need any processing to be offered as animal feeding, hence the desirability of these by-products. Some of the ruminants feeding produced under

a.Crop by-products such as garlic leaves, onion leaves, cocoa husks, coconut meal, cracked rice, sugarcane bagasse, molasses, tapioca discards, oat straws,

b.Perennial crops, seeds, and leaves of shrubs and trees like *Leucaena*, gua-

Some farmers are still not aware of the nutritional value contained in byproducts or in the form to be included into the productivity chain of animal feeding in an efficient way. In this regard, Asia and Africa are heavily focused on attempting to reach this goal. Due to the nature of some agricultural by-products, these tend to decompose in a short time. Hence, some techniques should be used to preserve and increase their shelf life. Therefore, some of the preservation techniques commonly

I. *Silages*: Many of the agricultural by-products are obtained in huge quantities due to the nature of the crops. However, the high humidity contents contribute to a short lifetime due to the rapidly appearance of fungi and, eventually, a decomposition. Therefore, the silage elaboration is a recommended preservation method due its large periods of storage, and it can always be offered fresh and with certain aroma provided from the fatty acids synthetized in the lactic fermentation which will add palatability for

II.*Chemical treatments*: Some agricultural residues obtained from cereal crops are treated with chemicals to increase their digestibility. Thus, by-products with high lignocellulosic complexes could be treated with ammonia in anaerobic conditions to enhance lignin and fiber hydrolysis, which will improve their digestibility [46]. However, this process requires special plastic sheets that increases costs and could become an unaffordable process for small producers. In this way, previous researches have reported diverse alternatives using mud and eliminating the use of plastic sheets [47].

III.*Multi-nutritional blocks*: Another conservation technique which involves the utilization of high humidity agricultural by-products is the elaboration of multi-nutritional blocks [48]. This technology is very flexible and allows the producer to use ingredients considered as indispensable in animal feeding. Additionally, important nutrients could be available for longer periods of time since the useful life of these blocks is very extensive. Although in dry seasons, drought decreases considerably the nutritional quality of forages. These blocks are generally offered as supplementation in livestock feeding in rangelands as part of an extensive feeding system, and they are commonly

and some aquatic crops like water hyacinth and azolla [38–41]

muchil, mesquite, mango, ebony, etc. [42–44]

**124**

In the last years, ruminants have been target of several feeding strategies aiming to reduce ruminal methane production and emissions; most of them have been stated earlier in this chapter. However, the use of secondary metabolites arises as a viable and newer alternative in this concern. There is evidence which proves certain secondary metabolites, such as condensed tannins, saponins, and alkaloids, reduce methane production in in vivo and in vitro assays [54]. Generally, the mechanisms of action of these compounds point out to certain metabolic pathways:

I.**Tannins**: Tannins are water-soluble polyphenol polymers with a high and diverse molecular weight. They can form complexes with proteins, mainly, and metal ions, amino acids, and polysaccharides in a lesser extent. These metabolites are normally synthetized in shrubs, trees, legumes, fruits, cereals, and grains [55]. Tannins are divided into two groups: condensed tannins (CTs) and hydrolysable tannins (HTs).

*Hydrolysable tannins*: These are complex molecules attached to a polyol group as a central core which are partially or fully esterified with a phenolic group (e.g., gallic acid). The remaining phenolic groups could be later esterified or oxidized to produce more complexes with HTs [55].

*Condensed tannins*: These compounds are also known as proanthocyanidins and are mainly polymers of the flavan-3-ol units which are bind by interflavonoids C3-C8 and C4-C6 linkages, such as catechin and epicatechin. The methanogenic activity conferred to tannins is mostly due to the condensed tannins; CTs attach to proteins and avoid their degradation in rumen. Additionally, CT decrease methanogenesis through a reduction in fiber digestion [56]. Some studies affirm that CT enhances acetate formation via acetogenesis; this metabolic pathway uses hydrogen for acetate synthesis and reduces methanogenesis [57].

II. **Saponins**. According to their chemical structure, they are divided into two groups: steroids and triterpenoids. Steroids are predominantly in

plants and are composed of 27 carbon atoms in the central skeleton of its molecule (e.g., spirostanol and furostanol). Otherwise, triterpenoids are composed mainly of aglycones with 30 carbon atoms in its molecule (e.g., oleanane) [58]. These are the most common types of saponins, especially in legumes [59]. Methanogenic action of saponins occurs by protozoa defaunation which is associated to methanogens. Moreover, saponins enhance production of propionate, a natural competitor of methane in hydrogen capture [58]. Nevertheless, some studies affirm that methane inhibition action by saponins is dose and time dependent and not conclusive [59].

III.**Flavonoids**. Flavonoids are phenolic compounds (like tannins); however, these contain only 15 carbon atoms linked to 2 aromatic rings connected through a 3-carbon bridge [60]. These metabolites are particularly studied for human purposes, and their biological benefits to health correlated to their consumption [61]. Almost all flavonoids are conjugated to glycosides and are common to find hydroxyl groups in carbons with four, five, and seven positions [60]. In addition, flavonoids stimulate microbial metabolism and reduce methane production through enhancing acetogenesis pathway and increasing hydrogen capture in propionate anabolism, in a similar way as described earlier with saponins [28, 59].
