The mechanisms and effects of tannin-ruminant interactions are so varied that precise predictions of animal performance (i.e. milk, meat or wool production) as influenced by
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tannin consumption cannot be easily made. There are huge varieties of tannin-nutrients, tannin-micro-organisms and tannin-animals interactions that it would appear inaccurate to attempt to locate an explanation for these contrasting occurrences. A number of studies that report positive effects of tannin in terms of animal productivity can be found in the literature. That may be due to moderate levels of tannins, since better animal performance is associated with moderate levels of tannins in the feed which has been ascribed to the protection of dietary protein from microbe degradation in the rumen, thus an increase in flow of dietary protein to the small intestine and then an increase in the chance of the amino acid absorption into blood (Makkar, 2003; Patra & Saxena, 2011). Aerts et al.
(1999) concluded that moderate amounts of CT (20–40 g kg−1 DM) in forages may exert beneficial effects on protein metabolism. Driedger and Hatfield (1972) found increased daily weight gains (DWG) in lambs fed a corn based diet supplemented with tannin-treated (10% of Tara tannin) soybean meal (DWG: 217 g) as compared to lambs receiving supplemental urea (DWG: 112 g) or soybean meal with no tannin added (DWG: 117 g).
Wang et al. (1996b) reported that lambs grazing L. corniculatus obtained better wool growth and carcass gain compared to those grazing lucerne, that was attributed to the presence of CT (34 g kg-1) in Lotus corniculatus. Terrill et al. (1992a) found that tannins (4.0-5.0% of dry matter) in sulla (Hedysarum coronarium) increased wool growth rate in sheep during spring and early summer as compared with sheep grazing the same pastures but given a daily dose of polyethylene glycol while no differences in body growth or voluntary feed intake. When treated lucerne hay with quebracho tannins 20 g kg-1 (equivalent to 15 g CT kg-1) fed to lamb, observed increase in body weight gain and feed conversion efficiency compared with control fed untreated lucerne hay (Al-Dobaib, 2009).
Moreover, when L. corniculatus fed to lactating ewes, milk yield was increased by 21%
during mid and late lactation (weeks 6-11) (Wang et al., 1996a). Similar result was found with dairy cows (milk yields (kg cow-1 day-1)) fed on L. corniculaus (21.24) compare to those fed on L. corniculatus plus PEG (to bind tannins) (18.61), ryegrass (15.53) and ryegrass plus PEG (15.49), representing that CT in L. corniculatus resulted in increased milk yield (Woodward et al., 2000). West et al. (1993) reported that an increase in DM intakes, milk yield and milk fat content when lactating dairy cows fed diets containing 8–
16% peanut skins (180 g CT kg−1).
Tannins seem to have more advantage in the control of nematode internal parasites. Ahmed (2010) conducted two grazing experiments to evaluate dosing tannins rates for controlling
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parasite in sheep. In both experiments, all dosing rates reduced the larvae counts compared to the control sheep. Ramírez-Restrepo et al. (2005a, b) recorded increase in body weight gain, decrease in parasite, improved reproductive performance and wool production in lambs grazing L. corniculatus than perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) pasture. That could be probably due to increased metabolisable protein supply, by dissociated tannin-protein complexes in the small intestine. Besides, the potential biological effects of CT on gastrointestinal parasites.
Another field that has much consideration concerning tannin-rich forages is to prevent bloat in cattle. Grazing on alfalfa and grass pastures has been frequently associated with the occurrence of bloat in cattle. The soluble proteins in these pastures are involved as surfactants responsible for the stable foams that grow in the rumen of animals suffering from bloat (Jones & Mangan, 1977). This foam captures rumen fermentation gases and the free gas space generally occur in the dorsal sac of the rumen is changed by a frothy volume of rumen digesta. Unless this condition is relieved, the animal suffers severe compression of heart and lungs, leading to anorexia and rapid death. However, forages release great amounts of soluble protein into the rumen by binding it with their content of condensed tannins (Patra & Saxena, 2011). Li et al. (1996) reported that most legume species are bloat-safe, since tannins in the rumen seem to reduce the solubility of protein by forming tannin-protein complexes, accordingly limiting the growing of bloat foam. In summary, it has been reported that an amount of 1.1 g CT kg-1 DM in the plant is sufficient to prevent the occurrence of bloat (Stockdale, 1994; Li et al., 1996).
A number of studies that report negative effects of tannin in terms of animal productivity can be found in the literature. Aerts et al. (1999) reviewed that high dietary CT contents (60–120 g kg−1 DM) in forages may reduce voluntary feed intakes, digestive efficiency and animal productivity. A decreased rate of body weight gain and wool growth has been reported by Barry (1985) on sheep fed L. pedunculatus (76–90 g CT kg−1 DM), which could be possibly due to the presence of high concentrations or different types of CT in forages (Waghorn & Shelton, 1997; Mueller-Harvey, 2006). Kaitho et al. (1998b) reported a decrease in the live weight gain of sheep with increased tannin levels, when they used six accessions of the tropical legume Sesbania sesban. Wiegand et al. (1996) reported that the high concentration of CT in Carissa edulis and Dichrostachys cinerea led to lower intake, digestibility and gain rate (6 g/day) whilst fed with maize and high weight loss (-63 g/day)
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without maize. Prasad et al. (1997) found high wool production from sheep fed tannin- containing forage with PEG compared to those without PEG. Barry (1984) assumed that the decrease in wool production due to tannins may be for two reasons: firstly, reduced protein absorption, which restricts the quantity of sulphur amino acids accessible for wool growth; secondly, an increase of plasma growth hormone concentration which would divert amino acid away from wool synthesis. However, Wang et al. (1994) appeared to suggest that tannins increase the availability of sulphur for body synthetic reactions.
Similarly, other researchers have reported no changes in the levels of growth hormone because of tannins (Waghorn et al., 1994b; Wang et al., 1996a). Grainger et al. (2009) reported that dosed daily with 163–326 g CT from Acacia mearnsii to dairy cows led to decrease in milk yield (from 33.0 to 31.8-29.8 kg cow-1 day-1), fat and protein percentage of milk as well as reduced feed intake and digestibility. Certainly, a better understanding of tannin effects in ruminants is required to offer opportunity to make adequate use of the property of tannins in improving animal production.