Potential and challenges of tannins as an alternative to in-feed antibiotics for farm animal production

Huang, Qianqian; Liu, Xiuli; Zhao, Guoqi; Hu, Tianming; Wang, Yuxi

Tannins especially CT are widely distributed in nutritionally important forages, trees, shrubs and legumes, which are commonly consumed by ruminants. Therefore, the effects of CT on ruminant nutrition, health and production have been extensively studies and reviewed (Frutos et al., 2004; Mueller-Harvey, 2006; Waghorn, 2008; Patra and Saxena, 2011; Wang et al., 2015) . Condensed tannins can have beneficial or detrimental effects on ruminants, depending on their amount consumed by animals, their type and chemical structure as well as the composition of the rest of the diet, especially CP concentration of the diet (Mueller-Harvey, 2006) . It is generally believed that CT in temperate forage in low to medium (<50 g/kg DM) concentration benefit ruminants in terms of improving protein utilization without negatively affecting feed intake and nutrient digestion (Barry and Mcnabb, 1999; Waghorn, 2008) , depending on CT source and analytical method/standard used to determine concentration. Protein precipitation capacity, anti-microbial, anti-parasitic and anti-oxidant activities are the most relevant properties of tannins to be considered for their uses in ruminant animals. By summarizing numerous researches, Waghorn (2008) has concluded that when forages are fed as a sole diet, the CT in L. corniculatus (about 30 g CT/kg DM) have been beneficial for ruminant production, but the CT in sainfoin, Hedysarum coronarium and L. pedunculatus (concentration generally greater than 50 g/kg DM) do not appear to benefit productivity other than by mitigating the impact of parasites. In contrast to temperate farming, the CT in browse, typical of warm and hot climates, are nearly always detrimental to ruminants, except for reducing internal parasite numbers (Waghorn, 2008) . Condensed tannins at low to medium concentrations benefit ruminant production efficiency because CT reduce protein degradation in the rumen, increase the amount of dietary protein reaching small intestine for absorption (Wang et al., 1994 (Wang et al., , 1996 . At high concentration, however, CT would impede feed intake due to their astringent nature and reduce protein and other nutrients digestion by "over" protecting protein, decrease rumen microbial activity and inhibit endogenous digestive enzyme activities thereby negatively affect animal performance. The dietary concentrations CT that exert the negative effective on animal performance again depend on CT sources (i.e., chemical compositions and structures). Therefore, the majority of the researches have focused on screening and evaluation of different tannins sources and to define their optimum concentrations in ruminant diets. These include to screen and identify the potential tannin-containing forages (e.g., L. corniculatus, L. pedunculatus, sainfoin) that could be incorporate into animal diets (Barry and Mcnabb, 1999; Berard et al., 2011; Acharya et al., 2013) in temperate farming, to define the optimum supplementation rates of varying external tannins (e.g., quebracho tannin, tannic acid) on animal performance (Dschaak et al., 2011; Anantasook et al., 2015; Rivera-M endez et al., 2017) in intensive feeding operation, and to develop technologies (e.g., alkaline treatment, tannin-binding agent, diet mixing) in utilizing the tannin-rich tree leaves and shrubs in tropical and other areas where feed sources are limited (Murdiati et al., 1990; Smith et al., 2005; Wina et al., 2005; Brown and Ng'ambi, 2017) .